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WITH 476 ILLVSTBATIOm vc^ NEW YORK D APPLETON AND COMPANY LONDON: CAXTOM H008B, PATEKNOSIBB SdOAEB 1890 r^ COPTBIOHT, 18fl0, Bt d. appleton and company AH rights reserved. PREFACE. Some years of contact with students of comparative (com- monly called veterinary) medicine, and a fair knowledge of the actual needs of the practitioner of this department of the medical art, have convinced me that the time has fully come when the text-books of physiology provided for students of human medicine, and which the former classes have hitherto been compelled to use, should be replaced by works written to meet their special wants and possibilities. In fact, so dif- ferent from man are most of the animals which the veterma- rian is called upon to treat, and therefore to understand, in health as well as in diseac.,that only the absence of suitable works of a special character can justify the use of those that confessedly treat of *man alone. Unfortunately, till within the past year the English-speak- ing student of comparative medicine has been without a single work in his own language of the special character re- quired. Within that period two have appeared— the excel- lent but ponderous Physiology of. the Domestic Animals, by Prof. Smith, and my own Text-Book of Animal Physiology. It has, therefore, occurred to me that a somewhat smaller work than the latter, embodying the same plan, but with greater specialization for the domestic animals, would com- mend itse:. "^o both the students and the practitioners of comparative i^edicine. In my other work I have endeavored to set before the student a short account of what has been deemed of most importance in g6neral biology; to furnish a full account of reproduction; to apply these two depart- ments throughout the whole of the rest of the work ; tobnng iv COMPARATIVK FlIYblOLOUY. I before tho studoiit enough of comparative physiology in Hh widest HuiiHe to improHs liim witli tlio imijorlunce of recog- nizing tluit iill medicine like nil science is, when ut itn best, eompurutive ; and to show timt tho doctrines of evolution must apply to physiology and medicine as well as to morphology. Comparative medicine is essentially broad. It will not do to measure all tho animals tho veterinarian is called upon to treat by the equine standard. This has been too much the case in the past for the good even of the horse himself ; while otheis, that fall to the practitioner's care, like tho dog, have been much neglected and misunderstood. There is no more reason, theoretically, why tho veterinn- rian should overlook man than that tho practitioner of human medicine should disregard the lessons to be learned from our domestic animals ; hence the attempt has been made to exclude references to the human subject from tho volume. Tho stu- dent of comparative medicine may learn, by careful observa- tion on himself, to understand much that would otherwise never become realized knowledge; and this conviction has been at the root of a large part of the advice given the stu- dent as to how to study throughout tho work. All that relates to reproduction and breeding is, in "these days of vast stock interests, of so much practical importance, that on this account alone the fullest treatment of the subject seems justifiable. But, apart from this, it has become clear to me that function as well as form can be much better and morfe easily grosped when embryology is early considered. This I have tested, with the happiest results, with my own classes. Usually those taking up physiology for the first time are, of course, not expected to master all the details of embryology, but the main outlines prove as helpful as inter- esting ; nevertheless, it is my experience that a considerable number of first-year men are not content to be confined to the merest rudiments of this or any other department of physiology. That a work written on so new a plan as ray Text-Book of Animal Physiology should have met with a reception al- most universally favorable, both in Britain and America, in u PREPACK. >logy in its CO of rocoj?- ttt itH ItUHt, liilioii niuHt •rphology. will not do led upon to much thi' iself ; while e dog, hnvi! lie voterinii- irof human }d from our e to exclude . The 8tu- tul observa- 1 otherwise viotion has en the stu- 80 rihort a space of time, encouragos me to ho|)e for one equally favorable for this book, which im ollored to a pru- fuHHiun from which I look for great things in the interests both of maik and the lower animals within the next few years. The time has certainly come when medicine must leave the narrow ruts within which it has been contined, and become essentially comparative. To hasten that consumnui- tion, so devoutly to be wished, has been the object with which both my earlier ami the present work have been written. Un- less the student is infused with the broad comparative s])irit in the earliest years of his studios, and guided accordingly, there la no sure guarantee of tlnal success in the widest sense. My publishers again deserve my thanks for the efforts they have made to present this work in their best form. Weslky Mills. FHVSIOUMUCAL liAHURATOBY, MoGh^L UNIVGRRITV, Montreal, Canada, Augu»t, 1800. is, in these mportance, the subject me clear to better and considered, ith my own r the first e details of 'ul as inter- onsiderable confined to artment of Text-Book ception al- Vmerica, in '?S!?sr ' ti CONTENTS. OiNIRAL DlOLOOT 1 Introduction 1 Tabular Htatcracnt of thu subdlviRions of Biology ... 4 Tlio Cell ,. . » Animal and vegetable cells 8 Structure of celU Cell-contentfi 1 The nucleus "... 8 TIbsucb 8 Summary 9 Unioellular OrganUros (Vegetable) 9 1. Yea»t » Morphological 9 Chemical tt Phyiiologtoal 10 Conclusions , . . 10 2. Protococcua 11 Morphological IS PhyBiological 13 Conclusions 12 Unicellular Animals . ..,,..... 18 The proteus animalcule 18 Morphological 18 Physiological 18 Conclusions It Parasitic Orgaulams . IS Fungi . . . . . .IB Huoormucedo 17 The Bacteria 18 Un'.nelluiar Animals with Differentiation of Structure ... 21 The bell-animalcule 21 Structure 21 Functions 88 h 1^ Vlll COMPARATIVE PHYSIOLOGY. PAOB Multicellular Organisms • • . ^» Tlie f resli-water polyps '^^ Tlie Cell reconsidered ^' The Animal Body— au epitomized account of the functions of a mam- mal 28 Living and Lifeless Matter— General explanation and comparison of their properties ^'^ Classification of the A !iimal Kingdom . . . . '. .34 Tabular statement ^* Man's place in the animal kingdom 36 The Law of Periodicity or Rhythm in Nature— Explanations und illustrations 37 The Law of Habit " .... 41 Its foundation *^ Instincts • • . ■ *' The Origin of the iorras of Life *2 Arguments from : Morphology *^ Embryology ^^ Mimicry . . . . • *'^ Rudimentary organs . ^^ Qeographical distribution *^ Paleontology ** Fossil and existing species . . • • • • • *•* Progression Domesticated animals * ' Summary RgPRODCOTIOS General . . • "l The ovum ' ' ' ^t The origin and development of the ovum . . > • . 67 Changes in the ovum itself . . . . . • • "8 The male cell ** The origin of the spermatozoon ''* Fertilization of the ovum *' Segmentation and subsequent changes . . . . , . . 64 Thegastrnia ** The hen's egg -I! The origin of the fowl's egg '® Embryonic membranes of birds •* The fcBtal (embryonic) membranes of mammals .... 78 The allantoic cavity . ow The placenta The discoidal placenta .88 CONTENTS. ix PAOB The metadiscoidal placcntn ..,.,.. 84 The zonsry placenta 80 The diffuse placenta 89 The polyootyledonaty placenta 89 Tabulation of placentation 90 Hicrosoopic structure of the placenta 90 IlluBtrations 91, 98 Evolution ~ . . 98 Summary 93 Thb Detklopment or the Embryo Itself 96 Germ-layers 98 Origin of the vascular systeai 102 The growth of the embryo . . . . . . 105 Development of the Vascular System in Vertebrates . 108 The later stages of the fcetal circulation 109 Development of the Urogenital l:3y stem , . . . .112 The Physiological Aspects of Development 118 Ovulation 120 Oestrum 121 The nutrition of the ovum 123 The foetal circulation 126 Periods of Gestation . ' 127 Parturition 128 Changes in the circulation after birth 129 C!oitu8 129 OcoANic Evolution reconsidebed W The Cbemioal Constitution or THE Animal Boot . . U2 Proximate principles 144 General characters of proteids 144 Certain non-crystalline bodies 146 The fats • M6 Peculiar fats 146 Carbohydrates • • !*• Kitrogenons metabolites W Non-nitrogenous metabolites 147 PuTStOLOaiCAL ^»IARCH AND PhYSIOLOOIOAL REASONING . . . 148 Tub Blood IM Comparative IM Corpuscles 166 Histoiy of the blood-cells 168 Chemical composition of the blood 140 Composition of serum 161 Composition of the corpuscles 162 The quantity and diatribution of the blood . . .168 i X COMPARATIVE PHYSIOLOGY. PAflB The coagulation of the blood .168 Clinical and i)athologioal 167 Summary 169 The Contractile Tissurs 171 General 171 Comparative 172 Ciliary morementB .178 The irritability of muscle and nerve '176 Tbk Graphic Method and the Study or Muscle Pbtbioloot . .176 Chronographs and various kinds of apparatus . . .176 The apparatus used for the stimulation of muscle . . . . 179 A single muscular contraction 186 Tetanic contraction 187 The muscle-tone 189 The changes in a muscle during contraction 189 The elasticity of muscle 189 The electrical phenomena of muscle 191 Chemical changes in muscle 192 Thermal changes in the contracting muscle . . . . .196 The physiology of nerve 196 ElectrotonuB 196 Pathological and clinical 197 Electrical organs 197 Muscular work 198 Circumstances influencing the character of musular and nervous ac- tivity 199 The influence of blood-supply and fatigue 199 Separation of muscle from the central nervous system . 201 The influence of temperature 801 Unstriped muscle 208 General 202 Comparative 802 Special considerations 808 Functional variations 804 Summary of the physiology of muscle and nerve ... 205 Till Nertohs Stbtem— GnncRAL Comsidbratiohs .... 208 ExperimenUl .810 Automatism 811 Condudons ....*. 812 Nerroos inhibition 218 Thi Oircclation of the Blood 214 General 814 The mammalian heart 816 Circuiatioa in the mammal 819 ous ac- PAfll 168 167 169 m 171 172 178 ■ 176 176 176 179 185 187 189 189 189 191 192 196 196 196 197 197 198 199 199 201 201 202 202 ao2 208 204 206 208 210 211 212 212 214 214 216 819 1 1 CONTENTS. ^ PAOB The action of the mammalian heart 222 The velocity of the blood and blood-pressure . . . . .228 General . . . ^ 228 Comparatire 224 The circulation under the microscope 224 The characters of the blood-flow 226 Blood-pressure 227 Thb Heart . 281 The cardiac movements 231 The impulse of the heart 232 Investigation of the heart-beat from witliin 238 The cardiac sounds . . 234 Causes of the sounds 236 Endo-cardiac pressures 236 The work of the heart 288 Variations in the cardiac pulsation 289 Comparative 240 The pulse 241 Features of an arterial pulsc-traoing . . .242 Venous pulse 244 Pathological . . _ 244 Comparative . . . , 244 The beat of the heart and its modifications 248 The nervous system in relation to the heart 249 Influence of the vagus nerve on the heart 268 Conclusions 267 The accelerator nerves of the heart 268 The heart in relation to biood-pressuro 260 The influence of the quantity of blood . , . . .261 The capillaries 264 Special considerations 266 Pathological . , .266 Personal observations . . see Comparative . . , . . . . 267 Evolution. . 268 Summary of the physiology of the circulation . . .269 DioBsnoN OF POO0 . 2Y4 FoodstuJTs, milk, eta ■" . .274 Embryoiogical jgo Comparative . . . ... . 286 Structure, arrangement, and significance of the teeth . 286 The digestive juices . . . • 297 Saliva and its action 297 Secretion of the diiferent glands 298 tr-s^s^-XiESSi?-^^ xii COMPARATIVE PHYSIOLOGY. PAOI Comparative • < 298 Gastric juice ^'^ Bile 801 General 301 Pigments 802 Digestive action 808 Comparative 304 Pancreatic secretion 806 SuccuB entericus 807 Comparative 810 Secretion as a physiological process 811 Secretion of the salivary glands '811 Secretion by the stomach 816 The secretion of bile and pancreatic juice 316 The nature of the act of secretion . . . . 818 Self-digestion of the digestive organs . . . . 888 Comparative 824 The alimentary canal of the vertebrate 881 The movements of the digestive organs 882 Deglutition 338 Comparative 386 The movements of the stomach 836 Comparative . . . • 887 Pathological • • .887 The intestinal movements . . . . • • 837 Defecation 388 Vomiting 88» Comparative 340 The removal of digestive products from the alimentary canal . . 841 Lymph and chyle , • • 348 The movements of the lymph — comparative . . . . 844 Pathological 862 Fteoes . 862 Pathological 868 The chains produced in the food In the alimentary canal . 864 General . . . . ' -864 Comparative .867 Pathological 369 Special considerations • • •• 369 Various 869 Evolution 868 Summary 864 Thb Respiratort Stbtbh 866 General . « 866 i 'i 824 881 882 338 386 SSff 88? 887 889 888 889 840 841 848 844 862 8B2 868 864 364 867 869 869 869 868 864 866 866 CONTENTS. xiii PAtIB Anatotniool 867 The entrance and exit of air . 870 The muBcIca of respiration 378 Types of respiration .......... 87b Comparative * . . . . 876 Personal obHcrvation . , , 876 The quantity of air respired , . , 878 The respiratory rhythm . 379 General 379 Pathological . . . . .879 Respiratory sounds sgi CJomparison of the inspired and the expired air . ... . 882 Respiration in the blood sss HoBmoglobin and its derivatives ....... 386 General 886 Blood-spectra , . 887 Comparative ' . . 889 The nitrogen and the carbon dioxide of the blood . .389 Foreign gases and respiration 891 Respiration in the tissues . . 891 The nervous system in relation to respiration . . . . 898 The Influence of the condition of the blood on respiration . . . 398 The influence of respiration on the circulation 896 General . •( 396 Comparative . 898. The respiration and circulation in asphyxia .... 399 Pathological 401 Peculiar respiratory movements 401 Coughing, laughing, etc. . . . . . . . . 401 Comparative 402 Special considerations , . . 408 Pathological and clinical 403 Personal observation 404 Evolution 404 Summary of the physiology of respiration 406 Protkotivx and Exorktort Functions ot tbb Sun .... 408 General - . . . 408 Comparative 40g The excretory function of the skin 411 Normal sweat ' . , 411 Patholo^cal , 4II Comparative— Respiration by the skin 412 Death from suppression of the functions of the skin . .412 The excretion of perspiration .412 f^ xiv COMPARATIVE PHYSIOLOGY. PAoa Experimental *' Absorption by the skin ....••••'*** Comparative * " Summary *^' EZORKTION BY THE KlDNKY ^'^ Anatoiuicai *** Comparative .416 Urine conaidered pliysloally and cliemioally . . • . . *1* Specific gravity *'• Color 4»9 Reaction . *^* Quantity *^* CompOBition : Nitrogenous crystalline bodies .... 480 Non-nitrogenous organic bodies 420 Inorganic salts *20 Abnormal urine *^1 Comparative *21 The secretion of urine *-^ Theories of secretion 421 Nervous influence 423 Patholi^cal *28 The expulsion of urine **** General *24 Facts of experiment and of experience 424 Pathological 426 Summary of urine and the functions of the kidneys . . . • 48* Comparative *** The Metabolism of the Boot 428 General remarks . • 428. llie metabolism of the liver • . 428 The glycogenic function 429 The uses of glycogen 429 Metabolism of the spleen 429 Histological . 480 Chemical *'** The construction of fat *^^ General and experimental 432 Histological *^^ Changes in the cells of the mammary gland . . , • . 434 Nature of fat formation 484 Milk and colostrum 436 Pathological ^^'^ Comparative • • • " The study of the metabolic processes by other methods . . . 486 COMPARATIVE PHYSIOLOGY. GENERAL BIOLOGY. IMTUODnonON. Biology (fitos, life ; Xoyor, a dissertation) is the science which treats of the nature of living things ; and, since the properties of plants and animals can not be explained without some knowl- edge of their form, this science includes morphology (/m/m/ii;, form ; Xoyor, a dissertation) as well as physiology {iftwns, na- ture ; Xoyof). Morphology describes the various forms of living things and their parts ; physiology, their action or function. General biology treats neither of animals nor plants exclu- sively. Its province is neither zoology nor botany ; but it at- tempts to define what is common to all living things. Its aim is to determine the properties of organic beings as such, rather than to classify or to give an exhaustive accoimt of either ani- mals or plants. Manifestly, before this can be done, living things, both animal and vegetable, must be carefully compared, otherwise it would be impossible to recognize dififerences and resemblances ; in other words, to ascertain what they have in common. When only the highest animals and plants are contem- plated, the differences between them seem so vast that they appear to have, at first sight, nothing in common but that they are living : between a tree and a dog an infant can discrimi- nate ; but there are microscopic forms of life that thus far defy the most learned to say whether they belong to the animal or the vegetable world. As we descend in the organic series, the lines of distinction grow fainter, till they seem finally to all but disappear. 2 COMPARATIVE PHYSIOLOGY. But let us flrat inquire : What are the determining oharac- teristica of living things as such ? By what barriers are the animate and inanimate worlds separated ? To decide this, falls within the province of general biology. Living things grow by interstitial additions of particles of matter derived from without and transformed into their own substance, while inanimate bodies increase in size by superficial additions of matter over which they have no power of decompo- sition and recomposition so as to make them like themselves. Among lifeless objects, crystals approach nearest to living forms ; but the crystal builds itself up only from material in solution of the same chemical composition as itself. The chemical constitution of living objects is peculiar. Car- bon, hydrogen, oxygen, and nitrogen are combined into a very complex whole or molecule, as protein ; and, when in combina- tion with a large proportion of water, constitute the basis of all life, animal and vegetable, known as protoplasm. Only living things can manufacture this substance, or even protein. Again, in the very nature of the case, protoplasm is continu- ally wasting by a process of oxidation, and being built up from simpler chemical forms. Carbon dioxide is an invariable prod- uct of this waste and oxidation, while the rest of the carbon, the hydrogen, oxygen, and nitrogen are given back to the in- organic kingdom in simpler forms of combination than those in which they exist in living beings. It will thus be evident that, while the flame of life continues to bum, there is constant chemical and physical change. Matter is being continuoiisly taken from the world of things that are without life, trans- formed into living beings, and then after a brief existence in that form returned to the source from which it was originally derived. It is true, all animals require their food in organized form— -that is, they either feed on animal or plant forms ; but the latter derive their nourishment from the soil and the atmos- phere, so that the above statement is a scientific truth. Another highly characteristic property of all living things is to be sought in their periodic changes and very limited dura- tion. Every animal and plant, no matter what its rank- in the scale of existence, begins in a simple form, passes through a series of changes of varying degrees of complexity, and finally declines and dies ; which simply means that it rejoins the in- animate kingdom : it passes into another world to which ii formerly belonged. ng oharac- ira are thu e this, falls [)articles of their own superficial f decompo- hemselyes. to living material in iliar. Car- into a very Q oombina- basis of all )nly living nn. is continu- ilt up from riable prod- the carbon, k to the in- than those be evident is constant tntinuously life, trans- xistence in I originally 1 organized forms ; but [ the atmoB- li. ring things nited dura- ■ank-in the through a and finally >ins the in- to which ii GENERAL BIOLOGY. 8 Living things alone give rise to living things ; protoplasm alone can beget protoplasm ; cell begeto cell. Omne animal (anima, life) ex ovo applies with a wide interppjtation to all living forms. From what has been said it will appear that life is a condi- tion of ceaseless change. Many of the movements of tht priP toplasm composing the cell-units of which living beings are made are visible under the microscope; their united effects are open to common observation— as, for example, in the move- ments of animals giving rise to locomotion we have the joint result of the movementa of the protoplasm composing millions of muscle-cells. But, beyond the powers of any microscope that has been or probably ever will be invented, there are molecular movements, ceaseless as the flow of time itself. All the pro- cesses which make up .the life-history of organisms involve this molecular motion. The ebb and flow of the tide may symboluw the influx and eiflux of the things that belong to the inanimate world, into and out of the things that live. It follows from this essential instability in living forms that life must involve a constant struggle against forces that tend to destroy it; at best this contest is maintained successfully for but a few years in all the highest grades of being. So long as a certain equilibrium can be maintained, so long may life con- tinue and no longer. The truths stated above will be illustrated in the simpler forms of plants and animals in the ensuing pages, and will become clearer as each chapter of this work is perused. They form the fundamental laws of general biology, and may be for- mulated as follows: 1. Living matter or protoplasm is characterized by ite chemi- cal composition, being made up of carbon, hydrogen, oxygen, and nitrogen, arranged into a very complex molecule. 2. Its universal and constant waste and its repair by inter- stitial formation of new matter similar to the old. 8. Ite power to give rise to new forms similar to the parent ones by a process of division. 4. Its manifestation of periodic changes constituting devel- opment, decay, and death. Though there is little in relation to living beings which may not be appropriately set down under zoology or botany, it tends to breadth to have a science of general biology which deals with the properties of things simply as living, irrespective ^ ■avnan COMPARATIVE PHYSIOLOGY. Biol- The, Hcience of liv- ing things: i. e., of matter in the living state. Mor- ogy. The science of form, struct- ure, etc. tially statical. Phyri. dogy Tlie science of action or func- tion. Essen- tially djrnam- ical. r Anatomy. ] Tho Holenco of Btructuro; , thoterm being uiuully , applied to tha ooaraer and mora obvioua I ooinposltion of plants or anlmolii. UUtology. Mlcroaooplcal anatomy. The ultimate optical analyaiii of Htraoture by tho aid of the ml- oroMopo ; aeparated nroro anatomy only oh a matter of oonvon- ienoe. Taxonomy, The classifloation of Ut- Kna thlnipt, bused ohlefly on phenomeni^ ofstruoture. IHttribution. Coniiders the position of livins thmgti in spaoe and time ; their distribution over the prosont face of'tiio earth : and their dis- tribution and suooes- sion at former po- riods, as displayed in fossil remains. Emhryology. The soience of dovolop- mont from tho gorra : inoludes many mixed problems pertaining both to morphology and physiology. At present largely mor- phological. PhytMogy. The special science of the t\inotionii of the individual in health and in disease ; hence including Ibtholoffy. Ayehology. ' The science of mental phenomena. Sociology. The science of social life, i. e., the life of communities, wheth- er of men or of lower animals. Botany. The science of veg- et^ living matter or plants. Bid. Zo»U ogy. The science of animal living matter or ani mals. le science of liv- ing things: i. e., of matter in the living state. i/ ORNFRAL BIOLOGY. B. Bidp The science of liv- ing ihings: i. e., oi matter in the living state. very much a« to whether they belong to the realm of animals or pluntH. Tlio relation of the wioncofl which may bu regarded UM Hulxlivisions of general biology is well shown in the accom- panying table. * TBB 01IZX.f All living things, great and small, are composed of cells. Animals may be divided into those conHisting of a ningle cell {Protozoa), and those made up of a multitude of relk (Metazoa) ; but in every case the animal begins as a single cell or ovum from which all the other colls, however different Anally from one another either in form or function, are derived by processe* of growth and division ; and, as will be seen later, the whole organism is at one period made up of cells practically alike in structure and behavior. The history of each individual animal or plant is the resultant of the conjoint histories of each of its cells, as that of a nation is, when complete, the story of the total outcome of the lives of the individuals composing it. It becomes, therefore, highly important that a clear notion of the characters of the cell be obtained at the outset ; and this chapter will be devoted to presenting a general account of the cell. The cell, whether animal or vegetable, in its most complete form consists of a mass of viscid, semifluid, transparent sub- stance (pi'oti^lasm), a cell wall, and a more or less circular body (nttcleua) situated generally centrally within; in which, again, is found a similar structure (nttcletdits). . This description applies to both the vegetable and the ani- mal cell; but the student will find that the greater proportion of animal cells have no cell widl, and that very few vegetable cells are without it. But there is this great difference between the animal and vegetable cell: the former never has a cellulose wall, while the latter rarely lacks such a covering. In every case the cell wall, whether in animal or vegetable cells, is of greater consistence than the rest of the cell. This is especially true of the vegetable cell. It is doubtful whether there are any cells without a nucleus, whUe not a few, especially when young and moat active, poa- * Taken from the General Biology of Sedgwick and Wilson, f The illustrations of the sectiona follow*ng will enable the stodent to form a generalized mental picture of the cell in all ita parts. "VPI 6 COMPARATIVE PHYSIOLOGY. I ■em ieverBl. The circular form may be regarded an the typical form of both ccllri nnd nuclei, and their infinite variety in nize and form may l)o conaidered aa in great jMirt the r«Hu1t of tJie action of mechanical forces, such a8 mutual pressure ; this is, of I'ourHc, more especnally true of shape. Retlucetl t4) its greatest simplicity, then, the cell may be simply a mass of protoplasm with a nucleus. It Hei'mH probable that the numerous researches of reoent years and others now in progress will open up a new world of Fra. 1.— NnciSAii Diriiiow. A-H. ktryoklnetU of a tiMiie cell. A, nuclesr retlcnlum In lU onltnary lUte. B, preparing for diviilon ; the contour ii leM (IpHned, and the flbere tUlcker and lei Inftlcate. C. wroathitjije ; the ctwomatln ■ arranged in a complicated looping ronnd the equator of the achroroatin ■pindlu. D, mo- BMter-ttatfc ; the ciuomatin now apjpean aa centripetal eoaatorial y>a,eachof whteh fhSuld be reprc«.nted aa don6le: K, » migration of flie half of each chrc matin loop towarda oppoalte polee of the aplndle. V, dlaater-atage ; the chroma- ™fonM a »tar. round eacli pole of a aplndle, each aater being connected by itrandi of achromatln. Q. dauKter-wrcath atage :«»'»"«*'' IS«2S*,HhL'LS? paaalng through their retrogreaaire development, which is completed In the reat- fi^'tiwe/U. a-f, karyoklnlala of •» ew-cell. fhowlngthe .mailer amount of cfiomatln than In the tliane-cell. The atageed. #./, correapond to D, B, f, re- apectlvely. The polar »Ur at the end of the aplndle la e2'"'S?^i"'»IL'','iS'*'^'K manulee of the cell itaelf, and mnat not be mlatekenforthedlaaterm. The OMuae llnea repreeent the chromatin, the Sne lines the achromatln, aud tlie dotted ItaeaM ll-BranulS?^ (Chiefly modlfled from Plemmlng.) X-Z, direct nucLiar dlvla- Ion in thfcells of the embryonic Integument of the KatofMn acorplon. After Blochmann (HadtUm). cell biology which will greatly advance our knowledge, espe- cially in the direction of increased depth and accuracy. (y OENKRAL BIOLOGY. the typical i«ty in Nize Hiilt nt the ; thii it, of tH (greatest )rutopUuiin I of recent w world of clear retlcninm •a (loaned, and itin li arransgd imdiu. D, mo- lal V'a. eacb of If or each chiO' I ; tbe chroma- It connected by rmed nuclei are )ted In the reat- Uler amoant of 1 to D, B, r, re- of prtttoplaam- aaterlF). The aud the dotted ;tnacii)ardivla- iorploii. After ledge, espe- cy. Thoufrh many i>oiuii are Htill in dinputo, it may he iiafely Miid that the nucleus ])liiyii, in nitml coIIh, u r6le of thu hiifhuHt iinportatK'«s >'< ''^'^ it w^tiit* an though wu might rKgdnl tho nuoleuH as the directive brain, ro to spealc, of the individual cell. It frequently liu|)|)«nH that tlie behavior of thu body of the cell tH foreshudowt'd by tlwt of the nucleus. Thus fre- quently, if not always, division of the body of the nucleus pre- cedes that of the coll itself, and is of a most complicated char- acter (karyokinesia or mitoais). The cell wall is of subordinate importance in the processes of life, though of great value as a mechani(«l support to the protoplasm of the cell and tlie aggro- gutions of cells known as tissues. The greater part of a tree may be said to be made up of the thickened walls of the colls, and these are destitute of true vitality, unless of the lowest order; while the really active, growing part of an old and large tree constitutes but a small and limited zone, as may bo learned from the plates of a work on modem botany representing sec- tions of the wood. Animals, too, have their rigid ports, in the adult state espe^ cially, resulting from the thickening of a part of the whole of the cell by a deposition usually of salts of lime, as in the case of the bones of animals. But in some cases, as in cartilage, the cell wall or capsule imdergoes thickening and consolidation, and several may fuse together, constituting a matrix, which ia also made up in part, possibly, of a secretion from the cell pro- toplasm. In the cuter parts of the body of animals we have a great abundance of examples of thickening and hardening of cells. Very well-known instances are the indurated patches of skin (eptYAe/tum) on the palms of tbe hands aud else- where. It will be scarcely necessary to remark that in cells thus altered the mechanical has largely taken the place of the vital in function. This at once harmonizes with and explains what is a matter of common observation, that old animals are less act- ive — have less of life within them, in a word, than the young. Ohemioally, the cellulose wall of plant-cells consists of carbon^ hydrogen, and oxygen, in the same relative proportion as exists in starch, though its properties are very different from those of that substance. Turning to cell contents, we find them everywhere made up- •f a clear, viscid substance, containing almost alwajrs granules of varying but very minute size, and differing in oonsistenoft m0Lk itti COMPARATIVE PHYSIOLOGY. I not only in diiferent groups of cells, but often in the same cell, so that we can distin^ish an outer portion (ectoplaam) and an inner more fluid and more granular region (endoplasm). The nucleus is a body with very clearly defined outline (in Bome cases limited by a membrane), through which an irregular network of fibers extends that stains more deeply than aay other part of the whole cell. Owing to the fact that it is so readily changed by the action of reagents, it is impossible to ascertain the exact chemical com- position of living protoplasm ; in consequence, we can only infer its chemical structure, etc., from the examination of the dead substance. In general, it may be said that protc-lasm belongs to the class of bodies known as proteids — that is, it consists chomically of carbon, hydrogen, a little sulphur, oxygen, and nitrogen, ar- ranged into a very complex and unstable molecule. This very instability seems to explain at once its adaptability for the man- ifestation of its nature as living matter, and at the same time the readiness with which it is modified by many circumstances, so that it is possible to understand that life demands an incessant adaptation of internal to externa^ conditions. It seems highly probable that protoplasm is not a single pro- teid substance, but a mixture of such ; or let us rather say, fur^ nishes these when chemically examined and therefore dead. Very frequently, indeed generally, protoplasm contains other substances, as salts, fat, starch, chlorophyl, etc. From the fsct that tha nucleus stains di£PerentIy from the cell contents, we may infer a difference between them, physi- cal and especially chemical. It (nucleus) furnishes on analysis nuclein, which contains the same elements as protoplasm (with the exception of sulphur) together with phosphorus. Nuclei have great resisting power to ordinary solvents and even the digestive juices. Inasmuch as all vital phenomena are associated with proto- plasm, it has been termed the "physical basis of life" (Hux- ley). TiMaes. — A collection of cells performing a similar physio- logical action constitutes a tissue. (Generally the cells are held together either by others with that sole function, or by cement material secreted by them- selves. An organ may consist of one or several tissues. Thus the stomach consists of muscular, serous, connective, and gland- GENERAL BIOLOGY. 9 ime cell, ) and an I). tline (in irregular ban any ne action ical com- lan only m of the gg to the lenucally gen, ar- rhis very the man- i time the tances, so incessant uagle pro- [• say, f ur^ id&oA. Bins other 'from the jin, physi- D analysis asm (with 8. Nuclei 1 even the rith ppoto- ife"(Hux- ar physio- thers with L by them- ues. Thus and gland- "^.SJ' '^r'tlSfStu »o„«* of a wall, p«M.«»i« !^^'. « IclST The vegetable ceU hae a Umitmit definite purposes. „-nf««lasm is hiffhly complex SlTS'he ita^d t-Vbe ^ntial to its perfect deve^ opment a«d greatest physiological efficiency. ^ UNIOBUiUl'AR P1.AMT8. YbIst (Toruto, Saccharomycea Cerevism). — *• 1 ~.-* r.f the common substance, yeast, may be The essential part of t^e ^"^ j ^^^ studied to adv^itage, ^^^^.^^^^'Z Jd^nt of physiology '''ijS.S^'-Sir^cles of which yeast is composed JS^TTt.^or'Z form, of an average diameter of ote) QUed with more fl'*i'\««^*«°?-, , ^^inir mav remmn united The various cells produced by buddmg may reiiii« ine vart""" r ^ ,, .• „* masses composed of four J^dT:::!. ehiefly o. »1U of ,»t.»u„. e.U.»,^ and magnesium. 10 COMPARATIVE PHYSIOLOGY. k I The elements of which yeart is composed are C, H, O, N, S, P, K, Hg, and Ca; but chiefly the first four. PhyiiologioaL— If a little of the powder obtained by drying yeast at a temperature below blood-heat be added to a solution of sugar, and the lat- ter be kept warm, bubblew of carbon di- oxide will be evolved, causing the mixture to become frothy ; and the fluid will acquire an alcoholic charac- ter (fermentation). If the mixture be raised to the boiling- point, the process de- scribed at once ceases. Fio. 9.— VarioMitaiiiM in the derelopment of brewer's It maybe further yeaat, eeen, with the exception of the Urst in the _ . . j iu x • ii. aeries, with «i ordinary high power (ZeiM,D. 4) of notioea that in the the microscope. The flrst la greatly magnified favmanHnir aa/wlia (Gnndhuih'aAImmeriionions). The seconfieriea »rmeilw|»g saCClMr of four represents etages In the division of a single fine solution there is a gradual increase of turbidity. All of these changes go on per- fectly well in the to- tal absence of sun- light Yeast - cells are found to grow and reproduce abundant- ly in an artificial food solution consisting of a dilute solution of Fia. 4.— Ftether development of the forms represented CCTtain Salts, together with sugar. O onelm i oa i.— What are the conclusions which may be Inti- mately drawn from the above facts ? That the essential part of yeast consists of cells of about the size of mammalian blood-corpuscles, but with a limiting wall of a substance different from the inclosed contents, which latter is composed chiefly of that substance common to all living things— protoplasm; that like other cells they reproduce their cell ; and the third series a branching colony. Bverywheie the light areas indicate vscaoles. Fio. S.— The cndogonidi* (aaeospore) phase of repio- dnction— i. e., endogenous division. !, H, O, N, S, id by drying to a solution , and the lat- kept warm, )f carbon di- ll be evolved, the mixture e frothy; and will acquire lolic charac- \entation). ) mixture be the boiling- i process de- b once ceases, ybe further that in the ng saocha- tion there is I increase of . All of these go on per- ill in the to- ice of sun- - cells are > grow and B abundant- irtificial food sonmsting of solution of dts, together ir. lay be l^ti- of about the miting wall which latter o all living roduce their GENERAL BIOLOGY. 11 kind, and in this instance by two methods: gemmation giving rise to the bead-like aggregations aUuded to above; and in- ternal division of the protoplasm (endogenoua divmon). From the circumstance^ under which growth and reproduc- tion take place, it will be seen that the original protoplasm of the ceUs may increase its bulk or grow when supplied with suitable food, which is not, as will be learned later, the same in all respects as that on which green plants thrive; and that this may occur in darkness. But it is to be especially noted that the protophasm resulting from the action of the living cells is whoUy different from any of the substances used as food. This power to construct protoplasm from inanimate and unorgan- ised materials, reproduction, and fermentation are all proper- ties characteristic of living organisms alone. It will be further observed that these changes all take place within narrow limits of temperature; or, to put the matter more generaUy, that the life-history of this humble organism can only be unfolded under certain well-defined conditions. Pkotocoooub {Protococcus pluvialia). The study of this one-celled plant will afford instiructive comparison between the ordinary green phmt and the colorless plants or fungi. Via. 8b ne e.ir. Fis. 7. PiM. 5to 7 npieMnit McceMlve stages observed in the IWe-htotory of Frotoeoeol Fw.'^i^gJESSSttodwSaSlS^lUa.tmttng nijrtUotf of dWlsIon. •aT nSSISS ff^otod by n«^ the cell wSF by c.w ; and the coJoring-instter by ttl^dS* J^t oSfttSleflt of B^. 7 >n IndlTldiu may be seen that Is^evold of a cell wall. mmmBmmmm B t I i t V ti; U 19 COMPARATIVE PHYSIOLOGY. Like TonUa it is selected because of its simple nature, its abundance, and the ease with which it may be obtained, for it abounds in water-barrels, standing pools, drinking-troughs, etc. KorphdlogiflaL— Frotococcus consists of a structureless wall and viscid granular contents, i. e., of cellulose and protoplasm. The protoplasm may contain starch and a red or g^^een color- ing matter (cKUyrophyl). It probably contains a nucleus. The oeU is mostly globular in form. PhyuologioaL — It reproduces by division of the original cell (fianon) into similar individuals, and by a process of budding and constriction {gemmation) which is much rarer. Under the influence of sunlight it decomposes carbon dioxide (COi), fixing the carbon and setting the oxygen free. It can flomrish per- fectly in rain-water, which contains only carbon dioxide, salts of ammonium, and minute quantities of other soluble salts that may as dust have been blown into it. There is a motile form of this unicellular plant, and in this stage it moves through the fluid in which it lives by means of extensions of its protoplasm (ct7to) through the cell wall ; or the cell wall may disappear entirely. Finally, the motile form, withdrawing its cilia and clothing itself with a cellulose coat, becomes globular and passes into a quiescent state again. Much of this part of its history is common to lowly animal forms. ConolviiOlU.— It win he seen that there is much in common in the life-history of Torula and Protococcm. By virtue of being living protoplasm they transform unorganized material into their own substance ; and they grow and reproduce by analogous methods. But there are sharply defined differences. For the green plant sunlight is essential, in the presence of which its chloro- phyl prepares the atmosphere for animals by the removal of carbonic anhydride and the addition of oxygen, while for Torula neither this gas nor sunlight is essential. Moreover, the fungus (Tonda) demands a higher kind of food, one more nearly related to the pabulum of anim.al8 ; and is absolutely independent of sunlight, if not actually injured by it ; not to mention the remarkable process of fermentation. : i I? a£NERAL BIOLOGY. 18 le nature, its itained, for it king-troughs, :tureles8 wall protoplasm. r green color- lucleus. The ) original cell IS of budding •, Under the ){COi), fixing flourish per- dioxide, salts ible salts that it, and in this i by means of ceU wall ; or a motile form, cellulose coat, state again, lowly animal ;h in common Bj virtue of aized material reproduce by ^or the green lich its chloro- le removal of en, while for igher kind of anim,als ; and illy injured by nentation. UNXOBLXiUIJlR ANIMAIJI. The Proteus AsmAuovusi (Amoeba). In order to illustrate «nimal life in its simpler form we choose the above-named creature, which is nearly as readily obtainable as Protococcus and often under the same circum- stances. . , x__ MoxphologioaL— Amosba is a microscopic mass of transpar- ent protoplasm, about the size of the largest of the colorless blood-corpuscles of cold-blooded animals, with a clearer, more consistent outer zone {ectomrc), (although without any proper ceU wall), and a more fluid, granular inner part. A clear space (contractile vesicle, vacuole) makes its appearance at intervals in the ectosarc, which may disappear somewhat suddenly. This appearance and vanishing have suggested the term pulsating or contracting vesicle. Both a nucleus and nucleolus may be seen in Amoeba. At varying short periods certain parts of its body (paeudopodia) are thrust out and others withdrawn. PhytlolOgioal.— Amoeba can not live on such food as proves adequate for either Protococcus or Torula, but requires, besides inorganic and unorganized food, also organized matter in the form of a complex organic compound known as protein, which contains nitrogen in addition to carbon, hydrogen, and oxygen. In fact, Amoeba can prey upon both plants and animals, and thus use up as food protoplasm itself. The pseudopodia serve the double purpose of organs of locomotion and prehension. This creature absorbs oxygen and evolves carbon dioxide. Inasmuch as any part of the body may serve for the admission, and possibly the digestion, of food and the ejection of the use- less remains, we are not able to define the functions of special parts. Amoeba exercises, however, some degree of choice as to what it accepts or rejects. The movements of the pseudopodia cease when the temperar ture of the surrounding medium is raised or lowered beyond a certain point. It can, however, survive in a quiescent form greater depression than elevation of the temperature. Thus, at 36° 0., heat-rigor is induced; at 40° to 45° C, death results ; but though all movement is arrested at the freezing-point of water, recovery ensues if the temperature be gradually raised. Its form is modified by electric shocks and chemical agents, as well as by variations in the temperature. At the pres- ent time it is not possible to define accurately the functions fB^^ _0mfgm mum 14 COMPARATIVE PHYSIOLOGY. of the vacuoles found in any of the organisms thus far consid- ered. It is worthy of note that Amoeba may spontaneously assume a spherical form, secrete a structureless covering, and Vff ne ne FlG.0. ye- Fia. 10. Fie.ll. VUi.18. Fia.18. •fte. Fra.14. '•ne- ne. ,xe na.15. Vf-'^f Fia. 18. FtoB. 8 to 16, repraaent nicceMiTe phases In the lif e-Ustonr of an Amoeboid oiniiism> kept under constant observation for three days ; Ffs. 16 a simUar ornmlsm en; CTSted, which was a few honrs later set free by the olslntegration of the cyst (All the llenres are drawn under Zeiss. D. 8.) ,. ^ . . Fio. 8.— The locomotor phase ; the eutopiasm is seen protrudlnK to form a psendopo- dluni, Into which the endoplawm passes. Fio. 9.~A statre in the ingestfve phase. A vegetable organism, J)7, is nndeigolng in- Fie. 10.— A portion of the creatnre represented In Fig. 0, after complete taigeetlon of the food-particle. .... « .n Fio. 11, 18.-^nccesslve stages in the assimilative and excretory procesMS, Plg^ represents the organism some twenty hours later than as seen in Fig. II. Tfte nndigested remnants of the Ingested organism are represented undergoing ejec- tion (excretion) at/^, in Fig. 18. . . , ... ,_ Fios. 18, 14, 16, represent successive stages in the reproductive process of the same m- dividnai, observed two days later. It will be noticed (Fig. 13) that the nucleus di- vides first. . ... , In the above llgnres. vc, denotes the contracting vacuole ; nc, the nucleus ; p», pseu- dopodlnm ; <«, diatom ; fp, food-particle. mum iiiiiMHiii>iimMaiiMi B far consid- mntaneoudy overing, and Fio. 10. Fig. 18. Fia. 18. Qoebotd oiniiUm> tilar OTsanisin en' ttion of the cyet f onn a paeadopo- , is nndeigoing In- iplete bigeetion of iroceaaei. Fig. 18 I in Fig. 11. The 1 nndei^otng e]ec- e«8of theeamein- lat the naclens di- inclens ; pi, psen- GENERAL BIOLOGY. 15 remain in this condition for a variable period, reminding us of the similar behavior of Torula. Amoeba reproduces by fission, in which the nucleus takes a prominent if not a directive part, as seems likely in regard to all the functions of unicellular organisms. GonollllioilS.— It is evident that Amoeba is, in much of its behavior, closely related to both colored and colorless one-celled plants. All of the three classes of organisms are composed of protoplasm ; each can construct protoplasm out of that which is very different from it ; each builds up the inanimate inor- ganic world into itself by virtue of that force which we call vitdl, but which in its essence we do not understand ; each mul- tiplies by division of itself, and all can only live, move, and have their being under certain definite limitations. But even among forms of life so lowly as those we have been consider- ing, the differences between the animal and vegetable worlds appear. Thus, Amoeba never has a cellulose wall, and can not subsist on inorganic food alone. The cellulose wall is not, how- ever, invariably present in plants, though this is generally the case ; and there are animals (Ascidians) with a cellulose invest- ment. Such are very exceptional cases. But the law that ani- mals must have organized material (protein) as food is without exception, and forms a broad line of distinction between the animal and vegetable kingdoms. Amoeba will receive further consideration later ; in the mean time, we turn to the study of forms of life in many re- spects intermediate between plants and animals, and full of prac- tical interest for mankind, on account of their relations to dis- ease, as revealed by recent investigatious. PABASmO OROAMISBU. The Fdnqi. Molds (PmiciUium gUtumm and Mucor mueedo). Closely related to Torula physiologically, but of more com- plex structure, are the molds, of which we select for convenient study the common green mold (PenieiUium), found growing in dark and moist places on bread and similar substances, and the white mold (Mucor), which grows readily on manure. The fungi originate in ^)ore8, which are essentially like Torula in structure, by a process of budding and longitudinal extension, resulting in the formation of transparent branches Fia.aO. -«»' Fia. W. GENERAL BIOLOGY. 17 FiOK. IT to JH.- In the following flguroi, ha, donotea atrial hvpha; i/i, Rporangltim; -■orTum; my, mycelium; mc, mucilage; el, culuniollu; «n, jy, itygOHpore; «x etidogonidla. . , ,. • Vui. 17. -8poro-t)carinK hj , of Mncor. srowlng from horae-dung. Flu. 1 .—The lame, toaned out with neediea (A, 4). Kio*. tU, !90, 81.— SuccvhhWv utaKi'H in the development of the iporangiam. Kio, 8St.— Isolated iporon of Mncor. Kio. 83. — UomiinBtlnK Hporos of tliu aame mold. Kio. 84.— Snccc'Siilvo HtOKes In tlui germination of a ilnglo spore. Kioi. as, SB, 87. - HuccciiHlve phaiK-a In the conjuaatlve process of Mucor. Kio. 88.— SucceMlvo vtagea oWrved during ten houra In the growth of a conldiophorc of Penicllllam In an object^laM culture (U, 4). or tubules, filled with protoplasm and invested by cellulose walls, across which transverse partitions are found at regular intervals, and in which vacuoles are also visible. The spores, when growing thus in a liquid, gives rise to up- ward branches (aerial hyphoe), and downward branches or root- lets {submerged hyphce)^ These multitudinous branches inter- lace in every direction, forming an intricate felt-work, which supports the green powder (spores) which may be so easily shalcen off from a growing mold. In certain cases the aSrial hyphffi terminate in tufts of branches, which, by transverse division, become split up into spores (Conidia), each of which is similar in structure to a yeast-cell. The green coloring matter of the fungi is not chlorophyl. The Conidia germinate under the same conditions as Torula. Kvoor mvoeoo.— The growth and development of this mold may be studied by simply inverting a glass tumbler over some horse-dung on a saucer, into which a very little water has been poured, and keeping the preparation in a warm place. Very soon whitish filaments, gradually getting stronger, ap- pear, and are finally topped by rounded heads or spore-cases (Sporangia). These filaments are the hyphce, similar in struct- ure to those of Penicillium. The spore-case is filled with a multitude of oval bodies (sporea), resulting from the subdivision of the protoplasm, which are finally released by the spore-case becoming thinned to the point of rupture. The development of these spores take place in substantially the same manner as those of Penicillium. Sporangia developing spores in this fash- ion by division of the protoplasm are termed asci, and the spores aaoospores. So long as nourishment is abundant and the medium of growth fluid, this asexual method of reproduction is the only one ; but, under other ciroumstances, a mode of increase, known as cor^'ugatum, arises. Two adjacent hyphae enlarge at the ex- tremities into somewhat globular heads, bend ovm* toward each 2 WT. 18 COMPARATIVE PHYSIOLOGY. il i)ther, Aiul, meetinfr, their opponed faces become thinned, and tho cMiitciitM interminffle. The result of thiH union {zygospore) uudergoofi now certain further changefl, tho celluloHe coat being separated into two— an outer, darlcer in color {exoaporium), and an inner colorless one (endoaporiutn). Under favoring circumstances these coats burst, and a branch sprouts forth from which a vertical tube arises fhat terminates in a sporangium, in which spores arise, as before de- scribed. It will be apparent that we have in Mucor the exem- plification of what is known in biology as " alternation of gen- erations^^ — that is, there is an intermediate generation be tween the original form and that in which the original is again reached. Physiologically the molds closely resemble yeast, some of them, as Mucor, being capable of exciting a fermentation. The fungi are of special interest to the medical student, be- cause many forms of cutaneous disease are directly associated with >).oir growth in the epithelium of the skin, as, for exam- ple, con'mon ringworm ; and their great vitality, and the faoil- ity with which their spores are widely dispersed, explain the highly contagious nature of such diseases. The media on which they flourish (feed) indicates their great physiological differ- ences in this particular from the green plants proper. They are closely related in not a few respects to an important class of vegetable organisms, known as bacteria, to be considered forth- with. Thb BACrTERIA. The bacteria include numberless varieties of organisms of extreme minuteness, many of them visible only by the help of the most powerful lenses. Their size has been estimated at from Tjf „-T to Tiftrir o' ^"^ inch in diameter. They grow mostly in the longitudinal direction, and repro- duce by transverse division, forming spores from which new generations arise. Some of them have vibratile cilia, while the oause of the movements of others is quite unknown. As in many other lowly forms of life, there is a quiescent as well as an active stage. In this stage (zodgloea form) they are surrounded by a gelatinous matter, probably secreted by themselves. Bacteria grow and reproduce in Pasteur's sohition, rendering it opaque, as well as in almost all fluids that abound in pinteid '"*^ LVT».«l«.'WJfJy,i(lJ,, , :: U ORNEIIAI. MOLIO^ le thinned, and ion {zygospore} iloHe coat being ' (exoapon'um), 1 bunt, and a tube ariaea that »e, as before de- lucor the exem- mation of gen- Hfeneration be the original is » yeast, some of mentation, ical student, be- rectly associated in, as, for exam- ly, and the facil- i-sed, explain the media on which Biological differ- troper. They are iportant class of considered forth- of organisms of ly by the help of leen estimated at action, and repro- from which new the cause of the )re is a quiescent ^loea form) they jably secreted by ohttion, rendering Bkbound in pinteid matter. That such fluids readily piitre is uwing t ib« pres- ence of bacteria, the vital action of whx'i ifn(>«ii tn k,.,«k asim- {} t^tococctiN, bear drying, with- out 1n>nt groups of liuc-tttriu havu a Honiewhat different life-hidtury in evident from the fatit that the ]' i>>ten(»of one uheolu the other in the Hanie fluid, and that nikc omive Bwarnm of ditferont kindn may flourish where othon* have ceaaed to live. That thcHe organisnu are enemies of the constituent cells of the tissuuH of the highest mammals has now been abundantly demonstrated. That tliey interfere with the normal working of the organism in a great variety of ways is also clear ; and (wrtain it is that the harm they do leads to aberration in cell- life, however that may be manifested. They rob the tissues of their nutriment and oxygen, and poison them by the products of the decompositions they produce. But apart from this, their very presence as foreign agents must hamper and derange the delicate mechanism of cell-life. These organisms seem to people the air, land, and waters with invisible hosts far more numerous than the forms of life we behold. Fortunately, they are not all dangerous to the higher forms of mammalian life ; but that a large proportion of the diseases which afflict both man and the domestic animals uro directly caused by the presence of such forms of lifp, in the sense of being invariably associated with them, is now beyond doubt I'he facts stated above oxplnin why that should be so ; why certain maladies should be infectious ; bow the germs of dis ease may be transported to a friend wrapped up in the folds of a letter. Disease thus caused, it must not be forgotten, is an illustra- tion of the struggle for existence and the survival of the fittest. If the cells of an organism are mightier than the bacteria, the latter are overwhelmed ; but if the bacteria are too great in numbers or more vigorous, the cells must yield ; the battle may waver — now dangerous disease, now improvement— but in the end the strongest in this, as in other instances, prevail. I 1 '■■a..<,?«-^|L^,a.^«Wf^T"i,, ■ ^>;,VvA;-' ORNKRAIi BIOLOGY. 91 sins tliemBelve*. ar drying, wUh- iiewliat diiTeront l>«t«oiM!eof one i<<;o88iveswarin« huvo ceased to utituout oeUi of been abundantly nonnal working I alno clear ; and )«rrttti()n in cell- tib the tisBues of i by the product* •t from this, their and derange the land, and waters the forms of life [langerous to the i large proportion domestic animals rms of life, in the m, is now beyond liould be so ; why the germs of dis up in the folds of tten, is an illustra- rivid of the fittest, m the bacteria, the A are too great in Id ; the battle may ement— but in the 8, prevail. UmOBLLnLAR AimiALt WITH PPrmRnKTIATIOM OF BTRUOTUIUD. The BRLL-ANiMAtiOUUB (Vorticella). Aincnba is an oxumple of a ono-ef the bell is a lid, attached by a hinge of protoplasm to the body, which may be raised or lowered A wide, funnel-like depres- sion (ceaophagua) leads into the softer substance within which it ends blindly. The outer part of the animal (outicuki) is denser and more transparent than any other part of the whole creature ; next to this is a portion more granular and of inter- mediate transparency between the external and innermost por- tions (cortical layer). Below the disk is a space (contractile vesicle) filled with a thin, clear fluid, which may be Been to en- large slowly, and then to collapse suddenly. When the Vorti- cellA is feeding, these vesicles may contain food-particles, and in the former, apparently, digestion goes on. Such food vacu- oles (v^icles) may circulate up one side of the body of the ani- mal and down the other. Their exact significance is not known, but it would appear as if digestion went on within them ; and possibly the clear fluid with which they are filled may be a spe- cial secretion with solvent action on food. Situated somewhat centraUy is a horseshoe-shaped body, with well-deflined edges, which stains more readily than the rest of the cell, indicating a different chemical composition; and, from the prominent part it takes in the reproductive and other functions of the creature, it may be considered the nudeus (endoplaef). 22 COMPABATIVE PHYSIOLOGY. Multiplication of the species is either by gemmation or by flsaim. In the first case the nucleus divides and the frag- Bto.«. Fia.40. Fio. as. Fia. 80. FisB 84 to 40.— In the figures d denote* dtak ; », lieriBtomo; ve, contractile vacuole; e^. fo^; vacuole; vi, vestibule; rf, contractile fiber; e, cyst; «e, nucleui; «/, cTllum. Pio at.— A group of vortlcelliB ehowlng the crea- ture In various positions (A, 8). Fio 85— The same, In the extended and In the retracted state. (Surface views.) ^.^^„, . Fio. 86.^8howB food- vacuoles; one In the act or Fio'^-A'vorticella, in which the process of mnltlpllcatlon by fission U begun. Fio ffi.-The resulti of fission; ^e production of two Indivldnals of unequal size. Fio. 89. -Illustration of reproduction by conju- Fio.^.— An encysted yorticella. Eio.a6. ments toe transformed into locomotive germs; in the latter the entire animal, including the nucleus, divides longitudi- nally, each half becoming a similar complete, independent oiv ganism. Still another method of reproduction is known. A more or less globular body encircled with a ring of cUiaand of relatively small size may sometimes be seen attached to the usual form of Yorticella, with which it finaUy becomes blended into one mass. This seems to foreshadow the " sexual ^Si^^'-ft^iS?^"''" GENERA [i BIOLOGY. 98 emmation or by 18 and the frag- -If* Bto.V. •^ •*c r».«>. urea d denotes diik ; 0, ictile vacuole; vf, fooo- e: qf, contractile flber; «/, cillnm. icelln showing the crea- ona (A, 8). lie extended and in the rface views.) :aoles; one in the act of 1 which the process of lion is begun, nsaion; the production r unequal size, reprodnctlou by conju- irticella. 'ms; in the latter divides longitudi- «, independent or- ion is known. A a, ring of cilia and seen attached to it finally becomes badow the " sexual conjugation " of higher forms, and is of great biological «ig^ niilcance. Vorticella may pass into an encysted and quiescent stage for an indefinite period and again become active. The history of the Bell-animalcule is substantially that of a vast variety of one-celled organisms known as Infusoria, to which Amoeba itself belongs. It will be observed that the resemblance of this organism to Amoeba is very great; it is, however, introduced here to illustrate an advance in differentiation of structure; and to show how, with the latter, there is usually a physiological advance also, since there is additional functional progress or division of labor; but still the whole of the work is done with- in one cell. Amoeba and Vorticella are both factories in which all of the work is done in one room, but in the latter case the machinery is more complex than in the former; there are cor- respondingly more processes, and each is performed with greater perfection. Thus, food in the case of the Bell-animalcule is swept into the gullet by the currents set up by the multitudes of vibrating ^rms around this opening and its immediate neigh- borhood; the contractile vesicles play a more prominent part; and the waste of undigested food is ejected at a more definite portion of the body, the fioor of the oesophagus; while all the movements of the animal are rhythmical to a degree not exem- plified in such simple forms as Amoeba; not to mention its various resources for multiplication and, therefore, for its perpetuation and permanence as a species. It, too, like all the unicelhilar organisms we have been considering, is susceptible of very wide distribution, being capable of retaining vitality in the dried state, so that these infusoria may be carried in vari- ous directions by winds in the form of microscopic dust. MUIiTIOEZXUUlR OROANISMa The Frhsh-Water Polyps (Hydra viridis; Hydra fuaca). The comparison of an animal so simple in structure, though made up of many cells, as the Polyp, with the more complex organizations with which we shall have especially to deal, may be fitly undertaken at this stage. The Polyps are easily obtain- able from ponds in which they are found attached to various kinds of weeds. To the naked eye, they resemble translucent massoM of jelly with a greenish or reddish tinge. They range in size from one quarter to one half an inch ; are of an elongated GENERAL BIOLOGY. '25 /..--> PfiJ EYo.«. '•«e ■. 41 to 40.— In the flgunw « d«note« ectoderm; fn, endodcnn; <, tentacle; Ayi, hyiH»tome;/,foot; <», te«te«; or, ovary; m, pgendopodlnm; ec\ larger etto „,, Fio. 48.— The leading types of thread-cells, after liberation from the bodv (P, 8). The cells are represented in the active and the resting conditions; in the former all the parts are more distinctly seen in consequence of the necessanr everslon. Fis. 44.— Small (lortion of a transverse section across the body of a green hydra Flo. 45.— A large brown hydra bearing at the same time bads produced asexoally and Fio. 46.— Larger cells of the ectoderm isolated. Note the processes of the cells or Kleinenberg's fibers (F, 8). , . . ,, ...,-„,, All the cuts on pages 9 to 84 have been selected from Howes' Atlas of Biology. cylindrical form; provided at the oral extremity with thread- like tenacles of considerable length, which are slowly moved about in all directions; but they and the entire body may short- en rapidly into a globular mass. They are usually attached at the opposite (aboral) pole to some object, but may float free, or slowly crawl from place to place. It may be observed, under the microscope, that the tenacles now and then embrace some living object, convey it toward an opening (mouth) near their base, from which, from time to time, refuse material is cast out. It may be noticed, too, that a living object within the touch of these tenacles soon loses the power to struggle, which is owing to the peculiar cells (nettle-cells, urticating capsuUs, nemato- cysts) with which they are abundantly provided, and which se- crete a poisonous fluid that paralyzes prey. The mouth leads into a simple cavity (ccelom) in which digestion proceeds. The green color in Hydra viridis, and the red color of Hydra fusca, is owing to the presence of cMorophyl, the function of which is not known. Hydra is structiurally a sac, made up of two layers of cells, an outer (ectoderm) and an inner (endoderm): the tentacles being repetitions of the scructure of the noain body of the animal, and so hollow and composed of two cell layers. Speaking generally, the outer layer is devoted to obtaining information of the surroundings ; the inner to the work of preparing nutriment, and probably, also, discharging waste matters, in which latter assistance is also received from the outer layer. As digestion ti^es place largely within the cells themselves, or is intracellular, we are reminded of Vorticella and still more of Amoeba. There is in Hydra a general advance in development, but not very much individual cell specialization. That of the urticating capsules is one of the best examples of such specialization in this creature. tmT~yi:>r '.?3?v-aNBIDBRBD. Having now studied certain one-celled plants and animals, and some very simple combinations of cells (molds, etc.), it will be profitable to endeavor to generalize the lessons these humble organisms convey ; for, as will be constantly seen in the study of the higher forms of life of which tliis work proposes to treat principally, the same laws operate as in the lowliest living creat- ures. The most complex organism is made up of tissues, which are but cells and their products, as houses are made of bricks, mortar, wood, and a few other materials, however large or elab- orate. The student of physiology who proceeds scientifically must endeavor, in investigating the functions of each organ, to learn the exact behavior of each cell as determined by its own inherent tendencies, and modified by the action of neighboring cells. The reason why the function of one organ differs from that of another is that its cells have departed in a special direction from those properties common to all cells, or have become function- ally differentiated. But such a statement has no meaning un- less it be well understood that cells have certai|i properties in common. This is one of the lessons imparted by the preceding studies which we now review. Briefly stated in language now extensively used in works on biology, the common properties of cells (protoplasm), whether animal or vegetable, whether consti- tuting in themselves entire animals or plants, or forming the elements of tissues, are these : The collective chemical processes associated with the vital activities of cells are termed its metab- 6li8m. Metabolism is constructive when more complex com- pounds are formed from simple ones, as when the Protococcus- oell builds up its protoplasm out of the simple materials, found in rain-water, which makes up its food. Metabolism is destructive when the reverse process takes place. The results of this process are eliminated as excreta, or useless and harmful products. 28 COMPARATIVE PHYSIOLOGY. Since all the vital activities of cells can only be manifested when supplied with food, it follows that living organisms convert po- tential or possible energy into kinetic or actual energy. When lifeless, immobile matter is taken in as food and, as a result, is converted by a process otcusimilation into the protoplasm of the cell using it, we have an example of potential being converteyjijjJSj>MtBafe"-aj«V*-i'W. -.w;'-^-'* : L '* * GENERAL BIOLOGY. 99 ids of proto- lers in great t be too well i, neither by h theantion no sense dif- e relation of Mtic,ahaorp- "odttctive. ^mental prop- yed that cells .Uy. No two d there be no lality. Everj'^ Emism, difPem a certain de- ation leading; ' with advan* ip of a single > work; or, if as compared lings in such Lized conunu- !h individual tries to perform every office for himself, ho is at once carpenter, blacksmith, shoemaker, and much more, with the natural ro- sult that he is not efficient in any one direction. A community may be judged in regard to its degree of advancement by the amount of division of labor existing within it. Thus is it with the animal body. We find in such a creature as the fresh-water Hydra, consisting of two layers of cells forming a simple sac, a slight amount of advancement on Amoeba. Its extemaF surface no longer serves for inolosure of food, but it has the simplest form of mouth and tentacles. Each cf the cells of the internal layer seems to act as a somewhat improved or specialized Amoe- ba, while in those of the outer layer we mark a beginning of those functions which taken collectively give the higher ani- mals information of the surrounding world. Looking to the existing state of things in the universe, it is pUun that an animal to attain to high ends must have powers of rapid locomotion, capacity to perceive what makes for its in- terest, and ability to utilize means to obtain this when perceived. These considerations demand that an animal high in the scale of being should be provided with limbs sufficiently rigid to sup- jiort its weight, moved 1^ strong muscles, which must act in harmony. But this implies abundance of nutriment duly pre- pared and regularly conveyed to the bones and muscles. All this would be useless imless there was a controlling and ener- gizing systemt capable both of being impressed and originating impressions. Such is found in the nerves and nerve-centers. Again, in order that this mechanism be kept in good running order, the waste of its own metabolism, which chokes and poi- sons, must be got rid of— hence the need of excretory apparatus. In order that the nervous system may get sufficient informa- tion of the world around, the surface of the body must be pro- vided with special message-receiving offices in the form of modified nerve-endings. In short, it is seen that an animal as high in the scale as a mammal must have muscular, osseous (and connective), digestive, circulatory, excretory, and nervous tissues; and to these may be added certain forms of protective tissues, as hair, nails, etc. Assuming that the student has at least some general knowl- edge of the structure of these various tissues, we propose to tell in a simple way the whole physiological story in brief. The blood is the source of all the nourishment of the organ- ism, including its oxygen supply, and is carried to every part of COMPARATIVE PHYSIOLOGY. the body ihrousfh eUwtio tubes which, continiially branching and becomini; gradually smaller, terminate in yesselH of hair- like finoneew in which the current is very slow — a condition per- mitting that interchange between the cells surrounding them and the blood which may be compared to a process of barter, the cells taking nutriment and oxygen, and giving (excreting) in return carbonic anhydride. From these minute vessels' thn blood is conveyed back toward the source whence it came by similar elastic tubes which gradually increase in size and be- come fewer. The force which directly propels the blood in its onward course is a muscular pump, with both a forcing and suction action, though chiefly the former. The flow of blood is maintained constant owing to the resistance in the smaller tubes on the one hand and the elastic recoil of the larger tubes on the other ; while in the returning vessels the column of blood is supported by elastic double gates which so close as to prevent reflux. The oxygen of the blood is carried in disks of microscopic size which give it up in proportion to the needs of the tissues past which they ore carried. But in reality the tissues of the body are not nourished directly by the blood, but by a fluid derived from it and resem- bling it greatly in most particulars. This fluid bathes the tis- sue-cells on all sides. It also is taken up by tubes that convey it into the blood after it has passed through little factories (lymphatic glands), in which it undergoes a regeneration. Since the tissues are impoverishing the blood by withdrawal of its constituents, and adding to it what is no longer useful, and is in reality poisonous, it becomes necessary that new material be added to it and the injiuious components withdrawn. The former is accomplished by the absorption of the products of food digestion, and the addition of a fresh supply of oxygen derived from without, while the poisonous ingredients that have foimd their way into the blood are got rid of through processes that may be, in general, compared to those of a sew- age system of a very elaborate character. To explain this re- generation of the blood in somewhat more detail, we must first consider the fate of food from the time it enters the viouth till it leaves the tract of the body in which its preparation is car-, ried on. The food L> in the mouth submitted to the action of a series of cutting and grinding organs worked by powerful muscles ; mixed with a fluid which changes the starchy part of it into t MRNKRAL BIOLOGY. 81 |y branching iIh of hair- [ondition per- nding them of barter, |fir (excreting) ▼easels' the it came by size and be- blood in its forcing and low of blood the smaller larger tubes e column of Bo close as to ed in disks of > the needs of lot nourished it and resem- laihes the tis- I that convey ittle factories regeneration, dthdrawal of er useful, and new material idrawn. The B products of ly of oxygen redlents that i of through lose of a sew- plain this re- we must first ^e xuouth till .ration is car- m of a series f ul muscles ; irt of it into sugar, and prepares the whole to pass further on its course : when this has been accomplished, the food is grasped and squeezed and pushed along the tube, owing to the action of its own muscular cells, into a, sac (stomach), in which it is rolled about and mixed with certain fluids of peculiar chemical com- position derived from cells on its inner siu^ace, which trans- form the proteid part of the food into a form susceptible of ready use (absorption). When this saccalar organ has done its share of the work, the food is moved on by the action of the muscles of its walls into a very long portion of the tract in which, in addition to processes carried on in the mouth and stomach, there are others which transform the food into a con- dition in which it can pass into the blood. Thus, all of the food that is susceptible of changes of the kind described is acted upon somewhere in the long tract devoted to this task. But there is usually a remnant of indigestible material which is finally evacuated. How is the prepared materitd conveyed into the blood ? In part, directly through the walls of the minutest blood-vessels distributed throughout the length of this tube ; and in part through special vessels with appropriate cells cov- ering them which act as minute porters {villi). The impure blood is carried periodically to an extensive sur- face, luiually much folded, and there exposed in the hair-like tubes referred to before, and thus parts with its excess of car- bon dioxide and takes up fresh oxygen. But all the functions described do not go on in a fixed and invariable manner, but are modified somewhat according to circumstances. The for- cing-pump of the circulatory system does not always beat equally fast ; the smaller blood-vessels are not always of the ,same size, but admit more or less blood to an organ according to its needs. This is all accomplished in obedience to the commands car- ried from the brain and spinal cord along the nerves. All movements of the limbs and other parts are executed in obe- dience to its behests; and in order that these may be in accord- ance with the best interests of each particular organ and the whole animal, the nervous centers, which may be compared to the chief officers of, say, a telegraph or railway system, are in constant receipt of information by messages carried onward along the nerves. The command issuing is always related to the information arriving. All those parts commonly known as sense-organs— the eye, 9ftfe".S-.!8Sa8-':. H'i COMPARATIVE PFIYSIOLOOY. niir, noiio, tonffue, and the entire niirfatn) of the btxly — are faith- ful reportera of fact«. They put the inner and outer worlds in conuiiunicutinn, and without thvni all lii^rher life at leant munt fflttMe, for the oriyraniam, like a train directed by a conductor that disregards the danf^r-sifrn&ls. muRt work itn own destruction. Without laroing into further details, sufHce it t«i say that the pni- cesaes of the various cells are subordinated to the general good through the nervous system, and that susceptibility of proto- pkisni to stimuli of a delicate kind which enables each cell to adapt to its surroundings, including the inHuence of remote as well as neighboring cells. Without this there could be no marked advance in organisms, no differentiation of n pro- nounced character, and so none of that physiological division of labor which will be inferred from our brief description of the functions of a mammal. The whole of physiology but illustrates this division of labor. It is hoped that the above account of the working of the ani- mal body, brief as it is, may serve to show the connection of one part functionally with another, for it is much more impor- tant that this should be kept in mind throughout, than that all the details of any one function should be known. * till UVIMO AMD LIFIILBSB BCATTBR. In order to enable the student the beti' i* to realize the na- ture of living matter or protoplasm, and to render clearer the distinction between the forms that belong to the organic and inorganic worlds respectively, we shall make some comparisons in detail which it is hoped may accomplish this object. A modem watch that keeps correct time must be regarded as a wonderful object, a marvelous triumph of human skill. Tltat it has aroused the awe of fiavag4)s, and been mistaken for a living being, is not surprising. But, admirable as is the result attained by the mechanism of a watch, it is, after all, composed of but a few metals, etc., chiefly in fact of two, brass and steel ; these are, however, made up into a great number of different parts, so adapted to one another as to work in unison and ac- complish the desired object of indicating the time of day. Now, however well constructed the watch may be, there are waste, wear and tear, which will manifest themselves more and more, until finally the machine becomes worthless for the pur- pose of its construction. If this mechanism possessed the power i i GRNRRAL BIOLOOT. 88 ily— are fnith- itor worlds in at least munt conductor that Ti deatruction. y that the pro- general giood ility of proto- « each cell to of remote on could be no on of u pro- )gical division description of hysiology but ing of the ani- connection of I more impor- \ than that all m. realiie the na- [er clearer the e organic and le oompariflons bject. 8t be regarded human skill, mistaken for a IS is the result ' all, composed "ass and steel ; erof different mison and ac- I of day. y be, there are Ives more and ss for the pur- ised the power of adapting from without foreign matter so as to construct it it into steel and brass, and arrange this just when required, it .would imitate a living organism ; but this it can not do, nor is Its waste chemically diffareht from its component metals ; it does not break up brass and steel into something wholly differ- ent. In one particular it does closely resemble living things, in that it gradually deteriorates ; but the degradation of a liv- ing cell is the consequence of an actual change in its compo- nent parts, commonly a fatty degeneration. The one is a real transformation, the other mere wear. Had the watch the power to give rise to a new one like itself by any process, especially a process of division of itself into two parts, we should have a parallel with living forms ; but the watch can not even renew its own parts, much less give rise to a second mechanism like itself. Here, then, is a manifest dis- tinction between living and inanimate things. Suppose, further, that the watch was so constructed that, after the lapse of a certain time, it underwent a change in its inner machinery and perhaps its outer form, so as to be scarcely recognisable as the same ; and that as a result, instead of indi- cating the hours and minutes of a time-reckoning adapted to the inhabitant* of our globe, it indicated time in a wholly dif- ferent way ; that after a series of such transformations it fell to pieces—took the original form of the metals from which it was constructed — we should then have in this succession of events a parallel with the development, decline, and death of living or- ganisms. In another particular o\a illustration of a watch may serve a useful purpose. Suppose a watch to exist, the works of which are so concealed as to be quite inaccessible to our vision, so that all we know of it is that it has a mechanism which when in action we can hear, and the result of which we perceive in the movements of the hands on the face ; we should then be in the exact position in reference to the watch that we now are as re- gards the molecular movements of protoplasm. On the latter the entire behavior of living matter depends ( yet it is abso- lutely hidden from us. We know, too, that variations must be produced in the mechanism of time-pieces by temperature, moisture, and other influences of the environment, resulting in altered action. The same, as will be shown in later chapters, occurs in protoplasm. This, too, is primarily a molecular effect If the works of 8 7' 'M 84 COMPARATIVE PIIV8I0L00Y. waUshwi were beyond obMnmUon, we should not be able to atat* exactly how the variationa obaerred in different kindii, or even different individual* of tho lame kind occurred, though thew differenoea might be of the moat marked character, luch oa any one could recogniie. Here once niorr* we refer the differ- ences to the mechaniani. 80 ii it with living beings : the ulti- nwte molecular mechanism is unknown to us. Could we but render these molecular movementa risible to our eyes, we should have a revelation of far greater scientific importance than that unfolded by the recent researches into those living forms of extreme minuteness that swarm every- where as dust in a sunbeam, and, as will be learned later, are often the source of deadly disease. Like the movements of the watch, the activities of protoplasm are ceaseless. A watch that will not run is, as such, worthless — it is mere metal — has under- gone an immense degradation in the scale of values ; ho proto- plasm is no longer protoplasm when its peculiar molecular movements cease ; it u at once degraded to the rank of dead matter. The student may observe that each of the four propositions, embodying the fundamental properties of living matter, stated in the preceding chapter, have been illtistrated by the simile of a watch. Such an illustration is necessarily crude, but it helps one to realize the meaning of truths which gather force with each living form studied if regarded aright ; and it is upon the realization of truth that mental growth ai well as practical efficiency depends. OZJUWIFIOATIOM OP THB AMIMAL XZNODOIC There are human beings so low in the scale as not to possess such general terms as tree, while tliey do employ names for dif- ferent kinds of trees. The use of such a word as " tree " im- plies generalization, or the abstraction of a set of qualities from the things in which they reside, and making them the basis for the grouping of a multitude of objects by which we are sur- rounded. Manifestly without such a process knowledge must be very limited, and the world without significance ; while in proportion as generalization may be safely widened, is our progress in the unifteation of knowledge toward which science is tending. But it also follows that without complete knowl- edge there can be no perfect classiflcation of objects ; henoey mm OENRRAri BIOLOOY. t be able to itate i kintlii, or even [|, thouifh theM ter, Buoh as any Dfer tbo differ- einga : the ultl- nenta riaible to reater aoientiflo reaearohea into it Bworm every- arned later, are jvementa of the A watch that )tal— has under- iluea ; ao proto- uliar molecular e nuik of dead ur propositions, g matter, stated i>y the simile of ide, but it helps kthor force with id it is upon the ell as practical [IN02X>M. IS not to possess Y names for dif- i as " tree " im- f qualities from 9m the basis for ich we are suiv :nowledge must ance ; while in widened, is our d which science omplete Icnowl- objects ; hence, any classiflcation must be regarded but as the temporary creed of wi«^nco, to be miKUflwl with the extensit)n of knowlMlge. As a nmtter of fact this has been the history of nil wiOlojfical and other systems of arrangement. The only purpose of grouping is to simplify and extend knowledge ; this being the case, it fol- lows that a method of grouping that acr€)mpIisheB this has value, though the system may be artificial that is based on resemblances which, though real and constuut, are associated with differences so numerous and radical that the total amount of likeness between objects thus grouped is often less than the difference. Such a system was that of Liniueus, who classified plants according to the number of stamens, etc., they bore. Seeing that animals which resemble each other are of com- mon descent from some earlier form, to establish the line of de- scent is to determine in great part the classification. Much as- sistance in this direction is derived from embryology, or the history of the development of the individual (ontogeny) ; so that it may be said that the ontogeny indicates, though it does not actually determine, the line of descent (phytogeny) ; and it is owing to the importance of this truth that naturalists have in recent years given so much attention to comparative embry- ology. It will be infoiTed that a natural system of classification must be based both on function and structure, though chiefly on the latter, since organs of very different origin may have a similar function ; or, to express this otherwise, homologoua structures may not be analogout ; and homology gives the better basis for classification. To illustrate, the wing of a bat and a bird are both homologous and analogous ; the wing of a Imtterfiy is analogous but not homologous with these ; manifestly, to clas- sify batH and birds together would be better than to put birds and insects in the same group, thus leaving other points of re- lationship out of consideration. The broadest possible division of the animal kingdom is into groups, including respectively one-celled and many-celled forms — i. e., into Protozoa and Metazoa. As the wider the grouping the less are differences considered, it follows that the more sub- divided the groups the more complete is the information con- veyed ; thus, to say that a dog is a metazoan is to convey a cer- tain amount of information ; that he is a vertebrate, more ; that he is a mammal, a good deal more, because each of the latter terms includes the former. 86 COMPARATIVE PHYSIOLOGY. Animal Kingdom. Invertc- brata. Vertebrata. ■ Protozoa (amoeba, vortioella, etc.). Coelenterata (spongea, jelly-flab, polyps, etc.). Eohinodermata (star-nsb, aea-uronioB, etc.)- Vermes (worms). Arthropods (orabs, insects, spiders, etc.). MolluHca (oysters, snails, etc. ). Molluscoidea (moss-like animals). Tunicata (ascidians). Pisces (flshes). Amphibia (frogs, menobranchus, etc.). Beptilia (snakes, turtles, etc.). | Aves (birds). Manunalia (domestic quadrupeds, etc.). The above classification (of Claus) is, like all such arrange- ments, but the expression of one out of many methods of view- ing the animal kingdom. For the deti«il8 of classification and for the grounds of that we have presented, we refer the student to works on zodlogy ; but we advise those who are not familiar with this subject, when a technical term is used, to think of that animal belong- ing to the group in question with the structure of which they are best acquainted. Man's Piaoe m the Animal Sinodom. It is no longer the custom with zoologists to place man in an entirely separate group by himself ; but Le is classed with the primates, among which are also grouped the anthropoid apes (gorilla, chimpanzee, orang, and the gibbon), the monkeys of the Old and of the New World, and the lemurs. So great is the structural resemblance of man and the other primates that competent authorities declare that there is more difference be- tween the structure of the most widely separated members of the group than between certain of the anthropid apes and man. The points of greatest resemblance between man and the anthropoid apes are the following : The same number of verte- brae ; the same general shape of the pelvis ; a brain distinguish- ing them from other mammals ; and posture, being bipeds. The distinctive characters are size, rather than form of the brain, that of man being more than twice as large ; a relatively larger cranial base, by which, together with the greater size of the jaws, the face becomes prominent ; the earlier closure of the sutiues of the cranium, arresting the growth of the brain ; more developed canine teeth and difference in the order of erup- tion of the permanent teeth ; the more posterior position of the foramen magnum ; the relative length of the limbs to each mmm GENERAL BIOLOOY. 87 , etc.). flsh, polype, etc.). a-urchinB, eto.)- spiders, etc.)- B.). mala). chus, eto.). I.). ipeda, etc.). Q such arrange- aethods of view- grounds of that irks on zodlogy ; ith this subject, animal belong- e of which they placeman in an classed with the anthropoid apes the monkeys of urs. So great is ler primates that re difference be- lted members of d apes and man. m man and the aumber of verte- irain distinguish- eing bipeds, than form of the rge ; a relatively le greater size of larlier closure of th of the brain ; he order of erup- ir position of the e limbs to each other and the rest of the body ; minor differences in the hands and feet, especially the greater f i-eedom and power of apposition of the great-toe. But the greatest distinctioti between man and even his closest allies among the apes is to be found in the development to an incomparably higher degree of his intellectual and moral na- ture, corresponding to the differences in weight and structure of the human brain, and associated with the use of spoken and written language ; so that the experience of previous genem- tions is not only registered in the organism (heredity), but in the readily available form of books, etc. The greatest structural difference between the race of men are referable to the cranium ; but, since they all interbreed freely, they are to be considered varieties of one species. TBB LAW OF PBBIODZOXTT OR BHTTBM IN NATORB. The term rhythm to most minds suggests music, poetry, or dancing, in all of which it forms an essential part so simple, pronounced, and uncomplicated as to be recognized by all with ease. The r^^ular division of music into bars, the recurrence of chords of lie same notes at certain intervals, of forte and piano, seeir. tf> be demanded by the very nature of the human mind. The k-une applies to poetry. Even a child that can not under- stand ihe language used, or an adult listening to recitations in an imknown tongue, enjoys the flow and recurrences of the sounds. Dancing has in aU ages met a want in human organi- zations, which is partly supplied in quieter moods by the regu- larity of the steps in walking and similar simple movements. But as rhythm runs through all the movements of animals, so is it also found in all literature and all art. Infinite variety wearies the mind, hence the fatigue felt by the sight-seer. Be- currence permits of repose, and gratifies an established taste or appetite. The mind delights in what it has once enjoyed, in repetition within limits. Repetition with variety is manifestly a condition of the growth and development of the mind. This seems to apply equally to the body, for every single function of each organism, however simple or complex it may be, exempli- fles this law of periodicity. The heart's action is rhythmical Q)&xta) ; the blood flows in intermitting gushes from the central pump ; the to-and-fro movements of respiration are so regular 88 COMPARATIVE PHYSIOLOGY. that their cessation would arouse the attention of the least in- structed ; food is demanded at regfular intervals ; the juices of the digestive tract are poured out, not constantly but period- ically ; the movements by which the food is urged along its path are markedly rhythmic ; the chemical processes of the body wax and wane like the fires in a furnace, giving rise to regular augmentations of the temperature of the body at fixed hours of the day, with corresponding periods of greatest bodily activity and the reverse. This principle finds perfect illustration in the nervous sys- tem. The respiratory act of the higher animals is effected through muscular movements dependent on regular waves of excitation reaching them along the nerves from the central cells which regularly discharge their forces along these channels. Were not the movements of the body periodic or rhythmical, instead of that harmony which now prevails, every muscular act would be a convulsion, though even in the movements of the latter there is a highly compounded rhythm, as a noise is made up of a variety of musical notes. The senses are subject to the same law. The eye ceases to see and the ear to hear and the hand to feel if continuously stimulated; and doubtless in all art this law is unconacwusly rc^. aized. That ceases to be art which fails to provide for th«: nj'. ' il repose and excita- tion of the senses. The eye will no- .,. a ; continuously one color, the ear the same sound. Why .eeze on a warm day so refreshing ? The answer is obvious. Looking to the world of animate nature as a whole, it is noticed that plants have their period of sprouting, flowering, seeding, and decline; animals are bom, pass through various stages to maturity, diminish in vigor, and die. These events make epochs in the life-history of each species; the recurrence of which is so constant that the agricultural and other arrange- ments even of savages are planned accordingly. The* vhe in- dividuals of each animal group have a definite period of dura- tion is ariothsr manifestation of the same law. Superficial observation suffices to furnish facts which show that the same law of periodicity is being constantly exemplified in the world of inanimate things. The regular ebb and flow of the tides; the rise and subsidence of rivers; the storm and the calm; summer and winter; day and night— are all recurrent, none constent. Elvents apparently without any regularity, utterly beyond ' -wft l Df the law of r recognition 7 ; it will, ac- « chapters of TBB I be the logical the universe i complex and ore homogene- nized or living B two views of roup of plants "creation "is GENERAL BIOLOGY. 48 simply meant that they came into being in a manner we know not how, in obedience to the will of a First Cause. The other view is denominated the theory of descent with modification, the theory of transmutation, organic evolution, etc., which teaches that all the various forms of life have been derived from one or a few primordial forms in harmony with the recog- nized principles of heredity and vambility. The most widely known and most favorably received exposition of this theory is that of Oharles Darwin, so that his views will be first presented in the form of a hypothetical case. Assume that one of a group of living forms varies from its fellows in some particular, and 1 M ..ag with another that has similarly varied, leaves progeny i aeriting this characteristic of the parents, that tends to be still further increased and rendered permanent by successive pairing Mrith forms possessing this variation in Hhape, color, or whatever it may be. We may suppose that the variations may be numerous, but are always small at the beginning. Since all animals and plants tend to multiply faster than the means of support, a competition for the means of subsistence arises, in which struggle the fittest, as judged by the circumstances, al- ways is the most successful.; and if one must perish outright, it is the less fit. If any variation arises that is unfavorable in this contest, it will render the possessor a weaker competitor : hence it follows that only useful variations are preserved. The struggle for exii^tence is, however, not alone for food, but for anything which may be an advantage to its possessor. One form of the contest is that which results from the rivalry of members of the same sex for the possession of the females ; and as the female chooses the strongest, most beautiful, most active, or the supreme in some respect, it follows that the best leave the great- est number of progeny. This has been termed sexual selection. In determining what forms shall survive, the presence of other plants or animals is quite as important as the abundance of food and the physical conditions, often more so. To illustrate this by an example : Certain kinds of clover are fertilized by the visits of the bumble-bee alone ; the numbers of bees exist- ing at any one place depends on the abundance of the field-mice which destroy the nests of these insects ; the numbers of mice will depend on the abundance of creatures that prey on the mice, as hawks and owls ; these, again, on the creatures that specially destroy them, as foxes, etc. ; and so on, the chain of connections becoming more and more lengthy. ^ J 44 COMPARATIVK PHYSIOLOGY. • l»o« (H), c«lf (O. wSbU (H), Md •man (M). The condlUon* ofthe thfMdiaM^ •",* ^Jf? "' deretoprnwit, which the three craw-iow* (I, n, IH) lepieMnt. are ■dec^ to correqwDd u ezaetlr u powlble. The flnt, 'or nppir oSSmow^. renjMenU • yery ewhr eti^e, with gill-openlugi, and witbirat limbi. The second (middle) cnMa-row, 11, ahowe a aomewhat later ataKe. with the Irat mdlmenta of limba, while the gttl-openincs are yet retained. The third (lowest) cross-row, HI. shows a still later stage, with the limbs more developed and the glll-openioga GENERAL BIOLOGY. 45 mdincitagMt of t the thne difluh II) tepKMnt, are •per croH-iow, I, lb*. TTw Mcond Int radlBMnt* of It) croM-row, in, the gill-openings iMt. The memhranm and Appondagce of tho cmliryr.nic body (the •mninn, rclk- Mc, allantoli) are omitted. The whole twolvn ligntvn are Klluhtly magnlfleil, the iipiKT ones mora then tho lower. To facilitate tho comparlion, thcv are all re- diiced to iivnrly thv name ilie in the cnti. All tho umbryoe are leen from tho loft aide ; tho hoaa extronlty la above, the tall extremity below ; the arched back turned to the riuht The lettera Indicate the fame parte In all the twelve flsure*, namely: v, fore-brain; (, twixt-brain; m, mld-brain; A, hInd-braIn; n, after-ornin; r, aplnal marrow: e, noae; f species have id. e former ezist- gh all the in- >t been found, the nature of ireserved ; and rom mud that Dne with hard while if these ould, owing to itamorphosed), earliest forms 2rved at all, be in consequence ve undergone. ! in the earth's ) it is explored, organic evolu- oups constitut- nks. als and plants GENERAL DIOLOOY. 4T now peopling the earth were entirely different from those that llouriHhed ii» the past, the objections U» the doctrine of descent would be greatly Bt.-«mgthened; but when it is found that there is in some cases :i scarcely broken succession of forms, great force is added to the argument* by which we are led to infer tho connection of all forms with one another. To illustrate by a single instance: the existing group of horses, with a single toe to each foot, was preceded in geological Vin M — nnnM of the feet of the dlkerent K«'ncr» of Egvida (after Manh). a. Uot "»• 1" — '!':"™ "' J"*!*^: i ."_. „# j_-*ini..j„m (l^w«r Hlocenet: c. foot of Hilh r the dlSerent K«'ncr» of Efuiaa («rier jtannj. a, ion 'of 0»wA*p»-M (Koccne ); 6, foot of AncMihMi>m(lA>^er Miocene); e, foot of mp- paritm (Pliocene); d, fool of the recent genu* Equui. time in America by forms with a greater number of toes, the latter increasing according to the antiquity of the group. These forms occur in succeeding geological formations. It is impossible to resist the conclusion that they are related gene- alogically (phylogenetically). 8. Piogrwwlail.— Inasmuch as any form of specialiBation that would give an animal or plant an advantage in the struggle for existence would be preserved, and as in most cases when the competing forms are numerous such would be the case, it is possible to understand how the organisms that have appeared have tended, on the whole, toward a most pronounced pro- gression in the scale of existence. This is well illustrated in the history of civilization. Barbarous tribes give way before civiliied man with the numberless subdivisions of labor he in- stitutes in the social organism. It enables greater numbers to flourish, as the competition is not so keen as if activities could be exercised in a few directions only. 9. DomestiiOSted Animftll.— Darwin studied our domestic ani- mals long and carefully, and drew many important conclusions 48 COMPARATIVK PHYSIOLOGY. 1^:1 from his rcMwrchni. He waa uoiiviticed that they had all limn derived from a few wild repreaentativea, in accordance with the principle* of natural lelection. Breedera have lioth conicioiialy and unoonacioualy, formed race* of animaU from Htoclu which the new groups have now Rupplanted ; while primitive man had tamed various speciea which hu kept for food and t4) otwiat in the chase, or as beasts of burden. It is impossible to believe that all the different races of dogs have originated from dis- tinct wild stocks, for nuiny of them have been formed within recent periods; in fact, it is likely that to the jackal, wolf, and fox, must we look for the wild progenitors of our dogs. Dar- win concluded that, as man had only utilized the materials Nature provided in forming his races of domestic animals, he had availed himself of the variations that arose spontaneously, and increased and fixed them by breeding those possessing the same variation together, so the like had occurred without his aid in nature among wild forms. Evolutionists are divided as to the origin of man himself ; some, like Wallace, who are in accord witlt Darwin as to the n \A M Fia, 46.— Skeleton of hand or fore-foot of ilz mammali. I. man; IT, doK; ITI, pig; IV. ox; V, tapir; VI, hone, r, ladlna; u, nina: a, icaphold; b, leinl-lunar; e, triqaetmm (cnnciform); rf, trapeilniii: «, tmpeaold; f, capitatum (unciform pro- o«*a); g, hamatiim (unciform bone); p, pialform; 1| thumb; 8, digit; 8, miadle flnger; 4, rlng-flnger; B, little flnger. (AfMr Gogenbaur.) origin of living forms in general, believe that the theory of natural selection does not suffice to account for the intellectual ey had all been nknce with the nth conm-iously n Htoclu which priiniiive man Oil and t4) ombt Mible to believe lutod from dla- formod within tckal, wolf, and lur dogs. Dar- 1 the ninterials itio aniniab, he ■pontaneouflly, B poMMMing the red without hia ' man himself ; irwin ao to the in; IT, doft; ITT, pig: lid; 6, teml-lunar; c, ■turn (nnclfonn pro- i; », digit; 8, middle t the theory of the intellectual (IKNKUAI. niOLOOY. 49 1' 1 1, , If . ^ 'i i i j'lj j,( ^ i'l 1 1 I'M 1 ii lii i 1 1 1 ' i If i !{|| s 1 1 1 ' il ■ III ■ 1 1 1 1 i , - i 1 "^ 1 II 1 ^ 50 COMPARATIVE PHYSIOLOGY. and moral nature of man. Wallace believes that man's body has been derived from lower forms, but that his higher nature is the result of some unknown law of accelerated development; while Darwin, and those of his way of thinking, consider that mau in his entire nature is but a grand development of powers existing in minor degree in the animals below him in the scale. Bnmmary. — Every group of animals and plants tends to in- crease in numbers in a geometrical progression, and must, if unchecked, overrun the earth. Every variety of animals and rlants imparts to its offspring a general resemblance to itself, but with minute variations from the original. The variations of offsprings may be in any direction, and by accumulation constitute fixed differences by which a new group is marked off. In the determination of the variations that persist, the law of survival of the fittest operates. Eit man's body higher nature , development ; , consider that aent of powers m in the scale, its tends to in- i, and must, if f animals and lance to itself, The variations accumulation oup is marked persist, the law REPRODUCTION. As has been already noticed, protoplasm, in whatever form, after passing through certain stages in development, undergoes a decline, and finally dies and joins the world of unorganized matter ; so that the permanence of living things demands the constant formation of new individuals. Groups of animals and plants from time to time become extinct; but the lifetime of the species is always long compared with that of the indi- vidual. Reproduction by division seems to arise from an exi- gency of a nutritive kind, best exemplified in the simpler or- ganisms. When the total mass becomes too great to be supported by absorption of pabulum from without by the surface of the body, division of the organism must take place, or death ensues. It appears to be a matter of indifference how this is accom- plished, whether by fission, endogenous division, or gemmation, so long as separate portions of protoplasm result, capable of leading an independent existence. The very undifferentiated character of these simple forms prepares us to understand how each fragment may go through the same cycle of changes as the parent form. In such cases, speaking generally, a million individuals tell the same biological story as one ; yet these must exist as individuals, if at all, and not in one great united mass. But in the case of conjugation, which takes place some- times in the same groups as also multiply by division in its various forms, there is plainly an entirely new aspect of the case presented. We have already shown that no two cells, how- ever much alike they may seem as regards form and the cir- cumstances under which Uiey exist, can have, in the nature of the case, precisely the same history, or be the subjects of ex- actly the same experiences. We have also pointed out that aU these phenomena of cell-life are known to us only as adapta- tions of internal to external conditions; for, though we may not be always able to trace this connection, the inference is justi- 52 COMPARATIVE PHYSIOLOGY. flable, because there are no facts known to us that contradict such an assumption, while those that are within our knovledgf, bear out the generalization. We have already learned Utui ' iv- ing things are in a state of constant change, as indeed -a-e all things ; we have observed a constant relation between certain changes in the environment, or sum total of the surrounding conditions, as, for example, temperature, and the behavior of the protoplasm of plants and animals; so that we must believe that any one form of protoplasm, however like another it may seem to our comparatively imperfect observation, is diiferent in some i-espects from every other— as different, relatively, as two human beings living in the same community during the whole of their lives ; and in many cases as unlike as individuals of very different nationality and history. We are aware that when two such persons meet, provided the unlikeness is not so great as to prevent social intercourse, intercommunication may prove very instructive. Indeed, the latter grows out of the former; our illustration is itself explained by the law we aro endeavoring to make plain. It would appear, then, that con- tinuous division of protoplasm without external aid is not pos- sible; but that the vigor necessary for this must in some way be imparted by a particle (cell) of similar, yet not wholly like, protoplasm. This seems to furnish an explanation of the neces- sity for the conjugation of living forms, and the differentiation of sex. Very frequently conjugation in the lowest animals and plants is followed by long periods when division is the prevail- ing method of reproduction. It Js worthy of note, too, that when living forms conjugate, they both become quiescent for a longer or shorter time. It is as though a period of preparation preceded one of extraordinary activity. We can at present trace only a few of the steps in this rejuvenation of life-stuff. Some of these have been already indicated, which, with others, will now be further studied in this division of our subject, both because reproduction throws so much light on cell-life, and be- cause it is so important for the understanding of the physio- logical behavior of tissues and organs. It may be said to be quite as important that the ancestral history of the cells of an organism be known as the history of the units composing a community. A, B, and C, can be much better understood if we know something alike of the history of their race, their an- cestors, and their own past; so is it with the study of any indi- vidual animal, or group of animals or plants. Accordingly, REPRODUCTION. 58 lat contradict r knov'ledgfi imed tiibi 'iv- indeed ^-e a]l ween certain surrounding |e behavior of must believe lother it may is different , relatively, as ty during the as individuals re aware that mess is not so iinication may ws out of the le law we are hen, that con- aid is not pos- in some way >t wholly like, m of the neces- differentiation st animals and 1 is the prevail- note, too, that quiescent for a of preparation can at present n of life-stuff, h, with others, \r subject, both )ll-life, and be- of the physio- ' be said to be the cells of an s composing a understood if race, their an- ly of any indi- AccordingJy, embryology, or the history of the origin and development of tissues and organs, will occupy a prominent place in the vari- ous chapters of this work. The student will, therefore, at the outset be furnished with a general account of the subject, while many details and applications, of principles will be left for the chapters that treat of the functions of the various organs of animals. The more knowledge the student possesses of zoology the better, while this science will appear in a new light under the study of embryology. Animals are divisible, according to general structure, into Protozoa, or unicellular animals, and Metazoa, or multicellular forms — that is, animals composed of cell aggregates, tissues, or organs. Among the latter one form of reproduction appears for the first time in the animal kingdom, and becomes all but universal, though it is not the exclusive method ; for, as seen in Hydra, both this form of generation and the more primitive gemmation occur. It is known as sexual multiplication, which usually, though not invariably, involves conjugation of two un- like cells which may arise in the same or different individuals. That these cells, known as the male and female elements, the ovum and the spermatozoon, are not necessarily radically dif- ferent, is clear from the fact that they may arise in the one in- dividual from the same tissue and be mingled together. These cells, however, like all others, tell a story of continual progress- ive differentiation corresponding to the advancing evolution of higher from lower forms. Thus hermaphroditiam, or the coex- istence of organs for the production of male and of female cells in the same individual, is confined to invertebrates, among which it is rather the exception than the rule. Moreover, in such hermaphrodite forms the union of cells with greater differ- ence in experiences is provided for by the union of different in- dividuals, so that commonly the male cell of one individual unites with (fertilizes) the female cell of a different individual. It sometimes happens that among the invertebrates the cells produced in the female organs of generation possess the power of division, and continued development wholly independently of the access of any male cell {parthenogenesis) ; such, how- ever, is almost never the exclusive method of increase for any group of animals, and is to be regarded as a retention of a more ancient method, or perhaps rather a reversion to a past biologi- cal condition. No instance of complete parthenogenesis is known among vertebrates, although in birds partial develop- 54 COMPARATIVE PHYSIOLobv. ment of the egg may take place independently of the influence of the male sex. The hest examples of parthenogenesis are to be fouhd among insects and crustaceans. It is to be remembered that, while the cells which form the tissues of the body of an animal have become apecialized to discharge one particular function, they have not wholly lost all otliers ; they do not remain characteristic amceboids, as we may term cells closely resembling Amoeba in behavior, nor do they wholly forsake their ancestral habits. They all retain the power of reproduction by division, especially when young and most vigorous ; for tissues grow chiefly by the production of new cells rather than the enlargement of already mature ones. Cells wear out and must be replaced, which is effected by the processes already described for Amoeba and similar forms. Moreover, there is retained in the blood of animals an army of cells, true amoeboids, ever ready to hasten to repair tissues lost by injury. These are true remnants of an embryonic condition ; for at one period all the cells of the organism were of this un- differentiated, plastic character. But the cell (pvum) from which the individual in its entirety and with all its complexity arises mostly by the union with another cell ispermatozodn), must be considered as one that has remained unspecialized and retained, and perliaps increased its reproductive functions. They certainly have become more complex. The germ-cell may be considered unspecialized as regards other functions, but highly specialized in the one direction of exceedingly great ca- pacity for growth and complex division, if we take into account the whole chain of results ; though in considering this it must be borne in mind that after a certain stage of division each individual cell repeats its ancestral history again ; that is to say, it divides and gives rise to cells which progress in turn as well as multiply. From another point of view the ovum is a marvelous storehouse of energy, latent or potential, of coun^^, but under proper conditions liberated in varied and unexpected forms of force. It is a sort of reservoir of biological energy in the most concentrated form, the liberation of which in sim- pler forms gives rise to that complicated chain of events which is termed by the biologist development, but which may be ex- pressed by the physiologist as the transformation of potential into kinetic energy, or the energy of motion. Viewed chemi- cally, it is the oft-repeated story of the production of forms, of greater stability and simplicity, from more unstable and com- RKPRODUCTION. 55 the influence genesis are to lich form the specialized to t wholly lost »boids, as we avior, nor do all retain the in young and production of mature ones, fected by the juilar forms. Is an army of ,ir tiusues lost uic condition ; re of this un- {ovum) from ts complexity )ermatozoSn), unspecialized ve functions. ?he germ-cell Functions, but igly great ca- » into account g this it must division each n ; that is to ess in turn as he OTum is a ial, of cours*^, id unexpected }gical energy which in sim- events which a may be ex- a of potential iewed chemi- n of forms, of ble and com- plex ones, involving throughout the process of oxidation ; for it must ever be kept in mind that life and oxidation are con- comitant and inseparable. The further study of reproduction in the concrete will render the meaning and force of many of the above statements clearer. THB OVX7M. The typical female cell, or ovum, consists of a mass of pro- toplasm, usually globular in form, containing a nucleus and nucleolus. , The ovum may or may not be invested by a membrane ; the protoplasm of the body of the cell is usually highly granular, and may have stored up within it a varying amount of proteid material (food-yelk), which has led to division of ova mto classes, according to the manner of distribution of this nutri- tive reserve. It is either concentrated at one pole (telolecith- at); toward the center (centrolecithat); ct evenly distributed throughout {alecithal). During development this ^ material is converted by -^-^ the agency of the cells of the young organism {em- bryo) into active proto- plasm ; in a word, they feed upon and assimilate or build up this food-stuff into their own substance, as Amoeba does with any proteid material it appro- priates. The nucleus (germinal vesicle) is large and well defined, and contains with- in itself a highly refractive nucleolus (germinal spot). „ , . *„. These closely resemble in general the rest of the^cell, but stam more deeply and are chemically different in that they conuim nucleine (nucleopUism, chromatin). It will be observed that the ovum differs in no essential par- ticular of structure from other ceUs. Its differences are hidden ones of molecular structure and functional behavior. In ac- Fio. 55.— Semi-dt»P«nini«tic repreeen ration of » mammalian ovum (SchMcr). , Highly niM- niflod. a>. isonapfillacida; vi, vlteUu8(yelk); ffv, germinal vesicle; gs, germinal epot. ■y.¥'m. i 66 COM PA RATI VE I'U YSIOLOG Y. cordance with the diverse circumstances under which ova ma- ture and develop, certain variations in structure, mostly of the nature of additions, pi-osent themselves. Thus, ova may be naked, or provided with one or more cover- ings. Tn vertebrates there are usually two membranes around the protoplasm of the ovum : a delicate covering (Vitelline membrane) beneath which there is another, which is sieve-like from numerous perforations (zona radiata, or z. pellucida). The egg membrane may be impregnated with lime salts (shell). Between the membranes and the yelk there is a fluid albumi- nous substance secreted by the glands of the oviduct, or by other special glands, which provide proteid nutriment in different physical condition from that of the yelk. The general naked-eye appearances of the ovum may be learned from the examination of a hen's egg, which is one of eAJ Fiu. 56.— DiagMmmatic section of an unimpregnated fowl's egg (Poster and Balfonr, after AH n Thomson), bl, blastoderm or cicatricula; w. y, whlto yelk; y. y. yel- low yelk; eh.l, chalazs; i.g. m, Inner layer of shell membrane; ». m, outer layer of shell membrane; ». shell; a.c. k, air-Bpace: w. the white of the egg; ». /, vitel- line mcmbrauu ; x, the denser albuminous layer lying next the vltellino mem- brane. the most complicated known, inasmuch as it is adapted for development outside of the body of the mother, and must, con- sequently, be capable of preserving its form and essential vital properties in a medium in which it is liable to undei-go loss of water, protected as it now is with shell, etc., but which, r.t the REPRODUCTION. fhich ovu iim- niostly of the T more cover- tranes around ng {Vitelline I is sieve-like pellucida). salts (shell). fluid albumi- ct, or by other in different )vum may be ich is one of 'o«ter and Balfonr, itc yelk; y. y, yel- ; t. m, outer layer :bo eeg; v. t, vitel- he vitellino mem- i adapted for ad must, oon- essential vital idergo loss of which, r.t the same time permits the entrance of oxygen and moisture, and conducts heat, all being essential for the development of the germ within this large food-mass. The shell serves, evidently, chiefly for protection, since the eggs of serpents (snakes, turtles, etc.) are provided only with aVery tough membranous cover- ing, this answering every purpose in eggs buried in sand or otherwise protected as theirs usually are. As the hen's egg is that most readily studied and most familiar, it may be well to describe it in somewhat further detail, as illustrated in the above figure, from the examination of which it will be ap- parent that the yelk itself is made up of a white and yellow portion distributed in alternating zones, and composed of cells of different microscopical appearances. The clear albumen is structureless. The relative distribution, and the nature of the accessory or non-essential parts of the hen's egg, will be understood when it is remembered that, after leaving its seat of origin, which will be presently described, the ovum passes along a tube (oviduct) by a movement imparted to it by the muscular walls of the latter, similar to that of the gullet during the swallowing of food ; that this tube is provided with glands which secrete in turn the albumen, the membrane (outer), the lime salts of the shell, etc. The twisted appearance of the i-ope-like structures {chalaza) at each end is owing to the spircJ rotatory movement the egg has undergone in its descent. The air-chamber at the larger end is not present from the first, but results from evaporation of the fluids of the albumen and the entrance of atmospheric air after the egg has been laid some time. THB ORIOIN AND DBVBLOPBOZINT OF THB OVUM. Between that protrusion of cells which gives rise to the bud which develops directly into the new indiAddual, and that which forms the ovary within which the ovum as a modified cell arises, there is not in Hydra much difference at first to be observed. In the mammal, however, the ovary is a more complex struct- lure, though, relatively to many organs, still simple. It consists, u the main, of connective tissue supplied with vessels and nerves inclosing modifications of that tissue {Oraafian follicles) within which the ovum is matured. The ovum and the follicles arise from an inversion of epithelial cells, on a portion of the body / ■'. 68 t'OMPARATlVR PIIVSIOLOGY. cavity (germinal ridge), whicli give rise to the ovum itself, and the other cells surrounding it in the Graafian follicle. At Hrst these inversions form tubules {egg-tubes) which latter become broken up into isolated nests of cells, the forerunners of the Graafian follicles. The Graafian follicle consists externally of a fibrous capsule {tunica flbroMi),ia close relation to which is a layer of cap- illary blood-vessels {tu- nica vasculoaa), the two together forming the gen- eral covering {tunica propria) for the more delicate and important cells within. Lining the Fio. 57.-Sectlon through portion of theovaryof . . • i_^-_ „» ama\} tnammal. illiiKtrntlng n. .10 of development of tumc IS a layer Of smau, the Oraaflan follicleii (Wledirshcim). />, dig- -nmnwliftf Aubioal cells CUB prollgcnw ; Ei, ripe ovum; 0, follicular SOmewnai CUDlcai ctJiiB cellnof germinal epithelium; fir, blood-vog»elH; (^igjn&rona oranulofM), K, germinal vesicle (nucleus) and germinal V"«"*"' """ V '1 Ppot (nucleoluH) ; KE. germinal epithelium; yrhlch at one part mvest jy, liquor foUieull; il/f/, membrana or tunica luTo jy; liquor foUleull; iWfir.memorana or luuicB , . i„_.««. gTanuTm-a. or follicular epithelium; Mp. Mna the ovum several layers l?^;!ia?^.aiiTZi!;S/b7mel'n".Sr'ra deep {discm proUgerus), ■onif of the neste retain their connection with while the remainder of the epithelium; S. cavity which apiicani with- wuiio wio »^iji««»* « in the Graafian follicle; So, stroma of ovary; ^q space IS filled by a 77, theca folllculi or capsule ; t'. primitive „ -Ji n- *^JU^.].'\ ova. When an ovum with its giirroundlng "— "^ "— "~ f^iu^.M cells has become separated from u nest, it ii known as a (iraaflan fullicle. fluid {liquor folliculi) probably either secreted by the cells themselves, or resulting from the disintegration of some of them, or both. In viewing a section of the ovary taken from a mammal at the breeding-season, ova and Graafian follicles may be seen in all stages of development— those, as a rule, nearest the surface being the least matured. The Graafian follicle appears to pass inward, to undergo growth and development and .again retire toward the exterior, where it bursts, freeing the ovum, which is conducted to the site of its future development by appropriate mechanism to be described hereafter. Change! in the Ovum itedf.— The series of transformations that take place in the ovum before and immediately after the RKPRHDUCTION. 59 vum itself, nnd Hide. At tlntt reraions form jy-ttibea) which Dine broken up ated nests of forerunners of ian follicles, raafian follicle ixternally of a apsule {tunica n close relation su layer of cap- Mxl-vessels (tu- uloaa), the two ormingthegen- ering (tunica for the more and important in. Lining the layer of small, ; cubical cells na granulom), one part invest 1 several layers cua proligerus), e remainder of 3 is filled by a quor fcHliculi) either secreted Ellis themselves, bhem, or both, n a mammal at may be seen in rest the surface I appears to pass and again retire ) ovum, which is ; by appropriate transformations liately after the accefw of the male element is, in the opinion of many biologists, of the highest significance, as indicating the course evolution 0*: (.J. n/- ot. .V/. v; ^'i'.^i'H .tri J^ '•yyj-v;;! hyii ■1>/- b.«- f-m m m [^■■1 wm f.c- ^-iS^^-iZ.^:i:i inm ^!^ f:>^r;A e.t. Fio. I«.-S«plttal iecMon of the ovnry of an adult bitch (after Waldeycr). o.f.ov^ rian eiilthenmn; o.t, ovarian tube*; y.f, vonngcr fplllclc»; o.f, older follicle, d. p. diDcua prollgcrua, with the ovnm: «, epithelium of a Becond oviim In the Mme follicle: /.eT fibrong ooat of the follicle; p. c, proiwr coat of the fuH'c e; j/j ep'" thellum of the follicle (membrana granuloaa); a./, col lapred atrophied foUlcle. ft.D.blood-voseelH; c.t, ccll-tnbea of the imrovarinm diviaed longitiidlnally and tranaverwly; (. d. tiibniar depreialonof the ovarian epithelium In the tlwue of the ovary; ft! «, beginning of the ovarion epithelium, close to the lower border of the ovary. has followed in the animal kingdom, as well as instructive in illustrating the behavior of nuclei generally. 60 COMPARATIVK PHYSIOLOGY. The germinnl voiicle may acquire iKtworeof hI«)w iiiovemotit (amceboid), ami the (yerininul BiM)t disapiM-ar : tlio f«»nner paasea U) OHO surface ([)*>/♦') of the ovum ; both these Htructuros may undergo that peculiar form of rearrangement {karyokineaia) which may occur in the nuclei and nucleoli of other cells prior to divinion ; in other words, the ovum has featuren common to it and many other cells in that early stage which precedoH the com- plicated transformationB which constitute the future history of the ovum. A portion of the changed nucleus (aster) with somo of the protoplasm of the cell accumulates at one surface {pole), which is termetl the upi)er pole because it is at this region that the epithe- lial cells will be ultimately developed, and is separatwl. This pro- cess is repeated. These bodies (polar cells, polar globulea, elfi.>, ■ • ^iii'irli ; - ■■■.^'^'^- Fio BO -Formation of polar cell* In a «tar-flih (.Atteriat glaHnlit) (from. OfW**- A-K after Fol. L afler O. HertwlR). A. r |)e oynm with ec"'"'"" 8«™ "j' ^'• clo andVpot; B-D. Kradual metamorpliosle of Kcrmlnsl vesicle and ipotMaeen In the llXi Vkk, into two asten.; F. formation of flrat polar cells and withdrawal of remalnlns partof nuclear spindle within the ovum: V'J'I.":f,".'=r»''u.i!j ii^e^ Xvnm in thiTflrnt Dolarcell; II. comp oton of second polar cell; I, a later stage, lowing the "ena nlng Interna half of the spindle In tihe form of two clear vesl- rr«*I^ ovum witl two polar cells and radial atrln round female pronucleus. a» ovum'lnThrnrit'iwlarceriT'lircomplot^^^^^ ' • ' temal half of the spindle In the mlar ' -^'-' -— e)K(E.F,H, of the first irolafcell. (Haddon.) then, are simply expelled ; they take no part in the development of the ovum •, and their extrusion is to be regarded as a prepar- ation for the progress of the cell, whether this event follows or precedes the entrance of the male cell into the ovum. It is wor- thy of note tliat the ovum may become amceboid in the region from which the polar globules are expelled. The remainder of the nucleusC/emafepronMctetw) now passes inward to undergo further changes of undoubted importance, possibly those by virtue of which all the subsequent evolution of the ovum is determined. This brings us to the consideration of another ceU destined to play a brief but important rdle on the biological stage. RKPRODL'CTION. ei ilow iiiovemont former paaaes HtructureH mfty ikoryokineaia) ther coIIb prior OH coiumou to it •eccdoH tho i'om- iture hiatory of th soino of the 36 (po/c), which 1 that the epithe- mtwl. ThiHpro- r globules, etc.), HalU) (from Geddes, pi'iitrlc gormliial veal- ilrlcandipot, aa seon r ct- 111* and withdrawal surface view of living cull; I. a later stage, irni of two clear veil- female pronucleus, as rations); L, vxpuliion the developmeut xled as a prepar- jvent foUowB or 3vum. It is wor- oid in the regfion cleua) now passes bled importance, equent evolution the consideration ortantrdteonthe THB MALB OBLL (BPBRMATOZOdN). This cell, almost without exception, consists of a nucleus (head) and vibratile oiliuni. However, as indicating tlu»t the 6M F,o. «..-4,perm«to.oa (after Haddon) Not d^^^^^^ ?;.";n'SSy"'^riiSo2.".i ?u •jSSl'.'Tk^dtlSiTxtrlm'et delicate' vlbratUe band is present. latter is not essential, spermatozoa without such an appendage do occur The obvious purpose of the cilium is to convey the male cell to the ovum through a fluid medium-either the water in which the ova are discharged in the case of most invertebrates, or through the fluids that overspread the surfaces of the female generative organs. The Origin of the Spennatoioai.— The structures devoted to the production of male cells (teatea), when reduced to their e^ aentials, consist of tubules, of great length in mammals, Imed J CH' COMl'ARATIVK PIIVHIOIAKJY. V'i :!' . aelpauxl opitlivliul coUh, from which, by a Horien of Ciiniigefl flifuivd above, a ({eneral idea of their tloTelopmnnt may be obtuinml. It will )m^ r)lMerveil that throughout the aeries the nuclega of the cell in in every eaae preserved, and Anally becomes the head H> Via. 61.— SMniwtoMDMls. A— H, bolated mnn-celli of the rat, ithowing the devel- opment of the RpermatocoOn and the gndusl tiMMfonnatlon of the nncieoi into the epermatoEoSn head. In O the Mminal grannie ii bcint; cast off (after H. H. Brown). I— M, ipermcellB of an Elaamobranch. The nucleiiR of each cell dlv/des Into a large nnmber of danghter-nuclei, each one of which in convcrtrd Into the rod-like head of '^ spermatozoon. N, tranivene wction of a ripe cell, ihowlne the bnndle of Rpermatosoa and the paetlve nucleus (I— N, after Semper). <>— 8. spermatogenesis In the earth-worm; O, jonng epcrm-ccll; F, the same divided Into fonr; Q, epermatosphero with the central sperm-blastophure; K, a later stage; 8, nearly matarc spermatozoa. (After Blomfleld.) t, nhowlng the devel- of the nacleni into csRt off (after H. H. iH of each cell divide* » converted Into the a ripe cell, ihowlng rter Semper). O— S P, the aame divided lore; H, a later stage; F.rm t'Ki. tH. UKrUOlU'CTloN. 68 «f th« nmlo cell. Onco more wo ar« Itxl to iwo the importar.co of thiH Htnicturc in the life «)f the toll. Fsrtilintlon of the Orum.— The H|)cnimt<)zo«»u, hwhiiiK itH way uloiiK. when it lueetH the ovum, enU^n* it either ihroujfh tt •liecial iniimtc Kiitevlruy (mi>w/»tf/«'), or, if thi« b«. not pronent- im it is not in the ova of all aniinaU-uctimlly |wnetruteH the memhruncB and Bub«tuiuoof the female «ell. and lontinucH uct ive till the female pronuclei^ iw reooheil. when the head cntera And the Uil Ib ab«)rbed or bleiulH with the female tell. The nu- cleiw of the male cell prior to union with the nucleui of the V.l'N: -M.pfr. o*.-Krrtillr.«tlon of ovnm of a mollnok {Khirta riridUl A. Ovnm tending np a nroliibemncc to meet the »iiermBloj!i«n. ft. Approach of m«l« oroniicleua lo m"i" iheftiiialepronuclBM. F.J'N, fimale proimclfua; M. VN. male pronucleut. S, ipcrmatozoOn. ovum undergoes chanpfes similar to those that the nucleus of the ovum underwent, and thus becomes flttef the polar celU; B, ; D, the naclena, m ; E, later stage; F, 9 clear nnclear epace reating stage of ap- of fonr spheres; K, ii first, but later I region at least, the surface, and at onoe become again be recog- , new nucleolus I brief period of of the nucleus, iges may be gen- 2 e o " .Sf) S -S* -w o* E jS -S a; ** a eS , • 09 e £ « £2 S 4) S " . ^ I ** '^ fe a. J2 <^ bb •- 5S , fcb ^ £ I ^ ;: V B - S " » -§ C 3 o .2 - ■ S 03 J- X be a; ^ ~ o a) , .. „ ^ 03 01 ■N be •5 £-- - .* OJ 05 ' -^ £ ^ ^ roduction, he appear- ff 08 above vhich they jroung but (atroma), Uliker). I hang ^6 opment, so f grapes ia an early stage. The ovum at first, in this case as in all others, a single cell, becomes complex by addition of other cells (dia- eua proligerua, etc.), which go to make up tlie yelk. All the other parts of the hen's egg are additions made to it, as ex- plained before, in its passage down the oviduct. The original ovum remains as the blastoderm, the segmentation of which may now be described briefly, its character being obvious from an examination of Fig. 68, which represents a surface view of the segmenting fertilized ovum (oosperm). A segmentation cavity apv^ears early, and is bounded above by a single layer of epiblast cells and below by a single layer of primitive hypoblast cells, which latter is soon composed of sev- eral layers, while the. segmentation cavity disappears. The blastoderm of an unincubated but fertilizei! egg consists of a layer of epiblastic cells, and beneath this a mass of rounded cells, arranged irregularly and lying loosely in the yelk, consti- tuting the primitive hypoblast. After incubation for a couple of hours, these cells become differentiated into a lower layer of flattened cells {hypoblast), with mesoblastic cells scattered be- Fio. 99.— Portion of section throngh an nnlneubated fowl's oOapenn tafter Klein), a. epIblMt composed of • single layer of colnninar cells; b, inegnlarly disposed lower lajrer cells of the primitive hypoblast; e, larger formative cells resting on white yelk; /, aichenteron. The segmentation cavity lies between a and b, and is nearly obliterated. tween the epiblast and hsrpoblast. It is noteworthy that, in the bird, segmentation will proceed up to a certain sta^ independ- ently of the advent of the male cell, apparen*'^ indicating a tendency to parthenogenesis. The fowl's ovum then belongs to the class, a portion of which alone segments and develops into the embryo (merobkutic), in contradistinction to what happens in the mammalian ovum, the whole of which undergoes division (holoblcutic) ; a distinction which is, however, superflcial rather than fundamental, for in reality in the fowl's egg the whole of the original ovum does segment. This holoblastic character of the mammalian ovum , lona pellucida; eel, octom cellR. We shall return to the deve later; in the mean time we prei ment in the bird. Remembering that the de^ takes place within the pellucid area opaca gradually extends o^ yelk, so that the original disk the rest of the ovum, has gro' of this area nearest the pelluci blood-vessels that derive the f blood as it is exhausted, from 1 UKrUODUCTION. 78 DO invertebrate t from an ex- Tli( first indications of future Btructural outlines in the em- bryo is the formation of the primitive streak, an opaque bantl -ent. ■•ap. he blastodermic veii- 'e 8ta^ci of develop- ', entomeree, or inner immalian ovum ures of develoj)- embryo proper >int out that the tn, inclosing the I watch-glass on . That portion nUoaa) develops sh replenish the lie area opaca. M.e- « f.< T>i...Mmn'^A The ycTof TheTf ml lv« m«mm.ill«n oO»i..t,u I. i.ow lo.t. B. pStlve l.ypoblMt; y. ; y«lk-ittc, or blo.torary aiid of ntial lo devel- BIRDS. a. / food-mi 1)- aud, as would -«» iprchenslon of the Ifour). A, B, C, D, innt periods, ihow- :-««c. I.IlJlI.IV nent. 1, li, ill, po«- n formation of the tt, vitelline mem- 7G«nai. i yelk, OP, as it lembranes, the liiijiir rr-----|- ■ -;-ie?»K.*A»i^l»ITi^i' :-;r"7t^' r ' 'r^' ' ^. IMAGE EVALUATION TEST TARGET (MT-3) .>.-'** y.*ti ^ 1.0 I.I 11.25 iai2.8 uT Iij2 12.2 lu lii :^ u£ |2.o S"'. I^H lifc.—— PhotDgraphk) Sciences Corporalion ■y 23 WEST MAIN STRKT WEBSTIR.N.Y. USM (716)S72-4503 ^ I CIHM/ICMH Microfiche Series. CIHM/ICIVIH Collection de microfiches. 4 \ *■■ Canadian Institute for Historicai iVIicroreproductions / Institut Canadian de microreproductions historiques s. • > i ! 76 COMPARATIVE PHYSIOLOGY. yeOc-aae (umbilical vesicle of the mammalian embryo). The manner in wiiich this takes place will appear upon an inspec- tion of the accompanying figures. Very early in the history of the embryo two eminences, the head and the tail folds, arise, and, curving over toward each Fio. 76.— Biagitmmatic longitadimU Mction throngli tbe azts of an embryo chick (after Fpater and Balfonr). N. C, Neural canal; Ch, notochord; Fg, forcgnt; F". So, «oinatoplenre; f". 8p, iplanchnopleare; Sp, aplanchnopleare, forming lower wall of foregut; Ht, Iteart; m plenroperitoneal cavity; Am, amniotic fold; E, eptblast; M, mesoblaat; H, bypoblaat other, meet after being joined by corresponding lateral folds. Fiision and absorption result at this meeting-point, in the inclosure of one cavity and the blending of two others. These folds constitute the amniotic membranes, the inner of which Fig. 77.— Diagrammatic longitndinal section of a cbick of the fonrth day (after Allen Thomson}, ep. epiblaat; Ay, hypoblast; «m, aomatoplenre; vm, aplanchnopleore; qf, pf, folds of the amnion; pp, pleoroperitoneal cavity; am, cavity of the am- nion: al. allantois; a, position of the fninre anas; h, heart; i, intestine; vi, vitel- line duct; y$, yelk; «, foregnt; m, position of the month; m», mesentery. forms the trite amnion, the outer the false amnion {serous memr brane, subzonal membrane). Within the amnion proper is the . amniotic cavity filled with fluid (liquor amnii), while the space between the true and false amniotic folds, which gradually in^ embryo). The upon an inspec- ) eminences, the er toward each of an embiyo chick ichord; Fg, fofcgnt; pleare, forming lower n, unniotic fold; B, ig lateral folds, ig-point, in the ) others. These inner of which nrth day (after Allen Dm, splanchnoplenre; N, cavity of the am- i, intestine; vi, vitel- I, meMnteiy. m («erotMm«m- on proper is the while the space ih gradually in- REPRODUCTION. creases in size as the yelk-sac diminishes, forms the pUwro- peritoneal cavity, body cavity, or coeUm. The amniotic cavity also extends, so that the embryo is surrounded by it or lies centrally within it The enlargement of the coelom and exten- sion of the false amniotic folds lead finally to a similar meeting and fusion like that which occurred in the formation of the true amniotic cavity. The yelk-sac, graduaUy lessening, is at last withdrawn into the body of the embryo. Fig. 76 shows how the amniotic head fold arises, from a budding out of the epiblast and mesoblast at a point where the original cell Uiyers of the embryo have separated into two folds, the 8tmatopleure or body fold and the «ptonc/mopfettre or vis- ceral fold, owing to a division or cleavage of the mesoblast toward the long axis of the body. Remembering this, it is always easy to determine by a diagram the composition of any one of the membranes or folds of the embryo, for the components must be epiblast^ mesoblast, or hypoblast ; thus, the splanchnoplenre is made up of hjrpoblast internally and mesoblast externally —a principle of great sig^ niflcance, since, as will be learned later, all the tis- sues of the body may be classified simply, and at the same time scientifi- cally, according to their embryological origin. The allantoia is » .DlagmnmatTc longitudinal action structure of much physi- thnitigh iJ» egg of a fowl (•»"» d>"^- , . , . . Ti. J3 cavity of alUmtoi«;a», albumen; o«.me8- ological unportance. It Steron;«m,cavityof amnion; «n6,embryo; arises at the same time as «*. ^"^^'^ "• »»• '"«"»»« '°*'"''"°*- the amniotic folds are . , . . x • * iu forming, by a budding or protrusion of the hm*gut mto the pleuio-peritoneal cavity, and hence consists of an outgrowth of mesoblast lined by hypoblast .v.m. 78 COMPARATIVE PHYSIOLOGY. TBB iHBTAXi (BMBRTONIO) MBBIBIUNBS OF The differences between the development of the eggf mem- branes of iTi i^mmftl* and birds are chiefly such as result from the absence in the '^ • ^ former of an eggnshell and its membranes, and of yelk and albumen. The mammalian orum is inclosed by a zona radiata (zona peUuci- da) surrounded by an- other very delicate cov- ering {vitettine mem- brane). The growth of the bkutodermic vesicle (yelk-sac) is rapid, and, being filled with fluid, the zona is thinned and soon disappears. The germinal area alone is made up of three layers of cells (Fig. 100), the rest of the upper part of the oSsperm being lined witii epiblast and hyp- oblast, while the low- er zone of the yelk-sac consists of epiblast only. Simple, non-vascular villi, serving to attach the embryo to the uterine walls, usually project from the epiblast of the subzonal membrane. In the rabbit they do not occur every- where, but only in that region of the epiblast beneath which the mesoblast does not extend, with the exception of a patch which soon appears and demarkates the site of the future plar centa. The amnion and allantois are formed in much the same way as has been described for the chick. At about the same period as these events are transpiring the vascular yelk-sac has become smaller, and the allantois with its abundant supply of blood-vessels is becoming more promi- Fio. 79. — DUgnunoutlc lonxitndtnal Mction of oOspenn of rabbit at an advanced itaRe of prec- nancy (KAIIiker, after Biacholt). a, amnion; t occur every- beneath which »tion of a patch the future pla- much the same transpiring the ! allantois with ig more promi- ,an. m.m. an. nent, and extending between the amnion and subsonal mem- brane. The formation of the chorion marks an important step in the development of mammals in which it plays an important functional part. It is the result of the fusion of the allantois, which is highly vascular, with the subional membrane, the villi of which now become themselves vascular and more complex in other respects. An interesting re- semblance to birds has been observed (by Os- bom) in the opossum (Fig. 83). When the allantois is small the blastodermic vesicle (yelk-sac) has vascular villi, which in all prob- ability not only serve the purpose of attach- ing the embryo to the uterine wall but derive nourishment, not as in birds, from the albumen of the ovum, but directly in some way from the uterine wall of the mother. It will be remembered that the opossum ranks low in the mammalian scale, so that this rtisemblanoe is the more significant from an evolutionary point of view. The term chorion is now restricted to those regions of the subional membrane to which either the yelk-sac or the allan- tois is attached. The former zone has been distinguished as the false chorion and the latter as the true chorion. In the rabbit the false chorion is very large (Fig. 79), and the true (placen- tal) chorion very small in comparison, but the reverse is the case in most mammals. It will be noted that in both birds and mammals the allantois is a nutritive oi^gan. Usually the more prominent and persistent the yelk-sac, the less so Fie. 80.— Dlagnunowtic donal view of an embno rab- bit with Tta membnuiM at the stage of nine io- mltes (Haddon, after Van Bcnedan and Jalln). at, alUntoli, showInK from behind the tall fold of tho embryo; am, anterior border of tme am- nion ; a. «. area vaacnioea, the outer border of which indieatea the farthest extension of tho mesobhwt; M, blastoderm, here consisting only of , ompliaio-mesen- „ . , proamnion ; pi, non-vascoiar qplbiastic ViOi of the future pUcen- ta ; «. f, Sinn* termlnalis. mesoblast; M, blastoderm, here cor epiblast and hypoblast; o. m. v, oi terio or Titeliino veins ; ». am, t non-vaseolar epibiastic viul of Inc ".Hi* 80 COMPARATIVE PHYSIOLOGY. the allantoia, and vice versa; they are plainly supplementary organs. Th« Allutoio Cavity. -The degree to which the various em- bryonic membranes fuse together is very variable for different groups of mammals, including our domestic species. In ruminants, but especially in solipeds, the allantois as it grows spreads itself over the inner surface of the subzonal Fi«. 81.-Embnro of dog. twentyflvo im old. opened on the vtntni side. Che«t •nd vontral waIIb havo been removca. a, noee-pito; ft, eyet; e, nnder-J«w (flnt gill-arch); d, oecoiid gill-arch; <>./, a, A. heart («. right,/ left auricle; gr, right, h, left ventricle); i, aorU (origin of); kL liver (in the middle between the two lobea is the cut yelk-'Veim: /, Rtomaeh; m, intestine; n. yelk-aac; o, urimitlve kidneys: p,allantol«: 9, fore-limbs; A, hlnd-llmbi>. The croiAedemhiyo has been stretched straight. (Haeckcl, after Bischoff.) membrane, often spoken of as the "chorion," while it also covers, though capable of easy detachment, the outer cniriace of the amnion ; and thus is formed the allantoic cavity. The por- tion of the allantois remaining finally within the foetus beoomea the bladder, which during embryonic life communicates by its contracted portion (uraehua) with the general amniotic cavity. i. ii .j. I i ii ,m i -i> ii «l l l|iii in i |[i ii»>>|.— DIamm of wi embryo ehowlns the relstloM of the ▼••calu- allMtole to the villi of the chorion (OadiM). «, embryo lying In the cavity of the unnlon; y«, yell(-ue: al, alluitoli; A. V, allantoic veeaele dipping into the vUli of the chorion; eh, chorion. In the mare especially these parts can be readily distin- gfuished. From the connection of the portion' that ultimately forms the bladder with the main sac, as indicated above, there is ground for regarding the allantoio fluid in the later stages of gestation, at all events, as a sort of urine. This fluid is at an ear- ly period abundant and colorless, later yellowish, and finally brown. Since at one thne it contains albumen and sugar, it inay serve some purpose in the nutrition of the foetus. When most suggestive of urine in the latest stages of gestation, it contains « Sto. 88.— Diagram of the fatal membnuiea of the Vlntlnian opoesam (Haddon, after Oa- bom). Two villi are shown greatly enlarged. The proceaeea of the cells, which have been ezamerated, donbtlcM coneapond to the peendopodia deaerlbed by Caldwell, al, allantob ; am, amnion; «. t, ainns tenni. nalia; «.a, anbaonal membrane: v, villi on the anbaonal membrane In the region of the yelk-aae ; ya, yelk-eac. The vaacolar aplanchnoplenre (hvpobbat and mesoblast) is indicated by the blMk line. -# 89 COMPARATIVE PBT8I0L001. i ; • oharaoteriitio body, aUantoin, reUtod to urio acid, una, etc. Certain bodiei, being probably intpiaiated allantoic fluid, have been termed "bippomanea." They may either float free in the fluid or be attached to the allantoii by a slender pedicle. The relation of the parta deaoribed above will become clearer after a study of the accompanying outa and thoae of preceding pagea, in which the allantois ii flgured. Fia. M.— Bst«rior of ohorU lae; mate. (CIuniTMn.) A, body; B. 0. oonitw. Tht TlMOlta.— Thia itmcture, which variea greatly in com- plexity, may be regarded as the result of the union of structures existing for a longer or shorter period, free and largely inde- pendent of each other. With evolution there is differentiation and complication, so that the placenta usually marks the site where structures have met and fused, differentiating a new or- gan; while corresponding atrophy, obliteration, and fusion take place in other regions. AU placentas are highly vascular, all are villous, all dis- charge similar functions in providing the embryo with nourish- ment and eliminating the waste of its cell-life (metabolism). In structural details they are so different that classiflcationB of wa mm fl l* have been founded upon their resemblances and dif- ferences. They will now be briefly described. In marsupials the yelk-sac is both large and vascular; the allantois small but vascular; the former is said (Owen) to be attached to the subaonal membrane, the latter not; but no villi, and consequently no true chorion, is developed. All mammals tm m I —I ■^ ■ ■1 ■■ ! ■ tm mx 'im^mmmmitmm* REPRODUCTION. 88 rio acid, una, alUmtoio fltiid, ither float free lender pedicle, become clearer « of preceding ly; B. 0. conn*. greatly in com- on of structures id largely inde- I di£Perentiation marks the site iating a new or- and fusion take villous, all dis- fo with nourish- fe (metabolism), slaarifloations of blances and dif- id vascular; the id (Owen) to be iot; but no villi, All mammals Fio. 86.— FottM of in«N with lit envelopM. (CbkOTMo.) A, chorion; 0, Mnitloii n- novad from allantold cavity and opened to ezpoM fcetw; D, infundlbnlamjDf nrachoa; B, atlantold portion of ambllical cord. other than the monotremes and maisupials have a true allan- toic placenta. Thib IMieaidal IlMWiita.— This form of placenta is that exist- ing in the rodentia, inseotivora, and cheiroptera. The condition found in the rabbit in that which has been most studied. The relation of parts is shown in Fig. 79. The uterus of the rodent is two-homed; so we find in gen- eral several embryos in each horn in the pregnant rabbit. They are functionally independent, each having its own set of wmm>* ■fw M COMPARATIVE PHYSIOLOGY. membranen. It will be obMnred from the figure that the true yilloua chorion ii oonttned to a comparatively Muall regiou; there is, however, in addition a falM chorion without villi, but highly vaacukr. Thii blending of forma of plaoetatation which exist aeparately in different groupa of animals is significant. In the rabbit at a later stage there is considerable interming- ling of foetal and maternal parts. rio. 8A.— Seriea of dlagnuni rapreMntlng the ralttloM of the dccldoa to the ovnin. at different periode, In the human enbiect. The dectdn* Me dark, the ovam ihadM trantvereely. In 4 and 5 the chorionic vaecniar proeeeiiec are flgnred (after Dal- ton). 1. Omm reatimr on the decidna eerotlna; >. Decldoa refleia growInK round the ovum: S. Complotion of the decidna aronnd the omm; 4. Villi, mwlng out all around the chorion; 6. The villi, epeclally developed at the site of the future placenta, having atrophied eieewhere. The MatadiiooidBl FlMOita.— This type, which; in general naked-eye appearances, greatly resembles the former, is found in man find the apes. The condition of things in man is by no means as well understood as in the lower mammals, especially in the early stages; so that, while the following account is that REPRODUCTION. •5 re that the true f aaiall ragioq; ithout villi, but uekitation which In ia Blgnifloant. «bl« interming- •cldna to the ornm. at lark, the ovam ihaM •re flgnred (after DaU reflexa Browlne roand 4. Villi, mWlnR oat t the site or the fntote rhioh; in general former, is found in man la by no nmals, especially ig account is that usually given in worka on embryology, the atudent may aa well underatand that our Icnowledge of human embryology in the very earlieat atages ia incomplete and partly conjectural. The reaaon of thia is obvious: s^jccimens for examination depending on accidents giving rise to abortion or sudden death, often not reaching the laboratory in a condition permitting of trust- worthy inferences. It is definitely known that the ovum, which ia usually fer- tilised in the oviduct (Fallopian tube), on entering the uterus becomes adherent to its wall and encapsuled. The mucous membrane of the uterus is known to undergo changes, its com- ponent parts increasing by cell multiplication, becoming in> tensely vascular and functionally more active. The general mucous surface shares in this, and is termed the deeidua vera ; but the locality where the ovum lodges is the seat of the great- est manifestation of exalted activity, and ia termed the deeidua aerotina; while the part believed to have invested the ovum by Fio. 87— Vancalar ayatem of the haman tatna, repreiented dUgrammatieally (Hnx- ley). J7, heart; TA, aoitle tmnk; e, eonmon carotid artery; «', eztamal carotM artery; e", Internal carotid artery; «, anbclavian artery; v. vertebral artery; 1. 9, S. 4. 5, aortic arehea; A', donal aorta; o. omphalo-meaenteric artery; ilv, Vitelltne duct; 0', omphalo-ineaenterU) rein; v', nmbillcal Tealele; vp, portal vein: X. liver; II. M, nmbllleal arterlea: u", u", their endlnga In the placenta; «', nmbillcal vein; Df), doetna vcnoana; vA. hepatic vein; «e, inferior vena cava; vU, Iliac velna; Of, vena aiygoa; vc', poaterior cardinal vein; DC, dnct of Covier; P, long. 86 COMPARATIVE PHYSIOLOGY. fused growths from the junction of the deoidua y«nt and aero- tina is the decidua reflexa. The decidua serotUia and reflexa thus become the outermost of all the coverings of the ovum. These and some other devel- opments are figured below. It is to be remembered, however, that they are highly diagrammatic, and repiresent a mixture of inferences based, seme of them, on actual obnrvation and others C2l analogy, etc. The figures will convey some information, though appear- ances in all such cases must be interpreted cautiously for the reasons already mentioned. During the first fourteen days villi appear over the whole surface of the ovum ; about this fact there is no doubt. At the end of the first month of foetal life, a complete chorion has been formed, owing, it would seem, to the growth of the allantois (its mesoblast only) beneath the whole surface of the subzonal membrane. From the chorionic surface vascular pro- cesses clothed with epithelium project like the plush of velvet. The allantois is compressed and devoid of a cavity, but abun- dantiy supplied with blood-vessels by the allantoic arteries and veins, which of course terminate in capillaries in the villi. Compare the whole series of figures. Flo. 6B.-Hniiu« on during Mriy ttagM of d«TC)opiiwnt A Md B, front and Bide view of an ovnm anppoaed to be abont thirteen day* old; «. embrronic am (Qoaln, after Helchert); C. o»»m of four io tm wedti. ahowlng the nmcral ■ atiractnte of the ovnm before formation of the plaeenta. ..«»*of the w^ITof the ovam ii removed to show the embryo in poeltlon (after Allen Thomaon). At this stage the condition of the chorion suggests the type of the difiPuse placenta which is normal for certain groups of animals, as will presentiy be learned. The subsequent changes are much better understood, for parts are in general no longer microscopic but of considerable sin, and their real structure less readily obscured or obliterated. The amniotic cavity continues to enlarge by growth of the walls of the amnion and is kept filled with a fluid; the yelk-sac REPEODUCTION. 87 ytm aadBero- i the outermost me other devel- Mred, however, it m mixture of ttion and others though appear- itioudy for the over the whole no doubt. At implete chorion growtli of the e surface of the ce vascular pro- plush of velvet, mty, but abun- oic arteries and Bs in the villi. ■ad B, front and side Id; «. embrronlc we* •howlng "»«.^I™*P' tetcrf thewwof ttM m TbomMm). luggests the type ertain groups of understood, for t of considerable ad or obliterated. >y growth of the aid; thes^elk-sao is now very small ; the decidua reflexa becomes almost non- vascular, and fuses finally with the decidua vera and the cho- rion, which except at one part has ceased to be villous and vas- cular ; so that becoming thinner and thinner with the advance of pregnancy, the single membrane, arising practically from Fia. 80.— HnniMi embrTo, twelve weeks old, with tto eoverinc*; lUttana siae. The navel-cord imutm mm the navel to the placenta, b, amnion; «, chorion; a, pla- cenU; d', lemalna of tufta on the amooth chorion; /, ut some rupture scularity of the I of this structure ' to great advan- method also the tmbranes can be to and ttom the placenta are reduced to three, two arteries and one vein. The villi of the placenta (chorion) are usually said to hang freely in the blood of the large irregular sinuses of the decidua sero- tina; but this is so unlike what prevails in other groups of animals that we can not refrain from believing that the state- ment is not wholly true. The Zonary FlMsenta.— In this tjrpe the placenta is formed along a broad equatorial belt, leaving the poles free. This form of placentation is exemplified in the camivora, hyrax, the ele- phant, etc. In the dog, for example, the yelk-sac is large, vascular, does not fuse with the chorion, and persists throughout A rudi- mentary discoid placenta is Brst formed, as in the rabbit; this gradually spreads over the whole central area, till only the ex- tremes (poles) of the ovum remain free; villi appear, fitting into pits in the uterine surface, the maternal and foetal parts of the placenta becoming highly vascular and closely approximated. The chorionic zone remains wider than the placental. As in man there is at burth a separation of the maternal as well as foetal part of the placenta— i. e., the latter is deciduate; there is also the beniming of a decidua reflexa. The Dimue PlMenta.— As found in the horse, pig, lemur, etc., the allantois completely incloses the embryo, and it be- comes villous in all parts, except a small wea at each pole. The Fdlyootyledoiiaxy FlMenta.— This form is that met with in ruminants, in which case the allantois completely covers the surface of the subconal membrane, the placental villi being gathered into patches {chorial cotyledons), which are equivalent to so many independent placoaitas. The component villi fit into corresponding pite in the uterine wall (uterine cotyledons), which is specially thickened at these points. When examined in a fresh oondi^on, under water, they constitute very beautiful objects. The pits referred to above into which the foetal villi fit are, as shown in the figures on page 91, essentially the same in structure as the villi themselves. In the cow the uterine cotyle- dons are convex ; but in the sheep and goat they are raised con- cave cups in which the openings for the foetal villi may be seen with the naked eye. The differences are not essential ones. Between the uterine cotyledons and the foetal villi which fit into them a thiokish, milky-looking fluid is found, the " uterine mUk " elaborated, no doubt, by the cells which line the cotyledonous pits. J 90 COMPARATIVH PHYSIOLOGY. The placentation of certain of our domeatio animals may be thus expressed in tabular form (Fleming) : ( General. Simple placenta. (Man. J Sow. ( Local and olnmUr. < B"<*- Multiple placenta. ICat. (CAw. i Sheep. (Goat Oomparing the formation, complete development, and atro* phy (in some cases) of the various foetal appendages in mam- mals, one can not but perceive a common plan of structure, with variations in the preponderance of one part over another here and there throughout. In birds these structures are sim- pler, chiefly because less blended and because of the presence of much food-yelk, albumen, egg-shell, etc., on the one hand, and thelibsence of a uterine wall, with which in the mammal the membranes are brought into close relationship, on the other ; but, as will be shown later, whatever the variations, they are adaptations to meet common needs and subserve common ends. BIZOROSOOFIO 8TRU0TUBS OF THB FZtAOSNTA. This varies somewhat for different forms, though, in that there is a supporting matrix, minute (capUlary) blood-vessels, and epithelial coverings in the fcetal and maternal tnrtaeea, the several forms agree. The jng ixMsesses the simplest form of placenta yet known. The vOli fit into depressions or crypts in the maternal uterine mucous membrane. The villi, consisting of a core of connective tisnie, in which capillaries abound, are covered with a flat epi- thelium; the maternal crypts correspond, being oompoMd of a similar matrix, lined with epithelium and permeated by oapillMy vessels, which constitute a plexus or mesh-work. It thus results that two layers of einthelium intervene between the maternal and foetal capillaries. The arrangement is substantially the same in the diffuse and the cotyledonary placenta. In tlie deciduate placenta, naturally, there is greater compli- cation. In certain forms, as in the fox and cat, the maternal tissue shows a Bjrstem of trabeculee assuming a meshed form, in which run dilated capillaries. These, which are covered with inimals may be «h. t. eep. at. nent,and atro* dagesin mam- n of Btruotttre, rt over another ctturesare sim- >f the presence the one hand, a the mammal p, on the other; itiona, they are common ends. 3LAOBHTA. hough, in that ) blood-veaeels, ialBurfooeB,the nta yet known, latemal uterine re of connective with a flat epi- ig compoeed of permeated by mesh-work. It Bnrene between . the diffuse and greater compli- maternal tissue eshed form, in recovered with M COMPARATIVB PHYSIOLOGY. In the placenta of the apes and of the human sub- ject the most marked depart- ure from simplicity is found. The maternal vessels are said to constitute' large intercom- municating sinuses; the villi may hang freely suspended in these sinuses, or be anchored to their walls by strands of tissue. There is believed to be only one layer of epithe- lial cells between the vessels of mother and foetus in the later stages of pregnancy. This, while closely investing Fia. M.— Placenta of • tiotb. Flat matemai the foetal vessels (capillaries), •pltheltal cellf) ihown In poaitlon on n v i x aiT i. rbriitaide; on left thqr an removed and really belongs tO the mater- feiere^S:ld.-'*' "'"^ '^ "'^' nal structures. The signifl- Via. 97.— Stractnre of bnman placenta; 0$, decidna seiotina; t. tr^Mcnla of MX^ln* paMbur to fatal vlUi; ea, corilnx arterr; vp, nteio-plaeenta] Teln; «, prolon gaUon of miMerAal tiMue im exterior of vUIm, ootaide celinlar lAyer r, whlcn mar npRK aent either endothelhun of matemai blood-veMele or delieato connaetite tlaane or tbe lerotina or both; e', matemai cell* of the seiotina. REPRODUCTION. 98 lacents of the ^e human sub- marked depart- plioity is found. vessels are said large intercom- inuses; the villi ely suspended in , or be anchored s by strands of B is believed to layer of epithe- reen the vessels id foetus in the of pregnancy, loeely investing lels (capillaries), pi to the mater- «. The signifl- ,tnbecDl««rfMK>Uii» 1 Teln; «, prolongation Br if, wUch nwT npre- ito conneetiTe tuane of canoe of this general arrangement will be explained in the chapter on the physiological aspects of the subject. It remains to inquire into the relation of these forms to one another from a phylogenetio (derivative) point of view, or to trace the evolution of the placenta. BfOLntion. — Passing by the lowest mammals, in which the placental relations are as yet imperfectly understood, it seems clear that the simplest condition is found in the rodentia. Thus, in the rabbit, as has been described, both yelk-sac and allantois take a nutritive part ; but the latter remains small. In forms above the rodents, the allantois assumes more and more importance, becomes laiger, and sooner or later predomi- nates over the yelk-sac. The discoidal, zonary, ootyledonary, etc., are plainly evolu- tions from the diffuse, for both differentiation of 8truct\u*e and integration of parts are evident. The human placenta seems to have arisen from the diffuse form ; and it will be remembered that it is at one period represented by the chorion with its villi distributed universally. The resemblance of the embryonic membranes at any early stage in man and other mammals to those of birds certainly suggests an evolution of some kind, though exactly along what lines that has taken place it is difficult to determine with exact- nefB ; however, aa before remarked, nearly all the complica- tions of the higher forms arise by concentration and fusion, on the one hand, and atrophy and disappearance of parts once functionally active, on the other. Snnunary. — ^The ovum is a typical cell ; unspecialized in most directions, but so specialized as to evolve from itself compli- cated structures of higher character. The s^mentation of the ovum is usually preceded by fertilization, or the union of the nuclei of male and female cells, which is again preceded by the extrusion of polar globules. In the early changes of the ovum, including segmentation, periods of rest and activity alternate. The method of segmentation has relation to the quantity and armngement of the food-yelk. Ova are divisible generally into completely segmenting (holoblastic), and those tiiat under- go s^^entation of only a part of their substance (meroblastic) ; but the processes are fundamentally the same. Provision is made for the nutrition, etc., of the ovum, when fertilized (ofisperm) by the formation of yelk-sac and allan- tois; as development proceeds, one becomes more prominent M COMPARATIVE PHYSIOLOGY. than the other. The allantoii may fine with adjacent mem- branea and form at one part a condensed and hypertropbied chorion (placenta), with corresponding atrophy elsewhere. The arrangement of the placenta yaries in different groups of animals so constantly as to furnish a basis for classification. Whatever the variations in the structure of the placenta, it is always highly vascular ; its parts consist of villi fitting into crypts in the maternal uterine membrane— both the villi and the crypts being provided with capillaries supported by a con- nective-tissue matrix covered externally by epithelium. The placenta in its different forms would appear to have been evolved from the diffuse type. The peculiarities of the embryonic membranes in birds are owing to the presence of a large food-yelk, egg-shell, and egg- membranes; but throughout, vertebrates follow in a common line of development, the differences which separate them into smaller and smaller groups appearing later and later. The same may be said of the animal kingdom as a whole. This seems to point clearly to a common origin with gradual diver- gence of type. L ' JMH « ft'W'H t» ' » | i l' adjacent mem- 1 hypertrophied phy elaewhere. ferent groups of tr olaaaiflcation. le placenta, it is rilli fitting into th the villi and orted by a con- pithelium. The r to have been nea in birds are If-shell, and egg- Mr in a common arate them into md later. The a whole. This h gradual diver- THB DEVELOPMENT OP THE EMBRYO ITSELF. We now turn to the development of the body of the animal for which the structures we have been describing exist. It is important, however, to remember that the development of parts, though treated separately for the sake of convenience, really goes on together to a certain extent; that new structures do not appear suddenly but gradually ; and that the same law applies to the disappearance of organs which are being super- seded by others. To represent this completely would require lengthy descriptions and an unlimited number of cuts; but with the above caution it is hoped the student may be able to avoid erroneous conceptions, and form in his own mind that series of pictures which can not be well furnished in at least the space we have to devote to the subject. But, better than any abstract statements or pictorial representations, would be the examination of a setting of eggs day by day during their development under a hen. This is a very simple matter, and, while the making and mounting of sections from hardened specimens is valuable, it may require more time than the student can spare; but it is neither so valuable nor so easily ac- complished as what we have indicated ; for, while the lack of sections made by the student may be made up in part by the exhibition to him of a set of specimens permanently mounted or even by plates, nothing can, in our opinion, take the place of the examination of eggs as we have suggested. It prepares for the study of the development of the m a mm a l , and exhibits the membranes in a simplicity, freshness, and beauty which impcut a knowledge that only such direct contact with nature can supply. To proceed with great simplicity and very little apparatus, one requires but a forceps, a glass dish or two, a couple of watch-glasses, or a broad section-lifter (even a case- knife will answer), some water, containing just enough salt to be tasted, rendered lukewarm (blood-heat). 96 COMPARATIVB PHYSIOLOGY. Holding the egg longitudinally, crack it aoroM the center tranivenely, gently and carefully pick away the ehell and ito 1 « 8 4 » ® # © ® • Fio. W.— VuioDi ttasM In the developmnit of the frog from Um en (wtOT I 1. The Mainentliic OAiiin, ihowlns Bnt clearage funow. «. Section of tfc •t risht angle* to the fnnow. 8. iSaaie, on •ppeMsnee of Moond farrow, plightlT from above. 4. •nee of first hciriaontal_fnrrow. •ection " " " • ■ — fifth fnrrowe above. The latter aeen from Deneath. 5. S. The fame, teen ffom above, (after Howea). ' the above viewed The Mune, on appear- 7. Longttmlinal 'Mrth and than the U. Later ph »'M'(aII oltiere K 6). W.' Ixmgitn3rn5Terticar_iS5iM'^ m^o rt 15. Caqgltudlnal vertical section of i than 14 a * 10). ne, nuclena; «. e, eteavage cavity; m>. epiblasf: LI, jrolk-bearing tower-larer cells; « bUstopore; at, aichenteron (mYd-gnt): *6, hypoblast; m$. nwUfleientlated mesoblast; «*, notochord; n. a, neuid (eerebro-spinal) asU. membranes, when the blastoderm may be seen floating upward, as it always does. It should be well examined in position, TUB DEVBLOPMENT OF THK BMHRYO ITSELF. 97 oroM the oentor he ihell and its n Um cog (aflOT Howm). 9. Section of the aboTe f Moond fnrrow, viewed 5. The Mune, on appear- above. 7. LongitMUniU ippaaranoe of fourth and hti)' later ttaoe than the m«te mptdljrlhan lower. 9 of It. 14. Becmentlns tlon of Mme. 18 and 15 >f embryo at a itaoe later iplblaan U, yolk-bearing lut): W, hypoblaat; m«. ^bio-sptnal) asU. n floating upward, oined in pontion, using a hand lens, though thia ii not eaential to getting a fair knowledge; in fact, if Uie exam- ination goes no further than tlie naked-eyo appearances of a doien eggs, selecting one every twenty- four hours during incubation, when opened and the shell and membranes well cleared away, such a knowledge will be sup- plied as can be obtained fnmt no books or lectures however good. It will be, of course, understood that the student approaches these examinations with some ideas gained from phites and previoiu reading. The latter will furnish a sort of biological pabulum on which he may feed till he can supply for himself a more natu- ral and therefore more healthful one. While these remarks apply with a certain degree of force 'to all the departments of physiolo- gy, they are of special impor> tance to aid the constructive fac- ulty in building up correct no- tions of the successive rapid transformations that occur in the development of a bird or mftmmal. Fig. 99 shows the embryo of the (bird at a very early poiod, when already, however, some of Fi«. ».— Kmbwo fowl 8 mm. long, of ., '',. . _, , about twMiCT-foarboata, lean mm the mam outlines of structure ^ - ' — — — - are marked out. Development in the fowl is so rapid that a few days suffice to outline all the principal organs of the body. In the mammal the process is slow- er, but in the main takes place in the same fashion. As the result of long and pa- 1 above. 1 K 10. (Haddon, aft«r KSIIiker.} JN, onion of the med- nllaiy folda In the realon of the htnd-brain ; fr, primlUve streak; A. parietal sone; I(f, posterior portfon of widely open nenral groove; H/', anterior part of neu- ral groove ; Bv, neaial ridge ; 81*, tmnk-sone ; vAf, anterior amni- otic fold ; vD, anterior umbilical ■inni showing throoch the blasto- derm. His divides the embryonic rudiment into a central .tmnk-ione and a pair of lateral or parietal ■ones. M COMPARATIVK PIIVSIOUKlY. tient nbwrvaUtni, it ii now Mttled that all tli( ]^tia of the moit coinplirutetl organUm ariw fmtn the three-la;yi>ivd blaatoderm provioualy flfrured ; every part may be traced buck ao ariaing in one or other of these layer* of cellit — the epiblunt mesubluMt, or hypoblaat. It frequently happens that an organ la made up ot cella derived from more than one layer. Rtruoture« rmiy, ac- cordingly, be oUuNifled um epiblastlc, meeoblaKi : , or hjrpoblastic ; for, when two atrata of oella unite in the formation itt any part, one ia alwaya of aubordinate importance to the other : thua the digeative organs are made up of meaoblaat aa well aa hypo- blaat, but the latter oonatitutea the eaaential aecreting cell mech- aniam. As already indicated, the embryonic membranea are alao derived from the aame aouroe. The epMaat gives rise to the akin and its appendages (hair, naila, feathers,* etc.}, the whole of the nervous system, and the chief parts of the organs of special sense. The tneadbkut originates the vascular syntem, the skeleton, all forms of conneotive tissue including the framework of glands, the muscles, and the epithelial (endothelial) structures covering serous membranes. The hypobkut furnishes the secreting cells of the digestive tract and its appendages— as the liver and pancreas— the lining epithelium of the lungs, and the cells of the secreting mucous membranes of their framework of bronchial tubes. It is difficult to ovenate the importance of these morpholog- ical generalisations for the physiologist ; for, once the origin of an organ is known, its function and physiologiaal relations gen- erally may be predicted with considerable certainty. We shall Fta. 100.— TnuMTMM Mction thfongh the uMdnllMy of a chiek of eltfhiMn hoan (fntter and Balfob;. a, vyin....^ ., •«« H, hypobbwt; nff, m edu lU ry foM; mg, iiMdollwjr groove; eh, nbtocbord. and half the bUwtodgnn K, epIblMt; M, nwMbhift; endeavor to qiake this prominent in the future chapters of thi^ work. Being prepared with these generalisations, we continue our study of the development of the bird's embryo. Before the end THE DKVKIiOPMBNT OF TIIK KMBRVO IT8KI.F. y9 irta of the mott rpd blaatuderm ok aa aruing in t, ine«ubliM«t, or L 18 made up ot lotureti rriftjr, ao- or hypoblastk' ; ion of any i»art, other : thus the I well aa hypo- sting cell mech- membranea are ipendagea (hair, tiyatem, and the m, the skeleton, I framework of lelial) structurea of the digestive sreaa-the lining ecreting mucoua ibes. heae morpholog- nee the origin of sal relations gen- linty. We shall mf tug eK ad hiUf the btartodsim iplMMt; M, mMoblMt; ; eh, nbtocbora. ■e chapters of this I, we continue our >. Before the end of the flmt twenty-four hours such an appearance as that repre- sented ill Fig. 100 is preaonted. The mounds of cells forming the metlullary foldn are sct>n coming in contact to form the medullary (neural) canal tia 101 -Tnnwvdrw lectlon of embryo chick at end of jjrit lUy (afU-r KOlllker). M, mSiibuitrW. hViM.blMt; m. roeduitary pl»t.;; ^..plblMt; m. j,, mwlullary irroove; mTm«SnlSry fl)ld; cA. chord* dorwIU ; l\ proioverlcbr.l pUte; »ffllve»ymcomplete cnvieri (preckvai veinin B. The events of the thtrd day are of ext»n>.i iUM«t«ie..: S.e, ^^ ^^^ ^^ ^^ extension of paHs already marked out rather than the formation of entirely new ones. The following are the principal changes : The bending of the head-end down- ward (cranial flexure) ; the turning of the embryo so that it lies on its the completion of the vitelline circulation ; the in- Fio. 108.— DIagnun at the em- bryonic vMGular ayitem (Wiedenhelm). A, atriooi; A', A'. doiMJ aorte ; Ab, bimneUal veeaelB: pocterior cardinal vein: 7«. common Iliac arteriea; K, L. Ml cleft* ; S. A, right and left roota of the aorta; S. S", branchial collecting tranks nr vein* ; 8b, subcUman ar- tery ; Sb', inbclavian Teln; Si, siona venoma; Fi'*eDtri- cle: VC, anterior cardinal vein; Km, vitelline velna. left side / :s*talmtiii^>tteiaeBaaMmMit m*a ii mma nmmim TT1" Y. at an early period Moblast cells near of the mefloblast r part of the sec- EU* system, includ- es great progress, uenoe of excessive ration of the rela- er parts, becomes that it presento which represents d one to the left, terial. The rudi- es also are to be iem is represented expanded portion 1 as the bulbuB ar- Ktensions from it, niting above the irm a single poste- From these great lesser ones arise, n constitute that >ree6nU!d diagram- )8, 109, from which irculation may be kting of the heart tie corpuscles have md while the tub- ti which the blood 11 very incomplete. le third day are of sxtension of parts t rather than the y new ones. The rincipal changes : i head-end down- ire) ; the turning hat it lies on its rculation ; the in- THB DEVBLOPMBNT OP THE BMBRYO ITSELF. 106 crease in the curvature of the heart and its complexity of struct- ure by divisions ; the appearance of additional aortic arches and of the cardinal veins ; the formation of four visceral clefts and five viaoend arohes ; a series of progressive changes in AAA Fi«. 10B.-Di«ai«m of ciicaUrtioii of yrik-Me at m« BUModiom Mm from below. ArteriM inMle blMk. Abewt: ^il.iM- ytSi M-or, right TiteUlne veto; 6. V, ■tonajrOMNai />•_£.«»»"» CuTleri; * A%. V, Mpwfor CMdtotf or Jugnlwr v^n; V.Ca, tnfwlor ewdta^ vein. the organs of the special senses, such as the formation of the lens of the eye and a secondary optic vesicle ; the dosing in of the optic veddo ; and the formation of the nasal pita. In the region of the future brain, the vesicles of the oerebtal hemi- spheres beoome distinct ; the hind-brain separates into cere- / 106 COMPARATIVE PHYSIOLOGY. Fia. HI. ri«.llS. dw (Forter and Balfour). nrcTnwinJ cmrt; pr. potterior root ^ f^ nwje irttti «ngU«r«r, witoHor root; A. O. C. Mteffor ar»J colmnn of S»«Mj^«»4s T. Jf. ftantwlor white colnnm to couwe of focinatioii; m. P< B»;^Pl^i„«ii notochoid: rA; WoMBm ridge ; AO, domU eorta : v.e. a. pwrterior ^dlB^ ?Sto; Til. W^toi duct; wr», WoiAm body, conttottag ol tabnlee and Hal- THE DEVELOPMENT OP THE BMBBTO ITSBLP. 107 PiS ^nS^ *!0, •om.lopleaw; HP, ■pUnehnoptonre ; V, blood • veweU ; ,'f{l^-^SS^^T^A dlgeitiw tnict of chick on fourth d.y (after G«to). *TT^iblick1i;"wpiS«.iiUh»poblMt; the .Uded P«rtlon. me«.blMtiJj. loiw df- vertlculnm. expimding at bMei Into primary lung veaicle; tt, •lomach; /, Hver; Fio.'ilS^uSdof chick of third day. viewed •Wewlw m a tfawp^nt obje^^^^^^ lev* /•. cerebral hemiapheree; A, veelcio of third Teptrtcla: II, mld-braln; III, h(;ia-brJni%opUcve.loIe; a.'naaal pit; b, otic yealcle: i, Infundlbnium : «. SlnMl bSy; A, notochoidiV, flfth nerve ; VU.ievmtii nenre ;Vni, united gloHophaivi^ and pnemnosutrle narvea. 1, S, >, 4, S, the flva viMeral foldi. bellum and medulla oblongata ; the nerves, both cranial and gpinal, bud out from the nervous centers. The alimentary ca- nal enlarge!, a fore-gut and hind-gut being formed, the former being divided into oecophagus, stomach, and duodenum ; the latter into the large intestine and the doaoa. The lungs arise from the alimentary canal in front of the stomach ; from simi- lar diverticula from the duodenum, the liver and pancreas orig- inate. Changes in the protovertebrsB and muscle-plates con- tinue, while the Wolffian bodies are formed and the Wolffian duct modified. Up to the third day the embryo lies mouth downward, but now it comes to lie on iti left side. See Fig. 106 with the ac- companying description, it being borne in mind that the view is from below, so that the right in tho out is the left in the em- KB. v.. 1M UmiI of ahiek of fourth day, viewed from below aa an opaque object (Foi- arohc8;^;lMaalptt. bryo itodf. Fig. 110 gives appearances furnished by a vertical transverse section. The relations of the parts of the digestive tract and the mode of origin of the lungs may be learned from Fig. HI. 108 COMPARATIVE PHYSIOLOGY. An examination of tlie figure* and subjoined deacriptioni must BufHce to convey a general notion of the lubMquent prog- a.ph. yjt. If. C.b. UK 'to. tfV -««. A. fit. •ehM. -CU. Wr' th. ^l. 'BJ^ Fio. 114.-Kmbi7o "t end of fonrth (toy. •«« w • tmiwpMent »I»J«!.» }'!"•»«' «*^ Balfonr). Off, cerebnU bemltpliere; r. B. fora-bmin, or vmIcIc of thIM ventricle (thalwnencephalon), with plne«l Blwid (/V.) projecting; JT. A MW-b>«lnv,,^*. cecebellam; VK. F, fonrth ventricle; t, lent; eh». choroid ullt; Cm. V, •udltorr vesicle; tm, raperior nuiilllMT prooM*; IF, «^etc., lint, Mcond. etc.. vlicend folda ; V, llfth nerve; 17/, eeventb nerve; a. Ph, KloHopluraisMl nerve: Pg, pnenmonitric. The dirtribntion of thiMe nervwle abio Indloited: e*. noto- chord; jKTheMt: MP. mi»cle.ptate.; W' *J»B-. H. L, }^vAA\mb. llie .innlon bM been removed. At, allantoli protmding f nnn cat end of aonuktlc (tiUk SS. resB of the embryo. Special points will be considered, either in a sepamte chapter now, or deferred for treatment in the body of the work from time to time, as they seem to throw light upon the subjects under discussion. DBTBUPOIIBIIT OF THB V AaOOIAR STBTBM Of VBBr TBBRATBS. This subject has been inddentally considered, but it is of such importance morpholopcal, physiological, and {lathological, as to deserve special treatment. In the earliest stages of the circulation of a vertelnato the arterial system is made up of a pair of arteries derived from the single buJbua arteriosus of the heart, which, after passing for- satass: SB ned deocriptioiM subsequent prog- ent object {Vimler and mlclc of third ventriele M. B, mid-bi«in: C.», d *llt; Clfii. V, audilorr It, MRoad, etc., viiceru DpharrngMl mnre; Pg, M Indlcatiid: eh, noto- lind-limb. The aronion of fomatlo atalk SS. osidered, either in nent in the body n to throw light imc nr lered, hut it is of and pathological, ' a vertebrate the I derived from the after paoring for- THE DEVELOPMENT OF THE EMBRYO ITSELF. 109 ward, bends round to the dorsal side of the pharynx, each giving off at right angles to the yelk-sao a mteUine artery ; the aortie un'*<< dorsally, then again separate and become lost in the pos- ter, md of the embryo. The so-called arohea of the aorta are large branches in the anterior end of the embryo derived from the aorta itself. The venous system corresponding to the above is composed of anterior and posterior pairs of longitudinal (cardinal) veins, the former (jugular, cardinal) uniting with the posterior to form a common trunk (duc(tM Cuvieri) by which the venous blood is returned to the heart The blood from the posterior part of the yelk-sao is collected by the viteiline veins, which terminate in the median m'ntM venoaus. TlM Lator BtagM of th* Festal Oironlatioii.— Corresponding to the number of visceral arches Ave pairs of aortic arches arise ; but they do not exist together, the first two having undergone more or less complete atrophy before the others appear. Figs. 115, 116 convey an idea of how the permanent forms (indicated by darker shading) stand related to the entire system of vessels in different groups of animals. Thus, in birds the right (fourth) aortic aroh only remains in connection with the aorta, the left forming the subclavian artery, while the reverse occurs in mammals. The fifth arch (pulmonary) always supplies the lungi. 1. Fia. 115.— Dlamma of the aortie archee of mammal (tandois and StIrllnB, after Bathke). I. Arterial tmnk with one pair of archee, and an Indleatlon where the •econd and third paira will develop, i. Ideal itago of Ave complete archee: the fonrth clefta are shown on the left fide. 8. The two anterior pain of archee have dlMppeaied. 4. Tranaltlon to the flnal at^te. A, aortic arch; ai, donal aorta; oar, anhcUiTlan or axillaij arterjr; Ot, external carotid; d. Internal carotid: dB, dnetna arteriaeuf Botallt; P, polmonaiy artery; 8, inbclavian artery; la, tnincus arterkwDs; c, vertebral artery. The arrangement of the principal vessels in the bird, mam- mal, etc., is represented on page 110. In mammals the two prim- Haaimw-J!/-!}.. ,..'-■ ■ -..i>^\v. ' .mfn ■ *:h..'---y>- ' 'i'!« ' ■■ • '■ > "Ji ' Mmj " I MWBWjK'i ' Wi 110 COMPARATIVE PHYSlOLOOy. iUve anterior abdominal (aUantoie) veimi develop e«rly and unite in front with the vitelline: but the right allantoic vein and the right vitelline veinanoon dimppear, while the long com- mon trunk of the allantoic and vitelline vefaw (duetu* venoaun) paMNfl through the liver, where it i» Mid the ductus venoeun gives oir and receive* branches. The ductus venosus Arantii penisU throughout life. (CompM« the various figures iUustrat- ing the circulation.) B *ia 116 — DlacnuB lllutratins tnuwformatioiif of aortk w^M tn • llMid. A ; • •iike, bT*S^ cTimimintl, D. .Seen fiom below. . (li«Woi. •'««» Rethke. ™i,tinii cMotid; \ exttrniU e«otld; «. emnmon owotM A. rf. doctw WmM b tween the third an^ fourth Mcbise; «. right •ortic jiKh^. •n»«'*»]"L?'*SBii •oita; A, loft aortic aich; I. pulmmiwy Mtenf; *• "«>'^' »LS^? SSSu .5*?^i between the palmonanr nrtery end the aortic mchee. B. Aright aortic areh.*. vMta^turt«n^:Tlen aortic aich: A, palmonarjr arteiy: <. dnctua Botalll of the Urtttr. C.TSlgta "aorta; «. fourth arch of the' right side (root of doiaal aorta) : / Tteht wbiSJCm; kdMMl aorta: *. left Mbetavlan tfouiih a«A of the left ■Idet • ( ralinon«7 aHenfTTaMl I. right and left dnctu Botal I of the imlmpaarT SteHellnTrfTwlKtacS aortoj i, ftnrth arch of the loft .Ide (roof of dotyia Srta)T>, dorwJ ao&; a, left mrtebral artery; », Wt •obclavtan; i.rijW w^: ffliWurth arch of'tK; right eMc); *. right vertebral artenr: '.«««"»•««»»' the right iubclavlan: m. pulmonary art»ry; », dnctua Boialll of the latter (usually termia dueUu artmomi). With the development of the placenta the allantoic circula- tion renders the vitelline subordinate, the vitelline and the larger mesenteric vein forming the portal. The portal vein at a later period joins one of the venae advehmtea of the allantoic vein. At first the vena cava inferior and the ductus venosus enter the heart as a common trunk. The ductus venosus Arantii be- comes a small branch of the vena cava. Y. THE I' ELOPMENT OP THK EM H\ ITSELF '\\ evelop early and ht allantoio vein bile the long oom- (duetus venoauM) ) ductus venmus yenomis Arantii LIS flgurw illustrat- iKbM ta • llMid. A ; • lllwld»n, after Rathlui.) tid. A. (/, doctM BoUIII i; /, labclAvlu; g, dmaal MM of tbo dnciM Botalll B. a, right aortic arch; «, rs i, doctM Botalll of th« lide (root of donal aorta) : (fourth arch of th« left Botalll of thcpalmonarT loft (Ide (root of dond mbclavian; i, right rab- artnrjr: /, contlnnation of >lalll of the latter (nanally le allantoio circula- Qine and the larger rtal Vein at a later le allantoic vein, ictus yenomis enter ^enosus Arantiibe- The allaiii< ' vc i' form as a solid nt«r history may be gathered from the following series of representations. In the fowl the heart shows the commencement of a division into a right and left half on the third day, and about the fourth week in man, from which fact alone some idea may be gaiK-xl as to the relative rate of development. The division is effected by the outgrowth of a septum from the ventral wall, which rap- Via. 118. Fm. 117. la-JIT.-Develaiiiiieiit of the heart In the haman embryo. fKn the fourth to the alsthweek, A. Rmbryo of four waeka (KOIIIker, after Coate). B, aatarior, 0, poeterior viewi of the heart of an embryo of ilx weeke (KMIIker. after Bcker). a. npper Umlt^of buccal cavity ; e, buccal cavity; A, Ilea between the ventral endi of aecond and third branchial archea; f the the blutid itself ture and ohangea plied to the blood lent of the r«apir- nTAL ■TtTHM. 1% the anatomical notion on the uuo nprehended; uor, le aame part may (urine) and at an- divided into three the primitive kid- and gradual eve- of which allusion not funotional at the anterior part . of primitive kid- A apparent on the ped in the ichthy- TIIR DRVRLOPMKNT OP THB EMBUYO ITSELF. 118 opeida (fliheii and amphibians). A vascular process from the |ierit(meum (glomerulus) projects into a dilated section of the bcxiy cavity, which is in part separated from the rest of this cavity (ccelom). ThiN process, together with the segmental duct, now coilral, and certain short tubes developed from the original duct, make up tho pronephros. The segmental duct opens at length into tho cloiioii. The metonephroa (Wolflian body), though largely developed in all vertebrates during foittal life, is not a persistent excretory organ of adult life. In the fowl recent investigation has shown that the Wolffian (segmental) tubes originate from outgrowths ef the Wolffian I In the fowl (Uaddon). lit (flattened) layer; Ay, ^, notochord; p. t., pro- rio. 1«1. i- lu. ISO,— Rndlmentarr prlmltlre kidnejr of embrjronte dof. The poaterlor portion of the hoij uf the embryo t« leen from the ventral side, covered by the Inteetinal layer or the yelk-*ac, which hae been torn away, and thrown biick In front in Older to ihow the primitive kidney dncta with the primitive kidney tubet (a). A, primitive vertebne; e, donal medulla; d, paaiage into the pelvic Inteetinal cavity. (Haeekel, after Blicboff.) Pio. ISI.— Primitive kidney of a hnman embryo, u, the urine-tnbes of tho primitive kidney; w, Wolfllan dnct; to', npper end of the latter dforMgnl'i hydatid); m, Mailerlan ductim', npper end of the latter (Faitoplan hydatid); ff, hermaphrwdita gland. (After Kobelt") duct and also from an intermediate cell-mass, from which latter the Malpighian bodies take rise. The tubes, at first not con* 8 K: j ^t/.ij. ' feSMiJtfa J Vf-y.t'JiKit '!' - ' 114 COMPARATIVE PHYSIOLOGY. nected with the duct, finaUy join it. This organ is oontinuous with the pronephros ; in fact, all three (pronephros, mt»one- phros, and metanephros) may be regarded as largely oontinuar tions one of another. ,1 *. * The metanephros, or kidney proper, arises from mesoblast at the posterior part of the Wolffian body. The ureter originates Fia M8-8ecH«m of the tatennedlate cell-mM* of fourth d«y (^^Mjo'JSiLPtJfTi atSi wSdewrt 1 X 180 m. metentery; L, ■omatoplenre; a', wwtlon ™ the SJiSJinll enlSeiinm from the d^of Mtlller U formeS by tavoluiton; a, thlA- SS ^rtlSi^SfgiSrepltheHu^^^^^ primitive ovj, Cwd o «» Mm; iETmodifledmesoblMt which wlU form the etrama of the ovwy, WK, WoOBan body; y, WoMBan duct. first from the hinder portion of the Wolffian duci In the fowl the kidney tubules bud out from the ureter as rounded eleva- tions. The ureter loses its connection with the Wolffian duct and opens independently into the cloaca. The following account will apply espedally to the higher vertebrates: The segmental (arohinephric) duct is divided horaontally into a dorsal or Wolffian (mesonephric) duct and a ventral or MtUlerian duct. The Wolffian duct, as we have seen, develops into both ureter and kidney proper. To carry the subject somewhat further back, the epithelium THE DEVELOPMENT OP THE EMBRYO ITSELF. 116 [an is oontinaouiii aephros, meaone- iargely oontinuar rom mecoblast at ureter originates •y (Foster and Balfoor, enre; a', pwtton ^ the by iBvoiuuon; a, tnwk- ImltWeova, Cando.we ima of the ovary; WK, duct In the fowl as rounded eleva- i the Wolffian duct ally to the higher vided horizontally ot and a ventral or lave seen, develops aok, the epithelium lining the ooelom at one region becomes differentiated into col- umnar cells (germinal epitheUum) which by involution into the underlying mesoblast forms a tubule extending from before backward and in close relation with the Wolffian duct, thus forming the Miillerian duct by the process of cleavage and separation referred to previously. Fig. 1».— Diacrammatic repreaentatlon of the genital oima of a human em^o m- ▼iomi to anual dirtinctfon (Allen Thontton}. W, Wolfflan body ; ae, ff »»»• e«2 m.M«leTlan duct; w, Wolfflan duct; wy, urogenital alnns: en. c"^ »' R™".' i, intestine; el, cloaca; to, part from which the acrotnm or ]»W» matow we derel- oped; ot, origin of the orary or teaticle reapectiveW; x, part of the wolfflan body derelived ItSer Into the eonl vaiculaei: ^ ureter; 4, bladder; 5, nrachua. The future of the Mfillerian and Wolfflan ducts varies ac- cording to the sex of the embryo. In the male the Wolffian duct persists as the vas deferens ; in the female it remains as a rudiment in the region near the ovary (hydatid of Morgagni). In the female the MWerian duct becomes the oviduct and related parts (uterus and vagina) ; in the male it atrophies. One, usually the right, also atrophies in female birds. The smus pocularis of the prostate is the remnant in the male of the fused tubes. The various forms of the generative apparatus derived from the Mfillerian ducts, as determined by diflierent degrees of fu- 116 COMPARATIVE PHT8I0L0 Y. „ „-jnoftlieiiiaimn8ltontypeofiii^eBeMalo«Mui(«tt«5^ Fio. 194.-pUe«n< vm\», cat ihort; «, caput epididymis; a, mi Morncnl. the penlatent anterior end «««SSi<2If15K^iSt «^m^ /Hiymen; I, rectum; /, labium; •».«?* '*»<»P'" *"~ ; flnet. orlCttleriandnet) nm; $e, vaacnlar bnib or satteiea lemaina of Wol- oUBandoct; 8. meter; 4, ■ant of (talk of allantoia. In both sezM the mort poBterior portiun of the WolfBaa duct gives rim to the metanephrot, or what becomes the permaoent kidn«y and ureter ; in the male also to the vaa deferens, testicle, vas aberrans, and seminal vesicle. The ovary has a similar origin to the testicle ; the germinal epithelium furnishing the cells, which are tnmsformed into Qraaflan follides, ova, etc., and the mesoblast the stroma in which those structures are imbedded. In the female the parovarium remains as the representative of the atrophied Wolffian body and duct The bladder and urachus are both remnants of the formerly extensive allantois. The final forms of the genito-urinary or- gans arise by differentiation, fusion, and atrophy : thus, the cloaca or common cavity of the genito-urinary ducts is divided AL Fia.UB. Via.tB, ^^' Fio. IM. Fia.U0. Fiei. U6 to !».— Diagiama iUnatnting the evdntion of the poateilar pasaagea (aflw i^M^m and stiriug). Fig. Ill— Allantoia eonttnnooa with reetom. Fio. 117.— Oloaea formed. Flo. UB.— Early eonditioa in male, befon the cloenre of the folda of. the groove on the poeterior aide of the penia. Fio. ]».— Bariy female oondltion. A, commencemaBt of pnetodMim; A IL, allantoia; B, btaddor: C, penia; Ct, elonea; M, Xttlerlan dnet; J{j«etam; r, urethra; 8, veatibole; aU, nrogenital anma; F, vaa def erena in Fig. US, vagina in Fig. M9. by a septum (the perineum externally) into a genito-urinary and an intestinal (anal) part ; the penis in the male and the ooneqKmding clitoris in the female appear in the region of the cloaca, as outgrowths which are followed by extension of folds of integument that become the scrotum in the one sex fnd the labia in the other. The urethra arises as a groove in the under suif aoe of fbo ^■Mtfi i wiMw.UHlWKiwii'iiuiiMh i B u i < m < ii» ^m. ji m i"mMmxm ■ 118 COMPARATIVE PHYSIOLOGY. peniB, which beoomeB a canal. The original opening of the urethra was at the base of the penis. In certain cases development of these parts is arrested at various stages, from which result abnormalities frequently re- quiring interference by the surgeon. The accounts of the previous chapters do not complete the history of development Certain of the renuuning subjects that are of special interest, from a physiological point of view, will be referred to again ; and in the mean time we shall con- sider rather briefly some of the phjrsiological problems of this subject to which scant reference has as yet been made. Though the physiology of reproduction is introduced here, so that ties of natural connection may not be severed, it may very well be omitted by the student who is dealing with embryology for the Fio. lao.— Varioni forms of ■ummaUan at«ri. A. Omlthochyiichiu. B. Did«lpliy» doniaera. C. Phalangtstavnlpiiuk. D. Double atenu and TMriiim;hniiiMi uooift- hr. B. Lepua cnnl'mlDa (mbblt). ntema dnplaz. F. Vtenis blconto. O. Utenu blpartitas. H.- Utenu liinplex (bmnu). a, una; el, cloaca; o.ryolog7 for the ?^ mchm. B. Dldelphj* while the reverse holds for those that Wtrn«.?ho;;^ usually give birth to but one. tw TMiom (Ugea The ovum in the fowl is fertilized in the of estmaion or _i. « i«_ .a ^ . .» . ova. (Cbwiveu.) upper part of tne oviduct; m the mammal mostly in this region also, as is shown hy the site of the embryos in those groups of animals with a two- homed uterus, and the occasional occurrence of tubal pregnao* , .,^..,-.^. . . M|| i|i| THE DEVELOPMENT OP THE EMBRYO ITSELF. 131 dog, and certain ^ear old ; and the tune mature and great regularity, unusual activity, of the ovary and on this, and that brate, seems to be nges. Ovulation \ie Graafian folli- of the ovary, be- lly and thus free tr at this period, ) emptied capsule organization and lergoes a certain on the condition ans, which varies been fertilized or sun the Graafian f a more marked ling a true eorpua follicles that ma- ova that escape at s, of course, with lal, and is not al- ear several young imber of ova must k about the same Ids for those that I. i fertilized in the ; in the mammal us is shown by the bnals with a two- I of tubal pregnao- cy in woman. But this is not, in the human subject at least, invariably the site of impregnation. After the ovum has been set free, as above describcNl, it is conveyed into the oviduct (Fallopian tube), though exactly how is still a matter of dis- pute : some holding that the current produced by the action of the ciliated cells of the Fallopian tube suffices; others that the ovum is grasped by the fimbriated extremity of the tube as part of a coH>rdinated act. It is likely, as in so many other instances, that both views are correct but partial; that is to say, both these methods are employ ed. The columnar ciliated cells, lining the oviduct, act so as to produce a current in the direction of the uterus, thus assisting the ovum in its passage toward its final resting place. (BttmilL— As a part of the general activity occurring at this time, the uterus manifests certain changes, ohiefiy in its inter- nal mucous lining, in which thickening and increased vascular- Fie. US.— Diagram of the hniium ntema jMt before meiMtnution. Tbe eluded pottion lepreeeate the macoaa mem- brMie'(HMt and Oarbonr, afUir J. Williams). Vio. in.~Utenw after menetmatioii haa Jnat eeaaed. The easlty of the bodjr of the uteroi is rappoeed to have been deprived of mocona membiaae (J. Wllllama). ' ity ai« prominent Aflowof blood from the uterus in the form of a gentle oonng follows; and as the superficial parts of the mucous lining of the uterus undergo softening and fatly d^;en- 122 COMPARATIVE PHYSIOLOGY. \ eration, they are thrown off and renewed at these periods {eata- taenia, menses, etc.), provided pregnancy does not take place. In mammals below man, in their natural state, pregnancy does almost invariably take place at such times, hence this exalted activity of the mucous coat of the uterus. In preparation for the reception and nutrition of the ovum, is not often in vain. In the human subject the menses appear monthly; pregnancy may or may not occur, and consequently there may be waste of na- ture's forces; though there is a certain amount of evidence that menstruation does not wholly represent a loss; but that it is largely of that character among a certain class of women is only too evident. As can be readily understood, the catamenial flow may take place prior to, during, or after the rupture of the egg-capsule. As the uterus is well supplied with glands, during this period of increased functional activity of its lining membrane, mucus in considerable excess over the usual quantify is dis- charged ; and this phase of activity is continued for a time should pregnancy occur. All the parts of the generative organs are supplied with muscular tissue, and with nerves as well as blood-vossels, so that it is possible to understand how, by the influence of nerve- oenters, the various events of ovulation, menstruation, and those that follow when pregnancy takes place, form a related series, very regular in their succession, though little prominent la the consciousness of the individual animal when normal. In all animals, without exception, the disturbance of the generative organs during the rutting season (oestrum) is accom- panied by unusual excitement and special alterations in the temper and disposition, while ,it may perhaps be said that the whole organism is correspondingly affected. The frequency of the season of heat or rutting is variable, as also its duration. In most of the domestic animaki it lasts but a few days; though in the bitch it may be prolonged for a month. It is not common for conception to occur in the human sub- ject while the young one is being suckled, and the same remark applies to the domestic animals, though leas so, and with con- siderable variation for different species. Naturally, the periods of oestrum will depend considerably on the occurrence of impregnation and the duration of gesta- tion. It is usual for the mare to be in season in spring and fall, THE DEVKLOPMENT OF THE EMBRYO ITSELF. 128 ese periods (eata- m not take plaoe. i, pregnancy does enoe this exalted eparation for the ften in vain. In pregnancy may y be waste of na- of evidence that m; but that it in n of women is d, the oatamenial he rupture of the nds, during this ining meml»mne, quantity is dis- I for a time should re supplied with blood-TOBsels, so fluence of nenre- lenstruation, and e, form a related I little prominent nrhen normal, sturbanoe of the Bstrum) is acoom- ilterations in the be said that the ing is variable, as malsitlastsbuta prolonged for a I the human sub- the same remark BO, and with con- end considerably duration of gesta- a spring and fall, but, of course, if impregnated in the spring, there will be no au- tumn oestrum on account of the prolonged period of gestation in this instance; and, similarly, in the case of the cow and other animals. It is important to recognize that rutting is only the evidence of the maturation of the Graafian follicle withiit the ovary and of correlated changes. In a state of nature— i. e., in the case of wild animals— the male experiences a period of sexual excitement corresponding with an increased activity of the sexual organs and at periods answering to the rutting season of the female. In some species the testes descend into the scrotum only at this season. This may be observed in the rabbit But in our domestic animals, as a class, the male, though capable of copulation at all times, is ex- cited only by the presence of a female in season. It is only at such periods that the approach of the male is permitted by the opposite sex. THB MUTHi ' i ' iO lf OF TBB OTUM (OdOnUI). This will be best understood if it be remembered that the ovum is a cell, undifferentiated in most directions, and thus a sort of amceboid organism. In the fowl it is known that the cells of the primitive germ devour, amoeba-like, the yelk-cells, while in the w^ftmmAlian oviduct the ovum is surrounded by abundance of proteid, which is doubtless utilized in a somewhat similar fashion, as also in the uterus itself, until the embryonic membranes have formed. To speak of the ovum being nour- ished by diffusion, and especially by osmosis, is an unnecessary assumption, and, as we believe, at variance with fundamental principles; for we doubt much whether any vital prooeeti is one of pure osmosis. As soon as the yelk-sac and allantois have been formed, nutriment is derived in great part through the vessel-walls, which, it will be remembered, are differenti- ated from the cells of the mesoblast, and, it may well be as- sumed, have not at this early stage entirely lost their amoeboid character. The blood-vessels certainly have a respiiratory func- tion, and BuflBce till the more complicated villi are formed. The latter are in the main similar in structure to the villi of the alimentary tract, and are adapted to being surrounded by sim- ilar structures of maternal origin. Both the maternal crypts and the foetal villi are, though complementary in shape, all but ■ '.MSHWWK'W^'.' 124 (X)MPARAT1VB PHYSIOLOOY. identicid in minuto ntruoture in moit inntanoM. In Moh oaM the blood-veaelB are covered miperfloially by oelii which we can not help thinking are eeeential in nutrition. The villi are both nutritive and respiratory. It in no more diffloult to under- stand their function than that of the celli of the endoderm of a polyp, or the epithelial coverings of lungs or gills. Experiment proves thnt iLure is a respiratory interchange of gases between the maternal and foetal blood which nowhere mingle physically. The same law holds in the respiration of the foetus as in the mammals. Oxygen passes to the region where there is least of it, and likewise carbonic anhydride. If the mother be asphyxiated so is the foetus, and indeed more rapidly than if its own umbilical vessels be tied, for the mater- nal blood in the first instance abstracts the oxygen from that of the foetus when the tension of this gas becomes lower in the maternal than in the foetal blood; the usual course of affairs is reversed, and the mother satisfies the oxygen hunger of her own blood and tissues by withdrawing that which she recently supplied to the foetus. It will be seen, then, that the embryo is from the first a panudte. This explains that exhaustion which pregnancy, and especially a series of gestations, entails. True, nature usually for the tinra meets the demand by an excess of nutritive energy : hence many animals are never so vigorous in appearance as when in this condition; often, however, to be fol- lowed by corresponding emaciation and senescence. The full and frequent respirations, the bounding pulse, are succeeded by reverse conditions ; action and reaction are alike present in the animate and inaninuito worlds. Moreover, it falls to the parent to eliminate not only the waste of its own organism but that of the foBtus; and not infrequently in the human subject the over- wrought excretory organs, especially the kidneys, fail, entailing disastrous consequences. The digestive functions of the embryo are naturally inact- ive, the blood being supplied with all its needful constituents through the placenta by a much shorter process ; indeed, the placental nutritive functions, so far as the foetus is concerned, may be compared with the removal of already digested mar terial from the alimentary canal, though of course only in a general way. During foetal life the digestive glands are developing, and at the time of birth all the digestive juices axe secreted in an efficient condition, though only relatively so, necessitating a special liquid food (milk) in a form in which i»iaii 'iw> ii < * .' i jHj ii»'i'i > " iWHwW i| l t» | ||» » i l' THE DEVBIiOPMBNT OF THK EMBRYO IT8BLF. J^S ea. In each oa«e ircelli which w« o. The villi are iiffloitlt to under- le «ndodenn of a Uk. lory interchange ~ which nowhere he respiration of w« to the region io anhydride. If uid indeed more )d, for the mater- izygen from that mee low«r in the course of affair* en hunger of her hioh she recently ut the embryo is izhaustion which s, entails. True, by an excess of rer so vigorous in lowever, to be fol- icenoe. The full , are succeeded by ike present in the falls to the parent 'anism but that of I subject the over- eys, fail, entailing 9 naturally inact- idful constituents loess ; indeed, the itus is concerned, ady digested mar coarse only in a istive glands are B digestive juices \i only relatively a form in which all the constituents of a normal diet are provided, easy of diges- tion, i Bile, inspissated and mixed with the dead and oastoiT epi- thelium of the alimentary tract, is abundant in the intestine at birth ; but bile is to be regarded perhaps rather in the light of an excretion than as a digestive fluid. The sldn and kidneyti, though not funotionless, are rendered unnecessary in great part by the fact that waste can be and is withdrawn by the placenta, which proves to be a nutritive, respiratory, and excretory organ ; it is in itself a sort of abstract and brief chronicle of the whole physiological story of foetal life. All the foetal organs, especially the muscles, abound in an animal starch (glycogen), which in some way, not well under- stood, forms a reserve fimd of nutritive energy which is pretty well used up in the earlier months of pregnancy. We may sup- pose that the amoeboid cells— all the undifferentiated cells of the body— feed ou it in primitive fashion; and it will not be forgotten that the older the cells become, the more do they de- part from the simpler habits of their earlier, cruder existence; htmce the disappearance of this substance in the later months of foetal Ufe. In one respect the foetus closely resembles the adult : it draws the pabulum for all its various tissues from blood which it- self may, perhaps, be regarded as the first completed tissue. We are, accordingly, led to inquire how this river of life is distrib- uted; in a word, into the nature of the foetal circulation. PMal OiMllktiOB.— The blood leaves the placenta by the umbilical vein, reaches the inferior vena cava, either directly (by theduohM venomtg), or, after first passing to the liver (by the veme advehentea, and returning by the vetue r«vehentea\ and proceeds, mingled with the blood returning from the lower extremities, to the right auride. This blood, though far from being as arterial in character as the blood after birth, is the best that reaches the heart or any part of the organism. After arriv- ing at the right auricle,being dammed back by the Eustachian valve, it avoids the right ventricle, and shoots on into the left auricle, passing thence into the left ventricle, from which it is sent into the aorta, anid is then carried by the great trunks of this arch to the head and upper extremities. The blood returning from these porta passes into the right auricle, then to the corre- sponding ventricle, and thence into the pulmonary artery; but, finding the branches of this vessel unopened, it takes the line of irvrvKgO^vt?--*^': tW" ■w'sr®™ v-rrt f wvRjirtJaifipsBW ■'" iWiMNary Art. ^ Formmm (halt. lAutMAUm Valrt. JUfht Awtie. - VmU. CJpmn^n^. N^xMe Vtim. Brtmehm of Uu UmbHieal Vtin, (othtLimr. -^■^•i^umonarg Art. •• Ltfi AurMt. ..Lfft Auri».-Vmt. Ojmting. THE DBVKLOPMENT OP THE KMUHYO ITSELF. 137 IfMwt rMiiUinoe through the duetu» arterioauit into thn aortio uroh beyond Uie point where ita great bnuichea emerge. It will be leen that the blood going to the head and upper part* of the body ii greatly more yaluableaa nutritive pabulum thait the reet, eapecially in the quantity of oxygen it contains ; that the blood of the foBtut, at beat, is relatively ill-vupplied with its vital even- tial; and ai a reault we And the upper (anterior in quadrupedn) parta of the fcetua beat developed, and a decided reaemblance be- tween the mammalian fcetua functionally and the adult forma uf reptilea and kindred groupa of lower vertebratea. But thia con- dition ia well enough adapted to the general enda to be attained at thia period— the nouriahment of atructurea on the way to a higher path of progreaa. Aa embryonic maturity ia being reached, preparation ia made for a new form of eziatenoe ; ao it is found that the Euatacbian valve ia leaa prominent and the foramen ovale amaller. €ht$ FtfMOtMa PBRIOIM OP OHMPATION. Aa a rule, the ahorter the period of geatation the more nu> meroua the offapring at a aingle birth and the greater the num- ber produced within the lifetime of the animal relatively to it« duration. Thua, on account of the number of young produced by the rabbit at one birth, the ahort period ot geatation, and the frequency with which impregnation octrura, there ia a much larger number of progeny, ahort as ia the animal's life uaually, than in the case of the cow, for example, that may bear young for a much longer period. The following table givea approximately the duration of the period of geatation of aome of our domestic animals and their wild allies : Ouinea-pig (cavy) 8 weeks. Babbit, squirrel, rat. 4 " Ferret 6 " Oat 8 " Dofi, fox lion ■* Sow 4 Sheep, goat 5 Bear 7 Reindeer 8 Cow, buffalo 10 monihti. It [Flint). f!('IS?W!W09HWS''<™W™fiSII'lftf'' 128 COMPARATIVE PHYSIOLOGY. Mare, ass, zebra 11 months. Camel 12 " Giraffe U " Elephant 22 to 25" The period of gestation in the human subject is nine months; in the monkeys and apes somewhat less. The incubation pe- riod of certain of our domestic birds and related species is about as follows : Canary 14 days. Pigeon 18 " Hen 21 " Duck, goose, pea-hen 28 " Guinea-hen 26 " Turkey 28 '• It is interesting to note that the smaller varieties of fowls, hatch out sooner than the larger ; and that the period of incu- bation of all of the above varies with the weather, the steadi- ness of the incubating bird, the date on which the eggs selected were laid, etc. With very recent eggs, an attentive sitter, and warm weather, the incubation period is shortened. PARTUBinON. All the efforts that have hitherto been made to determine the exact cause of the result of that series of events which make up parturition have failed. This has probably been owing to an attempt at too simple a solution. The foetus lies surroimded (protected) by fluid contained in the amniotic sac. For its ex- pulsion there is required, on the one hand, a dilatation of the uterine opening {oa uteri), and, on the other, an expulsive force. The latter is furnished by the contractions of the uterus itself, aided by the simultaneous action of the abdominal muscles. Throughout the greater part of gestation the uterus experiences somewhat rhythmical contractions, feeble as compared with the final ones which lead to expulsion of the foetus, but to be re- garded as of the same character. With the growth and func- tional development of other organs, the placenta becomes of less consequence, and a fatty degeneration sets iii, most marked at the periphery, usually where it is thinnest and of least use. It does not seem rational to believe that the onset of labor is referable to any one cause, as has been so often taught ; but rather that it is the final issue to a series of processes long ex- irtill«.teHllkiJn.!y^m>.^i«Mt^].t Y. THE DEVELOPMENT OP THE EMBRYO ITSELF. 129 . . 11 months. ..12 " ..14 " 22 to 25 " ct is nine months; he incubation pe- ed species is about 14 days. ....18 " ...21 " ... 28 " ...26 " 28 '• r varieties of fowls. ;he period of incu- reather, the steadi- h the eggs selected ttentive sitter, and xjned. made to determine events which make ibly been owing to stus lies surrounded ic sac. For its ex- , a dilatation of the an expulsive force, of the uterus itself, ibdominal muscles. i uterus experiences I compared with the foetus, but to be re- e growth andfunc- ilacenta becomes of ets in, most mark^ est and of least use. he onset of labor is > often taught ; but f processes long ex- isting and gradually, though at last rapidly, reaching that climax which seems like a vital storm. The law of rhythm affects the nervous system as others, and upon this depends the direction and co-ordination of those many activities which make up parturition. Wo have seen that throughout the whole of foetal life changes in one part are accompanied by correspond- ing changes in others ; and in the final chapter of this history it is not to be expected that this connection should be severed, though it is not at present possible to give the evolution of this process with any more than a general approach to probable correctness. OBANCHBS nr TBE OIRODLA^ON AFTER BIRTB. When the new-bom mammal takes the first breath, effected by the harmonious action of the respiratory muscles, excited to action by stimuli reaching them from the nerve-center (or centers) which preside over respiration, owing to its being roused into action by the lack of its accustomed supply of oxygen, the hitherto solid lungs are expanded ; the pulmonary vessels are rendered permeable, hence the blood now takes the path of least resistance along them, as it formerly did through the ductus arterioaua. The latter, from lack of use, atrophies in most instances. The blood, returning to the left auricle of the heart from the lungs in increased volume, so raises the pressure in this chamber that the stream that formerly fiowed through the foramen ovale from the right auricle is opposed by a force equal to its own, if not greater, and hence passes by on easier route into the right ventricle. The fold that tends to close the foramen ovale grows gradually over the latter, so thai it usually ceases to exist in a few days after birth. At birth, ligature of the umbilicsJ cord cuts off the placental circulation ; hence the duetu8 venosua atrophies and becomes a mere ligament The placenta, being now a foreign body in the uterus, is ex- pelled, and this organ, by the contractions of its walls, closes the ruptured and gaping vessels, thus providing against heemor- rhage. ooinra. In all the higher vertebrates congress of the sexes is essential to bring the male sexual product into contact with the ovum. « 130 COMPARATIVE PHYSIOLOGY. f > t i } Erection of the penis results from the conveyanoe of an ex- cess of blood to the organ, owing to dilatation of its arteries, and the retention of this blood within its caverns. Fio. 185.— Section of eraetUe titcue (Cadlat). the male and female generative organs, the student ik "•' ' .i-ad to works on this subject ; suffice it to say that it consists of erectile tissue, the chief characteristic of which is the opening of the capillaries into cavernous venous spaces (gintiaes) from which the veinlets arise ; with such an arrangement the circulation must be very slow— the inflow being greatly in excess of the outflow — apart altogether from the compressive action of certain muacles connected with the organ. The manner and duration of the act of copulation in the domestic animals varies witH the structure of the penis, the animal's nervous excitability, etc. In the stallion the act is of moderate duration, the penis long, and the glans penis highly sensitive. In the bull the penis is of a different shape. During erec- tion it is believed that the S-shaped curve disappears, and that the extremity of the organ enters the mouth of the utwus-itself. Copulation is of very brief duration. In the dog the penis is of very peculiar formation. Its an- C--T-. aiiMMtlMMM THB DEVELOPMENT OF THE EMBRYO ITSELF. 181 eyanoe of an ez- ti of its arteries, tenor part contaiiu a bone, while there are two erectile portions independent of each other. During copulation the largest (pos- ns. X ■^ject; sufBceitto )f characteristic of cavernous venous ise ; with such an slow— the inflow rt altogether from onnected with the ct of copulation in « of the penis, the dlion the act is of |rlans penis highly pe. During erec- aappears, and that >fiheuterusitBelf. brmation. Its an- Fia. 186.— Section of parta of three Mminiferone tnbolea of the rat (Schifer). a, with the ■pcrmatosoa leaet advanoed in development; h, more •dvaoced; t, containing f ally developed apermatosoa. Between the tnbalea are aeen etmnda of interetitiai cella, with blood-veaiela and iTmph-epacea. tenor) erectile region is spasmodically (reflexly) grasped by the sphincter cunni of the female, which is the analogu e of the bulbo-cavemosus, isohio-cavemosus, and deep transverse mus- cle of the perineeum, so that the penis can not be withdrawn till the erection subsides, an advantage, considering that the seminal vesicles are wanting in the dog, as well as Cowper's glands. In the cat tribe there is also an incomplete penial bone. The free portion of the organ is provided with rigid papillffi capable of erection during copulation. As previously expUuned, the spermatosoa originate in the seminal tubes, from which they find their way to the seminal vesicles or reoefrtacles for semen till required to be discharged. The spermatocoa as they mature are forced on by fresh addi- tions from behind and by the action of the ciliated cells of the epididymis, together with the wave-like (peristaltic) action of the vas deferens. Discharge of semen during coitus is effected by more vigorous peristaltic action of the vas deferens and the seminal ^^'f^^ •* tabe, exttnua face; 4. the wim. In- SSfSe/ShiwuKSiS «« *«°!a*>«i V'«'''»«!Sj!f f^ 1;i2lfitaSSn^ ntMSsrV, a horn throW«iopeni 8. bodj of ntentf, nW» *««: .TlJlSSf J«5SS« inTnMVix. with lu mncouYolaa; 11. cnl-de-tac of vagina with lU foWaof mncous i of vnlva. ' h^v«n.mi«#>Mn may be caused the oord has left 1 from the brain, thini; happening tr nature. If the 9 spinal cord) be le out this act be- ent or otherwise, cur during sleep, ise, with voluptu- r, it seems reasc.- inal cord contains thone influences to the generative hich take place in iiave seen, always influence, either ice— at all events )m (center) which cesses. It is corn- 's—as the erection iich doubt whether oal labor aa these misconception; ac- edge, we prefer to )nn in a somewhat >igan8 are not oon- ig impulses set up ly remote areas of cular changes; the into many streams of the outflowing n, and takes paths this fact in mind, a proteid and other f view, and consid- I the mind, it is not THE DEVELOPMENT OP THE EMBRYO ITSELF. 186 difHcult to understand that nothing so quicKly disorganizes the whole man, physical, mental, and morai, as sexual excesses, whether by the use of the organs in a natural way, or from n^asturbation. Nature has protected the lower animals by the strong bar- rier of instinct, so that habitual sexual excess is with them an impossibility, since the females do not permit of the approaches of the male except during the rutting period, which occurs only at stated, comparatively distant periods in most of the higher m a mm als. When man keeps his sexual functions in subjection to his higher nature, they likewise tend to advance his whole development Sviniliaiy.— Certain changes, commencing with the ripening of ova, followed by their discharge and conveyance into the uterus, accompanied by simultaneous and subsequent modifica- tions of the uterine mucous membrane, constitute, when preg- nancy occurs, an unbroken chain of biological events, though usually described separately for the sake of convenience. When impregnation does not result, there is a rotrogression in the utems (menstruation) and a return to general quiescence in all the reproductive organs. Parturition is to be regarded as the climax of a variety of rhythmic occurrences which have been gradually gathering head for a long period. The changes which take place in the placenta of a degenerative character fit it for being cast oB, and may render this structure to some extent a foreign body before it and the foetus are finally expelled, so that these changes may constitute one of a number of exciting causes of the increased uterine action of parturition. But it is important to regard the whole of the occurrences of pr^^ancy as a connected series of processes co-ordinated by the central nervous system so as to accomplish one great end. the development of a new individual. The nutrition of the ovum in its earliest stages is effected by means in. harmony with its nature as an amoeboid organism ; nutrition by the cells of blood-vessels is similar, while that by villi may be compared to what takes place through.the agency of similar structures in the alimentary canal of the adult mammal. The circulation of the foetus puts it on a par physiologically with the lower vertebrates. Before birth there is a gradual though somewhat rapid preparation, resulting in changes which speedily culminate after birth on the establishment of the per- manent condition of the circulation of extra-uterine life. 186 COMPARATIVE PHYSIOLOGY. The blood of the foetus (u in the adult) is the great store- house of nutriment and the common receptacle of all waste products ; these latter are in the main transferred to the moth- er's blood indirectly in the placenta; in a similar way nutri- ment is imported from the mother's blood to that of the foetus. The placenta takes the place of digestivo, respiratory, and excre- tory organs. Coitus is essential to bring the male and female elements together in the higher vertebrates. The erection of the penis is owing to vascular changes taking place in an organ composed of erectile tissue ; ejaculation of semen is the result of the peristaltic action of the various parts of the sexual tract, aided by rhythmical action of certain striped muscles. The sperma- toEoa, which are unicellular, flagellated (ciliated) cells, make up the essential part of semen ; though the latter is complicated by the addition of the secretions of several glands in connection with the seminal tract. Though competent by their own move- ments of reaching the ovum in the oviduct, it is probable that the uterus and oviduct experience peristaltic actions in a direc- tion toward the ovary, at least in a number of mammals. The lower part of the spinal cord is the seat in the higher mammals of a sexual center or collection of cells that receives afferent impulses and sends out efferent impulses to the sexual organs. This, like all the lower centers, is under the control of the higher centers in the brain, so that its action may be either initiated or inhibited by the cerebrum. ••»*.,* - «?,isf,--;rta •-».TOiDCTan»«*( 2SHi4*ai«-^!^WTB5TS«V#n«'3KI3WW.«W«»4**» r. ORGANIC EVOLUTION RKtONSIDERED. 189 1 its existpnoe be- the changm ; and ings to the other >f necemity, from great importance X the root of the 3 lower but inde- All organiams are id their product*. When a muaole- itter is not identi- mt cell was origi- rirtue of those ex- ad as a membei* of plexities of which >r may mov« in a ng upon it exactly ultant effect in the e case of heredity, m in the offspring, ted, augmented, or ludity in the other n connection with ng development, lings why acquired etc.) may or may ts is to be expected, ten gathering head i. Again, we urge, iation. cells of the body. a certain extent in I by reason of what I biological republic T sexual cells repre- cal story, though it By more than others «rsof specialization it along what paths e. Strong evidence is furnished for the above views by the his- tory of disease. Scar-tissue, for example, continues to repro- duce itself as such ; like produces like, though in this instance the like is in the first instance a departure from the normal. Gout is will known to be a hereditary disease ; noi only so, but it arises in the offspring at about the same age as in the parent, which iH equivalent to saying that in the rhythmical life of certain cells a period is reached when they display the behavior, physiologically, of their parents. Yet gout is a disease that can be traced to peculiar habits of living and may be eventually escaped by radical changes in this respect— that is to say, the behavior of the cells leading to gout can be induced and can be altered ; gout is hereditary, yet eradicable. Just as gout may be set up by the formation of certain modes of action of the cells of the body, so may a mode of behavior, in the nervous system, for example, become organized or fixed, become a habit, and so be transmitted to offspring. It will pass to the descendants or not according to the principles already noticed. If so Axed in the individual in which it arises as to predominate over more ancient methods of cell behavior, and not neutralized by the strength of the normal physiological ac- tion of the corresponding parts in the other parent, it will reap- pear. We can never determine whether this is so or not before- hand ; hence the fact that it is impossible, especially in the case of man, whose vital processes are so modified by his psychic life, to predict whether acquired variations shall become heredi- tary ; hence also the irr^larity which characterizes heredity in such cases ; they may reappear in offspring or they may not. In viewing heredity and modification it is impossible to get a true insight into the matter without taking into the account both the original natural tendencies of living matter and the influence of environment. We only know of vital manifes- tations in 8ome environment ; and, so far as our experience goes, life is impossible apart from the reaction of surroimd- ings. With these general principles to guide us, we shall at- tempt a brief examination of the leading theories of organic evolution. First of all, Spencer seems to be correct in regarding evolu- tion as universal, and organic evolution but one part of a whole. No one who looks at the facts presented in every field of nature can doubt that struggle (opposition, action and reac- tion) is universal, and that in the organic world the fittest to a 140 COMPARATIVE PIlYSIOIiOOT. given environment lurvivM. But Darwin hu pn>)>ably flxed hii attention ton fllt>iiely on this principle and altentpttnl t«> ex- plain t(M> much by it, oa well on foiled to see that there ore other deeper facta underlying it. Variation, which thia autlmr •oaroely attempted to explain, Heoms to ui to bo the natural re- sult of the very condition! under which living things have an existence. Stable equilibrium is an idea incompatible with our fundamental conceptions of life. Altered function implies al- tered molecular action, which sometimes leads to apprecinble structural change. From our conceptions of the nature of liv- ing matter, it naturally follows that variation nhould Ims great- est, as has been obterrvd, under the greatest alteration in thu surroundings. We are but very imperfootly acquainted as yet with the conditions under which life existed in the earlier epochs of the earth's history. Of late, deep-sea soundings and arctic explo- rations have brought surprising facts to light, showing that living matter can exist under a greater variety of conditions than was previously supposed. Thus it turns out that light is not an essential for life everywhere. We think these recent revelations of unexpected facts should moke us cautious in assuming that life tdways manifested itself under conditions closely similar to those we know. Variation may at one period have been more sudden and marked than Darwin suppooei; and there does seem to be room for such a conception as the "extraordinary births" of Mivart implies; though we would not have it understood that we think Darwin's view of slow modification inadequate to produce a new species, we simply venture to think that he was not justified in insisting so strongly that this was the only method of Nature; or, to put it more justly for the great author of the Origin of Species, with the facts that have accumulated since his time he would scarcely be warranted in maintaining so rigidly his conviction that new forms arose almost exclusively by the slow process he has so ably described. We must allow a great deal to use and effort, doubtless, and they explain the origin of variations up to a certain point, but the solution is only partial. Variations must ailse as we have attempted to explain, and use and disuse are only two of the facton amid many. Correlated growth, or the changes in >« part induced by changes in another, is a principle wL'- Vi though recognised hy Darwin, Cope, and others, has not, we ORGANIC EVOLUTION RECONSIDRRED. 141 iw protHibly fixed attempted to ex- )e that there are which this author n the natural re- g things have an upatible with our motion iuipliea ai- ds to apprecinble the nature of llv- Hhould be great- alteration in thu OH yet with the rlier epochs of the and arctic explo- fht, showing that iety of conditions ■ out that light is think these recent ke us cautious in under conditions may »t one period Darwin suppose*; , conception as the though we would Tin's view of slow ipecies, we simply udsting so strongly [)r, to put it more f Species, with the he would scarcely is conviction that ilow process he has Fort, doubtless, and , certain point, but Lst ailse as we have re only two of the the changes in « I principle wLi.ii >therB, has not, we think, receivetl the attention it deserves. To the mind of the phyNiologist, all changes must be correlated with othcns. In what sense has the line that evolution has takiii been prcdeterminetl » In the sense that all things in the universe are unstable, are undergoing change, leading to now forms and qualities of such a character tlu»t they result in a gradual prog- ress toward what our minds can not but consider higher niani- fostatiouN of being. * The secondary methods according to which thiH takes place lonstituto the Uws of nature, and as we learn from the prog- ress of science are very numerous. The unity of nature is a rt«lity toward which our conceptions are constantly leading lis. Evolution is a necessity of living matter (indeed, all matter) as we view it. THE CHEMICAL CONSTITUTION OF THE ANIMAL BODY. One visiting the ruins of a vast and elaborate building, which had been entirely pulled to pieces, would get an amount of information relative to the original structure and uses of the various parts of the edifice largely in proportion to his fa- miliarity with architecture and the varioiis trades which make that art a practical success. The study of the chemistry of the animal body is illustrated by such a case. Any attempt to determine the exact chemical composition of living matter must result in its destruction ; and the amount of inf oi-mation conveyed by the examination of the chemical ruins, so to speak, will depend a great deal on the knowledge already possessed of chemical and vital processes. It is in all probability true that the nature of any vital pro- cess is at all events closely bound up vrith the chemical changes involved ; but we must not go too far in this direction. We are not yet prepared to say that life is only the manifestation of certain chemical and physical processes, meaning thereby such chemistry and physics as are known to us ; nor are we prepared to go the length of those who regard life as but the equivalent of some other force or forces ; as electricity may be considered as the transformed representative of so much heat and vice versa. It may be so, but we do not consider that this view is warranted in the present state of our knowledge. On the other hand, vital phenomena, when our investiga- tions are pushed far enough, always seem to be closely asso- ciated with chemical action : hence the importance to the stu- dent of physiology of a soimd knowledge of dhemical princi- ples. We think the most satisfactory method of studying the functions of an organ will be found to be that which takes into consideration the totality of thw operations of which it is the seat, together with its structure and chemical composition ; — stswjjasB^uaii ■i.^«a»p*a)«!wa'«!»«sstiie8im'>iisK!»BB^^ CHEMICAL CONSTITUTION OP THE ANIMAL BODY. 148 N OF THE »laborate buUdini^, uld get an amount icture and uses of roportion to his fa- trades which make the chemistry of Any attempt of living matter of infoi-mation _ ruins, so to speak, Jready possessed of lase. n lUUti J re of any vital pro- le chemical changes i direction. We are the manifestation of aning thereby such nor are we prepared I but the equivalent y may be considered i heat and vice vena. lis view is warranted when our investiga- i to be closely asso- iportance to the stu- I of 6hemical princi- Ihod of studying the that which takes into OS of which it is the emical composition ; hence we shall treat chemical details in the chapters devoted to special physiology, and here g^ve only such an outline as will bring before the view the chemical composition of the body in its main outlines ; and even many of these will gather a signifi- cance, as the study of physiology progresses, that they can not possibly have at the present. Fewer than one third of the chemical elements enter into the composition of the mammalian body ; in fact, the great bulk of the organism is composed of carbon, hydrogen, nitro- gen, and oxygen ; sodium, potassium, magnesium, calcium, sulphur, phosphorus, chlorine, iron, fluorine, silicon, though occurring in very small quantity, seem to be indispensable to the living body ; while certain others are evidently only pres- ent as foreign bodies or impurities to be thrown out sooner or later. It need scarcely be said that the elements do not occur as such in the living body, but in combination form- ing salte, which latter are usually united with albuminous compounds. As previously mentioned, the various parts which make up the entire body of an animal are composed of living matter in very different degrees ; hence we find in such parts as the bones abundance of salts, relative to the proportion of proteid matter; a condition demanded by that rigidity without which an internal skeleton would be useless, a defect well illus- trated by that disease of the bones known as rickets, in which the limenudta are insufficient It is manifest that there may bo a very great variety of classiflcations of the compounds found in the animal body according as we regard it from a chemical, physical, or physiological point of view, or combine many aspects in one whole. The latter is, of course, the most correct and profitable method, and as such is impossible at this stage of the student's progress ; we shall simply present him with the following outline, which will be found both simple and com- prehensive.* OHBlflOAIi OUMtfriTUTlON OF THB BODY. Such food as supplies energy directly must contain carbon compounds. Living matter or protoplasm always contains nitrogenous carbon compounds. * Taken from the author's Outlines of Lectures on Physiology, W. Drys- dale k Co., Montreal % 144 COMPARATIVE PHYSIOLOGY. In consequence, C, H, O, N, are the elements found in greatr est abundance in the body. The elements 8 and Pare i-saoci ted with the nitrogenous carbon compounds ; they also for... metaUic sulphates and phos- ^ CI and F form salts with the alkaline metals Na, K, and the earthy metals Ca and Mg. Fe is found in hoemoglobin and its derivatives. Protoplasm, when submitted to chemical exammation, is kiUed. It is then found to consist of proteids, fats, carbohy- drates, salines, and extractives. It is probable that when Uving it has a very complex mole- cule consisting of C, H, O, N, S, and P chieEy. Proximate Pbinoipiss. ( (a) Nitrogenous. j Certun cryBUlline bodies. 1. Organic. \ (Carbohydrate*. ( (b) Non-nitrogenous. \ Ji^ta. ^ _ . S Mineral salts. 2. Inorgamc. | yfaier. Salts —In general, the salts of sodium are more characteris- tic of animal tissues and thoje ot potassium of vegetable tosues. Na CI is more abundant in the fluids of animals ; K and phosphates more abundant in tiie tissues. Earthy salts are most abundant in Uie harder tissues. The salts are probably not much, if at all, changed in their vassage through the body. tosome cLs tiiere is a change from acid to neutral or alkaline. . ,, i_iii The salts are essential <» preserve the balance of the nutntive processes. Their absence leads to disease, e. g., scurvy. OBNERAL CHABAOTERISnCM OF PKOTEIDB. They are the dhief constituents of most living tissues, inclad- coftstitution), and is formed of the elements C, H, N, O, B, and f. All proteids are amorphous. All are non-diffusible, the peptone* excepted. They are soluble m strong acids and alkalies, with change of nropertios or constitution. ,-, s.- £ general, tiiey are coagulated by alcohol, etiier, and heating. res. examination, is ds, fats, carbohy- I more characteris- r vegetable tissues. ' atiimn.la ; K and rder tissues. , changed in their icid to neutral or toe of the nutritive f,, scurvy. OTEIDS. ing tissues, inclad- of atoms (complex ,H,N,0,8,andP. ted. lies, with change of ether, and heating. ■ CHEMICAL CONSTITUTION OP THE ANIMAL BODY. 145 Coagulated proteids are soluble only in strong acids and alkalies. Classification and Distinguishiitg Characters of Proteids. 1. Native albumins : Serum albumin ; egg albumin ; solu- ble in water. 2. Derived albumins {albuminates) : Acid and alkali albu- min ; casein ; soluble in dilute acids and alkalies, insoluble in water. Not precipitated by boiling. 3. Globulins : Globulin (globin) ; paraglobulin ; myosin ; fibrinogen. Soluble in dilute saline solutions, and precipitated by stronger saline solutions. 4. Peptones: Soluble in water; diffusible through anima membranes; not precipitated by acids, alkalies, or heat. De- rived from the digestion (peptic, pancreatic) of all proteids. 6. Fibrin : Insoluble in water and dilute saline solutions. Soluble, but not readily, in strong saline solutions and in dilute acids and alkalies. OBATAIN NON-ORTSTALLUne BODIES. The following bodies are allied to proteids, but are not the equivalents of the latter in the food. They are all composed of C, H, N, O. Clhondrin, gelatin, keratin have, in addition, S. Chondrin : The oi^ganic basis of cartilage. Its solutions set into a firm jelly on cooling. Chlatin : The organic basis of bone, teeth, tendon, etc. Its solutions set (glue) on cooling. Elastin : The basis of elastic tissue. Its solutions do not set jelly-like (gelatinize). Mtusin : From the secretion of mucous membranes ; precipi- tated by acetic acid, and insoluble in excess. Keratin : Derived from hair, nails, epidermis, horn, feathers. Highly insoluble. Nuclein: Derived from the nuclei of cells. Not digested by pepsin ; contains P but no S. THE FATB. - The fats are hydrocarbons ; are less oxidised than the carbo- hydrates ; are inflammable ; possess latent energy in a high degree. Cihemically, the neutral fats are glyoerides or ethers of the 10 146 COMPARATIVE PHYSIOLOGY. fatty addB, i. e., the acid radicles of the fatty acids of the oleic and acetic series replace the exchangeable atoms of H in the triatomic alcohol glycerine, e. g. Glycerine, (OH Palmitic Mid, HO.OO.CitH. Glycerine triprimltate or palmltln. O.CO.CiiHii (OH HO.OO.C.H.. i RSS n'w 4. 1H.0 C5.H. } OH + HO.OC.C..H.. = C.H. ] O.OO.CuH.. + 3H.O ( OH HO.OC.C..H.. ( O.CO.CH.. Auoopisfonnedby the action of caustic alkaUeeon fats, e. g. : TrlptUmltm. Pota«lamp.lmltate. The soap may be decomposed by a strong acid into a fatty aoid and a salt, e. g. : C..H...CO.K + HCa = O..H...CO.H + KOI. PotaMlumi«amltate. P«lmlUc acid. The fata are insoluble in water, but soluble in hot alcohol, ether, chloroform, etc. The dUealine soaps are soluble in water. Most animal fata are mixtures of several kmds m varying proportions ; hence the melting-point for the fat of each species of animal is different. PBOUUAB FATS. Lecithin, Protagon, Cerdmn: They consist of O, H, N, O, and the first two of P m addi- tion. They occur in the nervous tissues. OABBOHTDBATES. General formula, C. (H,0).. « „ /^ jm 1 The Suoabs : Dextrose, or grape«igar, C,H.dO, reatttly undergoes alcoholic fermentation ; 'ess readily hwticfermen ^^'IblLstoee, milk-sugar, 0„H„0,. ; susceptible of the lactic acid fermentation. , .. 1 _*• *^__ Inowt, or muscle^raga^, C.H,.0. ; capable of the lactic fer- mentation. . ,. « ± a^ Maltoee, O.AdO„, capable of the alcohohc fermentation. The chief sugar of the digestive process. All the above are much lesa sweet and soluble than ordinary caneHnigar. lids of the oleic ns of H in the tate or palmitin. [!i.H.> CuH.. + 3H.0 0..H.. lies on fats, e.g. acid into a fatty + K01. ) in hot alcohol, kinds in varying at of each species ;wo of P in addi- *, C,H,A readily lily lactic fennen- 5 of the lactic acid 5 of .the lactic fer- olic fermentation. able than ordinary CHEMICAL CONSTITUTION OP THE ANIMAL BODY. 147 2. The Stabohes : Glycogen, CVHraO„ convertible into dex- trose. Occurs abundantly in many foetal tissues and in the liver, especially of the adult animal. Dextrin, C«H„0(, convertible into dextrose. Soluble in water : intermediate between starch and dextrose ; a product of digestion. Pathological : Grape-sug^.: occurs in the urine in diabetes mellittu. Certain substances formed within the body may be regarded as chiefly waste-products, the result of metabolism or tissue- changes. They are divisible into mtrogenoxis metabolites and non- nitrogenous metabolites. Nitrogenous Metabolitea. 1. Urea, luric acid and compounds, kreutinin, xanthin, hypo- xanthin (sarkin), hippuric acid, all occuring in urine. 2. Leuoin, tyrosin, tau.. •. aholic, and glycocholic acids, which occur in the digestive tract. 3. Ereatin, constantly found in muscle, and a few ot!iers of less constar ' occurrence. The above consists of C, H, N, O. Taurocholic acid contains alsoS. The molecule in most instances is complex. Non-Nitrogei/ume Metabolites. These occur in small quantity, and some of them are secreted in an altered form. They included lactic and sarcolaotic acid, oxalic acid, succinic add, etc. S^W^'^li'V^*^"^'-----^''^ PHYSIOLOGICAL RESEARCH AND PHYSIOLOGICAL REASONING. We propose in this chapter to examine into the methods employed in physiologcial investigation and teaching, and the character of conclusions arrived at hj physiologists as depend- ent on a certain method of reasoning. The first step toward a legitimate conclusion in any one of the indtictive sciences to which physiology helongs is the col- lection of facts which are to constitute the foundation on which the inference is to be based. , If there be any error in these, a correct conclusion can not be drawn by any reliable logical process. On the other hand, facts may abound in thou- sands and yet the correct conclusion never be reached, because the method of interpretation is faulty, which is equivalent to saying that the process of inference is either incomplete or in- correct. The conclusions of the ancients in regard to nature were usually faulty from errors in both these directions ; they neither had the requisite Tacts, nor did they correctly interpret those with which they were conversant. Let us first exanune into the methods employed by modem physiologists, and determine in how far they are reliable. First, there is Uie method of direct observation, in which no appara- tus whatever or only the simplest kind is employed ; thus, the student may count his own respirations, feel his own heart- beats, count his pulse, and do a very great deal more that will be pointed out hereafter; or he may examine in like manner an- other fellow-being or one of the lower animals. This method is simple, easy of application, and is that usually employed by the physician even at the present day, especially in private practice. The value of the results obviously depends on the reliability of the observer in two respects : First, as to the ac- curacy, extent, and delicacy of his perceptions ; and, secondly, on the inferences based on these sense-observations. Much a.i«rti«W M MHwi'ii i r»iiii i iiM >ii iiiiiwi:i