HABITS. "How shall I a habit break?" As you did that habit make. As you gathered, you must lose; As you yielded, now refuse. Thread by thread the patient hand Till they bind us, neck and wrist; Thread by thread the paitent hand Must untwine ere free we stand. As we builded, stone by stone, We must toil unhelped, alone, Till the wall is overthrown. But remember, as we try, Lighter every test goes by; Wading in. the stream grows deep Toward the center's downward sweep; Backward turn, each step ashore Shallower is than that before. Ah, the precious years we waste Leveling what we raised in haste! Doing what must be undone Ere content or love be won! First across the gulf we cast Kite-borne threads, till lines are passed And habit builds the bridge at last! JOHN BOYLE O'REILLY THE CELL BY THE SAME AUTHOR Text-book of the Embryology of JWan and fflammals Translated from the Third German Edition by EDWARD L. MARK, Ph.D., Hersey Professor of Anatomy in the Harvard University. With 339 Figures and 2 Lithographic Plates. 2is. ($5.25) LONDON: SWAN SONNENSCHEIN & CO NEW YORK : MACMILLAN & CO JHE CELL! OUTLINES OF GENERAL ANATOMY AND PHYSIOLOGY BY ^ DR. OSCAR HlTRTWIG Professor Extraordinarius of Anatomy ami Comparativt Anatomy, Director of the II. Anatomical Institute of the University of Berlin TRANSLATED BY M. CAMPBELL, AND EDITED BY HENRY JOHNSTONE CAMPBELL, M.D Assistant Physician to the City of London Hospital for the Diseases of the Chest and to the East London Hospital for Children Senior Demonstrator of Biology and Demonstrator of Physiology in Guy's Hospital ILLUSTRATIONS SWAN SONNENSCHEIN & CO NEW YORK : MACMILLAN & CO 1895 BUTLER & TANKER, THE SELWOOD PRINTING WORKS FROME, AND LONDON. TO HIS FRJEXD AXD COLLEAGUE W. WALDEYER AH C.3 AUTHOR'S PREFACE " Each living being must be considered a microcosm, a small universe, which is formed from a collection of organisms, which reproduce themselves, which are extremely small, and which are as numerous as the stars in heaven." Darwin. A GLANCE at the numerous text-books on histology shows us that many questions of great interest in scientific investigation are scarcely mentioned in them, whilst many branches of knowledge which are closely connected with histology are more or less excluded. The student is taught the microscopic appearances which are presented by the cell and the tissues, after these have been prepared according to the different methods which are most suitable to each, but he is taught very little of the vital properties of the cell, or of the marvellous forces which may be said to slumber in the small cell-organism, and which are revealed to us by the phenomena of protoplasmic movements, of irritability, of metabolism, and of reproduction. With regard to the different subjects which he studies, if he wish to be in touch with the progress of science, and to understand the nature and attributes of the cell-organism, he must read the works of specialists. It is not difficult to discover the reason for this ; it is chiefly due to the division of what was previously one subject into two, namely, into anatomy and physiology. This sub-division has been extended to the cell, and, it seems to me, with rather un- fortunate results ; for the separation which, in spite of the many disadvantages which are naturally attached to it, is in many respects a necessity in the investigation of the human body as a whole, is not practicable in the study of cells, and has in reality only brought about the result, that the physiology of the cell has been dogmatically treated as a part of descriptive anatomy, rather than as a science, and that in consequence much that the diligence of scientists has brought to light is barren of results. In this book I have avoided the beaten track, and in order to emphasise this Viii AUTHOR S PREFACE fact, I have added to the principal title of the whole work, "The Cell and the Tissues," the secondary title " Outlines of General Anatomy and Physiology." Farther, I ana able to say, as I said of my Text-book of Embryology : Man and Mammals, that it has been produced in close connection with my academical labours. The contents of the first part, in which I have endeavoured to sketch a comprehensive picture of the structure and life of the cells, were the subject of two lectures which I delivered at the University of Berlin four years ago, under the titles of " The Cell and its Life," and " The Theory of Generation and Heredity." Besides wishing to communicate to a larger circle of readers the views which I had often expressed verbally, I had the further desire of giving a comprehensive review of results obtained by private research, some of which were recorded in various Journals, whilst others appeared in the six papers on " The Morphology and Physiology of the Cell," which I wrote in conjunction with my brother. Finally, a third reason which induced me to write this book was, that it should supplement my Text-book of Embryology: Man and Mammals. In it I have endeavoured to state the laws which underlie animal formation, according to which cells, formed from the fertilised egg-cell by repeated division, split up, as a result of unequal growth, the complicated layers and outgrowths into germinal folds, and finally into individual organs. In addition to the distribution of cell-masses and to the arrangement of cells, that is to say, in addition to the morpho- logical differentiation, a second series of processes, which may be grouped together under the term histological differentiation, takes place during development. By means of histological differentia- tion, the morphologically separated cell material is capable of performing the different functions into which the vital processes of the developed collective organism may be divided. In my Text-book of Embryology, it was impossible to deal ex- haustively with the second or more physiological side of the pro- cess of development. The Anatomy and Physiology of the Cell, forms a necessary complement to it, as I mentioned above. This will be especially noticed by the student in the first part of the book, which deals with the cell alone. For not only is there, in the seventh chapter, a detailed description of the anatomy and physiology of reproduction, which is ultimately a cell pheno- menon, but at the end of the book, in the ninth chapter, there AUTHOR S PREFACE IX is a section entitled " The Cell as the Elemental Germ of an Organism," in which both the older and more recent theories of heredity are dealt with. The second part of the complete work, which is to deal with the tissues, will be of about the same length, and will form to a greater extent a supplement to the Text-book of Embryology. For in addition to a description of the tissues, especial emphasis will be laid upon their origin of histogenesis and upon the physiological causes which underlie the formation ; the other side of the process of development, that is to say, histological differentiation, will also be discussed. In the account, which I have endeavoured to make as intelligible as possible, scientific views have primarily guided me. What I have striven to do to the best of my ability is, to fix the scientific stand-point occupied at present by the doctrines of cell and tissue formation. Further, I have tried to delineate the historical course of the development of the more important theories. With regard to disputed points I have frequently compared various opinions. If, as is natural, I have placed my own views in the foreground, and, moreover, if I have occasionally differed from the views and explanations of prominent and highly-esteemed scientists whose opinions I value extremely, it is only due to them to say that I do not on that account consider the conceptions preferred by me to be unconditionally correct, still less do I wish to belittle the views from which I differ. Antagonistic opinions are necessary to the life and development of science ; and, as I have observed in studying the history of the subject, science progresses most rapidly and successfully in proportion to the diversity of the opinions held by different authorities. As is only human, almost all observations and the conclusions deduced from them are one- sided, and hence continually need correction. How necessary then must this be in the subject of the present inquiry, that is to say, in the cell, which is a marvellously complicated organism, a small universe, into the construction of which we can only laboriously penetrate by means of microscopical, chemico-physical and experi- mental methods of inquiry. OSCAR HEKTWIG. Berlin, October, 1892. EDITOR'S PREFACE THE translation of Professor Hertwig's book Las been no easy task. The extreme complexity of much of the matter treated, in addition to the large number of subjects referred to, has often rendered it extremely difficult to express the author's meaning in readable English. Of one thing there can be no doubt, and that is, that the subject matter is of very great importance ; moreover, it cannot but prove most useful to the student who does not read German fluently, to possess in English so comprehensive an account of the Anatomy and Physiology of the Cell, as the one contained in Professor Hertwig's book. In many cases it has been extremely difficult to find equivalents for terms used in the German. Amongst these the word " Anlage " may be specially mentioned. Various terms have been used by different translators to express the meaning of this word, but none of them seems to be applicable to all cases. Professor Mark has introduced the word "fundament," and Mr. Mitchell has suggested the term " blast," but neither of these appears to express the meaning of the German word sufficiently accurately to justify the use of either of them exclusively. Hence, we thought it best in some cases to employ the somewhat cumbrous expression, " elemental germ," although it is undoubtedly open to objection ; however, it frequently seemed to us to convey the author's idea most correctly. On other occasions we have thought better to make use of a paraphrase. Several additions have been made to the Bibliography of papery Xll EDITOR S PREFACE that the English student might wish to consult. The frequent quotations from English authors have in most cases been verified by reference to the originals ; but in some cases, despite careful search, we have been unable to find the passages in question. H. JOHNSTONE CAMPBELL. 54, Welbeck Street, London, W. CONTENTS CHAPTER I. PAGE Introduction 1 The History of the Cell Theory 2 The History of the Protoplasmic Theory 6 Literature 9 CHAPTER II. THE CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES OF THE CELL . 11 I. The Chemico-physical and Morphological Properties of the Proto- plasm 11 (a) Justification of the Use of the Term Protoplasm ... 12 (b) General Characteristics of Protoplasm 13 (c) Chemical Composition of Protoplasm 15 (d) The more minute Structure of Protoplasm .... 18 (e) Uniformity of Protoplasm. Diversity of the Cell ... 26 (/) Various examples of the Structure of the Cell-body . . 27 1. Cells consisting almost entirely of Protoplasm . . 27 2. Cells which contain several different substances in their Protoplasm 31 II. The Chemico-physical and Morphological Properties of the Nucleus 30 (a) The form, size and number of Nuclei 37 (b) Nuclear Substance 40 (c) The Structure of the Nucleus. Examples of its various Properties . ........ 45 III. Are there Elementary Organisms existing without Nuclei ? . . 54 IV. ') he Central or Pole Corpuscles of the Cell 55 V. Upon the Molecular Structure of Organised Bodies ... 58 Literature 61 CHAPTER III. TUB VITAL PROPERTIES OF THE CELL 65 The Phenomena of Movement 65 I. Protoplasmic Movements ........ 66 (a) The Movements of naked Protoplasm 66 (b) The Movements of Protoplasm inside the Cell-Membrane . 71 (c) Theories concerning Protoplasmic Movements ... 73 iv CONTENTS PAGB II. Movements of Flagella and Cilia 77 (a) Cells with Flagella 79 (b) Cells with numerous Cilia 83 III. The Contractile Vacuoles, or Vesicles, of Unicellular Organisms . 85 IV. Changes in the Cell during passive movement .... 88 Literature 89 CHAPTEE IV. THE VITAL PBOPEKTIES OF THE CELL ....... 91 Phenomena of Stimulation 91 I. Thermal Stimuli .94 II. Light Stimuli 99 III. Electrical Stimuli . 106 Phenomena produced by Galvanotropism . . . . . . 108 IV. Mechanical Stimuli 110 V. Chemical Stimuli Ill (a) Chemical Stimuli which affect the whole body . . . 112 (b) Chemical Stimuli which come into contact with the Cell- body at one spot only 115 1. Gases 115 2. Liquids 117 Literature . 123 CHAPTER V. THE VITAL PROPERTIES OF THE CELL 126 Metabolism and Formative Activity 126 1. Absorption and Excretion ........ 128 1. The Absorption and Excretion of Gaseous Material . . 128 2. The Absorption and Excretion of Fluid Substances . . 133 3. The Absorption of Solid Bodies . . . . .141 II. The Assimilative and Formative Activity of the Cell . . .145 1. The Chemistry of Assimilation ^ 146 2. The Morphology of Metabolism 154 (a) Internal Plasmic Products 154 (b) External Plasmic Products 166 Literature - .174 CHAPTER VI. THE VITAL PHENOMENA OF THE CELL 177 Reproduction of the Cell by division 177 I. History of Cell-formation 177 II. Nuclear Division . 179 PAGE 1. Nuclear Segmentation. Mitosis (Flemming) ; KaryokineBis (Schleicher) 179 (a) Cell division as it occurs in Salamandra maculata . . 179 First Stage. Preparation of the Nucleus for Division. 182 Second Stage of Division 185 Third Stage of Division 187 Fourth Stage of Division 188 (b) Division of the Egg-cells of Ascaris mcgalocephala and Toxopneustes lividus 189 (c) Division of Plant Cells 196 (d) Historical remarks and unsolved problems concerning Nuclear Segmentation 199 2. Direct Nuclear Division. Fragmentation. Amitosia . . 207 3. Endogenous Nuclear Multiplication, or the Formation of Multiple Nuclei . . .211 III. Various methods of Cell Multiplication 213 1. General Laws 213 2. Review of the Various Modes of Cell Division . . . 223 In. Equal Segmentation 224 Ib. Unequal Segmentation 225 Ic. Cell-Budding 228 2. Partial or Meroblastic Segmentation .... 230 3. So-called Free Cell Formation 232 4. Division with Reduction 235 IV. Influence of the Environment upon Cell Division. Degeneration . 239 Literature . 246 CHAPTER VII. THE VITAL PROPERTIES OF THE CELL 252 The Phenomena and Methods of Fertilisation 252 I. The Morphology of the Process of Fertilisation .... 256 1. The Fertilisation of the Animal Egg 256 (a) Echinoderm Eggs . . . . *. . .257 (b) Eggs of Ascaris megalocephala ..... 259 2. The Fertilisation of Phanerogamia 263 3. The Fertilisation of Infusoria 265 4. The various forms of Sexual Cells ; equivalence of partici- pating Substances during the Act of Fertilisation ; Con- ception of Male and Female Sexual Cells . . . 272 5. Primitive and Fundamental modes of Sexual Generation and the first appearance of Sexual Differences . . 278 II. The Physiology of the Process of Fertilisation .... 290 1. The Need of Reproduction of Cells 291 (a) Parthenogenesis 295 (6) Apogamy 300 2. Sexual Affinity 300 vi CONTENTS PAOF. (a) Sexual Affinity in general 301 (6) More minute discussion of Sexual Affinity, and its different gradations 305 a. Self-fertilisation . . . ' . . . .306 /3. Bastard Formation, or Hybridisation . . . 310 y. The Influence of Environment upon Sexual Affinity 313 S. Recapitulation and Attempted Explanations . . 316 Literature 320 CHAPTER VIII. METABOLIC CHANGES OCCURRING BETWEEN PROTOPLASM, NUCLEUS AND CELL PRODUCTS 323 I. Observations on the Position of the Nucleus, as an indication of its participation in Formative and Nutritive Processes . . . 324 II. Experiments proving Reciprocal Action of Nucleus and Protoplasm 330 Literature . 332 CHAPTER IX. THE CELL AS THE ELEMENTARY GEKM OF AN ORGANISM. THEORIES OF HEREDITY 334 I. History of the older Theories of Development .... 335 II. More Recent Theories of Reproduction and Development . . 339 III, The Nucleus as the Transmitter of Hereditary Elemental Germs . 344 1. The Equivalence of the Male and Female Hereditary Masses 345 2. The equal Distribution of the Multiplying Hereditary Mass 346 3. The Prevention of the Summation of the Hereditary Mass . 350 4. Isotropy of Protoplasm 354 IV. Development of the Elemental Germs .357 Literature 361 Index 363 THE CELL CHAPTER I INTBODUCTION BOTH plants and animals, although they differ so widely in their external appearance, are fundamentally similar in their anatomical structure ; for both are built up of similar elementary units, which, as a rule, are only to be seen with the microscope. These units, in consequence of a hypothesis which was once believed in, but is now discarded, are called cells ; and the view that plants and animals are built up in a similar manner of these extremely minute particles is called the cell-theory. The cell-theory is rightly considered to be one of the most important and funda- mental theories of the whole science of modern biology. In the study of the cell, the botanist, the zoologist, the physiologist, and the pathologist go hand in hand, if they wish to search into the vital phenomena which take place during health and disease. For it is in the cells, to which the anatomist reduces both plant and animal organisms, that the vital functions are executed ; they, as Vii'chow has expressed it, are the vital elementary units. Regarded from this point of view, all the vital processes of a complex organism appear to be nothing but the highly-developed result of the individual vital processes of its innumerable variously functioning cells. The study of the processes of digestion, of the changes in muscle and nerve cells, leads finally to the examination of the functions of gland, muscle, ganglion, and brain. And just as physiology has been found to be based upon the cell- theory, so has the study of disease been transformed into a cellular pathology. Hence, in many respects, the cell-theory is the centre around which the biological research of the present time revolves. Further, it forms the basis of the study of minute anatomy, now more commonly called histology, which consists in the exami- nation of the composition and minute structure of the organism. The conception or idea connected with the word " cell," used scientifically, has been considerably altered during the last fifty years. The history of the various changes in this conception, or the history of the cell-theory, is of great interest, and nothing could be more suitable than to give a short account of this history in order to introduce the beginner to the series of conceptions connected with the word " cell " ; this, indeed, may prove useful in other directions. For whilst, on the one hand, we see how the conception of the cell, which is at present accepted, has developed gradually out of older and less complete conceptions, we realise, on the other hand, that we cannot regard it as final or perfect ; but, on the contrary, we have every ground to hope that better and more delicate methods of investigation, due partly to improved optical instruments, may greatly add to our present knowledge, and may perhaps enrich it with a quite new series of conceptions. The History of the Cell-Theory. The theory, that organ- isms are composed of cells, was first suggested by the study of plant- structure. At the end of the seventeenth century the Italian, Marcellus Malpighi (I. 15), and the Englishman, Grew (I. 9), gained the first insight into the more delicate structure of plants ; by means of low magnifying powers they discovered, in the first place, small room-like spaces, provided with firm walls, and filled with fluid, the cells ; and in the second, various kinds of long tubes, which, in most parts of plants, are embedded in the ground tissue, and which, from their appearance, are now called spiral ducts or vessels. Much greater importance, however, was attached to these facts after the investigations, which were carried on in a more philo- sophical spirit by Bahn towards the end of the eighteenth century, were published. Caspar Friedrich Wolff (I. 34, 13), Oken (I. 21), and others, raised the question of the development of plants, and endeavoured to show that the ducts and vessels originated in cells. Above all, Treviranus (I. 32) rendered important service by proving in his treatise, entitled Vom inwendigen Bau der Gewachse, published in 1808, that vessels develop from cells ; he discovered that young cells arrange themselves in rows, and become transformed, by the breaking down of their partition walls, into elongated tubes ; this discovery was confirmed and established as a scientific fact by the subsequent researches of Mohl in 1830. THE HISTORY OF THE CELL-THEORY 3 The study of the lowest plants has also proved of the greatest importance in establishing the cell-theory. Small algae were observed, which during their whole lifetime remain either single cells, or consist of simple rows of cells, easily to be separated from one another. Finally, the study of the metabolism of plants led investigators to believe that, in the economy of the plant, it is the cell which absorbs the nutrient substances, elaborates them, and gives them up in an altered form (Turpin, Raspail). Thus, at the beginning of our century, the cell was recognised by many investigators as the morphological and physiological elementary unit of the plant. This view is especially clearly expressed in the following sentences, quoted from the Text-look of Botany (I. 16),. published by Meyen in 1830: " Plant-cells appear either singly, so that each one forms a single individual, as in the case of some algae and fungi, or they are united together in greater or smaller masses, to constitute a more highly-organized plant. Even in this case each cell forms an independent, isolated whole ; it nourishes itself, it builds itself up, and elaborates the raw nutrient materials, which it takes up, into very different sub- stances and structures." In consequence, Meyen describes the single cells as " little plants inside larger ones." These views, however, only obtained general acceptance after the year 1838, when M. Schleiden (I. 28), who is so frequently cited as the founder of the cell-theory, published in Muller's Archives his famous paper "Beitrage zur Phytogenesis." In this paper Schleiden endeavoured to explain the mystery of cell-formation. He thought he had found the key to the difficulty, in the discovery of the English botanist, R. Brown (I. 5), who, in the year 1833, whilst making investigations upon orchids, discovered nuclei. Schleiden made further discoveries in this direction ; he showed that nuclei are present in many plants, and as they are invariably found in young cells, the idea occurred to him, that the nucleus must have a near connection with the mysterious beginning of the cell, and in consequence must be of great importance in its life- history. The way in which Schleiden made use of this idea, which was based upon erroneous observations, to build up a theory of phyto- genesis, must now be regarded as a mistake (I. 27) ; on the other hand, it must riot be forgotten that his perception of the general importance of the nucleus was correct up to a certain point, and that this one idea has in itself exerted an influence far beyond the narrow limits of the science of botany, for it is owing to this that the cell-theory was first applied to animal tissues. For it is just in animal cells that the nuclei stand out most distinctly from amongst all the other cell-contents, thus showing most evidently the similarity between the histological elements of plants and animals. Thus this little treatise of Schleiden's, in 1838, marks an important historical turning-point, and since this time the most important work, in the building up of the cell-theory, has been done upon animal tissues. Attempts to represent the animal body as consisting of a large number of extremely minute elements had been made before Schleiden's time, as is shown by the hypotheses of Oken (I. 21), Heusinger, Raspail, and many other writers. However, it was impossible to develop these theories further, since they were based upon so many incorrect observations and false deductions, that the good in them was outweighed by their errors. It was not until after some improvements had been made in optical instruments, during the years from 1830-1840, that work justifying the application of the cell-theory to animal tissues was accomplished. Purkinje (I. 22) and Valentin, Joh. Miiller (I. 20) and Henle (I. 11), compared certain animal tissues with plant tissues, and recognized that the tissue of the chorda dorsal is, of cartilage, of epithelium and of glands, is composed of cells, and in so far is similar in its construction to that of plants. Schwann (I. 31), however, was the first to attempt to frame a really comprehensive cell-theory, which should refer to all kinds of animal tissues. This was suggested to him by Schleiden's " Phytogenesis," and was carried out by him in an ingenious manner. During the year 1838 Schwann, in the course of a conversation with Schleiden, was informed of the new theory of cell-formation, and of the importance which was attached to the nucleus in plant- cells. It immediately struck him, as he himself relates, that there are a great many points of resemblance between animal and vegetable cells. He therefore, with most praiseworthy energy, set on foot a comprehensive series of experiments, the results of which he published in 1839, under the title, Mikroscopische unter suchung en uber die Uebereinstimmung in der Structur wtd dem Wachsthum der Thiere und Pflanzm. This book of Schwann's is of the greatest importance, and may be considered to mark an epoch, for by its means the knowledge of the microscopical THE HISTORY OF THE CELL-THEORY 5 anatomy of animals was, in spite of the greater difficulty of observation, immediately placed upon the same plane as that of plants. Two circumstances contributed to the rapid and brilliant result of Schwann's observations. In the first place Schwann made the greatest use of the presence of the nucleus in demonstrating the animal cell, whilst emphasizing the statement that it is the most characteristic and least variable of its constituents. As before mentioned, this idea was suggested to him by Schleiden. The second, no less important circumstance, is the accurate method which Schwann employed in carrying out and recording his obser- vations. As the botanists by studying undeveloped parts of plants traced the development of the vessels, for instance, from primitive cells, so he, by devoting especial attention to the history of the development of the tissues, discovered that the embryo, at its earliest stage, consists of a number of quite similar cells ; he then traced the metamorphoses or transformations, which the cells undergo, until they develop into the fnlly-formed tissues of the adult animal. He showed that whilst a portion of the cells retain their original spherical shape, others become cylindrical in form, whilst yet others develop into long threads or star-shaped bodies, which send out numerous radiating processes from various parts of their surface. He showed how in bones, cartilage, teeth, and other tissues, cells become surrounded by firm walls of varying thickness; and, finally, he explained the appearance of a number of the most atypical tissues by the consideration that groups of cells become, so to speak, fused together ; this again is analogous to the development of the vessels in plants. Thus Schwann originated a theory which, although imperfect in many respects, yet is applicable both to plants and animals, and which, farther, is easily understood, and in the main correct. According to this theory, every part of the animal body is either built up of elements, corresponding to the plant cells, massed together, or is derived from such elements which have undergone certain metamorphoses. This theory has formed a satisfactory foundation upon which many further investigations have been based. However, as has been already mentioned, the conception which Schleiden and Schwann formed of the plant and animal element was incorrect in many respects. They both defined the cell as a small vesicle, with a firm membrane enclosing fluid contents, that is to say, as a small chamber, or cellula, in the true sense of the word. They considered the membrane to be the most important and essential part of the vesicle, for they thought that in consequence of its chemico-physical properties it regulated the metabolism of the cell. According to Schwann, the cell is an organic crystal, which is formed by a kind of crystallisation process from an organic mother- substance (cytoblastema) . The series of conceptions, which we now associate with the word "cell," are, thanks to .the great progress made during the last fifty years, essentially different from the above. Schleiden and Schwann's cell-theory has undergone a radical reform, having been superseded by the Protoplasmic theory, which is especially associated with the name of Max Schultze. The History of the Protoplasmic theory is also of supreme interest. Even Schleiden observed in the plant cell, in addition to the cell sap, a delicate transparent substance containing small granules ; this substance he called plant slime. In the year 1846 Mohl (I. 18) called it Protoplasm, a name which has since become so significant, and which before had been used by Purkinje (I. 24) for the substance of which the youngest animal embryos are formed. Further, he presented a new picture of the living appearances of plant protoplasm ; he discovered that it completely filled up the interior of young plant cells, and that in larger and older cells it absorbed fluid, which collected into droplets or vacuoles. Finally, Mohl established the fact that protoplasm, as had been already stated by Schleiden about the plant slime, shows strikingly peculiar movements ; these were first discovered in the year 1772 by Bonaventura Corti, and later in 1807 by C. L. Treviranus, and were described as " the circulatory movements of the cell-sap." By degrees further discoveries were made, which added to the importance attached to these protoplasmic contents of the cell. In the lowest algee, as was observed by Cohn (I. 7) and others, the protoplasm draws itself away from the cell membrane at the time of reproduction, and forms a naked oval body, the swarm- spore, which lies freely in the cell cavity ; this swarm-spore soon breaks down the membrane at one spot, after which it creeps out through the opening, and swims about in the water by means of its cilia, like an independent organism ; but it has no cell mem- brane. Similar facts were discovered through the study of the animal THE HISTORY OF THE PROTOPLASMIC THEORY 7 cell, which could not be reconciled with the old conception of the cell. A few years after the enunciation of Schwann's theory, various investigators, Kolliker (I. 14), Bischoff (I. 4), observed many animal cells, in ivliich no distinct membrane could be dis- covered, and in consequence a lengthy dispute arose as to whether these bodies were really without membranes, and hence not cells, or whether they were true cells. Further, movements similar to those seen in plant protoplasm were discovered in the granular ground substance of certain animal cells, such as the lymph cor- puscles (Siebold, Kolliker, Remak, Lieberkiihn, etc.). In con- sequence Remak (I. 25, 26) applied the term protoplasm, which Mohl had already made use of for plant cells, to the ground substance of animal cells. Important insight into the nature of protoplasm was afforded by the study of the lowest organisms, Rhizopoda (Amoebae), Myxomycetes, etc. Dujardin had called the slimy, granular, contractile substance of which they are composed Sarcode. Sub- sequently, Max Schultze (I. 29) and de Bary (I. 2) proved, after / most careful investigation, that the protoplasm of plants and i animals and the sarcode of the loivest organisms are identical. In consequence of these discoveries, investigators, such as Nageli, Alexander Braun, Leydig, Kolliker, Cohn, de Bary, etc., considered the cell membrane to be of but minor importance in com- parison to its contents ; however, the credit is due to Max Schultze, above all others, of having made use of these later discoveries in subjecting the cell theory of Schleiden and Schwann to a search- ing critical examination, and of founding a protoplasmic theory. He attacked the former articles of belief, which it was necessary to renounce, in four excellent though short papers, the first of which was published in the year 1860. He based his theory that the cell-membrane is not an essential part of the elementary organisms of plants and animals on the following three facts : first, that a certain substance, the .protoplasm of plants and animals, and the sarcode of the simplest forms, which may be recognised by its peculiar phenomena of movement, is found in all organisms ; secondly, that although as a rule the protoplasm of plants is surrounded by a special firm membrane, yet under certain conditions it is able to become divested of this membrane, and to swim about in water as in the case of naked swarm-spores ; and finally, that animal cells and the lowest unicellular organisms very frequently possess no cell-membrane, but appear as naked 8 THE CELL protoplasm and naked sareode. It is true that he retains the term " cell," which was introduced into anatomical language by Schleiden and Schwann ; but he defines it (I. 30) as : a small mass of protoplasm endowed with the attributes of life. Historical accuracy requires that it should be mentioned that in this definition Max Schultze reverted to the older opinions held by Purkinje (I. 22-24) and Arnold (I. 1), who endeavoured to build up a theory of granules and masses of protoplasm, but with- out much result, for the cell theory of Schwann was both more carefully worked out, and more adapted to the state of knowledge of the time. The term, a small mass of protoplasm, was not intended by Max Schultze and other investigators even then to mean so simple a matter as appears at first. The physiologist, Briicke (I. 6), especially came to the correct conclusion, gathered with justice from the complexity of the functions of life, which are inherent in protoplasm, that the protoplasm itself must be of a complex con- struction, that is must possess "an extremely intricate structure," into which, as yet, no satisfactory insight has been gained owing to the imperfections of our means of observation. Hence Briicke very pertinently designated the "ultimate particle" of animals and plants, that is the mass of protoplasm, an elementary organism. Hence it is evident that the term " cell " is incorrect. That it, nevertheless, has been retained, may be partly ascribed to a kind of loyalty to the vigorous combatants, who, as Briicke expresses it, conquered the whole field of histology under the banner of the cell-theory, and partly to the circumstance, that the discoveries which brought about the new reform were only made by degrees, and were only generally accepted at a time when, in consequence of its having been used for several decades of years, the word cell had taken firm root in the literature of the subject. Since the time of Briicke and Max Schultze, our knowledge of the true nature of the cell has increased considerably. Great insight has been gained into the structure and the vital properties of the protoplasm, and in especial, our knowledge of the nucleus, and of the part it plays in cell-multiplication, and in sexual repro- duction, has recently made great advances. The earlier definition, " the cell is a little mass of protoplasm," must now be replaced by the following : " the cell is a little mass of protoplasm, which contains in its interior a specially formed portion, the nucleus." The history of these more recent discoveries will be entered THE HISTORY OF THE PROTOPLASMIC THEORY into later, being only incidentally mentioned here and there in the following account of our present knowledge of the nature of the elementary organism. The enormous amount of knowledge which has been acquired through a century of investigation will be best systematically arranged in the following manner : In the first section the chemico-physical and morphological properties of the cell will be described. The second section will treat of the vital properties of the cell. These are, (1) its contractility, (2) its irritability, (3) the phe- nomena of metabolism, (4) its power of reproduction. Further, in order to complete and amplify our account of the nature of the cell, two sections more speculative in character will be added, one treating of the relationship between the proto- plasm, the nucleus, and the cell products, and the other of the cell considered as the germ of an organism. Literature I. 1. FK. ARNOLD. Lehrbuch der Physiologic des Mensclien. 2 Theil. Zurich. 1842. Handbuch der Anatomie des Menschen. 1845. 2. DE BARY. Myxomyceten. Zeitschrift f. wissenschaftl. Zool. 1853. 3. LIONEL S. BEALK. On the Structure of the Simple Tissues of the Human Body. 1861. 4. BISCHOFF. Entwicklungs-geschichte des Kanincheneies. 1842. 5. K. BROWN. Observations on the Organs and Mode of Fecundation in Orchidece and Asclepiadece. Transactions of the Linnean Soc., London. 1833. 6. BRUCKE. Die Elementarorganismen. Wiener Sitzungsber. Jahrg. 1861. XL1V. 2. Abth. CLELAND. On Cell Theories. Quar. Jour. Microsc. So. XIII. , p. 255. 7. COHN. Nachtrage z. Naturgeschichte des Protococcus pluviatilis. Nova acta. Vol. XXII., pp. 607-764. 8. BONAVENTURA CoRxi. Observazioni microsc. sulla Tremella e sulla circola- zione delfinido in una pianta acquaiola. 1774. DALLINGER and DRYSDALE. Researches on the Life History of the Monads. Month. Mic. Jonrn. Vols. X.-X1II, 9. GREW. The Anatomy of Plants. 10. HAECKKL. Die Radiolarien. 1862. Die Muneren. 11. HENLE. Symbols ad anatomiam villorum intestinalium. 1837. 12. OSCAR HERTWIG. Die Geschichte der Zellentheorie. Deutsche Rundschau. 13. HUXLEY. On the Cell Theory. Monthly Journal. 1853. 14. KULLIKER. Die Lehre von der thierisctien Zelle. Schleiden u. Ncigeli ll'is*en*chaftl. Botanik. Heft 2, 1845. KOLLIKER. Manual of Human Histology, trans. Sydenham Society. 1853. 10 THE CELL 15. MALPIGHI. Anatome plantarum. 16. MEYEN. Phytotomie. Berlin. 1830. 17. H. v. MOHL. Veber die Vermehrung der Pfianzenzellen durch Theilang. Dissert. Tubingen. 1835. Flora. 1837. 18. H. v. MOHL. Veber die Saftbewegung im Innern der Zellen. Botanische Zeitung. 1846. 19. H. v. MOHL. Grundziige der Anatomie und Physiologic der vegetabilischen Zelle. Wagners Handwdrterbuch der Physiologie. 1851. 20. J. MULLER. Vergleichende Anatomie der Myxinoiden. 21. OKEN. Lehrbuch der Naturphilosophie. 1809. 22. PUBKINJE. Bericht iiber die Versammlung deutscher Naturfarscher und Aertzte in Prag im September, 1837. Prag, 1838, pp. 174, 175. 23. POKKINJE. Uebersicht der Arbeiten und Veranderungen der schlesischen Gesellschaft fur vaterldnduche Cultur im Jahre, 1839. Breslau, 1840. 21. PORKINJE. Jahrbiicher fiir ivissenschaftliche Kritik. 1840. Nr 5, pp. 33-38. 25. REMAK. Veber extracellular -e Entstehung thierischer Zellen und i'tber Ver- mehrung derselben durch Theilung. Miillers Archiv. 1852. 26. EEMAK. On the Embnjological Basis of the Cell Tbeury (translated'). Q. J. M. S. II., p. 277. 27. SACHS. Geschichte der Botanik. 1875. 28. MATTHIAS SCHLEIDEN. Beitrtige zur Phylogenesis. Miillers Archiv. 1838. Principles of Scientific Botany, translated by Lankester. 1849. 29. MAX SCHULZE. Das Protoplasma der Rhizopoden und der Pflanzenzelle. 30. MAX SCHULZE. Veber Muskelkdrperchenund was man eine Zelle zunennen babe. Archiv fiir Anatomie und Physiologie. 1861. 31. TH. SCHWANN. Mikroscopische Untersnchungen iiber die Uebereinstimmung in der Structur und dem Wachsthum der Thiere und Pftanzen. 1839. SCHWANN und SCHLEIDEN. Microscopical Researches, trans. Sydenham Soc. 1837. 32. C. L. TKEVIBANUS. Vom inwendigen Ban der Gewachse, 1803. 33. E. VIKCHOW. Cellular Pathology as based upon Physiological and Patho- logical Histology, trans, by Chance. 1860. 34. CASP. FBIEDB. WOLFF. TJieorie von der Generation. 1764. CHAPTER II THE CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES OF THE CELL The cell is an organism, and by no means a simple one, being built up of many different parts. To ascertain with accuracy the true nature of all these constituents, which, for the greater part, elude our observation at present, will remain a problem for biological research for a long time. Our position, with regard to the cell, is similar to that of investigators towards the whole animal or vege- table body a hundred years ago, before the discovery of the cell theory. In order to penetrate more deeply into the secrets of the cell, optical instruments, and, above all, methods of chemical examination, must be brought to a much higher degree of perfec- tion than they have attained at present. It seems best to me to lay stress on these points to start with, in order that the student may have them always before his mind's eye in reading the follow- ing account. In each cell there is invariably to be seen one specially well- defined portion, the nucleus, which throughout the whole of the animal and vegetable kingdom is very uniform in appearance ; evidently the nucleus and the remaining portion of the cell have different functions to perform in the elementary organism. Hence the examination of the chemico-physical and morphological proper- ties of the cell becomes naturally divided into two sections, the examination of the protoplasm and of the nucleus. To these, three short sections are added. The first deals with the question, Are there cells which possess no nuclei ? The second treats of the pole or central corpuscles, which are at times found as special cell-structures in addition to the nucleus ; and in the third a short account is given of Nageli's theory of the mole- cular structure of organic bodies. I. The Chemico-physical and Morphological Properties of the Protoplasm. Some animal and plant-cells appear to differ so much from one another as to their form and contents, 12 THE CELL that, at first sight, they seem to have nothing in common, and that hence it is impossible to compare them. For instance, if a cell at the growing-point of a plant be taken and compared with one filled with starch granules from the tuber of a potato, or if the contents of an embryo cell from a germinal disc be com- pared with those of a fat cell, or of one from the egg of an Amphibian filled with yolk granules, the inexperienced observer sees nothing but contrasts. Nevertheless, all these exceedingly different cells are seen on closer examination to be similar in one respect, i.e. in the possession of a very important, peculiar mix- ture of substances, which is sometimes present in large quantities, and sometimes only in traces, but which is never wholly absent in any elementary organism. In this mixture of substances the wonderful vital phenomena, which are dealt with later, may very frequently be observed (contractility, irritability, etc.) ; and, more- over, since in young cells, in lower organisms, and in the cells of growing-points and germinal areas, it is in the cell-substance alone (the nucleus of course being excepted) that these properties have been observed, this substance has been recognised as the chief supporter of the vital functions. It is the protoplasm or "forming matter" of the English histologist, Beale (I. 3). a. Justification of the Use of the Term Protoplasm. In order to know what protoplasm is, it is advisable to examine it in those cells in which it is present in large quantities, and in which it is as free as possible from admixture with other bodies ; and amongst such the most suitable are those organisms from the study of which the founders of the protoplasmic theory formed their conception of the nature of this substance. Such organisms are, young -plant-cells, Amoebae, and the lymph corpuscles of vertebrates. After the student has learnt to recognise the cha- racteristic properties of protoplasm in such bodies, he will be able to discover it in others, in which it is only present in small quantities and is more or less concealed by other substances. It has been proposed (II. 10) to give up altogether the use of the term protoplasm, since it has been associated with such mistaken views; for the word has now come to be used in so indefinite and vague a manner, that it may be questioned whether it is not at present more misleading than useful. However, this proposition cannot be considered to be advisable or even justifiable in the present condition of affairs, for, although it must be admitted that the word is frequently used incorrectly ; ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 13 and that further, it is impossible in a short phrase to give an adequate definition of its meaning; and finally, that frequently it is difficult to determine what part of the cell really consists of protoplasm, and what does not; yet, in spite of all this, the necessity of the conception remains. Similar objections could be raised against a number of other words which we use for certain definite compounds present in organic bodies. For instance, to designate a certain portion of the nucleus we use the term nuclein or chromatin, which is considered fairly adequate by many people. And yet the microscopist is bound to admit that it is impossible to state exactly which part of a resting nucleus consists of limn, and which of nuclein, or to determine in any special case whether too much or too little has been stained. Now the term protoplasm is quite as necessary in speaking about the constituent parts of a cell. Only it must be stipulated that the word protoplasm must not be understood to designate a substance of definite chemical composition. The word protoplasm is a morphological term (the same is true in a greater or less degree of the word nuclein, and of many others) ; it is an expression for a complex substance, which exhibits a variety of physical, chemical, and biological properties. Such ex- pressions are absolutely necessary in the present state of our knowledge. Any one who is acquainted with the history of the cell knows what a number of observations and how much logical thought were necessary before this conception was arrived at, and further is quite aware that with the creation of this expression the whole theory of cells and tissues gained in depth and significance. How much wordy warfare was necessary before it was established that the cell contents, and not the cell membrane, constitute the essential portion of the cell, and further that amongst these cell contents a peculiar substance is invariably present, which takes part in the vital processes in quite a different way from the cell sap, the starch granules, and the fat globules. Thus we see that the use of the word protoplasm is not only justifiable from an historical point of view, but also from a scientific one, and we will now proceed to endeavour to explain what is meant by the term. 6. General Characteristics of Protoplasm. The proto- plasm of unicellular organisms, and of plant and animal cells (Figs. 1 and 2), appears as a viscid substance, which is almost always colourless, which will not mix with water, and which, in con- 14 sequence of a certain resemblance to slimy substances, was called by Schleiden the slime of the cell. Its refractive power is greater than that of water, so that the most delicate threads of protoplasm, although colourless, may be distinguished in this medium. Minute granules, the microsomes, which look only like dots, are always present in greater or less numbers in all protoplasm, and may be seen with a low power of the microscope to be embedded in a homogeneous ground sub- stance. Accord- ing to whether there are few or many of these microsomes in the protoplasm, it is more trans- parent (hyaline) or darker and more granular in appearance. The distribu- tion of these granules in the body of the cell is rarely regular. Generally a more or less thin outer zone remains free from granules. Now as this layer appears to be somewhat firmer in consistence than the more watery granula FIG. 1. Parenchyma cells, from the cortical layer of the root of Fritillaria imperialis ; longitudinal sections (x 650); after Sacbs([I. 33), Pig. 75 : A very young cells, as yet without cell-sap, from close to the apex of the root ; B cells of the same description, about 2mm above the apex of the root the cell- sap (s) forms in the protoplasm (p) separate drops, between which are the partition walls of the protoplasm ; C cells of the same description, about 7-8 mm. above the apex; the two lower cells on the right-hand side are seen in a front view, the larye cell on the left side is seen in optical section, the upper right- hand cell is opened by the section; the nucleus (ay) has a peculiar appearance, being distended with water which it has absorbed ; fc nucleus ; fcfc nucleolus ; h membrane. ITS CHEM1CO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 15 mass, it has been thought advisable to distinguish two kinds of protoplasm, the ectoplasm or hyaloplasm, and the endoplasm or granularplasm (Fig. 2, ek, en). Many investigators, such as Pfeffer, de Vries, etc., are inclined to consider that this peripheral layer is a specially differentiated organ of the cell and is endowed with special functions. The following experiment which I haA T e made seems to bear out this view. Some ripe e^rgs of Bana temporaria, which had entered the oviduct and were surrounded with a gelatinous coating, were care- fully pierced with the exceedingly fine point of a glass needle. The puncture thus made was not visible externally after the operation, nor was any yolk seen to exude through the holes. However, some time after fertilisation of the eggs had taken place, a fair quantity of yolk began to make its way out of all the punctured eggs, and to form a more or less large ridge (extraovat, Roux) between the membrane of the egg and the yolk. This welling out of the yolk substance was induced by the act of fertilisation, for the entrance of the spermatozoon stimulates the surface layer to con- tract energetically, as may be easily demonstrated under suitable conditions. Hence the puncture must have caused a wound in the peripheral layer, which had not time to heal before fertilisation took place, and through which the yolk was only pressed out after the contraction caused by the fertilisation had taken place. Now since between the piercing of the eggs and their fertilisation a fairly long interval, which however I did not accurately measure, had elapsed, this experiment seems to show that the peripheral layer possesses a structure differing somewhat from that of the rest of the cell contents, and also that it has properties peculiar to itself. c. Chemical Composition of Protoplasm. Our know- ledge of the chemical nature of protoplasm is most unsatisfactory. It has sometimes been described as an albuminous body, or as " living albumen." Such expressions may give rise to utterly incorrect conceptions of the nature of protoplasm. On this account I will recapitulate what I said in section a : Protoplasm is not a chemical, but a morphological conception; it is not a single chemical substance, however complex in composition, but is com- posed of a large number of different chemical substances, which we have to picture to ourselves as most minute particles united together to form a wonderfully complex structure. Chemical substances exhibit similar properties under different 16 circumstances (as, for instance, hasmoglobin, whether present as a constituent of the blood corpuscles, or dissolved in water, or in the form of crystals). Protoplasm, on the other hand, cannot be placed under different conditions without ceasing to be protoplasm, for its essential properties, in which its life manifests itself, depend upon a fixed organisation. For as the principal attributes of a marble statue consist in the form which the sculptor's hand has given to the marble, and as a statue ceases to be a statue if broken up into small pieces of marble (Nageli II. 28), so a body of protoplasm is no longer protoplasm after the organisation, which constitutes its life, has been destroyed ; we only examine the considerably altered ruins of the protoplasm when we treat the dead cells with chemical re- agents. It is possible that after a time our knowledge of chemistry may have advanced sufficiently to en- able us to produce albuminous bodies artificially by synthesis. On the other hand, the attempt to make a protoplasmic body would be like Wagner's en- deavour to crystallise out a homunculus in a flask. For, as far as we know at present, proto- plasmic bodies are only reproduced from existing protoplasm, and in no other way ; hence the present organisation of protoplasm is the result of an exceedingly long pro- cess of development. It is very difficult to determine the chemical nature of the sub- stances which are peculiar to living protoplasm. For setting aside the fact that the bodies are so unstable that the least inter- ference with them essentially alters their constitution, the difficulty in analysing them is considerably increased by the presence in each cell of various waste pi-oducts of metabolism, which it is not easy to separate from the rest of the cell contents. Amongst these complex substances the proteids, as the true sus- tainers of the vital processes, are of especial importance ; these FIG. 2. Amoeba. Proteus (after Leidy ; from Rich. Hertwig) : n nucleus; cv con- tractile vacuole; n food vacuoles ; en eiidoplasm ; ek ectoplasm. ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 17 proteids are the most complex of all known organic substances, but up till now very little has been determined as to their chemical structure. This complex structure depends, in the first place, upon the very remarkable chemical properties of carbon (Haeckel II. 15). In proteids carbon occurs combined with four other elements, hydrogen, oxygen, nitrogen, and sulphur, in proportions which, it has been endeavoured to express by the following formula : C 72 H 106 N 18 SO- 2 (composition of a molecule of egg-albumen). Amongst the various kinds of proteid bodies (albumins, globu- lins, fibrins, plastins, nucleins, etc.) plastin alone seems to be pecu- liar to protoplasm (Reinke II. 32; Schwarz II. 37; Zacharias II. 44) ; plastin is insoluble in water, in 10 per cent, salt solution, and in 10 per cent, solution of sulphate of magnesia ; it is pre- cipitated by weak acetic acid, whilst concentrated acetic acid causes it to swell up ; it is precipitated in concentrated salt solution ; it resists both pepsin and trypsin digestion. It is hardly, or not at all, stained by basic aniline dyes, but is stained by acid ones (eosin and acid fuchsine). In addition, globulins and albumins are present in smaller quantities ; these are also found in solution in the cell-sap of plants. Protoplasm is very rich in water, which, as Sachs (II. 33) states, is built up into the structure of its molecule, in the same sense as, for example, the water of crystallisation is a necessary constituent of many crystals, which lose their characteristic form if the water of crystallisation is withdrawn. Reinke (II. 32) found 71'6 per cent, of water and 28'4 per cent, of solid substances in fresh sporangia of the ^Ethaiium septicum (66 per cent, of this water could be squeezed out). Further, a number of various salts are present in protoplasm ; these remain as ash when the protoplasm is burnt ; in the case of the JEthalium septicum, the ash contains the following elements : chlorine, sulphur, phosphorus, potassium, sodium, magnesium, calcium, and iron. Living protoplasm is distinctly alkaline in reaction ; red litmus paper is turned blue by it, as is also a red colouring matter, which, is obtained from a species of cabbage, and which has been used by Schwarz. This is also the case with plants, although the cell-sap, as a rule, has an acid reaction. According to the investigations of Schwarz (II. 37) on plants, this alkaline reaction is due to the presence of an alkali, which is united with the proteid bodies in c 18 THE CELL living protoplasm. Eeinke (II. 32) states that the JEthalium septicum gives off ammonia after it has been dried. Moreover, the most different metabolic products are always to be demonstrated in protoplasm ; these are produced either by progressive or retrogressive metamorphosis. There is a great similarity shown between the substances occurring in plant and in animal cells. For example, the following substances are found in both, pepsin, diastase, myosin, sarcin, glycogen, sugar, inosit, dextrin, cholesterin and lecithin, fat, lactic acid, formic acid, acetic acid, butyric acid, etc. As an example of the quantitative composition of a cell includ- ing its nucleus, Kossel (II. 35) quotes in his text-book, the analysis of pus-corpuscles which was made by Hoppe-Seyler. According to this statement, 100 parts by weight of organic substance contain : Various albuminous substances 13-762 Nnclein 34-257 Insoluble substances 20-566 Lecithin and fat 14-383 Cholesterin 7-400 Cerebrin 5-199 Extractives 4-433 Phosphorus, sodium, iron, magnesium, calcium, phosphoric acid and chlorine were found in the ash. As regards the physical properties of protoplasm, streaming protoplasmic threads are sometimes noticed in which double re- fraction is seen, the movements being for the most part in a direction such that their optical axes coincide (Engelmann). d. The more minute Structure of Protoplasm. Proto- plasm was defined above as a combination of substances, the most minute particles of which we must picture to ourselves as united together to form a complex structure. Investigators have en- deavoured -to discover more about this marvellous structure, partly by speculation, and partly by microscopical observation. As to the first, Nageli has made some important suggestions, a more detailed account of which is given later in the section entitled " The Molecular Structure of Organised Bodies." As to the second, numerous investigators, amongst whom From- mann, Flemming, Butschli and Altmann are conspicuous, have recently been working at the subject. Living protoplasm, as well as that which has been killed by special reagents, has been ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 19 examined ; in the latter, its most minute structure has been rendered visible, by means of various staining reagents ; thus we have ah-eady a considerable amount of literature on the subject of the structure of protoplasm. Starting with the assumption that protoplasm consists of a mixture of a small quantity of solid substances with a large quantity of fluid, to which circumstance it owes its peculiar viscid property as a whole, the question might be raised as to whether it be possible, by using the strongest lenses, to distinguish optically the solid particles from the fluid which contains them, and to recognise their arrangement into special structures. A priori, it does not seem to be necessary to distinguish them from one another, since the solid particles are so very small, and since they differ so little from the fluid in their refractive power. Thus, according to Nageli's micellar theory, which will be de- scribed in detail later on, they are supposed to be arranged as a framework, which, hotvever, in consequence of the minute size of the hypothetical micellse, escapes our observation. In a word, it is possible that protoplasm may have a very complicated structure, although it appears to us to be a homogeneous body. Hence the expression homogeneous protoplasm does not necessarily imply that) protoplasm does not possess a definite structure or organisation. } Recent observations, for which powerful oil immersion lenses have been successfully used, point more and more to the conclusion that protoplasm possesses a structure which may be optically demonstrated ; however, individual microscopists differ so essen- tially in their views upon the nature of this structure, that it is impossible to come to any definite decision upon the subject. At the present time, at least four conflicting theories hold the field ; these may be described as the framework theory, the foam or honeycomb theory, the filament theory, and the granula theory. The framework theory has been advocated by Frommann (II. 14), Heitzmann (II. 17), Klein (II. 21), Leydig (II. 26), Schmitz (II. 30), and by others. According to this theory, protoplasm consists of a very fine network of fibrilloe or threads, in the inter- stices of which the fluid is held. Thus, roughly speaking, it is like a sponge, or, shortly expressed, its structure is spongiose. The microsomes, which are seen in the endoplasm (granular plasma), are nothing but the points where the fi brill se intersect. A glance over the literature on this subject shows the reader that very different appearances are sometimes described under the 20 THE CELL title, " The spongiose structure of protoplasm." Sometimes the description refers to coarser frameworks, which, being due to the deposition in the protoplasm of various kinds of substances, should not be considered as pertaining to protoplasm, nor should they be included in its description. This holds true, for example, of the description of the goblet cells of List (II. 48) (see p. 36, fig. 17). This subject is more fully discussed later on. Sometimes net-like structures are described and depicted, which, as they are evidently caused by coagulation (due to some pre- cipitation process), must be considered as artificial products. For instance, artificial framework structures may be easily pro- duced, if a solution of albumen or gelatine be caused to coagulate by the addition of chromic acid, picric acid, or alcohol. Thus Heitzmann (II. 17) demonstrates, in a somewhat diagrammatic manner, the presence of networks in the most various cells of the animal body, which does not correspond to actual fact. Biitschli also remarks in his abstract of the literature on the subject (II. 7b, p. 113): "Above all, it is frequently very difficult to determine whether the net-like appearances described by earlier observers are really delicate protoplasmic structures, or whether they are caused by coarser vacuolisation. Since the same appear- ance is produced in either case, it is only possible to form a fairly correct opinion by considering their relative sizes." Biitschli found that in all cases the spaces in the meshes of the protoplasm measured barely 1 p. Thus, although no doubt many statements may be legitimately questioned, yet it is undeniable that many investigators (From- mann, Schmitz, Leydig) have really based their descriptions upon the more delicate structures of the cell. In the explanation of these so-called net-work appearances, Biitschli takes up a position which is different from that of the other observers who have been mentioned, and which has caused him to advance a foam or honeycomb theory of protoplasm (II. 7a, 7b). He succeeded in producing a very delicate emulsion by mixing inspissated olive oil with K 2 C0 3 , common salt, or cane-sugar. This emulsion consists of a groundwork of oil, containing an exceedingly large number of spaces, which are completely closed in and filled with watery liquid ; if the emulsion is too fine to be seen except under the microscope, the diameter of the spaces is generally less than '001 mm. In appearance they are very like ITS CHEMICO-PHYSICAL AXD MORPHOLOGICAL PROPERTIES 21 the cells of a honeycomb, being in the form of very varying poly- hedra ; they are separated from one another by the most delicate lamellae of oil, which refract the light somewhat more strongly than the watery liquid does. As a result of physical laws, only three lamellae can touch at one edge. Hence it appears in optical section, that only three lines meet in any one point. If before the for- mation of the emulsion fine par- ticles of lamp-black are distributed throughout the oil, these collect at the point of intersection. Finally, the superficial layer is composed of a delicate froth, the frame- work of which is arranged in a peculiar fashion, the partition walls of oil, which touch the surface, being perpendicular to it, and thus appearing parallel to one another in optical section. Biitschli describes this as the alveolar layer (Fig. 3 alv.~). Butschli considers that the protoplasm of all plant and animal cells (Figs. 4, 5) possesses a structure which is similar to this. FIG. 3. Optical section of the edge of a drop of an emulsion made with olive oil and salt; the alveolar layer (air.) is very distinct, and relatively deep. ( x 1250 : after Biitgchli, PI. III., Fig. 4.) FIG. 4. FIG. 5. FIG. 4. Two living strands of plasma from a hair-cell of a Mtoliow. (x about 3,000 : after Butschli, PI. II., Fig. 14.) FIG. 5. Web-like extension, very distinct in structure, from the pseudopodic net of a Miliola from life, (x about 3.000 : after Butschli, PI. II., Fig. 5.) 22 THE CELL His opinion is based upon his experiments on living objects, which he treated with various reagents. In his opinion there is a frame- work of plasma corresponding to the lamellae of oil, which, in the artificial emulsion, separate the droplets of fluid from one another. Similarly here also granules (microsomes) are collected together at the points of intersection. Further the protoplasmic body is fre- quently differentiated externally to form an alveolar layer. The appearance, described by other observers as a thread or net-like structure with spaces which communicate and contain fluid, Biitschli considers to be due to the presence of a froth or honey- comb structure, in which the cavities are closed in on all sides ; he himself, however, remarks that, in consequence of the minuteness of the structures in question, it is impossible to decide finally, simply by the appearance under the microscope, whether a net-like or honeycomb structure really exists (II. 7b, p. 140), since " in either case the appearance under the microscope is the same." Now it seems hardly justifiable, that this similarity to an artificially prepared froth, although it has caused Biitschli finally to make up his mind, should be allowed to settle the question. Two objections to this theory of Biitschli's must be mentioned. The first is that it does not apply to nuclear substance, which without doubt is similar in its organisation to protoplasm. For during the process of nuclear division threadlike arrangements in the form of spindle- threads and nuclein-threads are so distinctly to be seen, that their existence certainly cannot be questioned by any one. The second objection is more theoretical in nature. The oil lamellae are composed of a fluid which does not mix with water. Now if the comparison between the structure of this emulsion and that of protoplasm is to depend upon something more than a mere superficial similarity, the plasma lamellae, corresponding to the oil lamellae, must be composed of a solution of albumen or of liquid albumen. Now this cannot be the case, for a solution of albumen is capable of mixing with water, and hence would of necessity mix with the contents of the spaces ; hence the albuminous froth would have to be prepared with air. In order to get over this difficulty, Biitschli assumes that the chemical basis of the framework sub- stance is a fluid, composed of molecules of albumen combined with those of a fatty acid (II. 7b, p. 199) ; this supposition, and especially the theory that the framework substance is a fluid, is ITS CHEM1CO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 23 not likely to meet with much support. For on many accounts it seems to be true that the structural elements of protoplasm, whether they form the threads of a net, or the lamellae of a honey- comb, or granules, or what not, must be solid in their nature. Protoplasm does not consist of two non-miscible fluids, such as water and oil, but of a combination of solid organic particles with a large quantity of water. Hence quite different physical condi- tions are necessarily present. (Compare section on molecular structure, p. 58.) The third of the above-mentioned views, or the filament theory, is connected with the name of Flemming (II. 10). Whilst examining a large number of living cells (cartilage, liver, connective tissue, and ganglion cells, etc.), Flemming observed in the protoplasm (Fig. 6) the presence of extremely delicate threads which have somewhat greater refractive power than the inter- vening ground substance. These threads vary in length, being longer in some cells than in others ; sometimes larger numbers are present than at others. It seemed im- possible to determine with certainty whether they are separated from one another all along their length, or whether they join together to form a net ; if they do form a net, then its meshes must be very uneven in size. Hence Flemming considers that two different substances occur in proto- FIG. e. -Living cartilage cell of a Salamander larva, much mag- plasm, a thread substance and an inter- nified, with clearly marked fila- stitial substance, or & filamentous and an mentous substance: after Fiem- . ,,, , / ., , ming (from Hatschek, Fig. 2). interjilamentous substance (mitome and paramitome) ; upon the chemical nature of these substances and upon their general condition Flemming does not enlarge. How much importance should be attached to this structure, about which at present nothing further can be stated, it remains for the future to reveal. In this section, "On the Structure of Protoplasm," the ray-like arrangement of the protoplasm which is observed at certain stages of the division of the nucleus, or the striated appearance which is exhibited by the protoplasm of secretory cells, might be more fully described. Since, however, such structures only occur under special conditions, it has been considered more advisable to defer their consideration to a later period. Fourthly, and finally, come the attempts of Altmann (II. 1) to 24 THE CELL demonstrate a still more minute structure of protoplasm (gramda theory). By means of a special method of treatment, this in- vestigator has succeeded in rendering minute particles visible in the body of the cell ; these he calls granula. He preserves the organ in a mixture of 5 percent, solution of potassium bichromate with 2 per cent, solution of perosmic acid; he then prepares thin sections of the organ and stains them with acid fuchsine, finally treating them with alcoholic solution of picric acid, by means of which the differentiation is rendered more distinct. The result of these staining reactions is to render visible a large num- ber of very minute dark-red granules. Sometimes they are seen to be isolated, sometimes more densely packed ; sometimes they are near together, sometimes further apart ; or they may be united in rows to form threads. In consequence of these observations, Altmann has propounded a very important and far-reaching hypothesis. He considers these granules to be still more minute elementary organisms, of which the cell itself is composed ; he calls them bioblasts, attributes to them the structure of organised crystals, and looks upon them as equivalent to the micro-organisms which, as individuals, arrange themselves in masses to form a zooglea, or in rows to form threads. " As in a zooglea the single individuals are connected together by means of a gelatinous substance secreted by them- selves, and at the same time are separated from one another by it, so in the cell the same might occur with the granula ; in this case also we must not consider that there is merely water and salt solution" surrounding the grauula, but similarly that a more gelatinous substance (intergranula substance) is present ; this is sometimes liquid, and sometimes fairly viscid in consistency. The great mobility, peculiar to most protoplasm, renders the former probable. If this intergranula substance becomes collected with- out granula at any point in the cell, a true hyaloplasm may be formed, which, being free from living elements, does not really deserve the name of protoplasm." Thus Altmann defines protoplasm as " a colony of bioblasts, the individual elements of which are grouped together either in a zooglea condition or in the form of threads, and which are con- nected by an indifferent substance." " Hence the bioblast is the much-sought-after, morphological unit of all organic substances, with which all biological investigation must finally deal." How- ever, the bioblast is not able to live alone, but dies with the cell ITS CHEM1CO-PHTSICAL AND MORPHOLOGICAL PROPERTIES 25 in which, according to Altmanii, it multiplies by fission (omne granulum e granule). Many objections may be raised to this hypothesis of Altmann's, in so far as it refers to the interpretation of recorded observations. Firstly, the most minute micro-organisms of a zooglea are connected by means of a great number of forms, which are intermediate as to size, with the larger fission and yeast fungi ; and since these are not to be distinguished from cells in their construction, they also must, according to Altmann, be colonies of bioblasts. Further, Biitschli has shown that the larger micro-organisms are most probably divided into nucleus and protoplasm, and hence are similar in structure to other cells. The flagella, also, which have been demonstrated in many micro-organisms, must be considered to be cell organs. Secondly, we have not been sufficiently enlightened upon the nature and function of the granula in the cell, excepting that for some reason or other we are to conclude that they are its true vital elements. According to Altmann's hypothesis, the relative importance which has been attached to cell-substances is completely reversed. The substance which he calls intergranula substance, and which in its physiological importance he considers to correspond to the gelatinous substance of the zooglea, is to all intents and purposes the protoplasm of the generally accepted cell theory, that is to say, the substance which is considered to form the most important generator of the vital processes ; on the other hand, the granula belong to the category of protoplasmic contents, and as such have had a much less important role ascribed to them. Thus Altmann designates the melanin granules of a pigment cell as the bioblasts, and the connecting protoplasm as the inter- granula substance. Similarly he completely reverses the physio- logical importance of the substances in the nucleus, as will be shown later on, in that he considers that his granula are con- tained in the nuclear sap, whilst his intergranula substance corre- sponds to the nuclear network, containing the chromatin. Under the term granula, Altmann has, according to our opinion, classed together substances of very different morphological im- portance, some of which should be considered as products of the protoplasm. However, he has rendered important service by faci- litating the investigation of protoplasm by means of new methods, although his bioblastic theory, which is based upon these experi- ments, is not likely to attract many supporters. (See the conclu- sion of the ninth chapter.) 26 THE CELL e. Uniformity of Protoplasm. Diversity of the Cell. A great uniformity of appearance is manifested by protoplasm in all organisms. With our present means of investigation we are unable to discover any fundamental difference between the proto- plasm present in animal cells and that in plant cells, or unicellular organisms. This uniformity is of necessity only apparent, being due to the inadequacy of our methods of investigation. For since the vital processes occur in each organism in a manner peculiar to itself, and since the protoplasm, if the nucleus be excepted, is the chief site of the individual vital processes, these differences must be due to differences in the fundamental substance, that is to say, in the protoplasm. We must therefore accept, as a theory, that the protoplasm of different organisms varies in its material, composition and structure. Apparently, however, these important differences are due to variations in molecular arrangement. In spite of the uniform appearance of the protoplasm, the in- dividual cell, of which after all the protoplasm forms only a more or less important part, when taken as a whole, may vary very much in appearance ; this is due partly to variations in external form, but chiefly to the fact, that sometimes one, and sometimes another substance is stored up in the protoplasm, in such a manner as to be distinguishable from it. Sometimes this occurs to so great a degree that the whole cell appears to be composed almost entirely of substances which under other circumstances are not present in protoplasm at all. If we imagine that these substances have been eliminated, a number of larger and smaller gaps would be naturally produced in the cell, between which the protoplasmic groundwork of the cell would be seen as partition walls and frame- works, which are sometimes extremely delicate. This arrangement of the protoplasm, as has been already mentioned (p. 19), must not be confused with the network structure, which, according to the opinion of many investigators, is inherent to protoplasm itself, and which was more fully described in the chapter on the structure of protoplasm. The names deutoplasm (van Beneden) and paraplasm (Kupffer, II. 24) have been proposed for these adventitious substances. Since, however, the idea of an albuminous substance is always con- nected with the word plasm and these substances may consist of fat, carbohydrates, sap, and of many other bodies the use of the above terms does not seem desirable, and it is better either to class them generally as intraplasmic products and adventitious cell contents ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 27 or, according to their significance, as reserve material and secretions, or indeed to specify them, as yolk granules, fat globules, starch granules, pigment granules, etc. The difference between the protoplasm and these substances, which may be classed together as cell contents, is the same as that between the materials of which the organs of our body are com- posed and those substances which in the first place are taken up as food by our bodies, and which later on are circulated in a liquid form as a nutrient fluid through all the organs ; the for- mer, which are less dependent upon the condition ot nourishment of the body for the time being, and hence are less subject to variations, are called in physiological language tissue substances, the latter circulating substances. The same distinction may be applied to the substances which compose the cell. Protoplasm is the tissue material, whilst the adventitious bodies are circulating substances. f. Various examples of the structure of the cell body. In connection with the chemico-physical and morphological pro- perties of the cell, a few especially pertinent examples may be of use in order to explain the general statements. For this pur- pose we will compare various lower unicellular organisms, both plant and animal, choosing first, cases in which the body consists almost entirely of protoplasm, and secondly, those in which the cells also contain considerable quantities of various adventitious substances, and hence are very much altered in appearance. Unicellular organisms, which live in water or on damp earth, such as Amoebae, Mycetozoa, and Reticularia, form very useful subjects for examination in studying the cell ; in addition, lymph corpuscles, the white blood corpuscles of vertebrates, and young plant cells are most suitable objects for investigation. 1. Cells consisting almost entirely of Protoplasm. An Amoeba (Fig. 7) is a small mass of protoplasm, from the surface of which, as a rule, a few short irregular processes (pseudopodia) -" or foot-like organs are extended. The body is quite naked, that is to say, it is not separated from the surrounding medium by any special thin coating or membrane ; the only differentiation being that the superficial layer of the protoplasm (ectoplasm), ek, is free from granules, and hence is transparent, like glass ; this ectoplasm is most marked in the pseudopodia; below the ectoplasm lies the darker and more liquid endoplasm (en), in which the vesicular nucleus (n) is embedded. 28 Very similar in appearance to the Amoeba, but much smaller in size, are the white blood corpuscles and the lymph corpuscles of the vertebrates (Fig. 8). If they are examined just after they have been taken from the body of the living animal, they are seen to be more or less globular masses of protoplasm, each one consisting of a scarcely visible hyaline layer, enclosing a granular internal portion in which the nucleus is situated. However, whilst the Specimen is fresh, this nucleus can hardly be distinguished, and sometimes even is quite invisible. After a time, the little body begins to push out from its surface, processes similar to the pseudo- podia of the Amoeba. 7. FIG. 8. FJG. 7. Amatba proteus (after Leidy: from R. Hertwig, Fig. 16): n nucleus ; cv con- tractile vacuole ; n food vacuoles ; en endoplasm ; ek ectoplasm. Fm. 8. A leucocyte of the Frog, containing a Bacterium which is undergoing the process of digestion ; the Bacterium has been stained with vesuvine. The two figures re- present two successive changes of shape in the same cell. (A.fter Metschnikoff, Fig. 54.) Myxomycetes and Eeticularia, which also consist of naked proto- plasm, are very different in appearance. The Myxomycete, which is best known to us, is the jEthalium septicum, which forms the so-called flowers of tan and grows over large portions of the surface of tan-pits, during its vegetative condition, like a thin coherent skin of protoplasm (plasmodium). Chondrioderma is another slime fungus which is nearly allied to the above. A small piece of its edge is represented in Fig. 9. ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 20 Towards its edge the plasmodium becomes broken up into a number of threads of protoplasm, which are sometimes exceedingly thin, and sometimes somewhat thicker, and which unite together to form a fine network. In the thicker threads it is possible to distinguish both a thin layer of homogeneous ectoplasm, and also the endoplasm which it encloses ; these cannot, however, be made out in the thinner ones. Throughout the whole mass of protoplasm, which is sometimes very extensive, a large number of minute nuclei are seen to be distributed. Amongst the Reticularia, of which many different kinds occur in fresh and salt water, Gromia oviformis (Fig. 10) is especially well known, in consequence of the experiments which have been made upon it by Max Schultze (I. 29). Part of the granular protoplasm, which contains a few small nuclei, lies within the oval shell, in which there is a wide opening at one pole, whilst the re- mainder protrudes through this open- ing, covering the surface of the shell with a thin layer. If the organism has not been disturbed, very delicate threads of protoplasm (pseudopodia) stretch out from this layer into the water in every direction ; sometimes these psendopodia are exceedingly longi many become forked, others break up into numerous minute threads, whilst yet others send off side branches, which unite with neighbour- ing pseudopodia. FIG. 9. Chondrioderma difforme (from Strasburger) : / part of a fairly old plasmodium ; a dry spore ; b the same, swollen up in water ; c spore, the contents of which are exuding ; d zoospore ; e amoeboid forms, produced by the transformation of zoospores which are commencing to unite together to form a plasmodium. (In d and e the nuclei and con- tractile vacuoles may be seen.) Dujardin gave the name of sarcode to the peculiar substance of which the bodies of the lower organisms, described above, are com- posed, because, like the muscle-substance of the higher animals, it is capable of exhibiting movements. Influenced by Schleiden and Schwann's cell theory, investigators attempted to prove that sarcode was composed of a number of minute cells, so that the sarcode organisms might be included in the cell hyp .thesis. However, the solution to the difficulty was found to be in quite another direction. Investigators like Cohn (I. 7) and Unger were the first to compare sarcode with the protoplasmic contents of a plant-cell, in consequence of the similarity of the vital phenomena. Finally, Max Schultze (I. 29), de Bary (I. 2), and Haeckel (1. 10) established THE CELL beyond a doubt the identity of sarcode with the protoplasm of plant and animal cells ; and this discovery was most helpful to Max Schultze in working out his cell theory, and in estab- > ' lishing his theory of pro- toplasm (p. 6). In Amoeba, lymph cells, Mjcetozoa, and Reticularia, we have learnt to recognise naked cells; those of plants on the contrary are almost invariably enclosed by a well- defined layer, which is sometimes very thick and firm ; this is also very frequently the case with animal cells (membrane, intercel- lular substance), and thus in such cases a little chamber, or cell, in the true sense of the word is formed. Young cells from the neighbourhood of the growing point of a plant, and cartilage cells from a Salaman- der larva, are very good examples of this. The cells at the growing point of a plant (Fig. 12^4), where they multiply very rapidly, are very small, and are very similar to animal cells. They are only separated from one FIG. lO.-Gromia oviformis. (After M. Schultze.) another by Very thin ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 31 cellulose walls. The small cell spaces are completely filled up with the cell-substance, which, with the exception of the nucleus and chlorophyll, consists solely of finely granular protoplasm. Flemming recommends cartilage cells from young Salamander larvae as affording the best and most reliable material for the study of the structure of living proto- plasm (Fig. 11). The cell-substance, which during life, as in the young plant-cells, completely fills the spaces in the cartilaginous ground-substance, is traversed by wavy threads of fairly high refractive power ; these are less than 1 /A in diameter, and are generally most numerous, and at the same time FlG - 1L -Living cartilage ceil . of a Salamander larva, much most wavy, in the neighbourhood of magnified, with distinctly marked the nucleus; sometimes the periphery threads. (After Flemming: from ,, ,, . i -c L- i Hatschek, Pig. 2.) or the cell is nearly, it not entirely, free from threads, but sometimes they are present in great num- bers here also. 2. Cells which contain several different substances in their protoplasm. In plants, and in unicellular organisms, the pro- toplasm frequently contains drops of fluid, in which salt, sugar, and albuminates are dissolved (circulating substances). The further we go (Fig. 12 A) from the growing-point of a plant, where the minute elementary particles of pure protoplasm as described above are grouped, the larger do the individual cells (c) appear, until they are frequently seen to be more than a hundred times as large as they were originally, whilst, in addition, their cellulose wall has become considerably thicker. However, this growth depends only to a very small extent upon any marked increase of the proto- plasmic substance. The cavity of such a large plant cell is never seen to be completely filled with granular protoplasmic substance. The increase in the size of the cell is due much more to the way in which the small amount of protoplasmic substance, which was originally present at the growing point, takes up fluid, which in the form of cell-sap separates out into small spaces in the interior, called vacuoles. By this means a frothy appearance is produced (Fig. ]2 B, s). More or less thick protoplasmic strands stretch out from the mass of protoplasm in which the nucleus is embedded. These strands serve to separate the individual sap vacuoles from one 32 THE CELL another, and in addition they unite together on the surface to form a continuous layer (primordial utricle), which adheres closely to the inner surface of the enlarged and thickened cellulose membrane. Two different conditions which are found in the fully grown plant cell are the result of this arrangement. Through the fur- ther increase of the cell-sap, the vacuoles are en- larged, and the partition wall at- tenuated. Finally the latter par- tially breaks down, so that the separate spaces are connected by openings, and thus form one continuous vaca- ole. Consequent- ly part of the protoplasmic sub- stance becomes transformed into a fairly thin layer lying close to the cellulose mem- brane, and the rest into more or less numerous strands and threads travers- ing the large con- tinuous vacuole which is filled with fluid (Fig. 12, right side, and FIG. 12. Parenchyma cells from the cortical layer of the root of Fritillariaimperialig (longitudinal sections, x 560: after Sachs II. 33, Fig. 75): A very young cells, as yet without cell-sap, from close to the apex of the root ; B cells of the same description, about 2 mm. above the apex of the root ; the cell- sap (o) forms in the protoplasm (p) separate drops between which are partition walls of protoplasm ; C cells of the same description, about 7-8 mm. above the apex ; the two lower cells on the right hand side are seen in a front view; the large cell on the left hand side is seen in optical section ; the upper right hand cell is opened by the section ; the nucleus (xy) has a peculiar appearance, in consequence of iOs being dis- tended, owing to the absorption of water; fc nucleus; fefe nu- cleolus ; h membrane. ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 33 Fig. 13). Finally, in other cases, even these strands of protoplasm in the interior of the cell may disappear. Then the protoplasmic substance is represented solely by a thin skin, which lines the interior of the little chamber, to use an expression of Sachs (II. 33), as the paper covers the walls of a room, and which con- tains one single large sap vacuole (Fig. 12 (7, left lower cell, and Fig. 59). In very large cells this coating is sometimes so thin that, except for the nucleus, the presence of protoplasm can hardly be demonstrated at all in the cell, even when a high power of the microscope is used, so that special methods of investigation are necessary in order to render it visible. FIG. 13. A cell from a hair on a staminal filament of Tradescantia virginica ( x 240 : after Strasburger, Practical Botany, Fig. 15). FIG. 14. (Edogonium, during process of form- ing zoospores (after Sachs; from R. Hertwig's Zoologie, Fig. 110): A a portion of the thread of the alga, with the cell contents just escap- ing ; C zoospore, which has reached the exterior ; D stationary spore undergoing germination. It was by the study of such cells, that the earlier investigators, such as Treviranus, Schleiden, and Schvvann, arrived at their conception of the cell. Hence it is not surprising that they con- sidered that the cell membrane and the nucleus constituted the essential portions of the cell, and quite overlooked the importance of the protoplasm. That this latter is the true living body in the plant-cell too, and that it is able to exist independently of the D 34 THE CELL membrane, has been proved beyond a doubt by the following observation, which has played such an important part in the history of the cell theory (I. 7). In many algae (CEdogonium, Fig. 14), at the time of reproduction, the protoplasmic substance becomes detached from the cellulose cell-wall, and, whilst pai-ting with some of its fluid contents, contracts up into a smaller volume, so that it no longer quite fills up the cavity ; it thus forms a naked swarmspore, which is either globular or oval in shape (A). After a time this swarmspore breaks down the original cell-wall, and, escaping through the opening it has made, reaches the exterior. It then develops cilia ((7) upon its surface, by means of which it moves about pretty quickly in the water, until after a time it comes to rest (D), when it differentiates a delicate new membrane upon its surface. Thus Nature herself has afforded us the best evidence that the protoplasmic body is the true living elementary organism. A similarly great formation of vacnoles and separation of sap, as is found in plant-cells, is also seen in the naked protoplasm of the lower unicellular organisms, especially in certain Reticularia and Radiolarians ; thus the Actinosphcerium, which is depicted in Fig. 15, presents quite a frothy appearance, resembling the fine froth which is produced when albumen or soap-suds are beaten up. An immense number of larger and smaller vacuoles, filled with fluid, are distributed throughout the whole body. These are only separated from one another by delicate partition walls of proto- plasm, which are sometimes too thin to be measured. The protoplasm consists of a homogeneous ground substance, in which granules are embedded. The result of this formation of vacuoles is that the protoplasmic substance becomes broken up, so that surfaces of it become exposed to the nutrient solutions in the vacuoles, in consequence of which diffusion can take place between them. Evidently the whole arrangement adds considerably to the facility with which materials are taken up and given out. This internal increase of surface may be compared with the external increase of surface, which is shown in the formation of many-branched pseudopodia (Fig. 10), and indeed it answers the same purpose. In animal-cells, on the contrary, the formation of vacuoles and the secretion of sap only take place extremely rarely, for instance, in notochordal cells; on the other hand, adventitious substances, such as glycogen, mucin, fat globules, albuminous substances, etc., ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 35 are more frequently found ; these either distend the cell or render it somewhat solid. When there has been a considerable develop- ment of such substances, the protoplasm may again assume a frothy appearance, as in Actinosphcerium (Fig. 15), or it may become transformed into a network structure, as in a Tradescantia cell (Fig. 13), the only difference being that the interstices are filled with substances denser than sap. FIG. 15. ActinosplKerium EicTihorni (after R. Hertwig, Zoologie, Fijr. 117) : M medullary substance, with nuclei (n) ; B peripheral substance, with contractile vacuoles (cv); Na nutrient substances. The most perfect examples are often seen in animal egg-cells The exceedingly large size, which is attained by many of these, is not so much caused by an increase of protoplasm, as by the storing up of reserve materials, which vary very much as to their chemical composition, being sometimes formed and sometimes unformed substances, and which are intended for future use in the economy of the cell. Very often the egg-cell appears to be almost entirely composed of such substances. The protoplasm only fills up the small spaces between them, like the mortar between the stones of 36 THE CELL a piece of masonry (Fig. 16) ; if a section be made of an egg, the protoplasm is seen to be present in the form of a delicate net- work, in the larger and smaller meshes of which these i-eserve substances are deposited. The only place where it is collected together into a thick, cohesive layer is on the surface of the egg, and in the neighbourhood of the nucleus. Another good example of a protoplasmic framework structure, caused by the deposition of various substances, is afforded us by the mucous cells of vertebrates (Fig. 17) and invertebrates. The section varies according as to whether it is taken from the epithelial surface, or from the base of the goblet. In the former case it is wider, and is seen to consist chiefly of homogeneous shining secretion, the mucilaginous substance, which is evacuated FIG. 16. An egg of Ascaris megalocephala, FIG. 17. Goblet-cell from the which has just been fertilised (after Van Bene- bladder epithelium of Squatina vul- den ; from O. Hertwig, Fig. 22) : sfc spermato- garis, hardened in Muller's fluid, zoon, with its nucleus which has just entered ; (After List, Plate I., Fig. 9.) / glistening fatty material of spermatozoon: fcb female pronucleus. from time to time by the cell, through a small opening at its free end, and transformed into mucin. The protoplasm traverses the mass of secretion in the form of fine threads, which join together to make a wide meshed network, only forming a compact body at the lower extremity of the cell, in which also the nucleus is situated. II. The Chemico-physical and Morphological Properties of the Nucleus. The nucleus is quite as important as the | protoplasm in the economy of the cell. It was first discovered, ! in 1833, by Robert Brown (I. 5), in plant-cells ; soon afterwards ( Schleiden (I. 28) and Schwann (I. 31) made it the foundation stone of their theory of cell formation ; after that the study of the nucleus remained for some time in the background, as the ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 37 interesting 1 vital phenomena of the protoplasm became more fully known. During the last thirty years, however, one discovery after another has been made about the nucleus, the result of which is that this neglected body has been shown to be of as much import- ance to the elementary organism as the protoplasmic substance. It is of interest that the history of the nucleus is analogous in some respects to that of the cell. The nucleus was also con- sidered at first to consist of a vesicle; indeed, it was even held to be a smaller cell inside the larger one. But just as it came to be recognised that the protoplasm is the vital substance of the cell, so by degrees it came to be seen that the form of the nucleus is of minor importance, and that its vitality depends far more upon the presence in it of certain substances, the arrangement of which may vary very considerably according as to whether the nucleus is in an active or a passive condition. Richard Hertwig (II. 18) was the first to enunciate this clearly in a short paper entitled, " Beitrage zu einer einheitlichen Auffassung der verschiedenen Kernformen," in the following words : "It is necessary to state at the commencement of my observations, as the most important point to be considered in classifying the various nuclear forms, that they all possess a certain uniformity in composition. Whether the nuclei of animals, plants, or Protista be under examination, it is invariably seen that they are composed of a larger or smaller quantity of a material which, like the earlier writers, I shall call nuclear/ substance (nuclein). We must commence with the properties of 1 this substance in the same way as he who wishes to describe the important characteristics of the cell must begin with the cell substance, i.e. protoplasm." Hence the nucleus is now defined, not> according to Schleiden and Schwann's idea, as a vesicle in the cell, but as a portion of a [ special substance which is distinct from the protoplasm, and to a , certain extent separate from it, and which may vary considerably, as to form, both in the resting ami in the actively dividing condition. We will now consider the form, the size, and the number of nuclei in a cell, and then the substances contained in the nucleus, and their various modes of arrangement (the structure of the nucleus). a. The form, size and number of Nuclei. As a rule the nucleus in plant and animal-cells appears as a round or oval body (Figs. 1, 2, 6, 16), situated in the middle of the cell. Since it is 38 THE CELL frequently richer in water than protoplasm is, it may be dis- tinguished from the latter even in the living cell, appearing as a bright spot with indistinct outlines, or as a vesicle or vacuole. But this is not always the case. In many objects, such as lymph corpuscles, corneal cells, and the epithelial cells of gills of Sala- mander larvae, no nuclei can be distinguished during life, although they immediately become visible when coagulation, induced either by the death of the cell, or by the addition of distilled water or weak acids, occurs. In many kinds of cells, and in the lower organisms, the nucleus may assume very various shapes. Sometimes it is in the shape of a horse-shoe (many Infusoria), sometimes of a more or less twisted FIG. 18. (After Paul Mayer, from Korschelt, Fig. 12.) A A piece of the seventh appen- dage of a young Phromma, 5 mm. in length (x 90). B A piece of the sixth appendage of a half-grown Phronimella (x 90). C A group of cells from a gland in the sixth appendage of a Phronimella ; the nucleus is only shown in two cells (x90). strand (Vorticella), and sometimes it is very much branched, stretching into the protoplasm in every direction (Fig. 18 B, C). This latter form chiefly appears in the large gland-cells of many insects (in the Malpighian tubes, in the spinning and salivary glands, etc.), and similarly in the gland-cells of the crustacean Phronima. The size to which the nucleus attains is generally proportional to the size of the mass of protoplasm surrounding it; the larger this is, the larger is the nucleus. Thus, in the great ganglionio ITS CHEM1CO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 39 cells of the spinal cord, extremely large vesicular nuclei are seen. Similarly, enormously large nuclei occur in immature egg-cells, which themselves are of a great size. Sometimes the nuclei of immature eggs of Fishes, Amphibians, and Reptiles are perceptible to the naked eye as small spots ; 'under these circumstances they can be easily extracted with needles and isolated. Yet there are exceptions to this rule ; for even these same eggs which, when immature, have such immense nuclei, when they are mature and fertilised contain such minute nuclei, that they can only be demonstrated with the greatest difficulty. The lowest organisms, when of a con- siderable size, frequently possess one single large nucleus. It is sometimes enormously large in the central capsules of many Radiolarians. As regards the number present, as a general rale there is only one nucleus in each cell in plants and animals. To this rule, however, there are some exceptions ; there are frequently two nuclei in liver cells, whilst a hundred or more have been observed in the giant cells of bone marrow. Osteoclasts and the cells of many tumours, the cells of several Fungi, and of many of the lower plants, such as Cladophora (Fig. 19) and Siphonese (Bo- try dium, Vaucheria, Caulerpa, etc.), are remarkable for this plurality of nuclei, as has been described by Schmitz. Similarly, a large number of the lowest organisms, such as Myxomycetes, many Mono- and Poly-thalamia, Radio- larians, and Infusoria (OpaMna ranarum\ possess many nuclei in each cell. Fre- quently in these cases the nuclei are so minute, and are distributed in such numbers throughout the protoplasm, that they have only been demonstrated quite recently by means of the most improved methods of staining (Myxomycetes). FIG. 19. dadophor ata. A cell from a thread in a chromic acid carmine prepara- tion (after Strasburger, Pract. Botany, Fig. 75) : n nucleus ; ch chromatophores ; p amyloid bodies (pyrenoids) ; a starch granules ( x 540). 40 THE CELL b. Nuclear Substance. As regards its composition, the nucleus is a fairly fixed body. Two chemically distinct proteid substances, which can be distinguished from one another with the \ microscope, are always present ; very often there are more. The two constant ones are nuclein or chromatin, and paranuclein, or pyrenin ; in addition, linin, nuclear sap, and amphipyrenin are generally to be found. <* Of these, NUCLEIN, or chromatin, is the most characteristic pro- teid of the nucleus, and it generally preponderates as regards quantity. When fresh it resembles non-granular protoplasm (hyaloplasm), but it can be easily distinguished from this substance by its behaviour towards certain staining solutions. After it has been caused to coagulate by means of reagents, it takes up the colouring matter from suitably prepared staining solutions (solu- tions of carmine, haematoxylin, aniline dyes), as has been discovered by Gerlach. This occurs to a more considerable extent during the stages preceding division, and during division itself, than when the nucleus is in a resting condition. Whether this is due to chemical or to physical causes has not yet been worked out. The art of staining is now so fully understood that it is quite easy to make the nuclein of the nucleus stand out clearly from the rest of the nucleus and the protoplasm, which are either quite colourless or are only slightly stained. In this manner even small particles of nuclein, only about as large as Bacteria, may be rendered visible in comparatively speaking large masses of protoplasm, as, for example, the minute heads of spermatozoa, or the chromosomes of the direction spindles in the centres of large egg-cells. The following fact, which is emphasised by Fol (II. 13), may at some future period prove to be of far-reaching importance : "that the staining of the nucleus with neutral staining solutions always produces the same shade of colour as the dye in question assumes when a small quantity of a substance of basic reaction is added to it. For example, red alum carmine becomes lilac when the solu- tion is rendered slightly alkaline, Bohmer's violet haematoxyliii becomes blue, red ribesia (blackcurrant juice) bluish-green, whilst the red dye made from red cabbage turns green. Now, it has been observed that nuclei of tissue-cells, stained with neutral solutions of these substances, exhibit a corresponding colouration; that is to say, they become lilac in alum carmine, blue in haematoxylin, light blue in ribesia, green in the colouring matter of red cabbage. That part of the nucleus which can be stained (the nuclein) behaves, ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 41 as a rule, towards the staining substance united to it, like a wealdy alkaline body " (Fol). Further, nuclein exhibits characteristic chemical reactions, which must not be forgotten in preparing nuclear structures for preservation (Schwarz II. 37, Zacharias II. 43, 45). It swells up in distilled water, in very dilute alkaline solutions, and in 2 or more per cent, solution of common salt, of sulphate of magnesia, or of monopotassium phosphate and of lime-water. If solutions of from 10 per cent, to 20 per cent, of the above-named salts are used, the nuclein, whilst swelling gradually, becomes quite dis- solved. Similarly, it dissolves completely in a mixture of ferro- cyanideof potassium and acetic acid, or in concentrated hydrochloric acid, or if it is subjected to pancreatic digestion. It becomes pre- cipitated in a fairly unaltered form if treated with acetic acid from 1 to 50 per cent, in strength, when it can be very clearly distinguished from the protoplasm by its greater refractive power, and by a glistening appearance which is peculiar to it. FIG. 20. A resting nucleus of a spermato-genetic cell of Ascaris megalocephala bivalens. B Nucleus of "a sperm-mother-cell from the commencement of the growth-zone of Ascaris megalocephala bivalent. C Resting nucleus of a sperm-mother-cell of the growth zone of .Ascaris megalocephala hivalens. D Bladder-like nucleus of a sperm-mother-cell of .Ascaris megalocephala bivalens, from the commencement of the dividing zone, shortly before division. In the nuclear vesicle (Fig. 20), the nuclein sometimes appears as isolated granules (A), or as delicate network (B, 0), or as threads (D). Miescher (II. 49) has attempted to obtain pure nuclein from pus corpuscles and from spermatozoa, in the heads of which it is present. An important ingredient in its composition is phosphoric acid, of which at least 3 per cent, is always present. Several facts seem to indicate that the nuclein of the nucleus " consists of a combination of an albuminous body with a complex organic com- pound containing phosphoric acid (Kossel II. 35). This latter has been called nucleic acid, and Miescher has calculated its formula to be C 29 H 49 N 9 P 3 22 . " If subjected for a long time to the action of weak acids or alkalies, or even if kept in a damp condition, nuclein becomes de- 42 THE CELL composed, albumen and nitrogenous bases being formed, whilst in addition phosphoric acid separates out. The two latter decom- position products are also formed from nucleic acid. The bases are: adenin, hypoxanthin, guanin, and xanthin." PARANUCLEIN, or pyrenin, is a proteid substance, which is always present in the nucleus ; however, the part it plays in the vital functions of the latter has not yet been worked out, much less being known about it than about nuclein. It occurs in the nucleus in the form of small granules, which are described as true nucleoli or nuclear corpuscles (Fig. 20). These paranuclein bodies resist the action of all the media (distilled water, very dilute alkaline solutions, solutions of salt, sulphate of magnesia, potassium phosphate, lime-water) in which nuclein substances swell up. Whilst the latter disappear from view in the nuclear cavity, which has become homogeneous in appearance, the former often stand out with greater clearness. They are invariably more easily seen after death than daring life. This explains the fact that these nuclear corpuscles were well known long ago to the older histologists, Schleiden and Schwann, who always examined their tissues in water. Osmic acid is a very useful reagent for rendering these corpuscles visible, for it very much increases their refractive power, whilst rendering the nuclein structures paler. Paranuclein and nuclein behave quite differently towards acetic acid (1 to 50 per cent.). Whilst the latter coagulates, and in- creases in refractive power, the nuclear coi'puscles swell up more or less, and may become quite transparent; however, they do not become dissolved, for if the acetic acid is washed away, they shrink up, and become visible again. In addition, it must be pointed out that paranuclein, in contradistinction to nuclein, is insoluble in 20 per cent, solution of common salt, in a saturated solution of sulphate of magnesia, in 1 per cent, and 5 per cent, solutions of potassium phosphate, of ferrocyanide of potassium plus acetic acid, and of copper sulphate ; finally, it is very resistent to the action of the pan- creatic juice. Further distinct differences are shown in their behaviour to- wards staining solutions. As Zacharias has observed, and as I can corroborate as a general rule from my own experience, nuclein bodies become especially clearly and intensely coloured in acid staining solutions (aceto-carmine, methyl green, and acetic acid), ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 43 whilst parannclein bodies remain almost unaffected ; on the other hand, the latter become better stained in ammoniacal staining solu- tions, such as ammonia, carmine, etc. Many substances, such as eosin, acid fuchsine, etc., have a greater affinity for paranuclein. Hence it is possible, by using two staining solutions at the same time, to stain the nuclein bodies a different colour from the para- nuclein ones, thus bringing about a so-called contrast staining (fuchsine and solid green, haematoxylin and eosin, Biondi's stain); however, since the nature of staining processes is as yet very im- perfectly understood by us, it is not possible at present to lay down general rules concerning the staining properties of these two nu- clear substances. I consider that nuclein and paranuclein are the essential constituents of the nucleus, and that its physiological action depends in the first instance upon their presence. They seem to me to be correlated in some way or other. Flemming (II. 10) has suggested, that the nucleoli may consist of nuclein in a special condition of develop- ment and density, thus representing a preliminary chemical phase of it. The material that we have at present for examination is not sufficient to enable us to decide these questions. The three other substances which may be distinguished in the nucleus, linin, nuclear sap, and amphipyrenin, appear to me to be of much less importance ; it is possible also that they are not always present. The name LININ has been applied by Schwarz (II. 37) to the material of which the threads, which frequently form a network or framework in the nuclear cavity, consist ; these threads are not affected by the ordinary staining reagents used for the nucleus, and can by this means, as well as by their different chemical re- actions, be easily distinguished from the nuclein, which is deposited upon them in the form of small particles and granules (Fig. 20 A, C). In many respects it resembles the plastin of proto- plasm, and indeed Zacharias has called it by that name. NUCLEAR SAP may be present in larger or smaller quantities ; it fills up the interstices left in the structures composed of nuclein, linin, and paranuclein. It may be compared to the cell-sap which is contained in the vacuoles of the protoplasm, and no doubt functions in a similar manner, by nourishing the nuclear substances, just as the cell-sap nourishes the protoplasm. By the action of several reagents, such as absolute alcohol, chromic acid, etc., finely granu- lar precipitates are caused to make their appearance in the nuclear 44 THE CELL sap ; these, being artificial products, must not be confused with the normal structures. Hence cell-sap must contain various substances in solution, amongst which albuminates are probably present; Zacharias has grouped these together under the common name of paralinin, a term which may well be dispensed with. The name AMPHIPYRENIN has been applied by Zacharias to the substance of the membrane which separates the nuclear space from the protoplasm, just as this latter is separated from the ex- terior by the cell membrane. In many cases it is as difficult to demonstrate the presence of this nuclear membrane, as to decide the vexed question whether a large number of cells are enclosed by membranes or no. It is most easily seen in the large germinal vesicles of many eggs, such as those of Amphibians, where it is at the same time somewhat dense in consistency. It is on this account that it is so easy to extract the nucleus quite intact from immature eggs with a needle. The nuclear membrane can be ruptured, as a result of which its contents flow out, and may be spread out in the liquid in which the examination is taking place. But it seems to me to be equally certain that, in other cases, a true nuclear membrane is absent, so that the nuclear sub- stance and protoplasm come into direct contact. Thus Flemming (II. 10), in the blood cells of Amphibians, and I myself, in the sperm-mother-cells of Nematodes at a certain stage of their develop- ment (Fig. 20 .B), have failed to discover a nuclear membrane. ALTMANN has endeavoured, by means of a special staining process with cyanin, to demonstrate a granula structure in the nucleus as well as in the protoplasm. By means of this process he has succeeded in intensely staining the sap which fills up the interstices in the nuclear network, and in thus showing up granula, whilst the nuclear network remains uncoloured, and is designated intergra'iu'a substance. In this manner Altmann has obtained a, so to speak, negative impression of the nuclear structure, as it becomes re- vealed by staining the nuclear network with the usual nuclear staining reagents. Since he considers that the granula form the most important part of the nucleus, his opinion of the relative importance of the nuclear sub- stances differs from the one which is generally accepted, and according to which the nuclear sap is of less importance than the nuclein and part" which lies in a net-knot of the frame- work, where the greatest number of linin threads intersect. In the enormously large germinal vesicles, for which the large eggs of Fishes, Amphibians, and Reptiles, which are so rich in yolk, are remarkable, the number of germinal spots increases consider- FIG. 28. Flo _ 29. FIG. 29. Immature egg from the ovary of an Echinoderm. In the large Kerminnl vesicle there is a network of threads, the nuclear net, in which the germinal spot can be seen. lO. Hertwig, Embryology, Fig. 1.) FIG. 29.-Germinal vehicle of a small immature egg from the Frog. In a dense nuclear net (fen) a very large number of germinal spots, mostly peripheral (fc/), are to be seen. (O. Hertwig, Embryology, Fig. 2.) ably during the growth of the cell, until finally they may number some hundreds ; whether this multiplication takes place by division or in some other fashion is not yet known. The position of the germinal spots varies at different times ; generally, however, they are situated on the surface of the vesicle, being distributed at even distances over the membrane, as is shown in Fig. 29, where the nucleus of a rather small immature egg of a frog is depicted. The shape of the germinal spots also varies ; they may be round this is especially the case when they are isolated or oval ; some- times they are somewhat extended, at others they are constricted in the middle ; occasionally they are irregular in outline, and when ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 51 they are very numerous, they show considerable differences in their size. Very frequently a few small vacuoles filled with fluid are to be seen. The examination of living 1 egg-cells shows that these vacuoles are not artificially produced. Additional vaeuoles may be formed after the death of the egg, whilst those already present may increase in size, as has been pointed out by Flemming (II. 10, p. 151). These germinal spots differ in their chemical properties from true nucleoli, which consist of paranuclein and do not become stained with the usual nuclear staining reagents. On the other hand, it has not yet been discovered whether their substance is quite iden- tical with the nuclein of the framework. Up to the present this point has not yet been satisfactorily worked out, in spite of the numerous experiments which have been made upon the nucleus. One thing alone can be accepted as certain that the more or less rounded bodies present in various plant and animal nuclei, which in scientific literature are classed together, for the most part incorrectly, under the name of nucleoli, show material differences amongst themselves. This has been proved beyond a doubt by the investigations made by Flemming (II. 10), Carnoy (II. 8), myself (II. 19a), Zacharias (II. 45), and others. Either such very different bodies should not be called by the same name, or if, merely on account of their similarity in form, the common name of nucleolus or nuclear body is retained for all round nuclear contents, at any rate in each case an accurate description of the chemical nature of the nucleolus in question should be given. Above all, as has been already remarked, in all examinations of the nucleus, more attention should be paid to the chemical properties of its individual constituents than to their form and arrangement, which are always of comparatively little importance. For the function of a framework in the nucleus composed of linin threads differs considerably from that of one consisting of nuclein, or of a combination of the two substances, and similarly the function of the nucleolus varies according to the material of which it is composed. I will conclude this discussion of nucleoli with the remark that germinal spots exist which are most evidently built up of tw<>\ different substances. This circumstance was first observed by Leydig in a lamellibranchiate Mollusc, and his statement has since been verified by Flemming (II. 10) from observations on the same animal, and by myself (II. 19) from those on other objects. I here quote the description as it is given by Flemming. In Cyclas cornea and in the Naiadese a pi-incipal nucleolus, in addition to a few smaller secondary nucleoli, is present in the germinal vesicle. " The former consists of two differently consti- tuted poi^tions ; these may be seen in Fig. 30 as a smaller, strongly stained more refractive part, and a larger, paler, less chromatic one, which swells up more in acids. In Anodon these two portions are closely coherent; in Unio they very frequently only just touch each other, or, indeed, may lie apart. The smaller secondary nucleoli, which lie in the meshes of the framework, show the same power of refracting light, of swelling up, and of becoming stained, as the larger portion of the principal nucleolus. If water is added, this larger portion disappears, as well as the small nucleoli, amongst the strands of the framework ; the small, strongly chromatic portion of the prin- cipal nucleolus alone remains ; this becomes more sharply de- fined, shrinking up somewhat, and developing a clearly marked outline. The addition of strong acetic acid (5 per cent, or more) causes the larger paler portion of the principal nucleolus to swell up rapidly and to dis- appear, whilst the smaller shin- ing portion, though also swell- ing up somewhat, remains visi- ble." "When nuclear staining reagents are used, both portions of the nucleolus, and also the FIG. 30.-(A.fier Flamming, Fig. E', p. 101.) o Nucleus ot an egg from the ovary of Uiiio : it has just emerged from the cell into the ovarian fluid. Nucleolus with two pro- tuberances. A small portion of the nuclear framework is visible; a a similar nucleus after 5 per cent, acetic acid has been added. The framework strands stand out more clearly; the larger paler portion of the- principal nucleolus, as well as the minor nucleoli, have similarly become swollen up and faded ; the smaller portion of the principal nucleolus is also swollen up, but to a less degree, b Nucleolus of an egg of Tichogonia p.ilymoi-pfia ; the principal glistening portion rests like a cap upon the In.rger one. ft Diagrammatic representation of an optical section of above. secondary nucleoli, become coloured to a considerable ex- tent; the most strongly refrac- tive part of the former, however, is especially intensely stained." "Such a differentiation of the principal nucleolus into two parts occurs in the egg-cells of many animals. In Dreissena polymorplia the strongly refractive chromatic portion covers the paler one like a hollow cap." ITS CHEMICO-PHYSCCAL AND MORPHOLOGICAL PROPERTIES 53 I have observed (IT. 19) that the germinal spot is composed of two substances in Helix, Tellina, and Asteracanthion, as well as in Anodon. Asteracanthion (Fig. 31) is of special interest, as the sepai-ation into two substances (p n, n n) only becomes distinctly visible when the germinal vesicle commences to break up and to form the polar spindle out of its contents. Finally, in the description of the structure of the resting nucleus, attention must be drawn to one other important point. According to tl into form B, and this during the process of development of the spermatozoon into form C ; the youngest sperm mother cells ( B) have naked nuclei containing dense nuclear frameworks, and superficially-placed nucleoli ; this form develops in older cells (C) into a vesicular nucleus with a distinctly marked membrane. In the vesicle a few linin threads are extended through the nuclear sap, the nuclein heaped up into one or two irregular masses, amongst which the more or less globular nucleolus is situated. In cells which are not yet mature, the nuclein is collected chiefly at one spot of the nuclear membrane in the form of a thick layer, whilst granules of varying size lie upon the surface of the linin 54 THE CELL threads, a few of which are extended throughout the nuclear space. A considerable time before division occurs, the nuclein FIG. 32. A Resting nucleus of a primitive sperm cell of Ascaris megaloceplwla bivnlens ; B nucleus of a sperm mother cell from the commencement of the growth zone ; C resting nucleus of a sperm mother cell from the growth zone; D vesicular nucleus of a sperm mother cell from the commencement of the dividing zone just before division. becomes arranged in definite threads (D). A nucleolus is always present in the meshes of the framework. III. Are there Elementary Organisms existing without Nuclei ? The important question, as to whether the nucleus is an indispensable portion of every cell, follows naturally on the description of the chemical and morphological properties of the nucleus. Are there elementary organisms without nuclei ? Formerly investigators were not at a loss to answer this question. For since, in consequence of the inadequacy of former methods of examination, no nuclei had been discovered in many of the lower organisms, the existence of two different kinds of elementary cells was assumed : more simple ones, consisting only of a mass of protoplasm, and more complex ones, which had developed in their interior a special organ, the nucleus. The former were A called cytodes by Haeckel (I. 10; II. 15), to the simplest, solitary forms of which he gave the name of Monera; the latter he called cellulas, or cytes. But since then the aspect of the question has become considerably changed. Thanks to the improvements in optical instruments, and in staining methods, the existence of organisms without nuclei is now much questioned. In many of the lower plants, such as Algre and Fungi, and in Protozoa, Vampyrella, Polythalamia, and Myxomycetes, all quoted formerly as examples of non-nucleated cells, nuclei may now be demonstrated without much trouble. Fui'ther, since the nucleus has been discovered in the mature ovum (Hertwig II. 19 a), we may safely say that, in the whole animal kingdom, "j there is not a single instance where the existence of a cell with- out a nucleus has been proved. I shall probably be confronted with the red corpuscles of Mammals. It is true that they contain no nuclei, but then neither do they contain any true proto- ITS CHEMICO-PHTSICAL AND MORPHOLOGICAL PROPERTIES 55 plasm, and hence the theory, more f ally described later, that the ) blood discs of Mammals are not true cells, but only the products^ of the metamorphosis, or of the development of former cells, may be defended for many reasons. The only remaining instance of cells in which, on account of their extreme minuteness, no differentiation into protoplasm and nuclear substance can be demonstrated, is furnished by Bacteria and other allied forms. However, even here Biitschli (II. 6) has endeavoured to prove the existence of a nnclear-like body. Thus in Oscillaria and in others (Fig 1 . 33 A, B), he has pointed out bodies which are not digested by gastric juice, and which contain a few granules, which stain intensely (probably nuclein granules) ; these make up the greater part of the cell substance, the protoplasm B being present only as a delicate envelope. Biitschli's views are for the most part shared by Zacharias (II. 47). Even if it is objected that the above statement is at present nn- proven, it cannot be denied that the supposition that Bactei-ia con- sist entirely, or principally, of nu- clear substance, seems at any rate as probable, if not more so, as the one that they are minute masses of pure protoplasm. The extra- ordinary affinity of these organisms for staining reagents is very much in favour of the first view. IV. The Central or Pole Corpuscles of the Cell. Long ago an exceedingly minute object, which, on account of its function, is of the greatest importance, was observed in addition to the nucleus in the protoplasm of some cells ; this is the central or pole corpuscle (centrosome) . This was first noticed during cell division (which is described later on in Chapter IV.), and here it plays a most important part, as it forms a central point for the peculiar radiated appearances, and above all functions as the centre of the cell, around which the various cell contents are, to a certain extent, arranged. As to size, it is only just visible, and is frequently much smaller m FIG. 33. A Otcillaria -. Optical section of a cell from a thread, killed with alcohol and stained with hsematoxylin (after Butschli, Fig. 12 a). B Bacterium Inieola (Cohn), in optical section, killed with alcohol and stained with haema- toxjlin (after Biitschli, Fig. 3 a). 56 THE CELL than the most minute micro-organism. As to its composition, it appears to consist of the same substance as the so-called neck or middle portion of the seminal thread, to which, further, during the process of fertilisation, genetic functions have been ascribed (vide Chap. VII., 1). When the ordinary methods for staining the nucleus are employed it does not absorb any of the dye ; if, however, special reagents, especially acid aniline dyes, such as acid fuchsine, safranin, and orange, are used, it becomes vividly coloured. This is the only way to distinguish the central cor- puscle from the other granules in the cell (microsomes) unless it is enclosed by a special radiation sphere or envelope. If we dis- regard the processes of cell division and of fertilisation, which are treated of in later sections, the central corpuscles have been, up till now, most frequently observed in lymph cells (Flemming II. 11, 12 6, and Heidenhain II. 16), in the pigment cells of the Pike (Solger II. 38), and in the flattened epithelial, endothelial, and connective tissue cells of Salamander larvae (Flemming II. 12 6). As a rule there is only one central corpuscle present in each lymph cell (Fig. 34) ; this can be seen without having been stained, since the protoplasm in its im- mediate neighbourhood assumes a distinctly ray-like appearance forming the radiation, or attraction sphere, which later on will occupy so much of our attention. The cen- tral corpuscle is sometimes situated in an indentation of the nucleus", or, if the latter has broken down into several pieces, a con- dition which is frequently seen in lymph cells, it lies between them and some portion or other of the protoplasmic body. In pigment cells (Fig. 35), Solger (II. 38) was able to make out the radiation sphere as a bright spot between the pigment gran- ules, and in consequence he concluded that the central corpuscle was present. In the epithelium of the lung, and in the endothelium and connective tissue cells of the peritoneum of Salamander larvce (Fig. 36 A, J>), Flemming found, almost without exception, that instead of a single central Pis. 34. Leucocyte from the peritoneum of a Salamander larva. For the sake of clearness in the figure, the central cor- puscle, surrounded by its radiation sphere, has been distinguished by a bright rinif, which is nt really present iu nature. (After Flemming, Fig. 5.) ITS CHEMICO-PHYSIOAL AND MORPHOLOGICAL PROPERTIES corpuscle, two were present, lying close together, either in the im- mediate neighbourhood of the resting nucleus, or in an indenta- tion of it, directly in contact with the nuclear membrane. As a rule no radiation sphere was to be seen in these cases ; some- times the two central corpuscles, instead of touching each other closely, were somewhat separated from one another, and under these circumstances the first com- mencement of a spindle formation between them was visible. FIG. 35. Pigment cell of the Pike, with two nuclei, and one pole corpuscle, sur- rounded by a radiatioa sphere. (After Solg.r, Fig. 2.) FIG. 36. A Nucleus of an endothelial cell from the peritoneum of a Salamander larva, vrith the pole corpuscle lying near (after Flemming, Fig. 2). B Nucleus of a con- nective tissue cell from the peritoneum of a Salamander larua, with the polo corpuscle lying near (after Fleminiug, Fig. 4). Van Beneden (II. 52) first advanced the theory that the central corpuscle, like the nucleus, is a constant orgun of each cell, and that it must be present in the cell in some portion of the protoplasm near the nucleus. The property possessed by the central corpuscle of being able to multiply itself by spontaneous division (vide Chap. VI.) seems to be in support of the first part of this view, as is also the role it plays in the process of fertilisation (vide Chap. VII. 1) ; but the second portion of this theory, although it is very generally accepted, that the central corpuscle belongs to the protoplasm, appears to me, on the contrary, less certainly true. 58 THE CELL I have for some time held the opinion, which, for reasons that I will state later (vide Chap. VI.), I still hold to be worthy of con- sideration, that the central corpuscles are generally constituent parts of the resting nucleus, since after division has taken place they enter its interior, and whilst it is preparing for division come out again into the protoplasm. Only in rare cases do the central corpuscle or corpuscles remain in the protoplasm itself, whilst the nucleus is resting, and then to a certain extent they represent a subordinate nucleus in addition to the principal one. This theory would explain the fact that, even with the more recent methods and most improved optical instruments, the central corpuscles as a rule cannot be demonstrated near the resting nucleus in the protoplasm of the cell. V. Upon the Molecular Structure of Organised Bodies. In order to explain the chemico-physical properties of organised bodies, Nageli (V. 17, 18 ; II. 27, 28) has advanced a micellar theory, which, although undoubtedly to a great extent hypothetical, is very useful in rendering many complicated conditions more easy of comprehension, and above all more easily pictured to the imagination. A short abstract of this micellar theory, which de- serves attention, if only on account of the strictly logical manner in which it has been worked out, will not be out of place here. One of the most remarkable properties of an organised body is its capacity of swelling up, that is to say, of absorbing into its interior a large, though not unlimited, quantity of water, with the substances dissolved in it. This may take place to such an extent that in an organised body only a small percentage of solid sub- stances may be present. The body increases in size in proportion to the amount of water absorbed, shrinking up again when the water is expelled. Hence the liquid is not stored up in a pre-existent cavity, which before was filled with air, as in a porous body, but becomes evenly distributed amongst the organised particles, which, as the body swells up, must become farther and farther pushed apart, being separated from one another by larger and larger envelopes of water. In spite of the absorption of so much water, none of the organised substance becomes dissolved. In this respect the phenomenon differs from that which takes place with a crystal of salt or sugar, which on the one hand does not possess the power of swelling up, and on the other becomes dissolved ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 59 in the water, its molecules separating from one another, and dis- tributing themselves evenly throughout the water. Its power of swelling up and its non-solubility in water are the most important properties of an organised body, without which it is inconceivable that the vital processes could proceed. Many organised bodies may be dissolved if treated according to special methods, as for example starch and gelatine-producing substances, which become dissolved when they are boiled in water. But even these starch and gelatine solutions differ very much in their chemical properties from solutions of salt or sugar. The latter diffuse easily through membranes, whilst the former either do not do so at all, or only to a very small extent, whilst their solutions are slimy or viscous. Graham distinguishes between the two groups of substances, which exhibit such different properties in solution, by calling them crystalloids and colloids. Now Xiigeli has attempted to explain all these phenomena as being due to differences in the molecular structure of the various bodies. As atoms combine together to form molecules, thus pro- ducing so great a variety of chemical substances, so he considers that the molecules unite together in groups to form still more complex units, the micella?, and that in this manner the complex properties of organised bodies arise. In comparison with that of the molecule, the size of the micella is considerable, although too small to be seen with the microscope ; it may be built up, not only of hundreds, but even of many thousands of molecules. ISTageli ascribes a crystalline structure to these micellae, in con- sequence of their power of double refraction, which further is ex- hibited by many organised bodies, such as cellulose, starch, mus- cular substance, and even protoplasm itself in polarised light. In addition, great differences may be present in their outward appear- ance as well as in their size. The micellae have an affinity for water as well as for each other; hence their power of swelling up. In a dry organised body the micellae lie close together, being only separated by delicate envelopes of water; as more water becomes absorbed, these envelopes increase considerably in size, since at first the micellae have a stronger affinity for water than for each other. Thus they become pushed apart from each other by the penetrating water as with a wedge ; " however, organised bodies cannot become really dissolved, for the molecular attraction of the micellae for the water diminishes with distance at a proportionally greater 60 THE CELL rate than that of the micellae for each other, and hence when the envelopes have reached a certain size a condition of equilibrium, the limit of the power of the body to swell up, is reached." When, however, by means of special methods of treatment, the attraction of the micellae for each other is quite overcome, a micellar solution is obtained. This solution is cloudy and opal- escent, which is an indication that the light is unevenly refracted. Nageli compares this with the slimy opalescent masses producid when Schizomycetes are crowded together in large numbers. Nageli explains the differences, which Graham has described as existing between crystalloids and colloids, by the statement that in the former isolated molecules are distributed amongst the particles of water, whilst in the latter crystalline groups of mole- cules or isolated micellae are so distributed. Hence numbers of the one group form molecular solutions, and those of the other micellar solutions (such as egg-albumen, glue, gum, etc ). The micellae themselves have considerable power of preventing the substance from breaking down into molecules. Such a breaking down is generally accompanied by chemical transformation. Thus starch, after it has been converted into sugar, is capable of forming a molecular solution, as is also the case with proteids and gelatine-yielding substances after they have been converted into peptones. In organised bodies the micellae unite together to form regu- larly arranged colonies, in which the individual micellae may consist of similar or different chemical substances, and may vary as to size and form ; further, they may unite in smaller or larger groups of micellae within the colony itself. The micdlte within these micellar colonies appear as a rule to hang together in chains, which further unite together to form a frame or network structure with more or less wide meshes. In the gaps or micellar interstices the water is enclosed. " Only in this manner is it possible to have a firm structure, composed of a large quantity of water and a small quantity of solid matter, such as is seen in a jelly." The water, which -is contained in organised bodies, may be found in three conditions, distinguished by Nageli under the names water of constitution or of crystallisatinn, water of adhesion, and capillary water. By the first are understood the molecules of water, which, as in a crystal, are united firmly to the molecules of the substance in a fixed proportion, thus entering into the structure of the micella. ITS CHEMICO- PHYSICAL AND MORPHOLOGICAL PROPERTIES 61 The water of adhesion consists of molecules of water, which are held closely to the surface of the micella by molecular attraction. ''The concentric layers of water, which compose the spherical envelope surrounding the micella, vary considerably as to their density and their immobility ; they are naturally most dense and firmly attached when they are in direct contact with the surface of the micella" (Pfeffer). The capillary water finally is outside the sphere of attraction of the individual micella? and fills up the gaps in the micellar net- work. " These three kinds of water show considerable variation as to the degree of motility shown by their molecules. The mole- cules of capillary water are as free in their movements, as those of free water; in the water of adhesion the progressive move- ments of the molecules are more or less diminished, whilst the molecules of the water of constitution are fixed and non-motile." Hence only the waters of capillarity and of adhesion can pass through a membrane by osmosis. Just as water particles may be firmly held upon the surface of the micella? by molecular attraction, other substances (calcium and silicon salts, colouring matter, nitrogenous compounds, etc.), having been taken up in solution into the organised body, may be deposited upon them. The growth of organised matter by intussusception is explained by Nageli, by the supposition that particles of material in solution make their way into the organised body, such as, for example, molecules of sugar into a cellulose membrane, where they may either become deposited upon the micella? which are already present, thus adding to their size, or to a certain extent they may crystallise out to form new micella? situated between the ones already present. As an example of this, the phenomenon of sugar molecules becoming converted into cellulose molecules may be quoted. This micellar hypothesis of Nageli is frequently referred to in later chapters, as it often is of great use in forming a mental picture of the complex arrangement of matter in the elementary organism. Literature II. 1. ALTMANN. Dig Elementarorganismen u. Hire Bczieliungcn zu den Zellen. Leipzig. 1890. 2. JUL. ARNOLD. Ueber feinere Structur der Zellen unter normalen imd pathologiifchen Bedingungen. Vircliows Arcliiv. Bd. 77, 1879, p. 181. 62 THE CELL 3. BALBIANI. Sur la structure du noyau des cellules salicaires chez lei larees de Chironomus. Zoologischer Anzeiger, 1881, p. 637. 4. VAN BENEDEN et NEYT. Kouvelles recherches sur la fecundation et la division mitotique chez Va*caride megalocephale. Leipzig. 1887. 5. BurscHLi. Einige liemerkungen uber gewisse Organisationnverhaltnisse der sogenannten Cilioflagellaten und der Noctiluca. Morph. Jahrbuch. Bd. X. 1885. 6. BurscHLi. Ueber den Bau der Bakterien und verwandter Organic men. Leipzig. 1890. ?A. BUTSCHLI. Ueber die Structur des Protoplasmas. Verhanillungen des Naturhist.-Mcd.-Vereins zu Heidelberg. N. F., Bd. IV., Heft 3. 1889. Heft 4. 1890. (See Quar. Jour. Mic. Sac., 1890.) 7u. BUXSCHLI. Untersuchungen uber mikroskopische Schdume u. das Proto- plasma. 1892. 8. CARNOY. Several papers in La Cellule. Recueil de Cytologie et d'histo- logie generale. La cytodierese chez les Arthropodes, T. I. La veaicule germinatiue et les glob, polaires chez divers nematodeK. See also Conference donnee a la societe beige de microicopie, T. II f. iSee also A. B. LEE. OH Camay's cell researches. Quar. Jour. Mic. Soc. Vol. XXVI., pp. 481-497. 9. ENGELMANN. Ueber den fas rig en Ban d. contractilen Su'istanzeu. Pjliigers Archiv. Bd. 2t5. Physiology of Protoplasmic Movement, tram. Quar. Jour. Mic. Soc. Vol. XXIV., p. 370. 10. FLEMMING. Zellsubstanz, Kern und Zflltlieilung. Leipzig. 1882. 11. FLEMMING. Ueber Tlieilung u. Kernformen bei Leukocyten und Uber deren Attractionsspharen. Archiv.f. mikroskop. Anat. Bd. 37, p. 249. 12A. FLEMMING. Neue Beitrage zur Kenntidss der Zelle. II. Theil. Archiv. f. mikroskop. Anat. Bd. 37, p. 685. 12n. FLEMMING. Attractionsspharen und Centralkorper in Gewebszell n und Wanderzellen. Anatomischer Anzeiger. Bd. VI. See also JOHN E. S. MOOBE. On the Relationships and Role of the Archoplasm during mitosis in the Larval Salamander. Quar. Jour. Mic. Soc. Vol. XXXIV., p. 181. 13. FOL. Lehrbuch der verglelchen mikroskop. Anatomie. Leipzig. 1884. 14A. FROMMANN. Zur Lehre von der Structur der Zellen. Jenaische Zeit- schriftf. AJed. und Naturiv. Bd. 9. 1875. 14s. FKOMMANN. Ztlle. Realencyklopadie der gesainmten Heilkunde. 2 Aufl. 1890. 15. HAECKEL. Generale Morphologic. 16. MARTIN HHIDENHAIN. Ueber Kern und Protoplasma. Festschrift fur Kolliker. 1892. See also W. D. HALLIBDRTON. Gulstonian Lectures on the Chemical Physiology of the Animal Cell. Brit. Med. Jour. Vol. I. 1893. 17. C. HEITZMANN. Untersucli. uber Protoplasma. Wiener Sitzungs'jer. math. naturw. Classe. Bd. LXV1I. 1873. 18. BICHARD HERTWIG. Beitrage zu ein r einheitlichen Auffassung der ver- schiedenen Kernformen. Morphol. Jahrbuch. Bd. 2. 1876. ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 63 19A. OSCAR HEKTWIG. Beitrage zur Kenntniss der Bildung, Brfruchtung und Theilung des Thierischen Eies. Morphol. Jahrbuch. Bd. I., II., IV. 19fi. OSCAK HERTWIG. Vergleich der Ei- u. Samenbildung bei Nematoden. Archiv. f. mikroskop. Anatomic. Bd. 36. 1890. 20. HOFMEISTER. Die Lehre von der Pflanzenzelle. Leipzig. 1867. 21. E. KLEIN. Observations on the Structure of Cells and Nuclei. Quar. Jour. Mic. Soc. Vol. XVIII., 1878, p. 315. 22. KOLLIKER. Ha'idbuck der Geicrbtlrhre. 1889. 23. KOSSEL. Zur Chemie dss Zellkerns. Zeitschrift fiir physiulog. Chemie von Hoppe Seyler. 1882. Pd. 1. Untersuchungen iiber die Nucleine und ihre Spaltungsproduktf. Strassburg. 1881. KANTHACK and HARDY. Proceedings of the Eoyal Society. Vol. LIT. 24. C. KUPFFER. Ueber Difffrenzirnng der Protoplasma an den Zellen thier- ischer Gewtbe. Schriften des naturwissenschaftl. Vereins fUr Schleswig- Hohtein. Bd. I., p. 229. Heft 3. 1875. 2-5. LEYUIG. Untersuchungen zur Anatomic u. Histologie der Thiere. Bonn. 1883. 26. LEYDIG. Zelle und Gewebe. Bonn. 1885. 27. NAGELI u. SCHWESDENER. The Microscope. Theory and Practice, trans. London. 28. C. NAGELI. Mechanisch-pht/siologische Theorie der Abstammungslehre. Milnchen und Leipzig. 1884. 29. PFITZNER. Beitrage zur Leln-e vom Ban des Zellkerns u. seinen The I- ungserscheinungen. Archiv. f. mikrosk. Anatomic. Bd. 22. 1883. J. PRIESTLEY. Recent Researches on the Nuclei of Animal and Vegetable Cells. Quar. Jour. Mic. &oc. Vol. XVI. , pp. 131-152. 30. v. RATH. Ueber eine eigenartige polucentrische Anordnung des Chromatins Zoolog. Anzeiger. 1890. 31. BAUBER. Neue Grundlegungen zur Kenntniss der Zelle. Morpli. Jahrb. VIII. 1882. 32. BEINKE u. H. RODEWALD. Studien iiber das Protoplasma. Untersuch- ungfii aits dein botanischen Institut der Universitat. Guttingen. Heft 2. 1881. 33. SACHS. Textbook of Botany, Morphological and Physiological, trans, by S. II. Vines. 18 *2. 34. SCHAFER and E. RAY LANKESTEB. Discussion on the Present Aspect of the Cell Question. Nature. Vol. XXXVI. 1887. See also SCHAFER in Quain'a Anatomy, I'ol. I., pt. 2. 1891. 35. SCIIIEFERDECKER u. KOSSEL. Geicebelelire mil besondere Beriicksichtigung des menschl. Knrpers. 36. SCHMITZ. Untersuchnngen iiber die Structnr des Protoplasmas und der Zellkerne der Pflanzenzellen. Sitz.-Ber. der Niedenh. Gesellsch. /. Natur u. Heilk. Bonn. 1880. 37. FRANK SCHWARZ. Die morphologische und chemische Zusainmensetzujig des I'rotoplasmas. Beitrfige zur Biologic der Pflanzen. Bd.IV. Breslau. 1887. 38. SOLGER. Zur Kenntniss der Pig meittzelleii. A natomischen Anzeiyer. Jahrg. VI. , p. 182. 64 THE CELL 30. STRASBURGER. Zellbildiinc/ und Zelltheilung. 2 Auf. Jena. 1876. 40. STKASBURGER. Studien ilber das Protoplasma. Jenaische Zeitschrift. 187C. Bd. X. 41. STRASBURGER. Practical Botany, trans, by Hillhouse. London. 42. WIESNER. Elementarstructur und Wachsthum der lebenden Substanz. 43. ZACHARIAS. Ueber den Zellkern. Botanisr.lie Zeitung, 1882, p. 639. 44. ZACHARIAS. Ueber hiiceiss, Nudein. und Plastin, Botanische Zeitunr/. 1883. 45. ZACHARIAS, Ueber den Nucleolus. Botanisclie Zeitiing. 1885. 40. ZACHARIAS. Beitrcir/e zur Kenntniss des Zellkerns u. der Sexualzellen. Botan. Zeitunii, 1887. lid. 45. 47. ZACHARIAS. Ueber die Zellen der Cyanophyceen. Botan. Zeitung. 1890. See also HALLIBURTON loc. cit. 43. LIST. Untersuch. iiber das Cloakenepithel der Plagiostomen. Sitzunr/sber. der kaiserl. Acad. der Wissensch. zu Wien, Bd. XCII. III., Abth. 1885. 49. MIESCHRR. Verhandl. der naturforschenden Cesellschaft in Basel.' 1871, 50. ANEBKCH. Organologische Studien. Heft I. 1874. CHAPTER III. THE VITAL PKOPEETIES OF THE CELL. I. The Phenomena of Movement. All the mysteries of life, which are exhibited by plants and animals, are present, as it were in a rudimentary form, in the simple cell. Each individual cell, like the whole complex organism, has an independent life of its own. If we wish to study more deeply the true nature of protoplasm, we must above all things investigate its most important properties, its so-called vital properties. However, life, even the life of the simplest elementary organism, is a most complex phenomenon, which it is most difficult to define ; it manifests itself, to use a wide generalisation, in this, that the cell in conse- quence of its own organisation, and under the influence of its environment, experiences continual changes and develops powers, by means of which its organic substance is being continually broken down and built up again. During the former process, energy is set free. The whole vital process, as Claude Bernard (IV. IA) expresses it, depends upon the continual co-relation of this organic destruction and restoration. It is most convenient to classify these most complex phenomena under four heads. Thus each living organism exhibits four different fundamental functions or properties, by .means of which its life is made manifest : it can alter its form, and exhibit move- ments ; it reacts to certain external stimuli in various ways, that is to say, it is irritable ; it has the power of nourishing itself, it can by absorbing and transforming food material, and by giving up waste products, form substances, which it utilises for growth, for building up tissues, and for special vital functions ; finally, it can reproduce itself. Hence we will discuss the vital properties of the cell in four chapters, which we will take in the following order : 1. Phenomena of movement. 2. Phenomena of irritability. 3. Metabolism and formative activity. 4. Reproduction. 66 THE CELL In addition there will be a special chapter on the process of fertilisation. The cell may exhibit several kinds of movement, as is seen if an extensive comparative study is made. We will here distinguish between : (1) true protoplasmic movements ; (2) ciliary or flagellar movements ; (3) the movements of the pulsating vacuole ; (4) the passive movements and changes of shape exhibited by cells. In addition to these four, there are a few special phenomena of motion, of which it will be best to treat in later chapters, for example, the formation of the receptive protuberance which appears in the egg-cell in consequence of fertilisation ; the radiation figures which are seen in the neighbourhood of the spermatozoon after it has penetrated into the ovum, and those which occur daring the process of cell division, when the cell body splits up into two or more parts. Protoplasmic Movements. Although it is probable that movements take place in all protoplasm, yet in most cases, with our present means of observation, they cannot be perceived on account of their great slowness ; hence in only a few objects in the plant and animal kingdoms can this phenomenon be studied and demonstrated. The movement manifests itself partly in changes in the external form of the cell, and partly in the arrange- ment of the structure enclosed in the protoplasm, the nucleus, the granules, and the vacuoles. These movements differ somewhat according as to whether they are manifested in naked protoplasm, or in that which is enclosed by a firm membrane. a. The Movements of naked Protoplasm. Small uni- cellular organisms, white blood corpuscles, lymph corpuscles, connective tissue cells, etc., exhibit movements which,* in con- sequence of their similarity to those seen in the Amoeba, are termed amoeboid. If a lymph corpuscle of a Frog (Fig. 37) is observed under suit- able ^circumstances, it is seen to undergo continual changes of form. Small processes of protoplasm, the foot-like processes, or pseudopodia, are protruded from its surface ; at first as a rule they consist of hyaloplasm alone, but after a time granular protoplasm streams into them. By this means the pseudopodia are increased in size ; they become broader, and may in their turn extend new, more minute processes from their surface. Or the protoplasm may THE VITAL PROPERTIES OF THE CELL flow back again, thus causing them to decrease in size, until finally they are com- pletely withdrawn, whilst new processes are being protruded from another portion of the body. By means of these alternate protrusions and retractions of their pseudo- podia, the small bodies of protoplasm are enabled to move from place to place, crawl- ing over the objects to whose surfaces they cling at a rate which can only be measured under the microscope. Amcebce are able to traverse a distance of \ mm. in a minute. In this manner the white blood cor- puscles during inflammation are able to pass through the walls of the capillaries and of the smaller vessels, and the lymph corpuscles make their way as wandering cells into the connective tissue spaces, such as the interlamellar spaces of the cornea, where the resistance to be overcome is not great, or they force their way between epithelial cells, and so reach the sur- face of an epithelial membrane. This extension and retraction of pseudopodia is most marked in a small Amoeba (Fig. 38), which was described as far back as 1755 by Roesel von Rosenhof, who on account of its energetic changes of form called it the small Pro- teus. Somewhat different movements take place in Myxomycetes, and in Thalamophora, Heliozoa, and Radiolaria. FIG. 38. Amoeba proteus (after Leidy ; The plasmodia of some species from R.Hert wig, Fig. 16): n nucleus; of Myxomycetes, such as the ct) contractile vacuole ; N food vacuoles ; , ,. . ,., * en endoplasm; efc ectoplasm. JStkolwm septicum, often spread FIG. 37. A Leucocyte of the Frog containing a Bac- terium which is undergoing the process of digestion. The Bacterium has been stained with vesuvine. The two figures represent two successive changes of shape in the same cell. (After Metschnikoff, Fig. 51.) themselves out over the object upon which they rest, in large masses about the size of a fist. In order to make a suitable pre- paration for observation of such a plasmodium, it is best to hold a moistened slide near to its edge in an oblique position, and to cause a stream of water by means of a special contrivance to flow slowly down the slide. The plasmodia of the JEthaUum possess the property of moving in a direction opposite to that of the stream of water (rheotropism) ; hence they protrude innumerable pseudopodia, and by this means crawl up on to the moistened slide, where they spread themselves out, and, by uniting neigh- bouring pseudopodia together by means of transverse branches, they form a delicate transparent net- work (Fig. 39). When this network is examined with a high power, it can be seen to exhibit two kinds of move- ments. At first the granular protoplasm which is present in the threads and strands, where it is surrounded by a thin peripheral layer of hyaline proto- plasm, is seen to have a quick, flowing movement, which is chiefly observable because of the movement of the small granules, and which resembles the cir- culation of the blood in the vessels of a living animal. There is no distinct boundary line between the motile endo- plasm and the non-motile ectoplasm, for the grannies at the edge of the stream move much more slowly than those in the centre; indeed, sometimes they may keep quite still for a time, to be later on agaiii caught up by the stream and carried along with it. In the thinner threads there is always only one stream flowing longi- tudinally, but in the thicker branches there are often two flowing along side by side in opposite directions. " In the flat membrane- like extensions " which are developed here and there in the net- work, " there are generally a large number of branched streams flowing either in the same or in different directions ; not infre- quently we find streams flowing along side by side in opposite FIG. 39. (Wiondrioderma dif- forme (after Strasburger). Part of a fairly old plasmodium. o Dry spore ; b the same, swollen up in water ; c spore, the contents of r,Vich are exuding ; d zoospore ; v moeboid forms produced by i^e transformation of zoospores, which are commencing to unite together to form a plasmodium. (In d and e the nuclei and con- tractile vacuoles may be distin- guished.) THE VITAL PROPERTIES OF THE CELL 69 directions." Further, the rate of movement may vary in different places, or it may gradually alter ; it may be so great that under a powerful lens the granules appear to travel so fast that the eye can scarcely follow them ; on the other hand, it may be so small that the granules scarcely appear to change their place. The second kind of movement consists of a change of form in the individual threads and in the network as a whole. As in the Arncebd, processes are protruded and withdrawn from various places, a mass of homogeneous protoplasm being first protruded, into which the granular protoplasm flows later on. Occasionally, when the streaming movements are very powerful, it appears as though the granular endoplasm is pressed forcibly into the newly formed processes. By this means the plasmodium can, like the Amceba, crawl slowly along over a surface in a given direction; new processes are continually being protruded from the one edge, towards which the endoplasm chiefly streams, whilst others are withdrawn from the opposite one. Gromia oviformis (Fig. 40) is a classical object amongst the Reticularia, for the study of protoplasmic movements (see p. 29). If the little organism has not been disturbed in any way, a large number of long fine threads may be distinguished stretching out from the protoplasm, which has made its way out of the capsule, and spreading themselves out radially in every direction into the water; here and there lateral branches are given off, and oc- casionally all the threads are united together into a network by such branches. Even the most delicate of these threads exhibit movements. As Max Schultze (1. 29) aptly describes it, " a glid- ing, a flowing of the granules which are imbedded in the threa' substance," may be seen with a high power; ''they move alol^N the thread, more or less quickly, either towards its periphery or in the other direction ; frequently streams flowing in both direc- tions may be seen at the same time even in the finest threads. When granules are moving in opposite directions, they either simply pass by each other, or else move round one another for a time, until after a short pause they either both go on in their original directions, or one takes the other along with it. All the granules in a thread do not move along at the same rate ; hence sometimes one may overtake another, either passing it or being stopped by it." Many evidently pass along the outermost surface of the thread, beyond which they can be plainly seen to pro- ject. Frequently other larger masses of substance, such as spindle- 70 THE CELL shaped seen to swellings or lateral accumulations in a thread, may be move in a similar manner. Even foreign bodies which adhere to the thread substance, and have been taken in by it, are seen to join in this movement, the rate of which may attain to '02^mm. per second. Where several threads over- lap each other gran- ules may be seen pass- ing from one into the other. At such places broad flat surfaces may be produced by the heaping up of the thread substance. A special kind of protoplasmic move- ment is described by Engelmann (III. 5, 7) under the name of gliding 'movement (Grlitschbewegung). It has been observed chiefly in Diatoms and Oscillaria. In the former the proto- plasm is surrounded by a siliceous shell, in the latter by a cellu- lose membrane. How- ever, outside this covering there is an exceedingly delicate layer of hyaloplasm, quite free from gran- ules, which cannot be FIG. 40.-Gromia ori/ormis. ( After M. Sdmltze.) Seen in the living ob- THE VITAL PROPERTIES OF THE CELL 71 jeet, but which may sometimes be demonstrated by means of reagents. Hence, since this layer moves in a certain direction over the siliceous shell, or cellulose membrane, the small organisms can " move in a gliding or creeping fashion over a solid surface " (Engelmann). b. The movements of Protoplasm inside the Cell Mem- brane. This kind of movement is chiefly seen in the vegetable kingdom, and as a rule is best observed in the cells of herbaceous plants rather than in those of shrubs and trees. According to de Vries (III. 25), these movements are never totally absent in any plant-cell, but frequently they are so slow as to escape direct observation. They are best seen in vascular tissues, and in those where materials have been stored up, and further at such times when considerable quantities of plastic substances are being transported in order to supply the material necessary for the continuation of growth, for local accumulations, and for special needs (de Vries). Hence this movement of the protoplasm ap- pears to be directly of importance during the conveyance of materials from one part of the plant to another. More rarely it may be seen in the lower organisms, and in the animal king- dom, as in Noctiluca in the vesicular cells in the centre of the tentacles of Coelenterata, etc. Two kinds of movements may be distinguished in plants, Hotation and Circulation. These movements of rotation were first observed in 1774 by Bonaventura Corti (I. 8); after that they were lost sight of for a time, but were re-discovered by Treviranus. The most suitable objects for studying them are afforded us by the Characeae ; root- hairs of the Hydrocharis morsus ranee, and of Trianea bogotensis, leaves of Vallisneria spiralis, etc., are also very convenient for observations. In the large cells of the Characece, the protoplasm, as has already been described on p. 33, is spread out as a thick cohesive layer upon the inner surface of the cellulose membrane, surrounding the large quantity of cell-sap like a closed sac. In this lining two distinct layers of protoplasm can always be dis- tinguished : an outer one, touching the cellulose membrane, and an inner one, in contact with the cell-sap. The former is always motionless ; in Hydrocharis it is very thin, in Characece it is somewhat thicker, and it also contains a greater number of chlorophyll grains, which remain motionless. This immotile layer gradually passes over into the inner motile one, which in 72 THE CELL Chara contains no chlorophyll corpuscles, but only nuclei and granules. The protoplasm of the inner layer, which, compared to that of the outer layer, appears to be richer in water, exhibits rotatory streaming movements, which take place in the following manner. The current passes up along the longitudinal wall of an elongated cell, then, turning round past a transverse wall, flows down the opposite longitudinal side, until, curving round again at the second transverse wall, it reaches the starting point, when the cycle recommences. Between the upward and downward streams there is a more or less broad neutral strip tvhere the protoplasm is at rest, and where as a rule it is reduced to a very thin layer. In Nitella there are no chlorophyll corpuscles along this neutral strip in the outer layer. A connecting link between the rotatory movement and true circulation is afforded us by the so-called "fountain-like rotation" (Klebs III. 14). This, which as a rule but rarely occurs, is found in young endosperm cells of Ceratophylhim, in young wood vessels of the leaf-stem of Ricinus, etc., etc. Here the protoplasm, in addition to spreading itself out in a thick layer over the inner surface of the cellulose wall, stretches itself in the form of a thick central strand along the longitudinal sap-cavity of the cell. Under these circumstances a single stream flows along this central strand, spreading itself out in all directions like a fountain upon the transverse wall, upon which it impinges ; then streaming down the sides of the cell, it collects again at the opposite trans- verse wall, where it re-enters the main axial stream. The motion which is described as circulation is observed in those plant and animal cells in which the protoplasm spreads itself out, both as a thin layer beneath the membrane, and also in the form of more or less delicate threads, which traverse the sap-cavity and are united together to form a net-like structure. The objects which have been most examined are the staminal hairs of the various kinds of Tradescantia, and young hairs of the stinging nettle, and of pumpkin shoots. The phenomenon of circulation resembles that observed in the protoplasmic nets of Myxomycetes, and of the delicate pseudo- podia of the Rhizopoda. Circulation consists of two kinds of movements. In the first place attention must be drawn to the streaming movements of the granules. In the thinnest threads they move more or less quickly over the surface of the walls in one direction, whilst in broader bands several separate streams may THE VITAL PROPERTIES OF THE CELL 7:3 circulate quite close together, sometimes in the same, sometimes in opposite directions. The nucleus, as well as the chlorophyll and starch grains, which lie embedded in the protoplasm, are car- ried slowly along by the current. Similarly in this case the most external hy- aline layer of protoplasm, which is in contact with the cellulose membrane, is, comparatively speaking, at rest. In the second place, the whole body of proto- plasm itself slowly moves along, in consequence of which it changes its form. Broad bands become nar- rowed, and may after a time disappear, delicate threads increase in size, and new processes are formed, just as new pseudopodia are protruded to the ex- terior by Myxomycetes and Rhi/opoda. Large masses of protoplasm become heaped up here and there upon the layer lining the cell-wall, whilst at other places the coating becomes thinner. c. Theories concerning Protoplasmic Movements. Attempts have lately been made by various investigators, Quincke ([II. 17), Biitschli (II. 7fl), Berthold (III. 2), and others, to com- pare these protoplasmic movements with those exhibited by a mixture of inorganic substances, and thus to explain them. Quincke has carefully investigated the movements which occur at the areas of contact of various fluids. He placed in a glass containing water a drop of a mixture of almond oil and chloroform, FIG. 41. A S, cells of a staminal hair of Tra- descantia virginica. A Undisturbed streaming movements of protoplasm. B Protoplasm which has run together into ball-like masses in con- sequence of irritation : a cell- wall, b transverse wall of two cells ; c d protoplasm which has massed itself together into small balls. (After Kiihne ; from Verworn, Fig. 13.) 74 THE CELL the specific gravity of which is slightly greater than that of water, and then, by means of a fine capillary tube, he caused a drop of 2 per cent, solution of soda to approach the globule of oil. This latter then exhibited changes in shape, which are similar to those observed with the microscope in cei-tain Amoebae. The explanation of this is that the soda solution gradually spreads itself out over the surface of the oil, forming a soap. Quincke is of opinion that the protoplasmic movements are analogous to these. In the plasmolysis of plant cells, the proto- plasm frequently breaks up into two or more balls, which spread themselves out, and then either re-unite, or remain separated from one another by an even surface, just as two soap bubbles of equal size which are placed in contact may touch each other, without uniting. In consequence of these appearances he is of opinion that, considering the physical pro- perties of delicate solid or fluid lamellae, the protoplasm must be surrounded by a very delicate fluid membrane, just as in the soap bubble the air is enclosed in a thin skin layer of soap solution. " The substance of the membrane surrounding the protoplasm," as Quincke proceeds to state, " must be a fluid which forms drops in water. Since of all the substances known in nature oil is the only one which possesses this pro- perty, the membrane must consist of an oil, that is to say of a fluid fat. The thickness of this layer may be most minute, less than "0001 mm., and hence it is not perceptible even with the micro- scope." Through the action of 'the albumen upon this oil, a substance is produced upon the areas of contact, which is soluble in water, and spreads itself out just like the soap produced by the combination of soda and oil. Hence it is called albuminous soap. Thus Quincke considers the cause of the protoplasmic move- ments to be a periodic spreading out of albuminous soap upon the inner surface of the envelope of oil surrounding the protoplasm. This soap, in being continually re-formed on the area of contact as fast as it is dissolved and diffused throughout the surrounding fluid, remains constant in quantity; thus, since the presence of oxygen is necessary in this chemical process, the fact is explained, that, in its absence, the protoplasmic movements are arrested, and similarly their cessation at extreme temperatures may be ascribed to chemico-physical conditions. Biitschli, being stimulated by these investigations of Quincke, THE VITAL PROPERTIES OF THE CELL 75 has undertaken some interesting experiments based on the assump- tion of his foam or emulsion theory of protoplasm, and these, as it appears to him, throw light upon the cause of the protoplasmic movements. He prepared frothy structures of oil in various ways. The most delicate and instructive masses were obtained by mix- ing a few drops of olive oil, which had been kept for some time in a warm chamber, with some finely powdered K 2 C0 3 , until a viscous mass was produced ; a small drop of this mixture was then introduced into water. The emulsion which is produced in this manner is milky white in appearance, and consists of minute vacuoles, filled with the solution of soap, which is formed at the same time : it may be cleared by adding to it a few drops of dilute glycerine. By this means active streaming movements are pro- duced, which, in a successful preparation, last for at least six days, and which are certainly surprisingly like the protoplasmic move- ments of an Amoeba. " From one place on the edge the current flows through the axis of the drop ; it then streams away from the edge down both sides, in order to unite again, gradually to form the axial current again. Here and there a blunt process is pro- truded and withdrawn, and so on; indeed, individual drops may exhibit fairly active locomotive powers for a time." Biitschli, in accordance with Quincke's experiments, explains these phenomena of movement in the following manner : " On some place on the surface some of the delicate chambers of the froth structure burst, and thus the soap solution at this region is able to reach the sur- face of the drop, which is composed of a very thin lamella of oil. The necessary consequence of this is a diminution of surface- tension at this spot, and hence a slight bulging and out-streaming occur. Both of these induce a flow of foam-substance from the interior to this spot. A few more meshes may be broken down by this current, and so on, the result being that a streaming, once induced, is persistent unless considerable obstacles present them- selves." Biitschli is quite convinced that the streaming move- ments seen in these saponified fat drops are identical in all essentials with amoeboid protoplasmic movements. These experiments of Quincke and Biitschli are of the greatest interest, for they prove that very complex phenomena of move- ment may be induced by means of comparatively simple methods. On the other hand, various objections may be raised against their deduction, that in protoplasmic movements similar processes occur. Even the hypothesis, that the protoplasmic substance is 76 THE CELL enveloped by a delicate lamella of oily substance, is exceedingly doubtful. For if we only take into account the single fact that protoplasm is composed of a great number of chemical substances, which, during the metabolic processes upon which its life depends, are continually undergoing chemico-physical changes, we cannot but think that conditions much more complex in their nature must be necessary for its movements, than those required for a moving drop of saponified oil, and, indeed, the complexity of these conditions must be proportionate to the immense difference in the complexity of the chemical composition and organisation of the two substances in question [cf . statements already made " on this subject on p. 22 ; and Die Beivegung der lebendigen Substanz by Verworn (III. 24)]. Further, all the protoplasmic movements the streaming movements, the radial arrangement round attrac- tion centres, the movements of cilia and flagella, and muscular contraction together constitute a single group of correlated phenomena which demand a common explanation. This, however, is not afforded us by the experiments of either Quincke or Biitschli. The movements, induced by them in a mixture of sub- stances, bear the same relation to the movements of living bodies, as the structure of the artificial cell produced by Traube does to the structure of the living cell. Fig. 42, which is taken from a paper by Verworn (III. 24), shows what very different appearances, closely resembling the various kinds of pseudopodic extensions, may be produced by the simple spreading out of a drop of oil upon a watery solution ; a-d is a drop of salad oil which has spread itself out upon soda solu- tions of different degrees of concentration ; in a it has assumed the form of Amoeba yuttnla, in b and c of Amoeba proteus, and in d of a plasmodium of a Myxomycete. Figs, e and /, which repre- sent drops of almond oil, resemble the formation of pseudopodia in Heliozoa and Radiolaria, whilst g is taken from Lehmann's Molecular Physics, and represents a drop of creasote in water, in which it has assumed a form resembling a typical ActinospJiserium (Verworn III. 24, p. 47). Other attempts to explain the protoplasmic movements (Engel- mann III. 6, Hofmeister II. 20, Sachs) lead us into the domain of theories upon the molecular structure of organised bodies, since the cause of the movements is supposed to lie in the changes of form of the most minute particles. A discussion of Verworn 's latest attempt (III. 24) would lead us too far in another direction. THE VITAL PROPERTIES OF THE CELL 77 Once for all, it must be admitted that none of the hypotheses which have, up till now, been propounded, are able to furnish us with a satisfactory conception of the causes and mechanical con- ditions of the plasmic movements, and that, therefore, we must confine ourselves to a simple description of observed conditions. This, however, is not to be wondered at, when we consider what FIG. 42. Different appearances assumed by drops of oil, which have spread themselves out. (After Verworn, Fig. 11.) a number of different opinions are held with regard to the ultimate structure of protoplasm itself (see pp. 18-26), and this must of course affect the explanations tendered of its movements. II. Movements of Flagella and Cilia. Unicellular organ- isms, by means of their flagella and cilia, are able to move from 78 THE CELL place to place much more rapidly than can be effected by means of pseudopodia. Flagella and cilia are delicate hair-like processes, which extend in greater or less numbers from the surface of the cell. They are composed of a homogeneous, non-granular sub- stance, and in this respect resemble short, thin pseudopodia, when these consist of hyaloplasm alone. However, they differ from pseudopodia in two respects : firstly, they move in a different and more energetic fashion, and secondly, they are not transitory, but permanent organs, being neither protruded nor withdrawn. Fundamentally, however, the movements of flagella and pseudo- podia are identical in kind, as is shown by the observations made by de Bary (I. 2) on swarmspores of Myxomycetes, and by Haeckel, Engelmann, R. Hertwig (III. 12s), and others on Rhizopoda. Many of the lower organisms reproduce themselves by means of small spores, which resemble Amoebae in their appearance and in their mode of movement (Fig. 43). After a time such spores usually protrude two thread-like pseudopodia (Fig. 43 a), which exhibit slow oscillatory movements, and develop into flagella, whilst the remainder of the body withdraws all its other pseudopodia, and so becomes spherical in shape. As the movements become stronger, the spore travels more and more rapidly, by means of its *- f^lla, through the water division, and has wandered from the (Fig. 43 6); thus a " S warm- colony; and which, having withdrawn all e has developed out o f t h e its pseudopodia, with the exception of two, _ r _ * which have developed into flagella, be- little amoeboid cell, comes transformed into a swarmspore (b). J^ m ay be Safely deduced from (Prom Hertwig, PI. 6, d find e.) . 77 these discoveries that flagella are developed from delicate protoplasmic processes, ivhich become especially contractile, and in consequence differ somewhat in their properties from the remaining protoplasm. Hence they may be considered as constituting a special plasmic product or cell-organ, composed of contractile substance. Flagella and cilia always arise directly from the body of the cell. If the cell is enveloped by a membrane, they protrude through pores in it. At their bases they are always somewhat thickened, frequently starting from the surface of the protoplasm THE VITAL PROPERTIES OF THE CELL 79 as small button-like protuberances, whilst at their free ends they gradually become reduced to fine points. Ciliary organs may occur in large or small numbers. In the latter case, when only from one to four are present, and when they are generally longer and more powerful, they are called flagella ; in the former case, they cover the whole surface of the cell in large numbers, thousands being frequently present, they are then smaller and more delicate, and are called cilia. a. Cells with Flagella. Flagella occur either at the anterior or posterior end of the body, producing a correspondingly different movement in the body. In the first case the flagella travel forwards, dragging the body along after them ; in the second they propel it from behind. The former mode of locomotion has FIG. 41. A Euglena viridis (after Stein): 71 nucleus; c contractile vacuole; o pigment- spot. B Hexamitus inflatu* (after Stein). C Chilomonas puramcecium (after Biitschli) : oe cytostome; v contractile vacuole; n nucleus. (From Hertwlg, Pigs. 130-132.) been chiefly observed in Flagellata and kindred organisms (Fig. 44 A, B, C), in many kinds of Bacteria (Fig. 33 fc), in antherozoids (Mosses, Ferns, Equisitaceae), and in swarrnspores, under which name the reproduction bodies of many Algae and Fungi are included ; the latter method of locomotion occurs in the spermatozoa of most animals (Fig. 45). The ciliary organs of unicellular organisms have a double 80 THE CELL FIG . 45. Mature human spermatozoon from two points of view. It is composed of Jc head; m middle portion ; and s tail. function to perform. Firstly, they have to keep the cell body afloat by means of their activity, since its specific gravity is some- what greater than that of the surrounding medium. This is proved by the fact that dead swarmspores and spermatozoa sink to the bottom of the vessel. Secondly, they have to propel the body in a certain direc- tion by means of their movements. Nageli (III. 16) has made most careful observations upon the mechanism of the move- ments of the motile cells of plants. Accord- ing to this investigator, the oscillations of the flagella impart a two-fold movement of the body a forward, and at the same time a rotatory movement. Hence the resultant motion resembles that of a ball shot out of a rifle. Such motions may be divided into three types : i " Many motile cells travel forwards in a straight or somewhat curved line, the anterior and posterior ends of their axes remaining exactly in the same direction ; these swim steadily forward, with- out deviating from a fixed path. With others it may be distinctly seen that they describe a straight, or somewhat bent spiral, in which one revolution round the axis always corresponds to one turn of the spiral (a given side of the cell always facing out- wards), whilst the axis of the cell is parallel to that of the spiral. Finally, there are other cells whose anterior ends describe spirals, whilst their posterior ends proceed in a straight line, or in a spiral of smaller diameter. The nature of the second and third of these movements can only be distinguished if they are very slow. If they are rather quicker, only a kind of wavering can be made out, which, especially in the third, is of a peculiar character." The direction in which the motile cells rotate about their longitudinal axis generally remains constant for each kind, species, or family; many rotate from south to west (Ulothrix) , others from south to east (antherozoids of Ferns), others are somewhat uncertain in their rotations, turning now from south to east, and now from south to west (Gonium). If motile cells strike against any object, they cease for a time their forward movements, but continue to rotate about their longitudinal axes ; then, " as a rule, they commence to retreat, their posterior ends being in advance, THE VITAL PROPERTIES OF THE CELL 81 and to rotate themselves in an opposite direction." This backward movement never lasts for long, and is always slower than the forward one ; however, the cell soon retm*ns to its normal mode of progression, which usually takes place in a somewhat oblique direction. In consequence of his investigations, ISTageli is of opinion that if zoospores and spermatozoa be quite regular in form, if their substance be evenly distributed throughout their mass, and further, if the medium be quite homogeneous, they must travel in a perfectly straight line, and hence that all deviations from this straight line, both as regards rotation round the axis and forward progression, must be ascribed either to the circumstance that they are not symmetrical in form, and that their centres of gravity are not in the centres of their bodies; or to the fact that the fric-tional opposition which they encounter is not equal in every direction. By means of flagella a far greater speed is attainable than by means of pseudopodia. According to ISTageli, zoospores usually proceed at the rate of one foot per hour ; the quickest, however, take only a quarter of an hour to traverse the same space ; whilst a man, at ordinary speed, traverses a distance of rather more than half his length in a second, a swarmspore in the same time covers a distance of nearly three times its own diameter. How- ever, although the rate at which they move appears, when they are seen under the microscope, to be very great, we must take into account the fact that the distance is also magnified, and that in consequence they appear to move much more rapidly than they do in reality. As a matter of fact, their movements are exceed- ingly slow. " Without magnification, even if the organisms could be plainly seen, no movement could be perceived on account of its slowness." Spermatozoa (Fig. 45) may be distinguished from the zoospores of plants by their possessing one single thread-like flagellum, situated at the posterior end of the body. The spermatozoon, being propelled by it, advances by means of snake-like move- ments, resembling those of many fishes. In some cases the structure is more complicated, a delicate contractile or undulating membrane, which may be compared to the edge of a fish's fin, being present. This is especially well developed on the posterior ond of the large spermatozoa of the Salamander and the Triton (Fig. 46). 82 If this undulating membrane be examined with a very high power of the microscope, waves are seen to travel continually over its surface, passing from the front to the back. "These," as Hensen explains, " are caused by each successive transverse portion passing one after the other from one extreme position (Fig. 47) to the other. For instance, if at the initial period a portion of the edge, which is seen from above, occupies position 1 to J 1 (Fig. 47), it is seen at the end of the first quarter of the period to have assumed position II to II 1 , or, which amounts to the same thing, position II x to II 2 . At the end of the second quarter the portion II 1 to 71 2 is in the position III to III 1 or, which is the same, III 1 to III 2 . At the end of the third quarter III 1 to III 2 has passed into the position IF to IV 1 , whilst, at the end of the whole period it has again taken up position I to I 1 . The movements follow after each other with a certain degree of force and speed ; it remains now to be seen how a for- ward motion results from them. Any one point on the surface of the undulating border (Fig. 47) moves, as is indicated by the arrow, from 8 to y with the force x = ay. This force can be resolved into its two components aft and fty. The force aft is exerted in the direc- tion of the border, compressing it, and ap- parently producing no further effect. Force fty may be again split up into yS and ye. ye exerts a direct backward pressui-e on the water, and hence, in consequence of the re- sistance of the water, propels the body in a forward direction. Force 78 would cause the body to rotate on its own axis ; but opposed to it is the opposite force, which is developed at all the places where the arrows point in an opposite direction (as for instance over D). Further the same force ye is present in Fig. D as in Fig. C, only the shaded por- tions of Fig. A develop the forces which are opposed to ye. It is seen, however, that the size of the surfaces in question, and hence FIG. 46. Spermato- zoon of Salamandra maculatn : k head ; w middle portion ; ef tail ; xp anterior end ; u un- dulating membrane. THE VITAL PROPERTIES OF THE CELL 83 of their force components, is invariably of minor importance " (Hensen III. 11). FIG. 47. Explanation of the mechanism of the movements of spermatozoa (after Hensen, Fig. 22). A The four phases of position assumed by the border of the flagelluin when an nndulation passes over it. I to I 1 , the first ; II to II l to II 2 , the second ; III to III 1 to I/J 2 , the third ; IV to IV l , the fourth stage of the bending of the border in a lonari- t idinal undulation. B Section of the thread-like tail and membrane, in its two positions of greater elongation. C and D resolution of forces. E Movement of an ordinary sper- matozoon ; a b c various phases of this movement. 6. Cells with numerous Cilia. The Infusoria are chiefly to be distinguished from other unicellular organisms by the large number of cilia they possess, on which account they are called Ciliata (Fig. 48). Cilia are much smaller than flagella, be- ing, as a rule, about '1 to '3 JJL thick, and about 15 p long. They may number many thousands. For example, it has been calculated, that the Paramwcium aurelia possesses approximately 2,500. As for the Balantidium elongatum, which is parasitic in the Frog, and which is very thickly ciliated, Butschli (III. 3) is of opinion that it has nearly ten thousand cilia ; these are generally arranged in several longitudinal rows, which either encircle the body in spirals, or ai-e con- fined to a certain portion of its sur- face. In addition to the cilia, many Infusoria possess special large organs of locomotion, cirri, and undulating membranes. The FIG. 48. Stylonychia my. tilus (after Stein; from Glaus" Zonlogy) seen from the ventral purface. Wz Adoral zone of cilia ; C contractile vacuole ; N nucleus ; N l nucleolus ; A anus. 84 THR CELL former may be distinguished from cilia by their greater thickness and length, and by the fact that they are somewhat wide at the base, whilst they taper off to a tine point (Fig. 48). Farther, like other special contractile tissues (muscular fibres), they exhibit a fibrillar differentiation, so that they may be split up into many delicate fibrils (Biitschli). These cirri occur with especial frequency in hypotrichous Infusoria, being situated chiefly around the mouth. The undulating membranes also terminate at the mouth cavity. They are locomotive organs which have been developed superficially ; they may frequently be seen to be dis- tinctly marked with delicate striae extending from the base to the free edge, and hence they, like the. cirri, must possess a fibrillar structure. Infusoria have various methods of locomotion. As a rule the body, when it moves freely through the water, revolves about its longitudinal axis. It has the power of changing the direction in which it travels; the rate at which the cilia move may-suddenly be altered, being either slowed or quickened ; the body may even keep still for a short time, without any apparent external cause. Hence various kinds of movements take place, suggesting the idea of volition. In addition, it is remarkable that the cilia, often thousands in number, of one and the same individual, always act together in a strictly co-ordinate fashion. " They do not only always oscillate at the same rate, and with the same amplitude of beat (rhythm), but they always strike the water in the same direc- tion, and in the same order" (Verworn). This co-ordination is carried out to such an extent, that two individuals which have been produced by the division of a parent cell always exhibit uniform and synchronous movements as long as they are united by a bridge of protoplasm. Hence it follows, that although the cilia possess the power of spontaneous contraction, yet their work- ing together is regulated by stimuli from the protoplasmic body itself. The ectoplasm seems to play an especially important part in the transference of these stimuli, as is shown by an experiment made by Verworn (IV. 40). He made a slight incision with a lancet in Spirostomum ambiguum (Fig. 49) and in Stentor cceruleus in the ectoplasm supporting the rows of cilia. "Under these circumstances it could be plainly seen that the ciliary waves did not cross the area of the incision, but were confined to the one side, and could not be seen on the other." Occasionally THE VITAL PROPERTIES OF THE CELL 85 also he observed that the mean position through which the cilia oscillated was different for a time in one half of the rows of cilia from that seen on the other side. III. The Contractile Vacuoles, or Vesicles, of Unicellular Organisms. Contractile vacuoles occur very frequently in Amoeba?, Reticularia, Flagellata (Figs. 7, 43, 44), and Ciliata (Fig. 50 cv). In the last, where they have been most accurately ex- amined, there is generally only one single vacuole in the whole body ; occasionally two are present (Fig. 50), or rarely a few more ; they are always situated just below the sur- face of the body, under the ectoplasm. They may be easily distinguished from the other fluid vacuoles, of which large numbers may be distributed throughout the body, by the fact, that at regular intervals they discharge their con- tents to the exterior, and then gradually fill up again. Hence they tem- porarily disappear (Fig. 50 cv) to reappear again in a short time (cv 1 ). The evacuation takes place through one or more special pores, which can be observed on the surface of the infusorian immediately over the vacuole. " Each pore appears as a rule as a minute circle, the border of which is dark, but which is bright in the centre ; this brightness of the centre is due to the re- fracbive power of the pellicular and alveolar layer. Sometimes each pore is connected to the vacuole by means of a fine excretory tube. In addition, it is not uncommon to find special conducting canals (1, 2, or more) regularly arranged in its neighbourhood. In V FIG. 49. Spirostomum ambiguum. The con- tinuity of the surface which bears the peri- stomatic cilia has been interrupted by an in- cision. (After Verworu (VI. 40), Fig. 25.) FIG. 5o. Paramceoium caudatum semi-diagram- matic (R. Hertwig, Zoo- logie. Fig. 139) : K nu- cleus ; life secondary nu- cleus ; o mouth aptrture (cytoatome); no. l food vacuole in process of for- mation ; no food vacuole ; cv contractile vacuole, contracted; cv 1 the same contractile vacuole, dis- teuded ; t trichocysts, t l the same with their threads ejected. 86 THE CELL Paramfecium aurelia and Paramfecium caudatum (Fig. 50), there is a system of conducting 1 canals, which have been known for a long time, and have been worked at more than any others ; from each of the two dorsal vacuoles about eight to ten fairly straight tubes radiate ; their course may be traced almost all over the whole body. However, the two systems remain independent throughout their whole extent." They are thickest in the neigh- bourhood of the vacuoles, becoming gradually finer distally. The Paramfecium affords us an excellent subject for a closer study of the working of this peculiar apparatus. When both the contractile vacuoles have attained their greatest size, their whole contents are suddenly and energetically ejected to the exterior through their efferent canals and pores, so that for a time the vacuole cavities quite disappear. This condition, as with the heart, is termed the systole, whilst the period during which the vacuoles become again filled with fluid, and hence distended and visible, is called the diastole. They become filled in the following manner : Even before the systole has commenced, the above-described conducting canals have collected fluid from the endoplasm of the body of the infnso- rian ; this fluid probably is charged with carbonic acid and other decomposition products. According to Schwalbe (III. 21) the process occurs in consequence of " the condition of pressure of the fluid in the animal's body, this pressure being due to the ever-in- creasing amount of water which is continually being taken in by the mouth." The conducting canals can be easily seen, at this time being full of water. They become swollen in the neighbour- hood of the contractile vacuole, which is now fully distended, so that they look like a circle of rosette-shaped vacuoles surrounding it ; these have been called formative vacuoles by Biitschli. In consequence of their being in this condition, the contractile vacuole cannot, during its systole, discharge its contents back through them, but only forwards to the exterior. As soon as the diastole again occurs, the distended formative vacuoles empty themselves into the contractile vacuole, which in consequence becomes visible again ; it then gradually distends itself until it reaches its maxi- mum size. Hence at the commencement of the diastole the emp- tied formative vacuoles disappear for a time ; however, they con- tinue to collect fluid from the parenchyma of the body until the commencement of the next systole. When several vacuoles are present they generally empty them- THE VITAL PROPERTIES OF THE CELL 87 selves in turn, with the result that the water is discharged as regularly as possible. The frequency with which these evacuations take place varies considerably in different species. According to the observation of Schwalbe (III. 21) the following law may be stated : that the smaller the vacuoles are, the more frequently are they emptied. For instance, in Chilodon cucullulus they contract about 13 to 14 times in two minutes, in Paramtecium aurelia, only 10 or 11 times in the same period, whilst in Vorticella microstoma, only once or twice. In Stentor and Spirostomum the contractions occur less frequently still. Of all the above-mentioned animals, the two last have the largest contractile vacuoles, next comes Vorticella^ then Paramtecium aurelia, and lastly Chilodon cucullulus, whose vacuoles are only half as large in diameter as those of Paramtec- ium, where the diameter is about '0127 mm. ; in Vorticella it is 0236 mm (Schwalbe). The interval which elapses between the two evacuations is very regular at the same temperature ; it is, however, considerably affected if the temperature is raised or lowered (Rossbach III. 19, Maupas). For instance, with Euplotes charon, the interval between the contractions is 61 seconds; at 30 Celsius, it has diminished to 23 seconds (Rossbach); that is tosay, the frequency has become nearly trebled. The amount of water which in this manner passes through the animal is extremely great. According to the computations of Maupas, Param&cium aurelia, for example, evacuates, in 46 minutes at 27 Celsius, a volume of water equal to its own volume. From the above-mentioned observations, it may be concluded that contractile vacuoles are not merely simple variable drops of water in the plasma, but that they are permanent morphological differentia- tions in the body of the Protozoon ; that is to say, true cell organs, which appear to perform an important function in the carrying on of breathing and excretion. The energy with which the vacuole dis- charges its contents, so that it completely disappears, indicates that its walls, which consist of hyaline substance resembling the flagel- lum substance, must be contractile to an exceptional degree, and by means of this property are to be distinguished from the endo- plasm of the infusorian body. It must, however, be admitted that no special membrane, clearly denned from the remainder of the body mass, can be seen microscopically, just as with smooth muscle fibres the contractile substance and the protoplasm are not sharply defined from one another, and further as flagella pass over imper- ceptibly at their base into the protoplasm of the cell. Therefore I agree with Schwalbe (III. 21) and with Engelmann, that the vacuoles possess contractile walls although they are not clearly denned from the rest of the protoplasm. In addition, it is well known that delicate membranes are often imperceptible with the microscope although they are undoubtedly present. In many plant cells it is impossible to see the so-called primordial utricle as long as it adheres closely to the cellulose membrane ; its exist- ence, however, cannot be doubted, as its presence can be proved by plasmolysing it. In this opinion, however, I find myself in opposition to Biitschli (III. 3). He considers that the contractile vesicle is simply a drop of water in the plasma. "Each vacuole after evacuation ceases to exist as such. The one that takes its place is a new formation, a newly created drop, which in its turn only exists until it has discharged itself." In his opinion they are due to the flowing to- gether of several formative vacuoles, which separate out as small drops in the plasma, where they increase in size until, by break- ing down the partition walls, they coalesce. However, the exist- ence of the conducting and afferent canals, described by Biitschli himself, the fact that the number of vacuoles present remains constant, and the circumstance that during the diastole the vacuole is seen to occupy the same position as during the systole, and moreover, that the frequency of contraction bears a fixed relation to changes of temperature, all appear to me to support the former view, and to be opposed to Biitschli's theory. The fact that at the end of the systole the vacuole, having evacuated its contents, is for a moment invisible, does not seem to weigh much against the theory of its constancy, especially if one considers that even large lymph spaces and capillary blood vessels in vertebrates elude per- ception in an uninjected condition. IV. Changes in the Cell during passive movement. In order to complete the subject of the movements of protoplasm, it is necessary to consider finally the changes of form which, to a certain extent, the cell may experience in consequence of passive movements. Under these circumstances, the cell is in the same condition as a muscle which, being excited by an external stimulus, becomes extended and then contracted again. In this manner the cells of an animal body may become con- siderably .altered in form, in adapting themselves to all the THE VITAL PROPERTIES OF THE CELL changes of shape which an individual organ experiences as a consequence of muscular action or of distension through a col- lection of fluid or nutriment. Thread-like epithelial cells have to become cylindrical, and cylindrical ones to become flat, when the surface increases in size through the distension of an organ, whilst, on the other hand, the reverse takes place when the whole organ, including its surface, decreases in size. How powerful and sudden may be the changes of form which the protoplasm of a cell, in consequence of passive movement, may experience without damage to its delicate structure, can be best seen in Ccelenterata, in which extended portions of the body, like palpocils, may sud- ^ j> denly shorten by about a tenth or more of their length, in con- sequence of sudden energetic muscular contraction (III. 12 a). The form which an epi- thelial cell assumes varies very considerably, according as to whether it has been taken from a portion of a body which is moderately or strongly con- tracted, as may be seen by comparing Fig. 51 A, B. The former was taken from the ten- tacle of an Actinia, which was only moderately contracted, since by means of chemical reagents it had been rendered non- sensitive before it was killed ; the second was derived from the ten- tacle of another individual which had contracted strongly in death. FIG. 51. Muscular epithelial cell from the endodermal surface of the tentacle of an Actinia (Sagartta parasttrca) (tifier O. arid R. Hertwijr, pi. vi., Fig. 1] ; from Hatschek, Fig. 108): A extended condition of tentacle; B strongly contracted condition of same. Literature III. DE BARY. Die Mycetozoen. Zeitschrift f. ivissenschaftl. Zoologie. Ed. 10. 1860. 2. G. BERTHOLD. Studien iiber Protdplasmamechanik. Leipzig. 1886. 3. BtrrscHLi. Protozoen. First Volume of Bronn's " Classen und Ordnungen dfs Thierreichs." 1889. 4. ALEX. ECKER. Zur LeJire torn Ban it. Leben der conlractilen Suhstanz der niedersten Thiere. Zeitschrift f. wissemchuftl. Zoologie. Bd. I. 1849. 5. ENGELMANN. Protoplasm and Ciliary Movement, trans, by Bourne from Hermann's " Handbuch der Physioloyie." Bd. I. Quar. Jour. Mic. Soc. 1880. 90 THE CELL 6. ENGELMANN. Contractilitdt und Doppelbrechung . ArcMv.f. die gesammte Physiologic. Bd. XL See also E. A. SCHAFER. On the Structure of Amoeboid Protoplasm, etc., with a Suggestion as tn the Mechanism of Ciliary Action. Proc. Hoy. Soc. 1891. J. CLAHK. Protoplasmic Movements and their Relation to Oxygen Pressure. Proc. Roy. Soc. 18o9. 7. ENGELMANN. Ueber die Bewegungen der Oscillarien und Diatomen. Pjiiigers Archiv. Bd. XIX. 8. ENGELMANN. Ueber die Flimmerbewegung. Jenaische Zeitschrift f. 3I((U- cin und Natunoi*senschaft. Bd. IV. 1868. 9. FROMMANN. Beobachtungen iiber Structur und Bewegungserscheinungen des Protoplasmas der Pflanzenzelle. Jena. 1880. 10. FROMMANN. Ueber neuere Erklarungsversuche d. Protoplasmastromungen u. ilber Sfhauinstructiiren BiitschlVs. Anatom. Anzeiger. 1890. 11. HENSEN. Physiologic der Zeugung. Handbuch der Physiologic. Bd. IV. 1881. 12A. 0. and R. HEBTWIG. Die Actinien. Jena. 1879. 12n. KICHARD HEUTWIG. Ueber Mikrogromla socialis, eine Colonie bi'dende Monothalamie des siissen Wassers. Archiv. f. tnikruskof>. Awt. Bd. X. 1874. 13. JURGENSEN. Ueber die in den Zellen der Vallisneiia spiralis stattfinden- den Beivegungxerscheinungtn. Studien des Physiol. liistituts zu Bres- lau. 1861. Heft 1. 14. KLEBS. Form und Wesen der Pflanzlichrn Protoplasmabewegung. Biologisches Certtralblati. Bd. I. 15. KOLLMANN. Ueber thierisches Protoplasm a. Biol. Centralblatt. Bd. II. 16. C. NAGELI. Die Bewegung im Pflanzenrtiche. Beitrage zur icissen- schaftiichen Botanik. Heft II. 1860. NAGELI. Rechts und links. Orlsbeicegungen die Bewegung der lebendigen Substanz. Jena. 18; 2. 25. DE VRIES. Ueber die Bedeutung der Circulation und der Rotation des Protoplasmas filr den Staff transport in der Pfanze. Botanische Zeitung. 1885. CHAPTER IV. THE VITAL PROPEETIES OF THE CELL (continued). Phenomena of Stimulation./ The most remarkable pro- perty of protoplasm is its power of reacting to stimuli : its Irritability. 1 By this is understood, as Sachs (IV. 32a) expresses it, "the power possessed by living organisms alone of reacting to the most various external stimuli in one way or another." It is chiefly through this irritability that living objects can be distin- guished from non-living ones, and in consequence the earlier natural philosophers considered that it was the expression of a special vital force which was only to be seen in organised nature. Modern science has discarded the theory of vitalism (vitalismus) ; instead of explaining irritability by means of a special vital force, it is considered to be a very complicated chemico-physical phenomenon, differing only in degree from other chemico-physical phenomena of inanimate nature. That is to say, the external stimuli come into contact with a substance very complex in structure, an organism, which is an exceedingly complicated material system, and in consequence they give rise to a series of very complex phenomena. However, care must be taken in accepting this mechanical conception not to fall into the very common mistake of trying to explain vital processes as being due directly to mechanical causes, in consequence of their analogy to many phenomena seen in 1 Claude Bernard (IV. la), in Lis lectures on vital phenomena, arrives at the same conclusion, hi-s opinion being based on a number of considerations : " Arrives an tt-rme de nos etudes, nous voyons qu'elles nous imposent une conclusion tres-geuerale, fruit de 1'experience, c'est, a savoir, qu entre les deux ecoles qui font des phenonemes vitaux quelque chose d'absolument distinct des phenouemes physico-chimiques ou quelque chose de tout a fait identique a eux il y a place pour une troisieme doctrine, celle du vitalisuje physique, qui tient compte de ce qu'il y a de special dans les manifestations de la vie et de ce qu'il y ade conforme a 1'action dea forces generales : 1'element ultime du pheuomene est physique ; 1'arraugement est vital ! " 92 THE CELL inanimate objects. It must never be forgotten that there is no substance in inanimate nature which remotely approaches the living cell for complexity of structure, and that bence the reactions of such a substance are of necessity correspondingly complex in character. The field of the phenomena of irritability is exceedingly wide, since it embraces all the correlations which take place between the organism and the outer world. The stimuli which act upon us from without are innumerable. For the sake of clearness, we will consider them under five heads : (1) thermal stimuli, (2) light stimuli, (3) electrical stimuli, (4) mechanical stimuli, (5) the almost infinite variety of chemical stimuli. The manner in which an organism responds to one of these stimuli is called its reaction. This may vary very considerably with different individuals even when they are exposed to the same stimulus. It depends entirely upon the structure of the organism, or upon its finer properties, although these may not be perceptible to us. Different organisms, to use a simile of Sachs (IV. 32a), may in this respect be compared with variously constructed machines, which, when set in motion by the same external force, heat, pro- duce different useful effects according to their internal structures. / Similarly, the same stimulus may produce quite different effects in I different organisms, according to their specific structure. We shall see later on that many protoplasmic bodies are to a certain extent attracted, whilst others are repelled, by light ; a similar difference will be seen when the action of chemical reagents, etc., on protoplasm is studied. The terms positive and negative heliotropism, positive and negative chemotropism, galvanotro- pism, and geotropism are used to describe these varying effects. Another phenomenon, in some respects the exact opposite of the ones described above, must also be explained by the varying specific structure of the stimulated substance ; the term specific energy has been used to describe this phenomenon. Whilst, as described above, we see that protoplasmic bodies, differing in structure, react in various ways to the same stimulus, we find, on the other hand, that similar effects are produced upon the same protoplasmic body by very different stimuli, such as light, electricity, or mechanical injury. A muscle cell responds to all kinds of stimuli by contracting, a gland cell by secreting ; an optic nerve can only experience the sensation of light, whether stimulated by light waves, electricity, THE VITAL PROPERTIES OF THE CELL 93 or pressure. Similarly, as Sachs has pointed out, plant cells also are furnished with their specific energies. Tendrils and roots bend themselves in a manner peculiar to themselves, whether stimulated by light, gravitation, pressure, or electricity. The effect of a stimulus bears the specific stamp, so to speak, of the special structure of the stimulated substance, or, in other words, irritability is a fundamental property of living protoplasm., but it manifests itself in specific actions according to the. specific structure of the protoplasm under the influence of the external world. The same idea is expressed by Claude Bernard (IV. la) in the following words : " La sensibilite, consideree comme propriete du systeme nei-veux, n'a rien d'essentiel ou de specifiquement distinct; c'est I'irritabilite speciale au nerf, comme la propriete de contrac- tion est I'irritabilite speciale au muscle, comme la propriete de secretion est I'irritabilite speciale a 1'element glandulaire. Ainsi, ces proprietes sur lesquelles on fondait la distinction des plantes et animaux ne touchent pas a leur vie meme, mais seulement aux mecanismes par lesquels cette vie s'exerce. Au fond tous ces me- canismes sont soumis a une condition generale et commune, I'irritabilite." In speaking generally of irritability, another peculiar pheno- menon deserves especial attention, namely the transmission or con- duction of stimuli. If a small portion of the surface of a protoplasmic body is stimulated, the effect produced is not limited to this point alone, but extends to far outlying ones. Hence the changes produced by the stimulus at the point of contact must be more or less quickly shared by the rest of the body. Stimuli, as a rule, are more quickly transmitted in animal than in vegetable bodies ; in human nerves, for example, the rate is 34 metres per second ; it is always slower in plant protoplasm. We imagine that the substance which is capable of receiving stimuli forms a system of exceedingly elastic particles in a condi- tion of unstable equilibrium. In such a system it is sufficient for one of the particles to receive a slight shock, in order to set all the others in motion, since each transmits its movement to another. This theory explains the phenomenon, that exceedingly great effects are often produced by very slight stimuli, just as a small spark, by setting on fire a single grain of powder, may cause a powder magazine to explode. Finally, another peculiarity of organic matter is its capacity of returning more or less completely to its original condition, after a 94 THE CELL period, varying in length, of rest or recuperation has elapsed since the cause of irritation was removed. I say advisedly more or less completely, for very often the organic substance is permanently altered in its structure and reacting powers by the application, for a considerable period, of a stimulus, or by the repeated action of the same stimulus. The phenomena thus produced are spoken of as the after-effects of stimulation. As a rule, we are not in a position to determine whether or no a body can be stimulated, that is to say, whether it reacts to changes in its environment, since -most of the effects due to stimulation are imperceptible to us. Sometimes the protoplasm responds by exhibit- ing movements, or by striking changes of form; but, as has been just remarked, such phenomena constitute only a small and limited portion of the results produced, although naturally they are the most important to the investigator, since they are apparent to his perception. In consequence, in the following pages, we will chiefly consider the way in which protoplasm responds, by means of move- ments, to the stimuli, which have been grouped into the above five classes. I have therefore decided to commence my considerations of the vital properties of the elementary organism with contrac- tility. I. Thermal Stimuli. One of the essential conditions for the vital activity of protoplasm is the temperature of its environment. This temperature can only vary between certain fixed limits; if it oversteps either of these, the protoplasm invariably dies immedi- ately. These limits, it is true, are not the same for all protoplas- mic bodies ; some are able to withstand extremes of temperature better than others. The maximum temperature for plants and animals is generally about 40 C. Exposure for a few minutes to such a temperature suffices to cause the protoplasm to swell up and become coagulated, and thereby its irritable structure and its life are destroyed. If an Amoeba is placed in water at 40, it dies immediately ; it draw's in its pseudopodia and " converts itself into a globular vesicle, whose sharply defined double contour encloses a large, turbid mass which, by transmitted light, looks brownish in colour" (Kiihne IV. 15). The same temperature causes " death from heat " in JEthalium gepttcum, coagulation being induced. In Actinophrys, however, instantaneous death occurs at a temperature of 45, whilst the cells of Tradescantia and Vallisneria are only killed by a tern- perature of 47-48 C. (Max Schultze I. 29). THE VITAL PROPERTIES OF THE CELL 95 The protoplasm of organisms which live in hot springs is able to sustain much higher temperatures. Colin found specimens of Leptothrix and Oscillaria in the Karlsbad springs at 53 C., whilst Ehrenberg observed Algae in the warm springs of Ischia. But even in these cases we have not arrived at the extreme limit of heat which can be sustained for a time by living substance. For endogenous spores of Bacilli, which are furnished with unusually resistent envelopes, remain capable of germination after they have been heated for a short time in a liquid at a temperature of 100. Many even can endure 105-130 (de Bary IV. 5/>, p. 4). It is only after a substance has been exposed to the action of dry heat of 140 x for a period of three hours that we can assume with certainty that * all life has been completely destroyed in it. It is even more difficult to determine the lower limit at which " death from cold " occurs. As a rule, temperatures below are less injurious to protoplasm than high ones. It is true that if the eggs of Echinodermata, which are about to divide, are placed in a freezing mixture at a temperature of from 2 to 3 C., the pro- cess of division is temporarily arrested (IV. 12) ; but division recommences and proceeds in a normal fashion when the eggs are slowlv warmed, even if they have been kept in the freezing mix- ture for a quarter of an hour. Indeed, the greater number of the eggs are found to be uninjured even if they have been kept at this temperature for two hours. Plant-cells may be frozen until ice crystals develop in the sap, and yet, after they have been thawed, they exhibit the streaming movements of protoplasm (IV. 15). Sudden exposure to temperatures below zero produces striking changes of form, in the protoplasm of plants; however, it reverts to its normal condition on being thawed. When Kiihne (IV. 15) ex- amined in water cells of Tradescantia, which had been kept for a little more than five minutes in a freezing mixture at 14 C., he fon-nd, in the place of the ordinary protoplasmic net, a large number of isolated, round drops and globules. These, after a few seconds, began to show active movements, and in a few minutes commenced to join themselves one to another, and thus to gradually become reconstructed into a network, in which active streaming movements soon commenced to take place. Kiihne describes in the following words another experiment : " If a preparation of Tradescantia cells is kept for at least one hour in a space which is maintained by means of ice at a tempera- 96 THE CELL ture of 0, the protoplasm is found to exhibit an inclination to break up into separate drops. Even where the network still per- sists, it is composed of extremely fine threads, which are studded here and there with large globules and drops ; several other glo- bules float about freely in the cell fluid, in which, without moving much from place to place, they revolve about their own axes with active, jerking movements. After a few minutes, the free globules are seen to unite themselves to the delicate threads, or to amalga- mate themselves with some of the globules hanging on to the threads, until the appearance of the streaming protoplasmic net- work is quite restored." In plants, as a rule, their power of resistance to cold is inversely in proportion to the amount of water they contain ; seeds which | have dried in the air, and winter-buds, the cells of which consist almost entirely of pure protoplasm, can withstand intense cold, whilst young leaves, with their sap-containing cells, are killed / even by frosty nights. However, the power of resistance to cold varies according to the specific organisation of different plants, or rather of their cells, as is proved by daily experience (Sachs IV. 32B). Micro-organisms are able to resist exceedingly low tempera- tures. Frisch has discovered that the spores, and indeed the vegetative cells of the Anthrax bacillus do not lose their capacity of development by being cooled down in a liquid to a temperature of 110 C., from which they were extracted after it had been thawed. Before reaching the above-mentioned extreme temperatures, at which death by heat or cold is produced, phenomena known as heat rigor or heat tetanus, and cold rigor, occur ; when the protoplasm is in either of these conditions, all the atti'ibutes which show it to be alive, especially those of movement, are arrested so long as the temperature in question is maintained ; but when this is either raised or lowered, as the case may be, after a period of rest, they again manifest themselves. Cold rigor generally occurs at a temperature of about C., whilst heat rigor sets in at a temperature only a few degrees lower than that at which immediate death results; in both cases the protoplasmic movements becomp gradually slower and slower, until at last they quite cease. Amnebfe, Reticularia, and white blood corpuscles draw in their pseudopodia and become converted into globular masses. Most plant cells assume the appearance described above by Kiihne. If the temperature is either slowly raised or THE VITAL PROPERTIES OF THE CELL 97 lowered, as the case may be, the vital appearances gradually become normal. It is true that if the condition of rigor produced by cold is maintained for a considerable time, death may ensue, although cold is better withstood, and for a longer time, than heat. When the protoplasm dies it becomes coagulated and turbid, whilst com- mencing to swell up and to decompose. At the temperatures lying between these extremes, the vital processes are performed in a manner which varies in intensity with the degree of temperature. This is especially true of the movements which take place at dif- ferent speeds, increasing in rate up to a certain point, as the tem- perature rises, until they reach a certain fixed maximum speed. This occurs at the so-called oftimum temperature, which is always several degrees below that at which heat rigor is produced. As the temperature passes this limit, the protoplasmic movements are seen to slacken, until at last rigor sets in. White blood corpuscles have been much used in studying the effects produced by heat ; for this purpose Max Schultze's warm stage, or Sachs' warm, cells, are most suitable. In a fresh drop of blood the corpuscles are seen to be motionless and globular in form. If the drop is warmed the necessary precautions being of course observed the corpuscles gradually commence to extend , pseudopodia, and to move about. As the temperature approaches the optimum for the time being, these changes of shape become more rapid. In Myxotnycetes, Rhizopoda, and plant cells, the effect produced by an access of heat is exhibited by an increase of rapidity of the streaming movements of the granules. Thus, according to the measurements of Max Schultze (I. 29), the granules in the hair-cells of Urtica and Tradescantia travel at ordinary temperatures at a rate of '004 '005 mm. per second, whilst if the temperature is raised to 35 C., their speed is in- creased to '009 mm. per second. In Vallisneria the rate of circulation may be increased to '015 mm., and in a species of Chara even to '04 mm. per second. The difference between the slow and accelerated movements may be so great that whilst with the former the length of a foot is traversed in fifty hours, with, the latter the same distance may be covered in half an hour. Nageli (III. 16) has expressed the acceleration produced by an accession of heat in the granular streaming movements in the cells of Nitella by the following figures : in order to traverse a distance of '1 mm. the granules require 60 seconds at 1 C. ; 24 seconds at 5 C.; 8 seconds at 10 C.; 5 seconds at 15 C. ; 3'6 seconds at 98 THE CELL 20 C. ; 2-4 seconds at 26 C.; 1 5 seconds at, 31 C.; and '65 seconds at 37 C. From these figures it is apparent that "each consecu- tive degree of temperature produces a corresponding slight acceleration " (Nageli, Velten). Finally, it is necessary to mention the remarkable behaviour of protoplasm towards sadden great fluctuations of temperature, and also towards partial or uneven heating. Fluctuations of temperature may be either positive or negative, that is to say, they may be caused by a raising or a lowering of temperature. The consequence of a violent thermal stimulation is a temporary cessation of all movements. However, after a time, the motion recommences at a rate corresponding to the tempera- ture (Dutrochet, Hofmeister, de Vries). The accuracy of these observations has been questioned by Velten (IV. 38). According to his experiments, fluctuations of temperature between the neces- sary limits produce neither a cessation nor a slackening of the protoplasmic movements, which, on the contrary, immediately proceed at a rate corresponding to the temperature which has been attained. . Stahl (IV. 35), in his experiments upon the plasmodia of J Myxomycetes, has made some very interesting discoveries concern- ing the effects produced by partial heating. If a portion of such a plasmodinm, which has spread its network out over an even sui-face, be cooled, the protoplasm is seen to travel gradually from 1 the cooler to the warmer part, so that the one portion of the net- work is seen to shrink up, whilst the other becomes swollen. The experiment may be conducted in the following manner : Two beakers, one filled with water at 7, and the other with water at 30, are placed quite close to one another ; a wetted sti-ip of paper over which a plasmodium has spread itself is then placed over their contingent edges, so that one of its ends dips into each beaker ; the temperature of the water in the beakers is not allowed to vary. After a time the plasmodium, by stretching out and drawing in its protoplasmic thread, succeeds in creeping over to the medium which is best adapted to it. No doubt most free-living protoplasmic bodies move somewhat in this fashion, for as a rule their movements are regulated by expediency, that is to say, they take place in order that the life of the organism may be maintained. For instance, flowers of tan sink down during the autumn to a depth of several feet into the warmer layers of the tan, in order to pass the winter there. THE VITAL PROPERTIES OF THE CELL 99 Then during the spring, as the temperature rises, they move in an * opposite direction, ascending to the warmer superficial layers. II. Light Stimuli. In many cases light, like heat, acts as a stimulus to animal and plant protoplasm. It induces character- istic changes of form in individual cells, and causes movements in fixed directions in free-living unicellular organisms. Botanists have obtained especially interesting results in this department. The plasmodia of JEthalium septicum only spread themselves out on the surface of the tan in the dark ; in the presence of light they sink down below the surface. If a small pencil of light is allowed to fall upon a plasmodinm which has spread its network upon a glass slide, the protoplasm is immediately seen to stream away from the illuminated portion, and to collect in the parts which are in shadow (Barenezki, Stahl IV. 35). Pelomyxa palustris, an organism resembling the Amoeba, is actively motile in shadow, extending and protruding broad pseudopodia. If a fairly powerful ray of light impinges upon it, - it suddenly draws in all its pseudopodia, and transforms itself into a globular body. Only after it has rested quietly in the shade for a time does it gradually recommence its amoeboid movements. " If, on the other hand, daylight is admitted gradually during a period of rather less than a quarter of an hour, no effects of stimu- lation are to be perceived ; this is also the case when, after a prolonged illumination, the light is suddenly withdrawn" (Engel- mann IV. 6 b). The star-shaped pigment cells of many invertebrates and verte- brates, which have been described under the name of chromatn- phores (IV. 3, 29, 30, 33), react very actively to light ; they are the cause of the changes of colour so often seen in many Fishes, Amphibians, Reptiles, and Cephalopods. For example, the skin of a Frog assumes a lighter shade of colour when under the influence of light. This is due to the fact that the light causes the black pigment cells, which extend their numerous processes through the ^ thick skin, to contract up into small black points. In addition, as , they become less prominent, the green and yellow pigment cells, which do not contract, become more easily seen. Further, the pigment cells of the retina become considerably / altered in form under the influence of light, both in vertebrates (Boll) and in invertebrates, for instance in the eyes of Cephalopoda (Rawitz IV. 31). It is a well-known fact that many unicellular organisms which 100 THE CELL propel themselves by means of cilia or flagella, such as Flagtl- lata, Ciliata, the swarm-spores of Alga, etc., prefer to collect either on that side of the cultivation dish which is nearest the window, or on the one which is away from it. This may be easily proved by means of a simple experiment described by Nageli (III. 16). A piece of glass tubing three feet in length is filled with water containing green swarm-spores of Algce (tetraspores), and. is placed perpendicularly. Then, if the upper part of the tube is covered with black paper, and light is allowed to fall upon the lower portion, it is seen after a few hours that all the spores have collected in this lower portion, leaving the upper part colourless. If now the upper portion is uncovered, and the paper is transferred to the lower part, all the swarm- spores ascend the tube, and collect on the surface of the water. Euglena viridis is exceedingly sensitive to light (Fig. 44 A, IV. 8). If a drop of water containing Euglence is placed upon a slide, and only a small portion of it is illuminated, all the individuals collect in this area, which, to quote an expression of Engelmann's, acts like a trap. This organism is especially interesting, because the perception of light is restricted to a definite portion of the body. Each Euglena consists of two portions, a large posterior one containing chlorophyll, and a colourless anterior, flagella-bearing one, in which there is a red pigment spot. Now, it is only when this anterior portion comes into contact with light, or is placed in shadow, that the organism is seen to react by altering the direction of its movements (Eugelmann). Heuce, in this case, a certain part of the body functions to a certain extent as an eye. Stahl (IV. 34) and Strasburger (IV. 37) have investigated most fully the action of light upon swarm-spores. The former sums up his results in the following words : " Light effects an alteration j in the direction of the movements of swarm-spores by causing j them to make their longitudinal axes coincide approximately with I the light. The colourless flagellated end may be directed either towards or away from the source of light. Either position may become exchanged for the other under otherwise unaltered external conditions, and, indeed, this occurs at very different degrees of light intensity. The intensity has the greatest influence j over relative positions. When the light is very intense, the anterior end is directed away from the source ; when it is less j strong, the swarm-spores move towards the light." THE VITAL PROPERTIES OF THE CELL 101 This sensitiveness towards light varies considerably both in different species and in individual members of the same species ; indeed, even in the same individual, considerable differences may be seen under different external conditions. This varying power of reaction in swarm-spores has been called phototonus or lio-ht- tone by Strasburger. Swarm-spores of the Botrydium and Ulothrix, which react some- what differently under the influence of light, are very suitable for experiments on this subject. If some swarm-spores of Botrydium are placed in a drop of water upon a coverglass, and are kept in shadow, they spread themselves out evenly in the water. If a light is allowed to fall on them, they are seen to immediately direct their anterior ends towards the source of light, and to hurry in fairly parallel paths towards it. After a short time, at most from one and a half to two minutes, almost all of them have collected at the illuminated side of the drop, which, for the sake of brevity, Strasburger has named the positive edge, to distinguish it from the opposite or negative edge. Here they are seen to intermingle and to conjugate in large numbers. If the slide is now turned round through an angle of 180, all the spores which are still capable of movement immediately forsake the edge of the drop, which is now turned away from the light, and hasten back towards the light. If . the microscope is fitted with a rotating stage, it is possible by turning the latter to make the swarm-spores continually keep I changing their course. They always travel in a straight line/ towards the light. Ulothrix zoospores behave in a somewhat different manner. " These also travel quickly, and in approximately straight paths towards the positive edge of the drop ; however, as a rule, they do not all move in this manner; on the contrary, it is generally the case that a larger or smaller number of individuals in each prepara- tion are seen to move rapidly in the opposite direction, that is to say, towards the negative edge. A most peculiar spectacle is thus produced, for the spores, since they go in opposite directions, appear to travel at double speed as they pass each other. If the preparation is turned through an angle of 180, the spores which had collected on the side which was positive are seen to hasten to the other edge, whilst the others, which were collected on the side which was negative, travel in the opposite direction, and having arrived at their destination, they commence to move about 102 THE CELL amongst themselves, keeping more or less close to the edge of the drop, according to the condition of the preparation. Continu- ally, individual spores are seen to suddenly forsake the side, either positive or negative, at which they were stationed, and to hurry through the drop to the opposite one. Such an exchange is continually taking place between the two sides. Indeed, it frequently occurs that certain individuals, which have just left one side and arrived at the other, hasten back to the one from which they originally came. Others become arrested in the middle of their course, and then return to their starting-point, in order eventually to oscillate backwards and forwards for a considerable time like a pendulum." The following experiment, described by Strasburger, shows how sensitively and quickly the zoospores react to light : " If a piece of paper is placed between the microscope and the source of light, just as the zoospores are on their way from one edge of the drop to the other, they immediately turn to one side, many rotating in a circle ; this, however, only lasts for a moment, after which they continue to move in the same direction as before (interruption movements)." Strusburger (IV. 37) has named those I zoospores which hasten towards the source of light light-seeking (photophylic), and those which travel from it light-avoiding (i>hoto- phobic). As has been already remarked, the way in which the spores collect at one or other side of the drop, thus indicating their special kind of j>hototomts, depends upon external circumstances, such as the intensity of the light, the temperature, the aeration of the water, and their condition of development. It is possible to entice spores, which under intense illumination have collected on the negative side, to come over to the other side. The intensity of the light must be gradually diminished in pro- portion to their phototonusby introducing one, two, three or more f screens of ground glass between the preparation and the source of light. The same result may be more easily obtained by moving the microscope slowly away from the window, and thus rendering the illumination less intense. t The temperature of the environment often has a considerable influence upon the degree of sensitiveness to light which is evinced by many zoospores. When the temperature is raised they become, so to speak, attuned to a greater degree of sensitiveness ; whilst, at the same time, their movements are rendered more active : the THE VITAL PROPERTIES OF THE CELL 103 reverse is the case when the temperature is lowered. In the first case they also become more photophylic (light-seeking), and in the latter- more photophobic (light-avoiding). " In addition, zoospores alter as regards their phototonus during the course of their development, for they appear to be able to withstand greater intensity when they are young than when they are old." As is shown by the experiments of Cohn, Strasburger, and Others, not all the rays of the spectrum are able to exert an influence upon the direction of the movements of the spores, it being only those which are strongly refracted (blue, indigo and violet) that produce stimulation. If a vessel containing a deep-coloured solution of ammoniated copper oxide, which only transmits blue or violet rays, be placed between the source of light and the preparation, the spores are seen to react just as if they came in contact with ordinary white light ; on the other hand, they do not react at all to light which has passed through bichromate of potassium solution, through the yellow vapour of a sodium flame, or through ruby-red glass. Another very important and complex manifestation of the effects due to light is seen in the movements of the chlorophyll , corpuscles in plant cells. The light acts as a stimulus to proto- plasm, which contains chlorophyll, causing the latter to collect by ' means of slow movements in suitable places within the cellulose membrane. The most suitable object for the study of these phenomena is the Alga, Mesocarpus, upon which Stahl (IV. 34) has made some most convincing observations. In the cylindrical cells, which are united together to form long threads, a narrow band of chlorophyll is extended longitudinally along the middle of the vacuole, which is thus divided into two equal parts ; the ends of this band pass over into the protoplasmic lining of the wall. Now this chlorophyll band changes its position according to the direction of the impinging light. If it is exposed directly from above or below to weak daylight, it turns its surface towards the observer. If, however, on the contrary, it is arranged so that only such rays as are parallel to the stage of the micro- scope are allowed to reach the preparation from one side, the green plates are seen to turn about through an angle of 90, so that they take up an exactly vertical position, assuming now an appearance of dark green longitudinal stripes, stretching them- JC4 THE CELL selves through the otherwise transparent cell. The band is able to assume every possible intermediate position in its endeavour t place its surface at right angles to the impinging light, On a warm summer's day this change- of position is effected in a very few minutes, being brought about by the active movements which the protoplasm makes inside the cell membrane. The effect produced varies in this case also, as with the zoo- spores, according to the intensity of the light. Whilst diffuse daylight has the effect described above, direct sunlight brings about a quite opposite result, for in this case the chlorophyll bands turn one of their edges to the sun. Hence we can educe the following: "Light exerts an influence upon the position of the chlorophyll bands of Mesocarpus. If the light is fairly weak, the bands turn themselves at right angles to the path of the rays; if, however, it is intense, they place themselves in the same direction as the rays." Stall 1 calls the first arrangement surface position, and the second, profile position. If illuminated intensely for a considerable period, the whole band contracts to form a dark green vermiform body; it is, however, under favourable conditions capable of resuming its original form. The purpose of all these various movements of the protoplasm \ under the influence of light is, on the one hand, to bring the . chlorophyll bands into a favourable position for the exercise of their functions ; and, on the other, to protect them from the in- jurious action of a too powerful illumination. Further, the plant-cells which contain chlorophyll granules, and which are connected to form tissues, are also subjected to the influence of light, as is so plainly seen in Mesocarpus. Only in this case the phenomena are somewhat more complex (Fig. 52). Sachs was the first to notice that the colour of leaves is lighter when they are exposed to direct sunlight, than when they are in shadow, or when the light is less intense. In consequence of this discovery, Sachs was able to produce light pictures upon leaves, by partially covering them with strips of paper, and exposing them to intense light (IV. 32a) ; after a certain time the strips of paper were removed, and it was then seen that the portions which they covered appeared as dark -green stripes upon a light-green background. This phenomenon may be explained by the law which was laid down in the case of Mesocarpus; this has been proved by the THE VITAL PROPERTIES OF THE CELL 105 investigation of Stahl (IV. 34), which he conducted on the lines laid down by Famintzin, Frank, and Borodin. When the illumina- tion is faint, or when the leaves are in shadow, the protoplasm moves so that the chlorophyll granules are arranged upon those external surfaces of the cells which are turned towards the light (Fig. 52^1), having completely forsaken the side-walls. On the other hand, the protoplasm, under the influence of direct sunlight;, streams away towards the side- walls, until the external surface is quite free from chlorophyll granules, that is to say, in the first FIG. 52. Transverse section through the leaf of Lemma Msulca (after Stahl) : A surface position (position assumed in diffused sunlight) ; B arrangement of chlorophyll granules under the influence of intense light ; C position assumed by chlorophyll granules in the dark. case, the whole chlorophyll-bearing substance, as in Mesocarpnfi, assumes a surface position towards the impinging light, and in the second, a profile position ; hence the varying colour of the leaves. 106 In addition, the chlorophyll granules themselves, iclien under the influence of intense light, alter their shape, becoming smaller and more globular. All these occurrences serve to accomplish the same end : " Chlorophyll granules protect themselves by turning on their axes (Mesocarpus} , by migration, or by altering their shapes from intense illumination." " If the illumination is weak, the largest surfaces are turned towards the light, in order that as much of it may be received as possible. The behaviour is exactly the oppo- site when the light is strong, a smaller surface being then exposed to the light." III. Electrical Stimuli. As has been shown by the experi- ments of Max Schultze (I. 29), of Kiihne (IV. 15), of Engelmann, and of Ver- worn (IV. 39), electrical currents, both constant and induced, act as stimuli upon protoplasm, when they flow directly through it. If some staminal hairs of Tradescantia (Fig. 3) are placed between non-polar- isable electrodes which are close together, and are then stimulated by means of weak induction shocks, the granular streaming move- ments can be seen to have been influenced in that por- tion of the protoplasmic net through which the current flowed. Irregular masses and globules develop upon the protoplasmic threads ; these separate off at the thinnest places, and become absorbed into neighbouring threads. After a short period of rest, the move- ments recommence, the PIG. 53.^, B cell of a staminal hair of Tra- descantiavirgimca. A Normal condition of proto- plasm before it has been disturbed. B The proto- plasm, in consequence of stimulation, has massed itself into balls ; a cell- wall ; b transverse wall of two cells; c, d balls of protoplasm. (After Kiihne; from Verworn, Fig. 13.) THE VITAL PROPERTIES OF THE CELL 107 masses arid globules being gradually taken up by the neighbouring streams of protoplasm, carried along by them, and finally split up. If strong shocks are repeatedly administered, so that the whole cell is affected, a return to the normal condition is impossible, for the protoplasmic body, by becoming partially coagulated, has been transformed into turbid flakes and masses. In Amoebce and white blood corpuscles the streaming motions of the granules and the crawling movements of the whole cell are both arrested for a time by slight induction shocks; after a while they are resumed and proceed in a normal fashion. If stronger induction shocks are administered, the result is that the pseudo- podia are quickly withdrawn, and the body contracts up into a ball ; finally, very strong shocks cause the bursting and consequent destruction of the contracted spherical body. If the induction current is applied for a considerable time to one of the loiver unicellular organisms, it can be gradually destroyed bit by bit, and thus diminished in size. In A ctinosphcerium the process is as follows : the pseudopodia, which are parallel to the current, soon exhibit varicosities ; they are gradually completely with- drawn, whilst the protoplasm becomes massed together to form little balls and spindles (Fig. 54) ; then at this place the surface of the body becomes gradually destroyed by a process resembling to a certain extent a kind of melting down, during which the vacuoles, which are con- tained in the protoplasm, burst. On the other hand, those pseudopodia which are at right angles to the cur- rent are unaffected. When the stimulus is removed, the body, which has thus been reduced to about a half or a third of its original size, gradually recovers, and re- produces the parts which have been destroyed. The action of the constant current upon the Actino- sphcerium (Fig. 55), Actino- j>l>rys, Pelomyxa, and Myxomycetes, is similar to this. When the circuit is closed, an excitation occurs at the positive pole or anode FIG. M.-Actinnsphm-ium Eiclihornii, action of an interrupted current. Progressive de- struction of protoplasm is equal at both poles. (After Verworn, Tab. 1, Fig. 5.) 108 (in Fig. 55, hich is manifested by the retraction of the pseudo- podia, and, if the stimulus lasts long, by the destruction of the protoplasm at the place where the current enters. When communication is broken, the destructive process at the anode immediately ceases, whilst, on the other hand, a -transitory contraction occurs at the sur- face which is turned towards the cathode. Perhaps even more interest- ing and important than these processes are the phenomena produced by Galvanotrop- ism, which have been observed by Verworn in a number of unicellular organisms (IV. 39, 40). Many organisms, in conse- quence of the influence of the constant current, are caused to move in certain fixed directions, just as they move when stimulated by a ray of light (heliotropism). " If a drop, containing as many Paramcecia aurelia as possible, is placed upon a slide between two non-polarisable electrodes, and the constant galvanic circuit is closed, it is seen that the Paramaecia immediately leave the anode in a mass, and hurry in a dense swarm to the cathode, where they collect in great numbers. After a few seconds the rest of the drop becomes completely free from Protista, whilst at the cathode there is a dense seething crowd of them. Here they remain as long as the current persists. When connection is broken, the whole swarm immediately forsakes the cathode to swim back in the direction of the anode. How- ever, they do not all collect at the anode, part of them re- maining scattered about in the drop ; at first they do not come near to the cathode, but after a time they gradually approach it, until finally all the Protista are again evenly distributed through- out the drop." If pointed electrodes are employed, the Paramcecia swarm inwards to form a galvanic Hgure around the cathode (Fig. 56.4). FIG. 55. Actinostihasrium Eichhornii, tween the poles of a constant current. A short time after the closing of the current, granular destruction of the protoplasm commpnces at the anode (+). At the cathode the pseudopodi* have become normal again. (After Verworn, Tab. 1, Fig. 2.) THE VITAL PROPERTIES OF THE CELL 109 An appearance similar to that produced when iron filings are attracted by a magnet is seen. " Under the circumstances," as B FIG. 56. On completing the circuit of the constant current all the Paramaecia in a drop of water swim within the curve of the electric current towards the negative pole (4) until after a time they collect on the other side of the pole (B). (After Verworn IV. 40, Fig. 20.) Vervvorn remarks, " it may be observed that after all the Paramcecia have wandered over to the negative pole, the largest collection is formed behind, that is to say reckoning from the positive pole on the other side of the negative pole, and that only a few remain on this side of the pole (Fig. 56 B). When the connection is broken the Protista swim back again, in the manner described above, towards the positive pole, keeping at first, just as before, well within the curve of the electric current, until gradually the movement, and with it the division into groups, becomes ir- regular again." In the same manner, a number of other Ciliata (such as Stentor, Colpoda, Halteria, Coleps, Urucentnm) and Flagellata (such as Trachelomonas, Peridinium) are galvanotropic. Amcebce react in a similar manner. At the first moment after the circuit of the constant current has been completed a cessa- tion of the streaming movements of the granules occurs ; very soon, however, the hyaline psendopodia are suddenly protruded from the end which is turned towards the cathode, and, whilst the remainder of the body substance Hows in the same direction, and keeps continually stretching out new pseudopodia, the Amoeba creeps towards the cathode. When the current is reversed it is seen that the granular streaming movements are also immediately reversed, and the Amoeba commences to creep in the opposite direction. The movement towards the cathode may be called negative galvanotropism. As there exist both negative and positive heliotropism and thermotropism, so we occasionally find isolated 110 THE CELL instances of positive galvanotropism. It has been observed by Verworn in Of>alina ranarum, and in a few Bacteria and Flagellata such as Cryptomonas and Ghilomonas. When the circuit is com- pleted the above-named species travel towards the anode instead of towards the cathode, and collect there. If Ciliata and Flagel- lata are present side by side in one drop, they are seen under the influence of the constant current to hasten in opposite directions, so that finally two distinct groups are to be seen, the Flagellata being at the anode, and the Ciliata at the cathode. If the current is now reversed they advance like two hostile armies upon one another, until they assemble again at the opposite poles. Each time the current was made it produced in a few seconds a dis- tinct sorting out of the crowd of Infusoria, which were otherwise in inextricable confusion. IV. Mechanical Stimuli. Pressure, violent shaking, crushing, all these act as stimuli to protoplasm. Weak mechanical stimula- tions only produce an effect upon the point of contact ; strong stimuli affect a larger area and produce a more rapid and more powerful effect than weak ones. If a cell of a Tradescantla or Chara or the plasmodium of an ^Ethalium be violently shaken, or pressed upon at one place, the granular movement is temporarily arrested, whilst swellings and knots may even appear on the protoplasmic threads, such as are produced by the electrical current. Hence it frequently occurs, that in preparing the slide for observation all the protoplasmic movements may be brought to a standstill, simply by putting on the coverglass. They gradually return after a period of rest. AmcebcK and white blood corpuscles withdraw their pseudopodia and assume a globular form when they are violently shaken. Reticularia, which have extended their long processes, often with- draw them with so much energy that the ends which were attached to the slide are torn off (Verworn). A localised stimulus can be produced at a given point with a fine needle. If the stimulus is weak the effect is confined to this point, a varicosity being formed and a shortening of the pseudopodium being pro- duced. Strong and repeated stimuli cause neighbouring pseudopodia, which were not directly touched, to contract (Fig. 57 B). If an Infusorian or other small animal comes in contact with an outstretched pseudopodium, it is firmly grasped by it, and becomes surrounded by the protoplasm. As the pseudopodium THE VITAL PROPERTIES OF THE CELL B 111 FIG. 57. Oi-bitolites. A portion of the surface with its psendopodia: A undisturbed ; B the whole has been stimulated by repeated shaking. (After Verworn III. 21, Fig. ".) This is of importance to Rhizopoda in absorbing food. gradually shortens itself, a motion in which the neighbouring threads eventually participate, the Infusorian is gradually drawn into the centre of the protoplasmic mass, where it undergoes digestion. V. Chemical Stimuli. A living cell is able to a certain extent to adapt itself to chemical changes in its environment. For this, however, one thing is most important, namely that the changes should be made gradually, not suddenly. JEthalium plasmodia flourish in a 2 per cent, solution of grape- sugar, if the latter is added in gradually increasing quantities to the water (IV. 35). If they were to be transferred straight from pure water into this chemically different environment, the sudden change would result in their death; this would also occur if they were to be suddenly placed back into pure water from the 2 per cent, sugar solution. It is evident that the protoplasm needs time to adapt itself to its altered condition, probably by increasing or diminishing the amount of water it contains. Marine Amoebae and Reticularia remain alive after the water which contains them, in consequence of being in an open vessel, has evaporated so much that it contains 10 per cent, of salt. Fresh water Amoebae can gradually accustom themselves to a 4 per cent, solution of common salt, whereas, if they are suddenly immersed in a 1 per cent, solution, they contract into balls, and in time become broken up into glistening droplets. During the pro- cess of adaptation to a new chemical environment, the individual 112 THE CELL cells may undergo greater or less changes in their structure and vital properties. When such changes are apparent to us, we speak of the effects of chemical stimulation. These phenomena, which are so exceedingly numerous, may vary considerably, according as to ivhether the whole, or only part, of the cell-body is affected by the stimulus. a. First group of experiments. Chemical stimuli which affect the whole of the body. In order to throw light upon this first group of phenomena, the behaviour of protoplasm towards certain gases, which are grouped under the common name of anaesthetics, must be investigated. The protoplasmic movements of a plant cell soon become arrested, if, instead of being put into water, it is placed in a drop of olive oil, by which means the air is excluded (IV. 15). After the oil has been removed, the movements are seen to gradually recommence. The streaming movements may in a similar manner be slackened and finally completely stopped, if the air is replaced by carbon di- oxide or hydrogen. For these experiments special slides with gas v chambers have been constructed through which a current of carbon dioxide or hydrogen may be conducted. If the plant cell is kept from 45 minutes to an hour in a current of carbon dioxide, the movements are as a rule completely stopped ; when hydrogen is used, a longer time must be allowed (III. 5). This protoplasmic paralysis may, if it has not been allowed to last too long, be removed by the addition of oxygen. "Apparently living protoplasm unites chemically with the oxygen of its environment. The definite oxygenated compound thus produced, of which under ordinary conditions a considerable amount must be assumed to exist in every protoplasmic body, is continually broken down during the movements, whilst carbon dioxide is probably given off" (Engelmann III. 5). Hence the removal of oxygen has a paralysing effect upon the irritability, and indeed upon all the vital activities of the protoplasm. Such anaesthetics, as chloroform, morphia, chloral-hydrate, etc., have a marked influence upon the vital activities of the cell. These substances do not affect the nervous system alone, as is frequently believed, but all the protoplasm of the body. The difference is only a matter of degree ; the irritability of the nerve-cells is more quickly lowered and finally destroyed than that of the protoplasm of other cells. Further, when narcotics THK VITAL PROPERTIES OF THE CELL 113 are employed medicinally, the attempt is made to act upon the nervous system alone, for if all the elementary cells were affected, a cessation of the vital processes Avould result, and death might ensue. However, the following examples will prove clearly that the irritability of animal and vegetable protoplasm may be temporarily destroyed without permanent harm. The sensitive plant, or Mimosa pudica, is very easily affected by mechanical stimulation. When a leaflet is shaken a little, it immediately closes itself up, and forsaking its upright position, droops downwards. In addition, it forms an example of the rapid manner in which a stimulus is conducted in plants, in which, since no nerves are present, it must be simply transmitted by each pro- toplasmic cell quickly conveying the impulse to its neighbour. In consequence of this, if the stimulus is sufficiently strong, not only do the leaves which were directly touched close up, but also those on the same branch, and eventually even the whole plant, are affected. In consequence of the stimulation, certain mechanical arrangements, not suitable for present discussion, come into play. In order to study the effect of anaesthetics, a sensitive plant, in a condition of normal irritability, should be placed under a bell- jar, and when the leaves are fully extended, a sponge soaked with chloroform or ether should be inserted (Claude Bernard IV. 1). After about half an hour it is seen that the chloroform or ether vapour has caused the protoplasm to lose all its irritability. When the bell- jar is removed, the leaves, which are spread out as usual, may be touched, or even severely crushed or cut, without any reaction being produced ; the result is the same as that pro- duced on one of the higher animals provided with nerves. And yet, if proper precautions have been taken, it is found that the protoplasm has not been killed, for after the sensitive plant has been for a short time in the fresh air, the narcosis gradually disappears ; at first, individual leaves gradually close up when they are roughly handled, until finally complete irritability is restored. Ova and spermatozoa may be subjected to the action of narcotics in a similar manner. When Richard Hertwig and myself (IV. 12a) placed the actively motile spermatozoa of a sea-urchin in a '5 per cent, solution of chloral-hydrate in sea water, we found that after five minutes, their motions were completely arrested; however, these soon recommenced, after the chloral solution had been diluted with pure sea water. Further, those spermatozoa which had been 114 THE CELL temporarily paralysed in this manner united with ova when they were brought to them, almost as quickly as fresh spermatozoa. When they were kept for half an hour in the chloral solution, a more marked paralysis was produced, which persisted for a long time after the noxious agent had been removed. It was not until some few minutes had elapsed that certain individual isolated spermatozoa commenced to exhibit snake-like movements, which gradually became more active. Even when they were brought into the neighbourhood of ova, it was observed, that after ten minutes none of these were fertilised, although several spermatozoa had attached themselves to their surfaces, and had bored their way in. But even in this case fructification and the subsequent normal division of the eggs took place finally. Similarly, egg-cells become affected, as regards their irritability, by a "2 to '5 per cent, solution of chloral hydrate or of some similar drug ; this may be recognised by the abnormal manner in which, after the seminal fluid has been added, the process of fertilisation takes place. For whilst under ordinary circumstances only one single spermatozoon penetrates into the ovum, with the result that a firm yolk membrane is immediately formed, which prevents the entrance of other spermatozoa, in Moralised eggs multiple fertilisa- tion takes place. It has been proved that, according to the inten- sity of the action of the chloral, that is to say, the stronger the solution, and the longer it is allowed to act, the greater is the number of spermatozoa which make their way into the ovum before the formation of the yolk and membrane. Evidently the effect of this chemical reagent is to lower the power of reaction of the egg plasma, so that the stimulus which is produced by the entrance of one spermatozoon is now no longer sufficient, but the ovum must be stimulated by the entrance of two, three, or even more spermatozoa, before it is sufficiently excited to form a mem- brane. Finally, another example will show that the chemical processes of 'the cell may also be hindered by anaesthetics. As is well known, the yeast fungi (Saccliaromyces cerevisiw) produce alcoholic fer- mentation in a solution of sugar, and during this process bubbles of carbon dioxide rise through the fluid. When Claude Bernard (IV. 1) added chloroform or ether to the solution of sugar, before adding the yeast, no fermentation took place, although in other respects the circumstances were favourable. But when the yeast, after having been filtered out from the chloroform THE VITAL PROPERTIES OF THE CELL 115 solution, and rinsed with clean water, was placed in pure sugar solution, he found that fermentation soon occurred; hence the yeast had recovered its power of converting sugar into alcohol and carbon dioxide, this power having, by the action of the chloroform and ether, been temporarily suspended. In a similar manner the functions which the chlorophyll per- forms in plants, and the dependent process of giving off oxygen in the sunlight, may be arrested by means of chloroform (Claude Bernard). b. Second Group of Experiments. Chemical Stimuli which come into contact with the cell-body at one spot only. Very interesting and varying phenomena are produced when chemical substances, instead of coming into contact with the body all round, only impinge upon it, at a definite fixed point. Such stimuli may produce changes in form, and movements in a definite direction, which phenomena have been classed under the name of Chemotropism (Chemo taxis). Chemotropic movements may be directed towards the stimulating source, or, on the contrary, away from it. In the first case the chemi- cal substance is said to attract, and in the second to repel, the protoplasmic body. This depends partly upon the chemical nature of the substance, partly upon the individual properties of the special kind of plasma, and, finally, upon the degree of conden- sation of the chemical substance. A substance, which when dilute may attract, may repel when the solution is strong. Here, as with strong and weak light, special differences are present. Just as heliotropism may be positive or negative, so may chemotro- pism be positive or negative. We will first examine the action of gases, - and next that of solutions ; at the same time we will become acquainted with a very ingenious method of investigation, for which we must especially thank the botanist Pfeffer (IV. 26). 1. Gases. Oxygen has great attractive powers for freely moving cells, as has been shown by the experiments of Stahl, Engelmann, and Verworn. Stahl has made experiments upon the plasmodia of ^Ethalium septicum (IV. 35). He half filled a glass cylinder with thoroughly boiled water, which, in order to exclude the air, he covered with a very thin layer of oil. He then took a strip of filter paper, over which a plasmodium had extended itself, and placed it along the side of the cylinder in such a manner that one half of it was 116 immersed in the water. The strands of protoplasm, which were placed in the non-oxygenated water, were seen to grow gradually thinner, until after a time all the protoplasm had crept up above the layer of oil, which, except in excluding the air, had no deleterious effect upon it, to the upper portion of the cylinder, where it could come into contact with the oxygen cf the air. Another method of performing the same experiment is to place a plasmodiurn in a cylinder which is quite full of thoroughly boiled water; to close the opening with a perforated cork, and then to place the cylinder upside down in a plate of fresh water. Very soon the plasmodium is seen to have wandered through the small hole in the cork into the medium which contains oxygen. Engelmann (IV. 7) has made some very interesting experiments upon the directing influence exerted by oxygen upon the move- ments of bacteria. He shows that many species of bacteria may be used as a very delicate, test for minute quantities of oxygen. . If into a fluid which contains certain bacteria, a small alga or diatom is introduced it is seen after a short time to be surrounded with a dense envelope of bacteria, which have been attracted by the oxy- gen set free by the action of its chlorophyll. Verworn (IV. 40) saw a dia- torn quite enclosed by a wall of motionless Spirochsetse whilst the rest of the preparation was quite free from them (Fig. 58). Suddenly the diatom moved a short distance away, getting out of the crowd of Bacteria. The Spirochsetfe, so suddenly left in the lurch by the producer of oxygen, remained quiet for a second, but soon commenced to move about quickly, and to swim after the diatom in dense masses. After a minute or two they had nearly all reassembled round about it, after which they remained motionless as before. This attractive power pos- FIG. 59. A large diatom (rinntila.-in) surrounded by a large number of S^ro- chcette plicatilis. (After Verworn IV. 10 Fig. U.) THE VITAL PROPERTIES OF THE CELL 117 sessed by oxygen explains the fact that in microscopic prepara- tions almost all Bacteria, Flagellata, and Ciliata are found collected together round the edges, or round any air bubbles which may be present in the water. Vervvorn describes a most instructive experiment (IV. 40). A large number of Paramrecia are placed in a test-tube, which is filled with water, poor in oxygen. The test-tube is then reversed and placed under mercury. Very soon the movements of the cilia commence to slacken, in consequence of the lack of oxygen. If now a bubble of pure oxygen is introduced through the mer- cury into the test-tube, it will be seen after a few seconds to be surrounded by a thick white envelope of Paramcecia, " which, driven by their thirst for oxygen, throw themselves energetically upon the bubble of this gas." 2. Liquids. Stahl and Pfeffer have made systematic experi- ments upon the stimulating action of fluid substances. Stahl (IV. 35) has again made great use of flowers of tan. Upon this organism even pure water has a stimulating effect, a phenomenon described by Stahl as positive and negative hydro- tropism. If a plasmodium is evenly spread out over a strip of damp filter paper, it is seen, as soon as the paper commences to dry, that the plasmodium makes its way to the dampest parts. If, whilst the drying process is going on, a slide covered with gelatine is held perpendicularly at about two mm. distance above the paper, a few branches are seen to extend themselves upwards to- wards the gelatine, attracted by the water vapour it gives off, until finally they reach it and spread themselves out upon it possibly, during the course of a few hours, the whole plasmodium may transfer itself to the damper surface. When Myxomycetes are about to fructify, negative instead of positive hydrotropism takes place. Under these conditions the plasmodia seek the driest portions of the environment, and withdraw themselves from any damp gelatine or moistened filter paper which may be brought into their neighbourhood. These phenomena of hydrotropism. are easily explained by the fact that protoplasm contains a certain quantity of imbibition water, which may fluctuate up to a certain extent, and may even increase or decrease during the development of the cell-body. The more saturated the protoplasm is with water, the more active as a rule are its movements. During the vegetative period the plasmodium of the ^thalium tends to increase its supply of water, 118 THE CELL and hence it moves towards the source of water ; when the re- productive period commences, it shuns moisture, because, at the time when spores are being formed, it diminishes its water supply. Many chemical substances attract, whilst others repel plasmodia. If a net of JEihalium, which has spread itself out upon a moist substratum, is brought into contact with a ball of filter paper, which is saturated with an infusion of tan, individual strands of plasma immediately commence to creep towards the nutrient medium. After a few hours all the spaces in the paper ball are filled up with the slime fungus. In order to study negative chemotropism, a crystal of common salt or of saltpetre, or a drop of glycerine, may be brought to the edge of the piece of damp filter paper upon which the slime fun- gus has spread itself out. It can then be seen how, as the con- centrated solution of salt or of glycerine gradually creeps along the filter paper, the protoplasm shrinks away from the source of stimulation in ever-widening circles. Hence the naked plasmodia, which are so easily destroyed, possess the marvellous property, on the one hand, of avoiding harmful substances, and, on the other, of searching all through the medium in which they are, for substances which are of value to them for purposes of nutrition, and of absorbing them. " For instance, if one of the numerous branches of a plasmodium, by chance comes across a place which is rich in nutriment, an influx of plasma immediately occurs to this favourable spot." Pfeffer has Very accurately examined the chemotropism of small, freely motile cells, such as spermatozoa, Bacteria, Flagellata, and Ciliata, in some pioneering investigations that he has made, and by this means has discovered a very simple and ingenious method of investigation. He takes some fine glass capillary tubes from 4 to 12 mm. long ; one end of each tube is closed, whilst at the other there is an opening varying in inside diameter from "03 to "15 mm., ac- cording to the size of the organism to be examined. He fills these tubes for about a half or a third of their length with the stimulating substance, there being a space filled with air at the closed end. In order to explain their use, we may quote the following ex- periment. Pfeffer has discovered that malic acid has a strong affinity for the antherozoids of Ferns, and that probably it is on this account that it is secreted normally by the archegonia. A THE VITAL PROPERTIES OF THE CELL 119 capillary tube is filled with '01 per cent, of malic acid, and after its surface has been most scrupulously cleansed, is reversed and care- fully placed in a drop of water containing a large number of Fern antherozoids. With a magnifying power of 100 to 200 diameters, it can be seen that some antherozoids immediately begin to make their way towards the opening of the tube, from which the malic acid commences to diffuse itself throughout the water. They soon force their way right into the tube itself, until after five or ten minutes several hundreds of them have collected there. After a short time there are only a few left outside of the tube. If experiments are made with solutions of malic acid of varying strengths, a law similar to that of the effect produced by various degrees of heat upon protoplasmic streaming movements may be deduced. Beyond a certain minimum concentration (about '001 per cent.) ivhich may be considered to constitute the stimulative starting point, every increase in concentration produces a corresponding in- creased effect, until a certain fixed point is reached, when the optimum or maximum result is produced; if the concentration is increased above this point the attraction of the malic acid for the anthero- zoids decreases, until finally the positive chemotropism is con- verted into negative chemotropism. Hence a very strong solution produces an exactly opposite effect to that produced by a weak one, the antherozoids being repelled instead of attracted. How small a quantity of malic acid is necessary to produce a result may be seen from the fact that in a capillary tube which contains a '001 per cent, solution only OOOOOU0284 milligramme, or -a-ooVooo of a milligramme, of malic acid is present. As has been already stated, if the chemical stimulus is to pro- duce movements in a certain direction, it must only be strongly applied at one point, or at any rate from one side. This is the case in the above experiment, for as ihe malic acid becomes dif- fused through the opening in the surrounding water, the anthero- zoids, passing through the opening and making their way up the tube, come into contact with solutions gradually increasing in strength. The diffusion causes an unequal distribution of the stimulus about the bodies of the antherozoids: "thus varying with its varying degrees of concentration, the malic acid exerts a stimulus which causes a movement in a fixed direction." The antherozoids, as might be expected, are distributed evenly throughout a homogeneous solution, yet even under these condi- 120 THE CELL tions a specific stimulative effect is exerted upon them. This, however, can only be perceived indirectly, and can only be ex- plained by the sapposition that the attitude, so to speak, of the antherozoids towards malic acid has experienced some modifica- tion. Pfeffer is able in this case to demonstrate a relation simi- lar to that expressed by the Weber-Fechner law for the mental perceptions of man: " Whilst the stimulus increases in geometrical progression, the perception or reaction increases in arithmetical progression." This ratio, which in many respects is very important, can be observed in the behaviour of antherozoids towards malic acid. To the fluid, containing the fern antherozoids, some malic acid is added in such a quantity that when the two are well mixed to- gether a solution of '0005 per cent, is produced. If now a capil- lary tube containing a solution of '001 per cent, is inserted, attrac- tive influence, as was the case when the antherozoids were in pure water, can be perceived. The tube must now contain a '015 per cent, solution in order to produce an effect, and if the water, in which the antherozoids are, contains '05 per cent, of malic acid, the solution in the tube must be 1'5 per cent, in strength. Or more generally expressed, the solution in the tube must be thirty times as strong as that from which the antherozoids are to be attracted. The sensitiveness to stimuli, or the stimulation tone of the antherozoids, is a/ected, if they are present in a liquid which contains a certain proportional amount of the substance which is to act as the stimulus. Thus it is possible in an artificial way to render them non- sensitive towards weak solutions of malic acid, which under ordinary circumstances constitute excellent stimuli, whilst on the other hand they may be made susceptible to attraction from strong concentrations of malic acid, which would repel antherozoids accustomed to living in pure water. Individual cell bodies behave very variously towards chemical substances, just as they do towards light. Malic acid, which exerts such a powerful attraction upon fern antherozoids, does not affect those of Feather-moss at all. For these, however, a 1 per cent, solution of cane sugar acts as a stimulus, whilst on the other hand neither of these substances has any effect on Liverwort or Characece. A 1 per cent, solution of meat extract or of Asparagin exerts a strong attraction upon Bacterium termo, Spirillum undula, and many other unicellular organisms. Even after a short period, THE VITAL PROPERTIES OF THE CELL 121 varying- from two to five minutes, a distinct plug of bacteria is seen to have collected at the mouth of a capillary tube, which has been placed in a drop of water containing these micro-organisms. On account of the different ways in which various cell bodies react towards different chemical stimuli, the method, which Pfeffer has perfected and used with various reagents, may be employed, not only to attract one individual organism sensitive to one special reagent, but also to separate different species which are mixed together, as has also been done by means of galvanotropism or heliotropism. Glass tubes provided with suitable attractive material, and inserted in fluids, may be used as traps for Bacteria or Infusoria. Farther, it follows from the above-mentioned experiments, that organisms which are specially sensitive towards a given chemical substance may be used as reagents to indicate the presence of this stimulating substance. Thus, according to Engelmann (IV. 7), certain Schizomycetes form an excellent test for oxygen, of which such a minute portion as one trillionth of a milligramme is sufficient to attract them. Not every substance which attracts an organism is useful to it as food, or is even innocuous to it ; many, such as sodium salicylate, saltpetre, strychnine, or morphia, even cause the immediate death of the organisms which they have enticed. However, as a rule the substances which are hurtful to protoplasm generally repel it ; this is the case with most acid and alkaline solutions. Even 2 per cent, solutions of citric acid and sodium carbonate exert a distinctly repellent influence. Hence, within the above-mentioned limitations, the general rule may be stated that organisms are, through positive chemotropism, enabled to seek suitable nutriment, whilst in consequence of negative chemotropism they avoid hurtful substances. These phenomena of chemotropism are of the greatest import- ance in understanding many processes in the bodies of man and of other vertebrates. Here also there are cells which react to chemical stimuli by changes of shape, and movements in special directions. These cells are the white blood corpuscles and lymph cells (leucocytes or wandering cells). The chemical irritability of leucocytes has been established as a fact by the experiments of Leber (IV. I7a, 6) ; Massart and Bordet (IV. 20, 21); Steinhaus (IV. 36); Gabritschevsky (IV. 10) ; and Buchner (IV. 2). If, in accordance with Pfeffer's 122 THE CELL method fine capillary tubes, filled with small quantities of some " irritating substance," are introduced into the anterior chamber of the eye or the lymph sac of a frog, they become filled in a short time with leucocytes, whilst tubes filled with distilled water exert no attractive power upon the leucocytes. When introduced into the subcutaneous connective tissue the tubes cause the out- wandering of the leucocytes from the neighbouring capillary vessels (diapedesis), and under certain conditions produce sup- puration. Amongst substances which will set up inflammation, many micro-organisms and their metabolic products are in the first rank. Thus, Leber found during his experiments that an extract of Staphylococcus pyogenes proved very effectual as an inflamma- tory agent. Hence the study of chemotropism is of the greatest importance in the investigation of the diseases produced by the presence of pathogenetic micro-organisms. Accurate knowledge of the former will no doubt explain many apparently contradictory phenomena, which are met with in the study of infectious diseases. It may be taken for granted at the outset, that if leucocytes can be stimulated by means of chemical substances produced by micro-organisms, such stimulation can only occur in accordance with Uws similar to those which have been established generally with regard to cells. Positive and negative chemotropism- ex- citation, and the variations which may occur in it owing to the even distribution of the existing agent the effects of stimulation all these must be taken into account. Hence the behaviour of the leucocytes towards the stimulating substance assumes the form of a complicated process, which may vary very considerably according to the special conditions. For the metabolic products excreted by micro-organisms may, accord- ing to their nature and state of concentration, exert an attractive or repellent influence. In addition, the effect produced may vary according as to whether these products are restricted to the region where they are produced, and from which they attack the leuco- cytes, or whether they are in addition evenly distributed through- out the blood. For in the latter case the presence of the bactei'ial products in the blood will modify the way in which the leucocytes react towards those which are collected in considerable quantities 'near the diseased spot; and as was the case with the antherozoids and malic acid (pp. 118-120), the result will depend upon the rela- THE VITAL PROPERTIES OF THE CELL 123 tire proportions of the stimulating substance which is present in each region. The numerous possibilities may be grouped under two heads. First group. The metabolic products are evenly distributed or approximately so throughout the blood and the diseased tissues. Since under these conditions there can be no special point of stimulation, it stands to reason that the leucocytes cannot wander away from the diseased spot. Second group.- The collections of products are unequal in con- centration, and further, the difference in their concentration is sufficient to give rise to an effective stimulation. Two alter- natives may occur. Either the higher degree of concentration is present at the seat of the disease, or in the blood-vessels. In the first case only will the leucocytes collect around the affected tissue. The consideration of these relative conditions appears to me to explain many interesting phenomena, which have been observed by certain French investigators, Roger, Charrin, Bouchard (IV. Ib), etc., during their various experiments with the catabolic products of the Bacillus pyocyaneus, of the Anthrax bacillus, etc. ; and by Koch in his observations upon the action of Tuberculin. I have endeavoured to explain such phenomena in a short popular paper : " Ueber die physiologische Grundlage der Tuberculin wirkung, eine Theorie der Wirkungsweise bacillarer Stoffwechsel- producte" (IV. 13), to which I refer the reader for information with regard to physiological experiments and the explanation of the special phenomena of disease. Literature IV. IA. CLAUDE BERNARD. Lemons sur Us phenomenes de la vie commune aux animaux et aux vegetaux. IB. BOUCHARD. Theorie de Vinfection. Verhandl. des A. intern, med. Con- gresses za Berlin. Bd. I. 1891. 2. BOCHNEK. Die chemische Reizbarkeit der Leukocyten und deren Beziehung zur Entziindung und Eiterung. Berliner kliitische Woclieschn. 1890. 3. BRUCKE. Untersuchungen iiber den Farbenwechsel des afrikan. Chameleons. Denkschrift d. math, nakurw., Clause der Akad. d. Witsentch. Bd. IV. 1851. T. LAUDER BRUNTON. Action of Drugs on Protoplasm. Pharmacology Therapeutics and Materia Medica. London. 4. BUNGE. Vitalizmus und Mechanismus. 5A. DE BABY. Vorlesungen iiber Bacterieii. 1885. 124 THE CELL 5s. DEHNECKE. Einige Beobachtungen iiber den Einfluss der Preparations- method* aufdie Bewegungen des Protoplasmas der Pfianzenzellen. Flora 1881. CA. ENGELMANN. Beitrfige zur Physiologie des Protoplasmas-Pjliigers Archiv. Bd. II. 1869. 6u. ENGELMANN. Ueber Reizung contraction Protoplasmas durch plotzliche Beleuchtung. Pfliigers Archiv. Bd. XIX. 7. ENGELMANN. Neue Methode zur Untersuchung der Saitersto/ausscheidung pflanzlicher u. thierischer Organismen. PJiiigers Archiv. Bd. XXV. 8. ENGELMANN. Ueber Licht u. Farbenperception niederster Organismen. PJiiigers Archiv. Bd. XXIX. 1882. 9. ENGELMAXN. Bacterium photometricum. Ein Beitrdg zur vergleichenden Physiologie des Licht und Farbensinnes. Pflttgers Archiv. Bd. XXX. 10. GABRITCHEVSKY. Sur les proprietes chimiotactiqu.es des leucocytes. Aunales de llnstitut Pasteur 1890. 11. RICHARD HEKTWIG. Erythropsis agilis, eine neue Protozoe. Morph. Jahrb. Bd.X. 12A. OSCAR u. EICHARD HERTWiG. Ueber den Befmchtungs und Theilungs- vorgang des thieritchen Eies unter dem Einfluss dusserer Agentien. 1887. 12B. OSCAR u. EICHARD HERTWIG. ExperimenteHe Studienam thierischen Eivor, icdhrend und nach der Befruchtung. 1890. 13. OSCAR HERTWIG. Ueber die physiologische Grundlage der TubercuUmcir- kung. Eine Theorie der Wirkungsweise bacilldrer Stojfwechselpruducte. Jena. 1891. 14. KLEBS. Beitrfige zur Physiologie der Pftanzenzelle. Untersuch aus dem botanischen Institut zu Tubingen. Bd. II. p. 489. 15. \V. KUHNE. U nter suchungen iiber das Protoplasma und die Contractilitiit. 1864. 16. KUNSTLER. Les yeux des infusoires flag ell ij "e res. Journ. Mic. Paris. Wth year. I?A. LEBER. Ueber die Entstehung der Entziindung und die Wirkung der entziindungserregendeii Schddlichkeiten. Fortschritte der Hedicin, 1888, p. 460. 17s. LEBER. Die Enstehung der Entziindung und die Wirkung der entziindung- si-rregenden Schddlichkeiten. Leipzig. 1891. 18. J. LOEB. Der Heliotropismus der Thiere und seine Uebereinstimmung mil dem Heliotropismus der Pfianzen. Wilrzburg. 1890. 19. J. LOEB. Weitere Untersuchungen iiber den Heliotropismus der Thiere. Pjlugers Archiv. Bd. XLVII. 1890. 20. J. MASSART et BORDET. Recherches sur Virritabilite des leucocytes et stir ^intervention de cette irritabilite dans la nutrition des cellules et dans I' inflammation. Journ. de la Soc. R. des Sciences medicates et natureilts de Bruxelles. 1890. 21. J. MASSART et BORDET. Annales de Vlnstitut Pasteur. 1891. 22. METCHNIKOFF. Lectures on the Comparative Pathology of Inflammation, trans, by F. A. andE. H. Starling. 1893. 23. W. PFEFFER. Handbuch der Pjlanzenphysiologie. Bd. I. 1881. THE VITAL PROPERTIES OF THE CELL 125 24. W. PFEFFEK. Locomotorische Richtungsbewtgimgen durch chemische Reize. Untersiich. aus d. botan. Institut zti Tiibingrii. Bd. I. 25. W. PFEFFER. Zur Kenntniss der Contaclreize. Untenuch. aus dem botan. Institut zu Tubingen. Bd. I. 26. W. PFEFFER. Ueber chemotactische Bewegungen von Bakterien, Flagellaten und Volvocineen. Untersuch. aus d. botan. Institut zu Tiibingtii. Bd. II. 27. GEOKGE POUCHET. D'un ceil veritable chez les Protnzoaires. C. R. soc. Biol. No. 36. 28. GEORGE POUCHET. Durole des nerfs dans les cliangements de coloration des 2)oissons. Journ. de Vanat. et de la phys. 1872. 29. GEORGE POUCHET. Note sur I'influence de I' ablation des yeux sur la colora- tion de certaines especes animates. Journ. de Vanat. et de la phys. T. A'. 1874. 30. F. A. POUCHET. Stir la mutabilite de la coloration des reiuettes et sur l 10-93 29 1-52 11-66 34 4-40 7-44 62 2-56 SO, . 28-16 26-69 21-06 13-26 SiU,. Cl . 1-35 15-24 1-20 12-24 43 11-39 1-56 17-23 I. 31 46 1-13 3-08 Marine plants show most clearly, in what very nnequal propor- tions, they absorb from the multitude of salts offered them in sea- water, the ones which are necessary to them. For instance, they only store up very small quantities of common salt, of which about 3 per cent, is present in the water, whilst, on the contrary, they take up relatively large amounts of potassium, magnesium, and calcium salts, of which there are only traces. And in a similar manner, the analysis of the ashes of different land-plants which have flourished side by side in the same earth yields very different results. Investigation of the metabolism occurring in the animal body leads to the same conclusion. Only certain cells have the tendency to take possession of the lime-salts, which are present in almost inappreciable amounts in the fluids of the body, and to deposit them in the osseous tissues ; other groups of cells, such as those in the kidneys, take up the substances from the blood, and excrete them in the form of urine ; others store up fat, etc., etc. The factors concerned in this absorption and non- absorption of matter are at present quite beyond our comprehension. It is curious that the need which is evinced by the economy of a cell for a certain substance does not always imply that this will be taken up. Cells may absorb materials which are either directly hurtful or completely useless to them. In this respect the very different ways in which living plant cells take up aniline dyes are very instructive (Pfeffer V. 22b). Although solutions of methylene blue, methyl violet, cyanin, Bismark brown, fuchsine and safranin, are absorbed, those of nigrosin, aniline blue, methyl blue, eosin, and congo-red, are not. THE VITAL PROPERTIES OF THE CELL 137 As to whether a given substance will be absorbed or not can, ac- cording to Pfeffer, who has carefully studied the subject, only be decided empirically. The substances excreted by cells also vary. Just as with absorption, excretion depends upon the special individual properties of the living cell body. The red or blue-coloured petals of phanerogamic flowers do not allow the concentrated solution of colouring matter which they contain to become diffused into the surrounding water as long as they are alive. However, as soon as the cells die, the colouring matter commences to pass through the cell-wall. In order to really understand all these complicated phenomena, it would be necessary to possess an exhaustive knowledge of the chemistry and physics of the cell. For the property, which I have designated above as the power of selection, must in the last instance be traced back to the chemical affinities of the very numerous substances which, being formed during the process of metabolism, are present for a time in the cell. The same thing, doubtless, occurs here as with the absorption of oxygen and carbon dioxide, which can only take place when, through metabolic processes, sub- stances with chemical affinities for them are set free. It is on this account that no carbon dioxide is taken up by plants in the dark, although it is immediately absorbed, if, under the influence of direct sunlight, the chemical process for which it is necessary is started. The same thing occurs when living cells absorb aniline dyes. Azolla, Spirogyra, the root-hairs of Lemna, etc., gradually draw up into themselves so much colouring matter out of a very weak solution of methylene blue, that they acquire a deep blue coloura- tion, such as is seen in a 1 per cent, solution. The methylene blue does not stain the protoplasm itself, but simply passes through it, thus forming in the cell sap a solution of ever-increasing strength. Hence the death of the cell, which would inevitably occur if the poisonous methylene blue were to be collected in such quantities in the protoplasin itself, does not ensue. This storing up in the cell sap is caused by the presence in it of substances which, with the aniline dye, form compounds, which osmose with difficulty. Pfeffer considers that the tannin which is so frequently found in plant cells is a substance of this nature. This tannin, with the aniline colour, forms compounds which are sometimes insoluble, and hence are precipitated in the cell sap (methylene blue, methyl 138 THE CELL violet), and sometimes are more or less soluble (fuchsine, methyl orange, tropaeolin). Further, animals afford us good examples of this storing up in living cells. Fertilised eggs of Echinoidea acquire a more or less intense blue colouration, if they are placed for a short time in a very dilute solution of methylene blue (Hertwig IV. 12b). A small accumulation of colouring matter does not arrest the process of segmentation, which still continues, although somewhat slowly, in a normal fashion, and in some cases may go on even until the gastrula is formed. Here the colouring matter is chiefly deposited in the endoderm cells, which points to the conclusion that it is by the agency of the yolk material that the accumulation takes place. Living Frog and Triton larvae become of an intense blue colour if they are left for from five to eight days in a weak solution of methylene blue. In this case the colouring matter combines with the granules in the cells (Oscar Schultze V. 44). After remaining for days in pure water they commence to become colourless again. If indigo-carmine is injected directly into the blood of a mammal, it is soon taken up both by the liver-cells and by the epithelium of the convoluted tubules of the kidney, and then is excreted either into the biliary ducts, or into the kidney tubules (Heidenhain V. 42). If methylene blue is injected into the blood, it combines with the substance of the nerve fibres, imparting to them a dark blue colouration (Ehrlich V. 41). Alizarin is stored up in the ground substance of the bones. Next to the chemical affinities, which exist between the par- ticles of matter within the cell and those outside of it, the study of the physical processes of osmosis is of the greatest importance for the comprehension of the absorption and rejection of matter. We must here observe whether the membrane, when present, is more or less permeable. As a rule it is much more permeable to dissolved substances than is the protoplasmic substance itself. This latter is separated from the exterior by a peripheral layer (c/. p. 15), which, according to Pfeffer, plays a most important part in the process of osmosis. If some substance in solution is to be taken up into the protoplasm, it must first be imbibed by the peripheral layer ; that is to say, its molecules must become deposited between the plasmic particles, and from there be trans- ferred to the interior. Further, a substance in solution can, even if it be not actually absorbed, produce an osmotic action by exerting an attraction upon the water contained in the cell, and by thus THE VITAL PROPERTIES OF THE CELL 130 inducing a flow of water towards the exterior. " Essentially osmosis consists in this, that two fluids simultaneously pass through a membrane in opposite directions ; with regard to an endosmotic equivalent (a term expressing the proportionate inter- change, upon which there is frequently too much stress laid), this cannot be spoken of in such cases where only water is diosmosed thi^ough a membrane " (Pfeffer V. 23). On account of their fragility and small size, experiments upon osmosis can only be made in animal cells with great difficulty. Hence the osmotic processes have been investigated chiefly by botanists in plant cells, which are much more suitable, and our 123 FIG. 59. 1. A young, at most half-grown, cell from the cortical parenchyma of the flower peduncle of Cephalaria leucantha. 2. The same cell immersed in a 4 per cent, solution. 3. The same cell in a 6 per cent, solution. 4. The same cell in a 10 per cent, solution (Nos. 1 and 4 are taken from nature, Nos. 2 and 3 are diagrammatic ; all in optical longitudinal section), h Peripheral layer ; p protoplasmic coating of wall; fc nucleus; c chlorophyll granules; s cell sap ; e salt solution which has penetrated into the interior. After de Vries (V. 36). knowledge has been especially advanced by the following experi- ments. If plant cells containing a large sap space are placed in a 5 to 20 per cent, solution of a suitable salt, or of sugar or glucose (Fig. 59), they are seen to diminish somewhat in size from having given up water from the interior to the exterior; in consequence, as this process of water abstraction proceeds, the protoplasmic coating becomes separated from the cellulose membrane, which, on account of its greater firmness, is unable to shrink any more (de Vries V. 36). 140 THE CELL Thus the salt or sugar solution must make its way through the cellulose membrane, after which it continues to abstract more water from the protoplasm, which shrinks more and more accord- ing to the concentration of the solution, so as to occupy a smaller and smaller space. The sap which it encloses becomes corre- spondingly more concentrated. In spite of these changes, which are grouped together under the same plasmolysis, the protoplasm may remain alive for weeks, and exhibit its usual streaming movements ; it may even surround itself with a new peripheral layer, although it remains in its contracted condition. Two conclusions may be deduced from the process of plas- molysis : (1) that the cellulose membrane is pervious to the salt solutions which were used ; (2) " that the amount of dissolved salt which diosmoses through the peripheral layer is not worth mentioning, for if a considerable quantity penetrated into the protoplasm, or into the cell sap, an increase in the quantity of the substances setting up osmosis would be produced within the proto- plasmic membrane, and thus an increase in the volume of the protoplasmic body would result " (Pfeffer). If the cells which have become flaccid through plasmolysis are carefully removed and placed in pure water, the reverse process occurs. The sugar solution which was enclosed within the cellu- lose membrane becomes diffused into the water. In consequence, the peripheral protoplasm layer becomes distended, because its cell sap is now richer in osmotolytic substances than its environ- ment, and so water is caused to flow in the opposite direction. This distension gradually increases, as the water becomes ab- sorbed, until the peripheral layer of protoplasm comes into close contact with the cellulose membrane, and until finally the cell has dilated to its original size. Other experiments have shown that the sap contained in the plant cell is under a considerable pressure, often of several atmos"- pheres. This produces the natural turgescence of certain por- tions of plants. The cause is, that powerfully osmotolytic sub- stances are present in the cell sap, such as saltpetre, vegetable acids, and their potassium salts, which have a strong affinity for water (Pfeffer V. 23 ; de Vries V. 36). Therefore under these conditions the protoplasmic coating con- taining the cell sap may be compared to a very elastic thin- walled bladder, which is filled with a concentrated salt solution. If such a bladder is put into pure water, the solution attracts the water, THE VITAL PROPERTIES OF THE CELL 141 and so produces a current, the result being that the bladder swells up in consequence of the increased pressure of its contents, and its wall grows thinner and thinner. The distension of the bladder only ceases when the external and internal liquids are in osmotic equilibrium. Thus the protoplasmic coating of many plant-cells would be very much distended in consequence of the internal pressure (turgor) were it not that a limit is set to its distension by the less elastic cellulose membrane. Equilibrium between the cell-sap and the surrounding fluid might be established, if the osmotic substances were to become diffused into the water, so as to remove the cause of the internal pressure. However, this is prevented by the properties of the living plasmic membrane. As the plasmic membrane, if the ex- pression may be allowed, decides whether a body may be admitted into the interior of the cell or no, similarly it has the important power of retaining in the cell-sap dissolved substances which otherwise would be washed out by the water bathing the cell ; of this property mention has already been made, and an instance cited (Pfeffer V. 23). That, in fact, the cell-sap exists under a pressure greater than that of its environment, for instance, that the pressure in aquatic plants is greater than that of the surrounding water, may be easily proved by some simple experiments, as has been shown by Nageli (Y. 16). If a cell of Spirogyra be opened by an incision, so that part of its contents flows out, the transverse walls of the two neighbouring cells bulge out towards the cavity of the injured one. Hence the pressure in the uninjured cells must be greater than that in the injured one, the tension of which has sunk down to the level of that of the surrounding water. 3. Absorption of Solid Bodies. Cells, which either are not surrounded by a special membrane, or possess apertures in their membranes, are able to take solid bodies up into their protoplasm, and to digest them. Thus Rhizopoda capture other small unicellular organisms with which their widely outstretched pseudopodia come into contact (Figs. 10, 60). The pseudopodia which have seized the foreign body contract, and so gradually draw it into the mass of the protoplasm ; here the nutrient sub- stances are extracted, whilst the indigestible remains, such as skeletal structures, are after a time ejected to the exterior. Even solid substances, which possess but small nutritive value, are taken up. If carmine or cinnabar granules are introduced into the water, 142 THE CELL the Rhizopoda eagerly seize upon them, so that after a short time their whole bodies are quite filled with them. Infusoria (Fig. 50) eat Flagellata, unicellular Algae and Bacteria, conveying them into their endoplasm through an opening in their cuticle which functions as a mouth. Here a vacuole filled with fluid forms itself round each foreign body, which undergoes digestion. Na cv FIG. W.-Actinoapharium Eichhorni (after R. Hertwig, Zool., Fig. 117) : M medullary substance with nuclei (n) ; K cortical substance with contractile vacuoles (CD) ; Na nutrient material. In a similar manner to that shown by unicellular organisms, mauy tissue cells of Metazoa devour solid substances offered to them, and digest them. Intracellular digestion, as it has been termed by Metchnikoff (V. 12), occurs very frequently in Invertebrates ; it may be best demonstrated by means of feeding experiments with easily recog- nisable substances, such, as granules of colouring matter, globules of milk, spores of fungi, etc. In some Ccelenterata the ectoderm as well as the endoderm takes up foreign bodies. The tentacular euds of Actinia may load themselves with carmine granules, which THE VITAL PROPERTIES OF THE CELL 143 may also be found distributed throughout the whole endoderm of Actinia larvae after suitable feeding. But white blood corpuscles, lymph cells and the migratory cells of the mesoblast, in both Vertebrates and Invertebrates, afford us the best material for observation, in consequence of their power of absorbing and digesting solid bodies. This important fact was first observed by Haeckel (V. 4a), who injected a mollusc (Tethys) with indigo, and found after a short time that indigo granules were present inside the blood corpuscles. Metchnikoff (V. 12) has further investigated the phenomenon most thoroughly. He found that if powdered carmine were injected tinder the skin of another species of mollusc (the transparent Phyllirhoe), the smaller granules were eaten up by some of the migratory cells, while the larger ones attracted a number of other migratory cells around them, which surrounded them like an envelope, and fused themselves together to form a plasmodium or multinucleated giant cell. That the same thing occurs in Vertebrates may be easily proved by injecting some carmine into the dorsal lymph sac of a Frog, and, after a short time has elapsed, removing some drops of lymph, and examining them with the microscope. Further, the eating process can be directly followed under the microscope if powdered carmine or a little milk be added to some fresh drops of lymph or of blood which have been carefully drawn off, certain precautions having been observed. If the blood has been taken from man or some other mammal, the preparation must be carefully heated on Max Schultze's warm stage until it has attained a temperature of 30-35 Celsius (V. 43). The white blood corpuscles now commence to show amoeboid movements ; they seize with their pseudopodia the carmine granules, or milk globules with which they come in contact, and draw them into their bodies. On this account Metchnikoff designates them as phagocytes, and the whole process as phagocytosis. This capacity of the amoeboid elements of the animal to take up solid substances is of great physiological importance ; for herein the organism possesses a means of ridding itself of foreign and noxious organic particles which are present in its tissues. There are three different conditions of the body, partly normal and partly pathological, when the phagocytes exercise this function. Firstly, during the process of development in many Inverte- brates and also in Vertebrates, certain larval organs lose their 144 THE CELL importance, and undergo fatty degeneration. Thus, during the metamorphosis of Echinoderm larvae and of Nemertines, certain portions disappear; and, similarly, the young Frog daring its development loses its conspicuous tail, which acted as a rudder. In all these cases the cells of these degenerating organs undergo a fatty metamorphosis, die and disintegrate. In the meantime a large number of migratory cells or phagocytes have collected in their neighbourhood, and these commence to devour and digest the degenerated tissue, as can be plainly seen during life in trans- parent marine animals. Secondly, just as during the normal processes of development, the phagocytes occupy themselves in reabsorbing particles, the death or disintegration of which has been brought about either by normal or pathological conditions. Red blood corpuscles become destroyed after they have circulated in the blood for a certain time. In splenic blood their remains have been seen in the bodies of white corpuscles, which here again fulfil their function of getting rid of dead material. When in consequence of a wound an effusion of blood occurs in the tissue, and thousands of blood corpuscles and elementary particles are destroyed, the migratory cells again set to work, and produce reabsorption and healing. Thirdly, and lastly, the phagocytes during infectious diseases constitute a body-guard to the organism, in opposing the spread of the micro-organisms in the blood and tissues. Metchnikoff has rendered great service in drawing attention to this circumstance (V. 13-15, IV. 22). He succeeded in showing that the Cocci of erysipelas, the Spirilla of relapsing fever, and the Bacilli of anthrax were eaten up by the wandering cells, and thus rendered harmless (Fig. 61). The micro-organisms, of which as many as from ten to twenty may be present in one cell, after a certain time show distinct signs of degeneration. If the micro- organisms are present in the blood, they are destroyed, especially in the spleen, liver, and red bone marrow. If they succeed in settling down in some place in the tissue, the body endeavours to get rid of the intruders by collecting as the result of inflammatory processes a large number of migratory cells to the spot. As Metchnikoff expresses it, between micro-organisms and phagocytes an active war is raging. This is settled in favour of one or other party, resulting, as the case may be, in the recovery or death of the affected animal. The power possessed by migratory cells of destroying certain THE VITAL PROPERTIES OF THE CELL 145 species of micro-organisms appears to vary considerably in different animals, and to depend largely upon the most varying 1 conditions ; for instance, chemical stimuli play an especially important part, as has been already mentioned on p. 121 (negative and positive chemotropism ; Herfcvvig IV. 13). Apparently it is upon this that the greater or less immunity of organisms from many infectious diseases depends. This discovery opens a wide vista in the tield of the comprehension and treatment of infectious diseases. IT. The Assimilative and Forma- tive Activity of the Cell. The gases, the fluids, and the solid substances, which are introduced into the protoplasm as food, and through respiration, compose the very varying raw materials which are elaborated FIG. 61. A leucocyte of a Frog, enclosing a Bac- in the chemical workshop of the cell, and tenum, which is nnderpo- ,. , . j / j-i ing digestion. The Bac- which are converted into an exceedingly large number of substances. Amongst terium is stained with vesuvine. The two figures represent two stages of one and the same cell. (After these the most important for both plants and animals are: carbo-hydrates, fats, pro- Metchnikoff, Fig. 54.) teids, and their numerous compounds. Similarly the ways in which they are utilised in the vital pro- cesses of the cell vary very considerably. They serve partly to replace the substances, which, during the vital process, become decomposed in the cell, such as the substance which is oxidised during respiration, and which thus furnishes the vital energy necessary for the activity of the cell. They are also utilised for that growth and increase of the protoplasm which is absolutely indispensable for the function of reproduction. Further, some of the substances formed in the chemical laboratory are stored up for future use in the cell-body in some form or other, thus consti- tuting reserve material. Finally they may be set aside to fulfil some function inside or outside the cell. Thus arise the different materials which, especially in the animal kingdom, are very numerous, and upon which the dif- ferentiation of tissues depends : glandular secretions, which are passed to the exterior, membranes, and intercellular substances of 146 THE CELL very varying chemical composition, and muscle and nerve fibres, which, in consequence of their peculiar organisation, are endowed in a special manner with contractility and the power of conduct- ing stimuli. In the last case the chemical activity of the cell assumes a character which Max Schultze has designated as its formative activity. The protoplasm makes use of the raw ma- terial which is brought to it, and prepares from it often very wonderfully constructed substances, which answer special pur- poses. In this activity the cell appears, to a certain extent, like a builder, or, as Haeckel (V. 4b) has it, like a modeller or sculptor. This formative activity of the cell, or, as it is better expressed, the power of the protoplasmic body to create different structures, is of extreme importance; for it is solely due to this power that there is so great a diversity of elementary particles, in consequence of which the animal body is able to attain to so high a degree of pei-fection. The division of labour, which is so successful amongst cells, is based solely upon this foundation, and by its means the capacity for work of the cell community is rendered much greater. Hence this subject of the assimilation of material must be examined from two points of view ; the first is a chemical one, in so far as it treats of the formation of innumerable substances by means of the protoplasm, whilst the second is more morphological, in so far as the various substances present in the protoplasm may be seen to differ from it, to occupy a definite position, to have a fixed form and structure, and to obey special laws of development. One of the most important tasks for the biological chemist of the future is to render accessible to morphological investigation the various substances distributed throughout the cell body by means of differential staining mixtures. 1. Chemistry of Assimilation. The chemical processes of the cell, which are at present shrouded in mystery, can only be treated here in so far as they are connected with fundamental problems, such as the synthesis of carbo-hydrates, fats, and pro- teids out of more simple elementary substances. The chemical processes in the animal kingdom appear to differ considerably from those occurring in the vegetable kingdom. Only that protoplasm present in plant cells, which contains chloro- phyll, is able to make high molecular ternary compounds out of carbon dioxide and water ; the protoplasm which does not contain chlorophyll, and which is present in animals and certain colourless portions of plants, is only able to undertake further synthesis THE VITAL PROPERTIES OF THE CELL 147 with this original material, and thus to produce quaternary com- pounds. It is as yet impossible to say what chemical processes occur in the green protoplasm, when, under the influence of the sun's vital energy, carbon dioxide and water are taken up, and oxygen is given off. The first product of assimilation, which can be definitely made out, is starch, or perhaps, as a preliminary stage, sugar. It is almost inconceivable that either of these could be formed by a direct synthesis of carbon and water ; apparently a number of intermediate substances are formed during the course of a complicated process. " Indeed, it is not impossible," as Sachs (IV. 32a) remarks, " that certain closely-connected constituents of the green plasma themselves participate in the process ; that, for example, the molecules of the green protoplasm become split np, and that certain atoms are given up and others substituted for them. The theory has a certain degree of probability from the observation that in many, though not all cases, the mass of chlorophyll substance gradually decreases, and finally quite dis- appears, whilst the starch granules which it contains become larger and larger." The carbo-hydrates (starch) which, by means of the chloro- phyll function, have accumulated in the body of the plant, form the material which is converted in the protoplasm into the vegetable oils. The ternary non-nitrogenous, organic compounds supply further the basis for the synthesis of quaternary albumin- ous substances, and thus assist in the completion and increase of the protoplasm. However, for these processes, nitrates and sul- phates are necessary, and these are obtained by the plants from the earth by means of their roots. That proteid substances can be formed by the living cell out of such material has been experimentally proved by Pasteur. He cultivated low Schizomycetes, such as Mycoderma aceti, Yeast, etc., in artificially prepared nutrient solutions. Thus he showed that Mycoderma aceti can multiply actively in the dark, if only a few cells are placed in a nutrient solution, composed of a salt of ammonia, phosphoric acid, potash, magnesia, water, and alcohol or acetic acid of suitable strength. Hence the fungi cells, if they have multiplied to a considerable extent, must have formed proteid materials by means of the decomposition of these substances, in addition to cellulose and fats. Thus plants, which by means of their chlorophyll produce carbo- 148 THE CELL hydrates, and convert these again into fats and albuminous sub- stances, supply to the animal organism the ternary and quater- nary substances which are necessary for its nutriment, and which it is unable to elaborate, as the plants do, from such simple sub- stances. In this manner the vegetable and animal kingdoms con- stitute a life cycle, in which they assume opposite positions and complement each other. This antithesis may be formulated as follows : In the green plant cell the organic substance is formed syn-> thetically from carbon dioxide and water, whilst the vital force which is obtained from the sunlight becomes potential ; on the other hand, the animal cell uses as nutriment the ternary and quaternary compounds formed in the vegetable kingdom, for the most part oxidising them. By this means it reconverts the potential energy stored up in the complex compounds into vital energy whilst performing work and evolving heat. The plant, whilst its chlorophyll is exercising its function, absorbs carbon di- oxide, and gives off oxygen ; the animal breathes in oxygen, and breathes out carbon dioxide. In the chemical processes of the plant reduction and synthesis predominate, whilst in those of the animal oxidation, combustion and analysis are most important. However, from this one example of antithesis occurring in the economy of nature between the animal and vegetable kingdoms, it must not be concluded that plant and animal cells are quite opposed in all their ordinary vital phenomena ; for this is not true. Close investigation shows that there is universal unity in the fundamental processes of the whole organic world. The above-mentioned difference is only due to the fact that the plant cell has developed a special faculty which is lacking in animal cells, namely, the power of decomposing carbon dioxide by means of its chlorophyll. With the exception of this one function, exer- cised by chlorophyll, many of the metabolic processes which are essential for the maintenance of life are performed in the protoplasm in a perfectly similar manner in both plant and animal cells. In both the protoplasm must breathe, take up oxygen, evolve heat, and give up carbon dioxide if the vital processes are to be carried on. In both plants and animals the decomposition and reconstruction of protoplasm follow one another, and complicated processes of correlated chemical analysis and synthesis occur. This similarity can be more easily understood when it is re- THE VITAL PROPERTIES OF THE CELL 149 membered that a large proportion of plant cells, namely all those which do not contain chlorophyll, ai*e in a position similar to that occupied by animal cells; these also, since they cannot assimi- late directly, must obtain from the green cells, the material neces- sary for the maintenance of their life, for their growth, and for their reproduction. Thus the same antithesis, which is present in the economy of nature between plants and animals, also exists in the plant itself between its colourless and its chlorophyll-con- taining cells. Claude Bernard has shortly and in a striking way expressed the relationship in the following words : " If, in the language of a mechanician, the vital phenomena, namely the construction and destruction of organic substance, may be compared to the rise and fall of a weight, then we may say that the rise and fall are accomplished in all cells both plant and animal, but with this difference, that the animal element finds its weight already raised up to a certain level (niveau), and that hence it has to be raised less than it subsequently falls. The reverse occurs in the green plant cells. In a woi-d, ' Des deux versants, celui de la descente est preponderant chez 1'animal ; celui de la montee, chez le vegetal ' " (Claude Bernard, IV. la, vol. ii. p. 514). Now, having placed the subject of the chlorophyll function in its true position, we will proceed to examine the important uniformity which exists in the chemistry of metabolism between plant and animal cells. We must first lay stress upon the fact that a large number of the materials made use of in progressive and retrogressive meta- morphosis are common to both plants and animals. Further, the means by which certain important processes in plant and animal cells are carried out appear to be similar. Carbo-hydrates, fats and albuminous substances are not adapted in every condition for direct use in the laboratory of the cell and for conversion into other chemical compounds. It is necessary to prepare them by transforming them into a soluble and easily diffusible form. This occurs, for instance, when starch and glyco- gen are converted into grape sugar, dextrose and levulose ; when fat is split up into glycerine and fatty acids, or when proteids are peptonised. Sachs (IV. 32a) describes the above-mentioned modifications of carbo-hydrates, fats and proteids as their active condition, in dis- 150 THE CELL tinction to their passive condition, when they either remain accumulated in the cell as fixed reserve materials starch, oil, fat, albumen crystals or are taken up as nourishment by animals. It is only when they are in the active condition that the plastic materials in both plant and animal bodies can accomplish their migrations, by means of which they reach the places where they are either to be temporarily stored up or immediately used. For instance, the starch, which is accumulated in seeds or in portions of plants which are underground, such as tubers, was not assimilated at these spots. It originated in the assimilating green cells, from which it was transported, often through long distances, by means of intermediate cells to the tubers or seeds. Now, since starch grains cannot pass through the cell-membrane, this migration can only occur when the substances are in a soluble form (sugar) ; when they reach the place where they are to be stored up, they are re-converted into the insoluble form (starch). If now the germ develops, either in the tuber or in the seed, the passive reserve materials assume the active form and make their way to the place where they are needed, namely, to the cells of the developing gerrn. Similarly the carbo-hydrates, fats and pro- teids which enter the body in the form of food, must be rendered soluble, so that they may be able to reach the place where they will be used, and the fats which are stored up in fatty tissues must be altered before they can be used in any part of the body. In plant and animal cells this important transformation of carbo-hydrates, fats and proteids from a passive into an active condition is efficiently accomplished by means of very peculiar chemical substances called ferments. These are allied to the albumens, and indeed are derived from them ; they are present in very minute quantities in the cell, but nevertheless produce powerful chemical effects, and induce chemical processes without being essentially altered themselves. This process of fermenta- tion is very characteristic of the chemistry of the cell. There are special ferments for carbo-hydrates, others for proteids, and others for fats. Whenever starch is rendered soluble in plants, the process is effected by means of a ferment, diastase, which can easily be ob- tained from germinating seeds. Its efficacy is so great, that one part, by weight of diastase is sufficient to convert in a short time 2,000 parts of starch into sugar. Another ferment, invertin, THE VITAL PROPERTIES OF THE CELL 151 which acts upon carbo-hydrates, is present in some fission fungi and moulds ; it splits cane sugar up into dextrose and levulose. The salivary ferment in the animal, ptyalin, which converts starch into dextrin arid maltose, corresponds to the diastase in the plant. Similarly the non-diffusible glycogen, which in conse- quence of its properties has been called animal starch, must, if it is to be utilised further, be converted by means of a sugar-form- ing ferment, wherever it occurs, into sugar (liver, muscles). Albuminous bodies are peptonised before they can be absorbed. In the animal body this takes place chiefly by means of a ferment, pepsine, which is secreted by the cells of the gastric glands. A small quantity of pepsine is able either in the stomach or in a test-tube to dissolve a considerable amount of coagulated albu- men in the presence of free hydrochloric acid, thus converting it into such a form that it is able to diffuse through membranes. Peptonising ferments have been also demonstrated in plant cells. For example, one has been extracted in the form of a digestive juice from those organs of carnivorous plants which are adapted for the capture of insects, such as the glandular hairs of the leaves of the Drosera ; in this manner the small dead animals are partially dissolved and absorbed by the plant cells. A fer- ment resembling pepsine has also been demonstrated in germi- nating plants, where it serves to peptonise the proteid bodies which are stored up as reserve material in the seed. The pepto- nising ferment from the milky juice of the Carica papaya and of other species of Carica is well known on account of its energetic action. Finally, a similar ferment has been discovered in the body of the Myxomycetes by Krukenberg. In the animal body fats are split up into glycerine and fatty acids. This result is effected mainly by the pancreatic juice. Claude Bernard endeavoured to trace this back to a fat decom- posing ferment secreted by the pancreas. Further, it is supposed that during the germination of fat-containing plant seeds the oils are split up into glycerine and fatty acids by means of ferments (Schiitzenberger) . Thus even from these few data it may be seen that, although at present so little is known about the subject, there appears to exist a far-reaching uniformity throughout the whole organic kingdom as regards the elaboration of material in the cell. One of the points which is least understood concerning the metabolism of the cell is the part played by the protoplasm. 152 THE CELL This is especially true of all the processes which are described above as belonging to the formative activity of the cell. What relationship does the protoplasm bear to its organised products, such as the cell membrane, the intercellular substance, etc.? Two quite opposite views have been suggested upon this sub- ject. According to the one, the organised substances are formed by the transformation of the protoplasm itself, that is to say, through the chemical rearrangement or splitting up of the proto- plasmic molecules ; according to the other, on the contrary, they are supposed to be formed of plastic materials, carbo-hydrates, fats, peptonised proteids, etc., which are taken up during meta- bolism by the protoplasm, conveyed to the place where they are required, and there brought into a suitable condition for secre- tion. This difference may be best explained by an example, such as the formation of the cellulose membrane of the plant cell. According to a hypothesis which has been strongly supported by Strasburger (V. 31-33) amongst others, the microsome containing protoplasm becomes directly transformed into cellulose lamellae ; that is to say, cellulose, as a firm organised substance, is formed directly out of the protoplasm. Another theory is, that some non-nitrogenous plastic substance, such as glucose, dextrin, or some other soluble carbo-hydrate, forms the materials from which the cell membrane is constructed. These materials are conveyed by the protoplasm to the place where they are required, and are here converted into an insoluble modification, cellulose. Since this cellulose acquires a fixed struc- ture from the beginning, the pi'otoplasm must, in a manner at present unknown to us, assist in its construction ; this process is described by the expression " formative activity." According to the first hypothesis, the cellulose membrane may be described shortly as a metabolic product of the protoplasm, and, according to the second, as a separation product of it. The question of the formation of chitinous skin, of the ground substance of cartilage and bone, of calcareous and gelatinous sub- stances, may also be regarded from the same two points of view ; in fact, all conceptions of the metabolism of the cell present the same difficulty. Claude Bernard (IV. la) described this relationship in the following words: "From a physiological standpoint it may be conceived that in the organism only one synthesis occurs, that of THE VITAL PROPERTIES OF THE CELL 153 protoplasm, which grows and develops itself at the expense of the substances which it absorbs. Then, from the splitting up of this most complex of all organised bodies, all the complicated ternary and quaternary compounds must arise, the formation of these being ordinarily ascribed to a direct synthesis. Hence Sachs was obliged to allow that it was possible, although he con- sidered it improbable, that in the assimilation of starch decompo- sition and restitution occur in the molecules of the green proto- plasm." These remarks show how difficult the whole subject is in so far as it concerns the chemical processes in question. If it is allowable to draw conclusions from analogous cases, I must certainly decide in favour of the second hypothesis, accord- ing to which the protoplasm participates more indirectly than in the first in the formation of the greater number of intercellular substances. For in the cases where organisms construct a sili- cious or calcareous membrane the nature of the substance itself distinctly shows that it could not proceed directly as a firm organised substance out of protoplasm. This latter in such a case, in consequence of its chemical composition, can only play the part of an intermediary, by selecting the substances from its environment, absorbing them, accumulating them at the places where they are required, and depositing them in a distinct form as firm compounds, which are invariably joined to an organic substratum. Such a conception appears to me to be nearer the truth in the case of the formation of the cellulose membrane also, if the facility with which various carbo-hydrates become transformed into one another is taken into account, as well as the complicated process, which would be necessary if protoplasm were to be converted into cellulose. And even those intercellular substances which are chemically more nearly related to protoplasm, such as chondrin, gluten, etc., may be governed by the same laws of construction. For, apart from the organised proteid substances, protoplasm and nuclear substance, there are always present in each cell a large number of unorganised proteids ; these serve as formative material, and occur in a condition of solution in the cell sap of plant cells, in the nuclear sap, and in the blood and lymph of animals. Instead of the protoplasm itself being directly seized upon and used up in the formation of nitrogenous intercellular substances, it is possible that the unorganised proteid materials 154 THE CELL may be utilised by the formative activity of the cell, in the same way as has been suggested above, that other substances are used for the formation of the cellulose membrane. In what way the protoplasm executes its above-mentioned function of adoption is quite beyond our comprehension at this present time, when the majority of the bio-chemical processes escape our observation. This function of the protoplasm, however, may consist in this, that certain particles of its substance may unite, through molecular addition, with particles of other sub- stances present in the nutrient solutions, and thus become trans- formed into an organic product. Thus soluble silicious compounds may unite with molecules of organic substance to form a silicious skeleton ; thus particles of cellulose may be formed through the influence of particles of protoplasmic substance from soluble carbo-hydrates, forming with them a compound (probably per- manent, but possibly only temporary), and becoming organised to form a cell-membrane. This conception is quite in accordance with the fact that in many objects freshly-formed layers of cellulose are found to pass imperceptibly into the neighbouring protoplasm. 2. The Morphology of Metabolism. The formative activity of the Cell. The substances which are formed during the meta- bolism of the cell may be included under the head of morpho- logy > i n so far as they can be optically distinguished from the protoplasm. They may be differentiated out in a formed or unformed condition, either in the interior of the protoplasm, or upon its surface ; according to their position they are distin- guished as internal or external plasmic products. However, as is so often the case in biological classifications, a sharp line of dis- tinction cannot be drawn between the two groups. a. Internal Plasmic Products. Substances dissolved in water may separate out as larger or smaller drops in the protoplasm, and thus cause cavities or vacuoles. These play a most important part, especially in the morphology of plants. As has already been described in detail on p. 31, a plant cell (Fig. 62) is able by secreting sap to increase its size in a short time more than a hundred-fold. It is by means of the simultaneous action of a large number of such cells that in spring-time certain organs of plants are able to grow to such a considerable size. The solid substance contained by a plant very rich in water may be as little as 5 per cent., or even only 2 per cent. THE VITAL PROPERTIES OF THE CELL The cell sap, however, is not pure water, but a very complex, nutrient solution containing 1 veget- able acids and their salts, nit- rates and phos- phates, sugar, and small quantities of dissolved pro- teids, etc. Thus between the pro- toplasm a.nd the sap material is interchanged to a considerable ex- tent, substances for use being ex- tracted from the one, which in return receives other substances in exchange. Since the sap re- presents a con- centrated solu- tion of osmotic substances, it ex- erts a powerful attraction upon water, and also an internal pres- sure, which is of- ten considerable, upon the envelope surrounding it, thus producing FIG. 62. Parenchyma cells from the cortical [layer of the root of Fritillaria imperialis (longitudinal sections, 650 rafter Sachs II. 33, Fig. 75): A very young cells, as y t without cell-sap, from close to the apex of the root; B cells o the same description, about 2 mm. above the apex of the roo the cell- sap (o) forms in the protoplasm (p) separate drop between which are partition walls of protoplasm ; Cecils o the same description, about 7-8 mm. above the apex ; the two lower cells on the right hand side are seen in a front view ; the large cell on the left hand side is seen in optical section ; the upper right hand cell is opened by the section ; the nucleus (.ry) has a peculiar appearance, in consequence of its beini? dis- tended, owing to the absorption of water; fc nucleus; kk nu- cleolus; h membrane. tense condition, which was described on p. 141 as turgor. Many botanists, especially do Vries (V. 35) and Went, consider the vacuoles to be special cell organs, which are not of accidental 156 THE CELL formation in the cell-body, bat which, can only be produced by division. Even in the youngest plant-cells, according 1 to their opinion, minute vacuoles are present, which multiply continually by fission, and which are distributed amongst the daughter cells when cell division occurs. Here all the vacuoles of the whole plant would originate from those of the meristem. This theory however is disputed by other investigatoi-s. Just as the proto- plasm is bounded externally by a peripheral layer, the vacuoles, in de Vries' opinion, possess a special wall (the tonoplast), which regulates the secretion and accumulation of the dissolved sub- stances present in the cell sap. Na cv Fir,. M.-Acti