TEXT-BOOK OF EMBRYOLOGY MA CM ILL AN AND CO.. LIMITKD LONDON • BOMBAY • « AI.CCTTA • MADRAS MELBOURNE THE .MACMILLAN COMPANY NKW YORK • BOSTON • CHICAGO DALLAS • SAN KRANCIsro TIIK MACMILLAN CO. OF CANADA. LTD TOHON ITo T-oo\ K TKXT-HOOK OF EMBRYOLOGY VOL. II VERTEBRATA WITH THE EXCEPTION OF MAMMALIA BY .1. (Jit A HAM KEEK EtEOIUS PROFESSOR <>K 7.011 i.oov IN TDK, UNMVEHSITV UK i;i,. \si.ow MACMILLAN AND CO, LIMITED ST. MARTIN'S ST1IKKT, LONDON 1919 COPYRIGHT TO THE MEMORY OF THREE CAMBRIDGE NATURALISTS CHARLES DARWIN, M.A., CHRIST'S COLLEGE FRANCIS BALFOUR, M.A., TRINITY COLLEGE ADAM SEDGWICK, M.A., TRINITY COLLEGE PREFACE THK object of this volume is to sketch in its main outlines the science of Vertebrate Embryology as disclosed by the study of the in Hi -mammalian vertebrates. It is not meant as a work of reference as regards details. The facts of embryology are dealt with as illustrating general principles: large masses' of data which have no particular bearing, in the present condition of knowledge, 'are deliberately omitted. It is believed that a volume upon the lines indicated is greatly needed — not merely for students intending to specialize in vertebrate morphology but also for students of medicine who desire to know something of the framework of morphological principles which serves l,o unite together the detailed facts of anatomy. The science of embryology, in fact the science of animal morphology as a whole, IK is suffered much through the patient but undiscrimiuating accumulation of masses of mere descriptive detail which have tended to obscure general principles and incidentally to smother interest in one of the most fascinating of sciences. It is hoped that the student who reads through portions of this book will have at least his suspicions aroused that behind the dull facts of anatomical structure there lies a very charming and living philosophy. It has again been one of the misfortunes of vertebrate embryology that its teaching has been dominated in great part by general ideas based upon insufficient data. In an evolutionary science like morphology the real fundamental principles are to be elicited by enquiry into the more archaic types of existing animal life. But the material for the earlier embryological investigations was chosen not for its archaicism but rather for purely practical reasons such as vii viii KMliK'YnUHJV OF THE LOWER VERTEBRATES accessibility or ease of investigation. It follows that at the present time when we have knowledge of the more archaic subdivisions of the vertebrata not accessible to the early builders of the science, it is necessary to regard the historical foundations of vertebrate embryology rather critically in the light of the fuller knowledge of to-day. In essaying the writing of this volume I have been fortunate in having at my disposal — for the first time in the history of embryology — developmental material of all three genera of Dipnoi as well as of Polypterus — in addition to the more accessible material of the other relatively archaic groups constituted by the Elasmo- branchs, Actinopterygian Ganoids, and Urodele Amphibians. This has rendered possible an all-round survey of the chief problems of vertebrate embryology which would otherwise have been quite impossible. As already indicated I do not intend this volume as a work of reference on the details of vertebrate embryology: that role is fulfilled by the wonderful and indispensable Handbuch edited by O. Hertwig — of which incidentally I have made constant use and to which I must express my sincere acknowledgments. Nor do I attempt to give full historical accounts of the development of various parts of the subject. The literature lists are merely guides to point the way to the student who desires to extend his reading to original papers. The dates given in these lists are as a rule the dates given on the title-page of the complete volume, and are merely to facilitate finding the particular paper : they must not be taken as giving the actual date of publication of the individual memoir. I have to express my grateful thanks to various friends. As regards the first three chapters I had the benefit of the wise counsel of Mr. Walter Heape, who unfortunately however found himself com- pelled by the exigencies of war work to withdraw from the Editorship. Various chapters ha\e benefited by the help and advice of my fri'-nd and colleague, Dr. W. E. Aj^ir. The entire volume has been read in proof by Mr. James ('Immley and l)r. Monica Taylor, to both of whom I am deeply indebted. To Dr. John Love and to Dr. .lam- L'obnison I am indebted for helpful criticism in regard to Special -cci n,ii of (he book. The illnslr it \m\< which form a marked feature of the volume I PREFACE ix owe tor the most part to the artistic skill, combined with high scientific accuracy, of Mr. Kirkpatrick Maxwell. Apart from the completely original figures it, will be noticed that there are many which have heen worked up from illustrations in original papers, hut which are practieally new figures. In all such cases, however, I have thought it, only ri^ht to make due acknowledgment of the author of the original ligure. Kor perm ission to borrow particular text-hook figures 1 have to thank 1'roi'essor Frank R Lillie, Mr. John Murray, Messrs. Masson \ Cie. and Messrs. Macmillan. The present unfortunate circuin- slanc.es of international strife call for a special acknowledgment of the generous way in which Professor Alf. Greil entrusted to me the originals of his valuable unpublished figures illustrating the development of the heart in the bird. They are reproduced on pages 384 and 385. I have included the name of Charles Darwin in the dedication of this volume to emphasize the fact that Embryology is primarily a branch of synthetic evolutionary science. While the fashion of the day in evolutionary research favours rather experimental research into the phenomena of inheritance and more or less speculative enquiry into the ultimate mechanism of inheritance or into the possible causes of evolutionary change — morphology, and more especially embryology, is steadily at work all the while, mapping out the paths along which the evolution of organisms and their con- stituent organs has taken place. Working away in comparative seclusion, unadvertised, and for the most part unnoticed, embryology is thus building up an important part of the framework of what will be the permanent edifice of evolutionary science. J. GEAHAM KEEE. February 3, 1919. CONTENTS CHAPTER I PACK SEGMENTATION, GASTRULATION, AND THE FORMATION OF THE GERM , LAYERS ........ 1 CHAPTER II 'I'm SKIN AND ITS DERIVATIVES ..... 69 CHAPTER III THE ALIMENTARY CANAL . . . . . .144 CHAPTER IV THE COELOMIC ORGANS . . . . . .197 CHAPTER V THE SKELETON . 288 * CHAPTER VI VASCULAR SYSTEM ... ... 360 CHAPTER VII THE EXTERNAL FEATURES OF THE BODY . . .429 CHAPTER VIII ADAPTATION TO ENVIRONMENTAL CONDITIONS DURING EARLY ST.V OF DEVELOPMENT . . . . . .455 xi xii EMBRYOLOGY OF THE LOWKK VERTEBRATES CHAPTER IX PAGE SOME OF THE GENERAL CONSIDERATIONS RELATING TO THE EMBRY- OLOGY OF THE VERTEBRATA 484 CHAPTER X THE PRACTICAL STUDY OF THE EMBRYOLOGY OF THE COMMON FOWL 508 CHAPTER XI HINTS REGARDING THE PRACTICAL STUDY O^ THE EMBRYOLOGY OF THE VARIOUS TYPES OF LOWER VERTEBRATES . . .558 APPENDIX THE GENERAL METHODS OF EMBRYOLOGICAL RESEARCH . 573 INDEX 583 CHAPTER I SEGMENTATION, GASTRULATION, AND THE FORMATION OF THE GEKM LAYERS THK Vertebrate begins its individual existence in the form of a single cell, the Zygote or fertilized egg, which in turn originates in the process of fertilization by the fusion or conjugation of two gametes.1 Of these the ruicroganiete or spermatozoon, derived from the male parent, is of relatively insignificant bulk as compared with the inarrogamete or unfertilized egg. As a consequence the more obvious features of the Zygote, such as shape, size, and so on, are simply taken over from the macrogamete — in other words, they are of maternal origin. Such maternal features may remain obvious for some time during early stages of development, so long in fact as the maternal protoplasm remains predominant in bulk as compared with that elaborated under the control of the Zygote nucleus, but it seems unnecessary to assume that this fact has the important bearing upon questions connected with Heredity which has been claimed for it by some workers on Invertebrates. The Zygote is a typical cell, composed, so far as its living substance is concerned, of cytoplasm and nucleus, the cytoplasm containing a lesser or greater amount of food-material or yolk. In shape it is in the vast majority of cases approximately spherical. In the Myxinoids it is elongated, almost sausage-shaped, and in a certain number of cases, for example Amia, its shape is literally " oval." The macrogamete — and therefore the Zygote- differs much in size in different Vertebrates, ranging from about -1 mm. in diameter in Ampliioxus to as much as 85 mm., or more, in the case of the African Ostrich. In some of the Sharks the size of the Zygote is also very great and this was doubtless the case too, with tbat of such extinct birds as Aepyornis? Such relatively huge Zygotes are of 1 A general account of the processes of gametogenesis and fertilization has already been given in Vol. I. and they are not further dealt with in this volume. 2 Assuming that the /ygote of Aepyornis bore the same ratio in size to its protective envelopes as doe.s that of the Ostrich it would measure about 160 mm. in diamet.-r. EMBRYOLOGY OF THE LOWER VEETEBKATES . Trigla yurnardus 1-5. Ayonus cataphractus 1-8. Trachinus vipera 1-3. Scomber scombrus 1-2. Gobius minutus 1 — 1-4. Cyclopterus lumpus 2-5. Anarrhicha* //'y /> -.-• 1 < i. Paludicold fuKomaculata \. Enyystoma ovale 1-25. Cornufer aalomonit ">. Rhacophorus n/nir,ir•' subdivision making "r'r:^iv*--/."'\'"*V»'";.: :l:: the yolk readily ^3$:?:g#sg$$:< assimilable and so available for meta- B.— Section through egg ot / r, showing more marked hnlin 'Prl« WhprP ten.U-n.-y lor the yolk -ranulos t., concentrate towards the abapical pole (Telolecithal condition). the yolk is com- J ,, n, nucleus. p, pigm.Mit. paiatlVely Small in Tne yolk grannies are Indicated In both flgnrea by dark doU. amount, as in Am- phioxus (Fig. 1, A), it may be distributed nearly equally through- out the egg substance; in other words there is an approach I EMBRYOLOGY OF THE LOWER VERTEBRATES CH. t" tlit' isolecithal erudition : but us a rule in the Vertebrate the yolk is large in amount and is concentrate 1 towards the lower or abapical pole df tin- egg. the protoplasm towards tin- upper or apical p<>le being comparatively poor in y«,lk Telolecithal condition). This segregation of the dead yolk and the living protoplasm towards opposite poles of tin- egg is well seen in the relatively huge egg of the bird where the protoplasm is concentrated in a, germinal disc containing practically no yolk and forming a cap at the apical pole of an enormous mass of yolk practically free from protoplasm. It has already been indicated that the egg may have a charact.-r- istic coloration due to the colour of the yolk. Such yolk coloration may be looked upon as accidental and without any special biological significance in itself. Many eggs on the other hand especially amongst the Ganoid fishes and the Aniphibians are given a dark colour by the presence within them of brownish-black pigments belonging to the melanin group. Such pigment appears to be of definite biological significance, providing as it does an opaque coat which protects the living protoplasm from the harmful influence of light. Eggs in which it occurs develop, as a rule, under conditions where they are exposed to intense daylight. The eggs of ordinary Frogs and Toads for example which are surrounded by clear trans- parent jelly have a well-developed pigment coat. On the other hand in the case of Frogs and Toads whose eggs are surrounded by light- proof foam (see Chapter VIII.) or are deposited in burrows under- ground they are commonly without pigment. In all probability this deposition of melanin pigment in the superficial protoplasm of the egg (normally in its upper portion) is to be interpreted as having been originally a direct reaction to the influence of light, the metabolism being so affected as to bring about the formal ion of this particular iron-containing excretory pigment. It may be objected that the tpigrneiit is produced before the egg is laid (e.ij. tin- < 'ommon Frog) and therefore before if is exposed to the action of Light, but as a matter of fact the body-wall of the adult is by no means opaque to light rays and even while still in the ovary the exposed to tin- influence of faint light. If \\r may take ma prol.ahlc, that the inlhieiiee of natural selection has gradually developed in such eases the particular type of sensit iveiiess to light \\hich lei. Is to the formation of melanin, on account of its protective Value, then there is nothing surprising in the developing of th 't earlier aud earlier periods until at laM it has r. -ulted in i in- pigmentation of the still intra-ovarian egg in response fo the I'eehle light Hi \ - \\llicll penetrate I lie hod\-Ua!l. The other possible explanation of this precocious pigment forma- tion is that the product inn of tin- pigment t hough originally taking reaction to light in the laid egg, ha< become so ,III»M| in t he const it ut ion of the Specie* that it mm ( nines about ffl the i Of the in ' lllllllllS. The object foil to this nation i- that H populates I lie inheritance .if an •'a i SEGMENTATION character,' 'Hid llint is unfurl imately no| justified by our knowled so far as it goes at present. SEGMENTATION The first important steps in the evolution of the unicellular Xygnte into the mult icellular adult are seen in the proc. Segmentation which is, in fact, a process of niitotic cell di\ision showing special peculiarities in different groups of the Vertebrata. During this process there appear in succession on the surface of the grooves which gradually deepen and eventually divide the egg incompletely or completely into dis- tinct, segments or Blastomeres. Before entering into the details of this process it will be convenient to describe it in outline and define the various technical terms used in its description. The first phase of segmentation is commonly marked by the appear- ance of a superficial groove which may conveniently be designated by the letter a, passing through both the upper and lower poles of the egg. Such a groove or furrow is termed meridional, as it marks a great circle on the surface of the egg corresponding to a meridian of longitude on a terrestrial globe. The single nucleus of the Zygote meanwhile divides by mitosis — a daughter nucleus passing into each hemisphere. From the known facts Fie. 2. — Diagram to illustrate technical terms used in describing Ihc pro. mentation. «./', apical poll1; iih./i. ak-ipicul pole ; <•, cqua- irial furrow; /, latitudinal furrow; ///, meri- dional furrow ; /•. vertical furrow. of fertilization we have reason to believe that the Zygote nucleus contains exactly equivalent amounts of chromatin from each of the two parents. In the process of mr this maternal and paternal chromatin is again shared equally between the two daughter nuclei. The first meridional furrow gradually deepens so that the becomes completely divided into two blastomeres or segments each representing a hemisphere of the Zygote. A second meridional furrow (/?) now appears in a plane perpendicular to that of the first and by the deepening of this the egg becomes divided into four equal blastomeres. The next furrow to appear may lie one running round the equator of the egg (equatorial). In eggs. howe\er. which are not absolutely isolecithal — and this holds for all the lower \ i apical pole of the egg, their lower ends gradually extending down- wards towards the abapical pole. This phenomenon appears to In- due to the retarding influence of the dead and inert yolk. The proportion of this to the living protoplasm becomes greater and greater as the distance from the apical pole is greater, and in correlation with this the retarding effect becomes more and more pronounced. After segmentation has reached the stage indicated its tnrther progress tends to become irregular. New furrows make their appearance — latitudinal, and vertical or meridional — and tlxe surface of the egg takes on the appearance of a mosaic- work, while its substance becomes cleaved or split apart into corresponding blast o- meres as the superficial furrows gradually deepen into slits. At somewhere about this period there begins a new type of initoiic division in which the individual hlastmneiv becomes split in .1 din-ciioii parallel to a plane tangential to its outer surfa< that it divides into an outer hlastomeiv visible in surface \ie\\ and an inner one concealed in the interior of the egg. With tin- further progress of segmentation the blastnm- divide over and over aga in, so that e\eiil uallv the egg is eonverled into a very large number of small cellular elements. The rapidity wilh which tin- e^lls divide bears a rough Inverse relation t" the richness of 1 heir conlenl s in yolk. head inert \olk lends to cause the '-ell to lag behind in the prooeSB of division, and the resiill of tllH in tclnlecit h;i| eggS is I hat the ditfeiVlice ill si/e between micronjei-es and macro meres becomes more and more marked as segment at ion 'joes on ih,- |..\\er and mmv richly ynlked segments i SEGMENTATION 7 tending in lag. in their mil.. lie division, more and more Kehind the I olkv upper elements. This inequality is found at its iiiaxiniuin in the large eggs of Elasmobranchs, Eeptiles, and Birds, where the main mass of the egg has its proportion «>!' protoplasm reduced so nearly to vanishing point that it does not divide at all. It is only a small portion of the egg in the neighbourhood of the apical pole that is rich enough in protoplasm to carry out the process of segmentation into separate cells. This is known as the germinal disc or, later on, when it has segmented into a mass of cells, blastoderm. An egg of such a type, showing partial or incomplete segmentation, is termed meroblastic in contrast with the more primitive holoblastic type in which the egg segments as a whole. The blastomeres into which the egg divides being composed of protoplasm — a somewhat viscous fluid — tend under the physical laws of surface tension to assume a spherical shape except when flattened by pressure against their neighbours. There thus exist normally chinks between the blastomeres filled with watery fluid. As the process of segmentation proceeds . this intercellular fluid increases in amount and the process normally culminates in the stage known as the blastula. The blastula consists of a more or less spherical mass of cells surrounding a relatively considerable volume of fluid which is for the most part no longer distributed in small chinks but collected together into a large space — the blastocoele or segmentation cavity. In the simplest case, that of Amphioxus, the wall of the blastula is composed of a single layer of cells — the. cells towards one pole being larger and containing fine granules of yolk or food material. In holoblastic Vertebrates above Fishes it is however, as a rule, no longer composed of a single layer, the roof of the segmentation cavity being frequently composed of two layers while the floor is composed of a thick mass of large heavily yolk-laden cells. The details of the segmentation process may now be followed out as it occurs in the various types of lower Vertebrates. AMPHIOXUS Amphioxus is, of all the lower Vertebrates, that in which developmental processes are least interfered with by the presence of yolk, and for this reason the phenomena shown during its seg- mentation must form the basis for the comparative study of the corresponding phenomena in the Vertebrata in general. The process of segmentation in Amphioxus was described first in two works which are now amongst the classics of morphological science: the first by A. Kowalevsky (1867) and the second by B. Hatschek (1881). The process begins (Fig. 3) with the appearance of a depression of the surface in the region of the apical pole. This depression takes an elongated groove-like form and extends outwards at each 8 EMBKYOLOGY 0* THE U>\VKi; VERTEBRATES GH. end until finally it forms a wide meridional valley encircling the entire egg (Fig. 3, B). This valley gradually deepens dividing the egg into two halves. Finally after about 5 minutes from the commencement of the process the protoplasmic bridge connecting the two halves snaps across and the egg is now completely divided ^ . .;. Illustrating tin- proci's> of s.-im-nUti.ui • >!' tin- 1-1:1; <>t .!//// :il,o\c in i-:iHi li^nir. Th,. .s.-r.-ml |,.,I;n Ix.ilv is si't-n in prMximily t«> it. into t\\«» Mastnmeres, each ot \\hieli assumes a splierieal shape in .il.K-r tension. The l\\<» |.la.slnmeres he.-i.nie slightly tl.iltein-d \sheiv Lln-v ;iiv in euntaet i.e. in 1 he plain- <>!' I he iirst iMilial flllTMU Fig, :',, C). Tin- lllture Cdiirse nf de\ -elnpnu-ilt nhoWH that this plane (-(.nvsj.onds tn the si^il lal J'lane «»f the enihryo (< Vrl'uiil a i ne. IIMHJ): in nther \\nrds the t\\«» 1 ila^l mm-ivs represent ih«- ri.:_rlii and i.-n halves of the developing individual, i SEGMENTATION 9 After ;i resting period of about ;ui hour a seeond meridional furrow develops in a manner simil;ir to the first and in a plane perpendicular to the plane of the first furrow. This gradually deepens and each hemisphere becomes divided into two blastomeres, eaeh of \\hirli as In-fore assumes a spherical shape and then becomes flattened out slightly against the other. Of the four blastomeres \\hieh art- m»\v present two, shown by subsequent development to In- anterodorsal in position, are according to Cerfontaine normally smaller than the other two. The two meridional furrows (a and /?) are followed after an interval of about a quarter of an hour by a latitudinal furrow slightly above the equator and this divides each of the four segments into two. The egg now consists of eight blastomeres — four smaller micromeres on the apical side of the latitudinal division plane, and ,*•* I I M Fi*;. 1. — Apical view of Amphioxus i-ggs at the eiglit-blastomere stage. (After E. B. Wilson, 1893.) A, <; Radial " type ; B, " Spiral " type ; and C, " Bilateral " type. four larger macromeres upon its abapical side. Each micromere lies, according to Hatschek, exactly over the corresponding macromere so that the apical side of the egg as seen from above looks like A in Fig. 4. Wilson (1893), followed by Samassa (1898), has however drawn attention to the fact that in a considerable proportion of cases the cap of four micromeres is, as seen from above, rotated in a clockwise direction through, an angle varying from 0° to 45° (Fig. 4, B) thus conforming to Wilson's " spiral " typa of segmentation or cleavage. Again in a still smaller percentage of eggs at this stage the blasto- meres are arranged according to Wilson's " bilateral " type (Fig. 4, C) the eight blastoineres being arranged symmetrically on. each side of the first division-plane but either two or all four macromeres being displaced outwards somewhat from this plane. Fourth division. After another short interval (less than a quarter of an hour) a new set of furrows appear bisecting each of the already existing blastomeres so that the embryo comes to consist of sixteen blastomeres arranged in two tiers, eight micromeres above and eight macromeres below (Fig. 3, F). Hatschek described this fourth set of furrows as being meridional (Fig. 3, F) while according to Cerfontaine (1906) the division planes are when first 10 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. indicated meridional in position luit become displaced somewhat so as to be in the case of the micromeres perpendicular to the first (sagittal) division-plane or in that of the macromeres slightly oblique. Fifth division. Each blastomere divides again,1 the smaller blastonieres towards the apical pole dividing rather earlier than the others, and the result is that there are now thirty-two blastonieres in all, arranged in eight meridional rows of four cells each, the cell at the lower (abapical) end being decidedly larger than the others. Between these four large elements a wide opening is present (Fig. 3, G) leading into a space which made its appearance as a little chink between the blastonieres of the four-cell stage but which has since then increased greatly in size. This space in the interior of the egg is the blastocoele or segmentation cavity. From this period onwards the segmentation process becomes less regular. There has already shown itself a tendency for the larger blastonieres towards the lower pole to lag behind somewhat. And the arrangement of the blastomeres becomes less regular as they become smaller and fit more closely together. In particular the bilateral symmetry in the arrangement of the blastonieres which is conspicuous in most of the eggs during the earlier stages (Cerfontaine) ceases to be apparent. To summarize the later phases of segmentation it may simply be said that the blastomeres go on dividing, the segmentation cavity increases in size, its communication with the exterior closes up, and there is formed eventually a blastula of approximately spherical shape. The wall of the blastula is composed of a single layer of cells those towards the apical pole being smaller and less rich in yolk than those on the opposite side (Fig. 3, I). RAN A In the case of the Frog we have an egg in which as compared with that of Amphioxus there is present a much greater proportional amount of yolk and which in consequence serves well to illustrate tin nature of the influence of yolk upon segmentation. The process of segmentation begins with the appearanet . in the n <»f the apical pole, of a small dimple on the surface of th which -radiially lengthens <>ut to form the first meridional furrow (a). The fin-row gradually extends downwards over the surface of the egg (I A) and becomes completed by reaching the lower pole after about an hour and a <|iiarter." It also extends inwards from the B1U&06 and finally biseets the egg into two hemispheres. Tin- S.M-OJH] I'm row (fl) is ;(}><> meridional and is in a plane 1 A 'i "I diaeiWMt&oiei between tin- .-H-i-omits xi\'-" I'.v «liii'i-ivnt observ. \-l.ik-- il M prODftblfl tlial lln-n- i- ."h M.i.iM, \.ni.-iliililyiii tin- • l.-l.iil- «.! * S<-« ii«.\\. \« -i, I. it., r,,i oration in reftri QM '" ''"• 'i''"1 rwtm- '" «l«i\«il«'i'iin-iit. SEGMENTATION 11 perpendicular to I hat of the first. It appears about t hree-.|iia i h-i > of an hour after the latter and, like it. extends d<»\\ nwards and inwards so that the egg becomes divided into lour approximately e<|iial segments. The third furrow is latitudinal in position being situated Kig 5, 0) roughly about l_;() above the equator. It extends inwards and the egg is now converted into eight blastomeres, four mien niM'ivs towards the apical pole and four inacromeres towards t he lower pole. Closer study of these first three cleavages in the case of the 1'Yog brings out a number of im- portant points. It will be noticed in Fig. 5 that the circular area of the egg-surface which is free from pigment is placed somewhat eccen- trically so that at one edge it approaches the equator of the egg much more nearly than it does at the opposite edge. It will be noticed further that the egg as judged by the distribution of pigment is arranged symmetric- ally about the plane of the first furrow. This furrow seems to cor- respond, under normal conditions, with the sagittal plane of the embryo, and therefore the two hemispheres separated by the first furrow correspond to the right and left halves of the embryo. The study of later stages will bring out the fact that the point in the boundary of the unpigmented portion which lies nearest to the equator marks what will become the posterior end of the embryo. From the time of appearance of the third furrow onwards wide differences occur between different eggs. Occasionally one may be found in which matters proceed with diagrammatic regularity. T\\o new meridional furrows appear intersecting the angle between " and ft and like the latter they gradually extend downwards, halving each of the existing blastomeres and thus giving rise to sixteen blastomeres — in two tiers of eight, micromeres above, macro- meres below. Then a latitudinal furrow appears dividing the mieromeres, and later a similar furrow dividing the macroineres ; so that there are now four tiers of eight blastomeres each. Commonly however there is no such regularity either in the arrangement or in the time of appearance of the furn»\\s. The meridional furrows in particular tend to be replaced by vertical FIG. 5. — Illustrating segmentation of Frog's _•. (After Schtdtze, 1S99.) L2 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. n furrows which intersect u or (3 at some distance from the poles. As regards the variation in order of development of the various furrows a good idea will be got from Fig. 6. Whatever be the case with the divisions immediately succeeding the eight cell stage, from now onwards there is little regularity. All that can be said is that each individual blastomere goes on dividing over and over again, the length of time elapsing between successive divisions bearing a rough relation to the amount of yolk present in the particular blasto- mere. Already at the third cleavage the eight blastomeres have «a distinct chink — the com- mencing blastocoele — between their inner ends and as segmentation goes on this space becomes larger. The thirty- two-cell stage is a blastula which in a meridional section (Fig. 7, A) is seen to correspond in its general character with the blastula of Amphioxus but to differ from it in three features: (1) it is of larger size, (2) it is composed of fewer cells and (3) the difference in size between the less richly yolked cells towards the apical pole and the more heavily yolked cells towards the opposite pole is more marked. As development proceeds a farther differ- ence becomes apparent. In the various mitotic divisions during the preceding phases of segmentation the ax is of the spindle has been arranged more or less tangentially but now divisions begin to take place in which the spindi(1 ;IXrS are arranged radi- -n , . . . ally and the division -planes bangentially. When this happens one of the t\\o result- ing daughter cells is nearer the centre. the other nearer to the surface of the Pw, 6.— tion in tin- order of ;ipjip;ii ancc of the first (Alter .lurdiiii and Ky<-Ics liym.-r. IS'.. |. i III time, of the >f tha forrbwi fe In. blastula a.nd the effect ot repeated d:\ in • of this type is that the blast ula -\\all loses ~~; its original cbararter of beinu; com; only ot ,i singje layer of cell-; and becomes .1 oellfl tlnci i; • I ;i> follow •IIS Tip .inv ordinary KlaHinobraQOh such as a 1 loutish, Sk or TnrpiMln. illuslralr I In- 1 \ p.- . -illation that, takes place SEGMENTATION in an r-j^ in which the proportion of yolk present approaches the maximum. In this case tin- /y^ote nucleus commonly undergoes two mitolie divisions be-fmv there is any external symptom of segmentation of the cytoplasm. Usually a single furrow makes its appearance first, incising the surface of the germinal disc I ml m.t r\ tending to its periphery (Fig. 8, A). Occasionally a second regular furrow make- its appearance intersecting the first at right angles Ki<;. 7. — Vertical (meridional) sections through blastulae of Frog. (From Morgan, 1897.) l It, commencing imagination ; SG, segmentation cavity. and it is a curious point that it is sometimes this second furrow which corresponds to the first nuclear division. These first two furrows apparently represent the ' first two meridional furrows of the holoblastic egg though in the Elasmobranch the first to appear may be either a or /3. More usually, in place of a second regular furrow developing, irregular branches of the first furrow, or even independent furrows, appear and an arrangement of somewhat radiating furrows is brought about which gradually converted into a network (Fig. 8, B, C, D). It should be noticed in regard to these segmentation furrows 14 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. that the first latitudinal furrow cannot be identified in the Elasmo- b ranch and further that the study of sections shows that the furrows sometimes cut into the germinal disc obliquely instead of being perpendicular to the egg surface. The nuclei of the blastoderm divide synchronously and after four di \isions have taken place, when there are sixteen nuclei in place of the original single zygote nucleus, the segmentation furrows (Fig. 8, C) form a network dividing up the blastoderm into smaller central and larger peripheral blastomeres. These blastouieres are, however, not completely isolated from one another but are still in continuity B i-'n,. 8, •Surface views or tin- blastoderm of Elanmobrancha illusti-Mtin- tin- ]>i of s,. -,,,,. ,,t;l{ iOI1. (A I! IT kuekert, 1899.) D i -njiimn. H& B* showB an abortive fragmentation whioh ',>•t the p-i niiii.-il disc | M.-nu-v !•• se-tuent s ot rf|. at tlu-ir bases, ami. in the rase of the ].ln-ral Mast oincivs. a! tli.-ir (.llt.T (Mr l'|. In Ihr lil'lli lnitntic ilivisinFI lllc a\CS <•!' lllr lllilnlic Spilldli'S have l«-rn appnixiiiiMh-ly parallel to the surface Imt im\\ l.lasl(.nicivs begin 1« «livi«l«- \\illi lln-ii- spindlr axes perpendicular t<> llie surface K> thfi "f supel lieial seLHiienls 1). -ci ni les sepa I'a I e. I (ill'. I'.elieatll tliiid aceuiiiulatcs inlereelliilai'ly and a se^ineiitat inn cavity I-) Muring lli«- -ixlh di\i-i"M «.me nf the Mast miieivs 1'ni-iniii^ the ii.-ntatinn Oavitj l.cn.nie sepai-aled .ill' IVnni the SEGMENTATION 15 underlying, unflegmented, yolfc ( Ki.^. '.', <"> ;md in surface view the blastoderm a-sumes the appearanc-e shown in Fig S, 1). I'p t«i a.nd including the seventh division mitosis takes place B $&'. ..:-;;i§;- £'?•"•' Fie. !». — \'ci tical sections tlirnu^li Kl:isni(il.ran<-h blastoderms illustrating tin1 process of segmentation. (After Riickert, 1899.) \. 0, I', K, Torpedo ; 1!. . ; F, (i, I1, -latin ru*. \'., F, and G are sagittal sections with j>osterior edge of blastoderm tn th-1 ri-ln. practically synchronously throughout tin* blastoderm. In Torj'»/<> Riickert (1899) -found that even in the ninth division the majority of the nuclei still divided synchronously ,-nid lliat in some eggs the same was the ease with the tenth division but in any case appmxi- 16 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. inately about this period individual nuclei lair behind others and the regular rhythm becomes lost. This rhythm of nuclear division is of interest in relation to the size of the individual blastomeres. It is often noticeable in an Elasmobranch blastoderm that the blastomeres are somewhat smaller in what is shown by later development to be its posterior half i.e. the half next the side on which the embryonic rudiment makes its appearance later. It would be natural to suppose that the smaller size of the blastomeres is due to their having gone through a greater number of divisions but this explanation is rendered less satisfac- tory by the synchronism of the mitotic divisions. Apparently the inequality is at least to some extent due to the zygote nucleus, and, later on, the first segmentation furrows, being not quite central in position in the germinal disc but situated slightly towards its posterior edge (Riickert). The stage up to which mitosis remains synchronous varies amongst individuals of one species and a fortiori amongst those of different species and genera. Thus in Pristiurus it is, .commonly, regular only up to the fifth mitosis according to Riickert. While segmentation has been proceeding, important changes have been taking place in the segmentation cavity. About the time of the seventh division the rounded inner blastomeres fill up most of the cavity so that it becomes reduced to chinks between the individual blastomeres. These chinks are filled with fluid secreted by the egg substance, and in the yolk beneath the blastoderm the activity of this process of secretion is indicated by the appearance of fluid vacuoles. As development goes on the amount of fluid increases greatly and about the tenth division it begins to collect especially between the blastoderm and the yolk, forming the "germ cavity" of Riickert (Fig. 9, D, E, F). This cavity is best marked towards the posterior side of the blastoderm and in ground-plan is crescentic in shape. It varies greatly in its degree of development in different individuals. Whether it is advisable to use a separate name for this cavity is \. -i v questionable. When a broad view is taken of the relations of blastomeres and segnientat inn cavity in the Elasmobranchs these seem t<» be similar in kind to those which hold in the case of the LIIIILT fishes. In these fishes, as will he shown later, the blasto- whieh originally formed the floor of the segmentation cavity become later on shifted in position towards its roof hut the leMillant change in the I opo-raphical relat ions and form of the it y would clearly alf<.rd no valid reason for ^ivin- new name. The Yolk Syncytium. The layer of substance immediately underlying tin- blastoderm and segmentation- ,,r ^erm-ca vily is dMiirjui-lied from the main : yolk upon \\hich it in turn i.y the !im-r -jiMim-d character of its yolk granules, and by its SKC MENTATION 17 richness in pmii»pl;isin. This layer slmw.s no division into cells and is therefore termed 'the yolk-syncytium1 (H. Virchou : Kiickert's term " merocytes " is synonymous). The marginal portion round the edge of the blastoderm is sometimes termed the germ- wait Functionally the yolk-syncytium is apparently concerned especi- ally with the digestion and assimilation of the yolk. Scattered ahout in it are nuclei, often of enormous size and irregular form. Concerning the origin and fate of these, nuclei much discussion has raged and the matter cannot yet be regarded as satisfac- torily settled. The question is complicated by the fact that, as shown by Elickert (1890), polyspermy appears to be a normal occurrence in Elasmobranchs. In addition to the single micro- gamete which takes part in the formation of the zygote-nucleus a variable number of extra spermatozoa make their way into the egg and give rise to accessory sperm-nuclei. Where such sperm-nuclei are situated in the coarse yolk they apparently soon degenerate but when, on the other hand, they are within the protoplasm of the germ-disc they remain during the early stages of development in a living and apparently healthy condition, even undergoing mitosis synchronously with the nuclei derived from the zygote-nucleus up to the fourth or even fifth or sixth division in the case of Torpedo. The importance of this fact should be noted in connexion with our ideas of the reciprocal physiological relations of nucleus and cytoplasm. It is fully recognized that the nucleus governs and controls cell metabolism : it is not always so fully recognized that conversely the cytoplasm exerts an important influence over the nucleus. Clearly the fact that the accessory sperm-nuclei "keep step " in their mitotic divisions with the embryonic nuclei must be due to some influence exerted on the former nuclei through the cyto- plasm. It should, in fact, never be forgotten that cytoplasm and nucleus are merely locally specialized portions of the same common living substance or protoplasm. At first the accessory sperm-nuclei are clearly distinguishable in the germinal disc from the time embryonic nuclei by their smaller size and reduced (haploid) number of chromosomes. After the zygote-nucleus has undergone two mitoses however — or even before the second mitosis — the accessory sperm-nuclei wander — or become transported by cytoplasmic movements— outside the limits of the germ-disc. They continue their mitotic rhythm for a time so that, for example, at the 8-nuclear stage of the blastoderm they may be seen in groups of eight lying in the yolk-syncytium. During early stages of segmentation numerous such obviously accessory sperm- nuclei may be seen in the syncytium but as time goes on the ;iuclei 1 Although Haeckel originally defined the term syncytium (Die Kalkschir<'i,nin<\ lid. I. p. 161) as a protoplasmic mass funned by the fusion of originally separate cells the word has come into such general use for a multinucleate mass of protoplasm which shows no subdivision into cells, whatever its origin may have been, that there seems no serious objection to the use of the term yolk-syncytium a.s suggested by Virchow. VOL. II C is EMBKYOLOGI or TIIK L<>\VKi; \TERTEBEATES OH. of the syncytium are seen to lie of a different character. They are no\\ of enormous size and of peculiar lobed appearance. The lulling becomes more complex as time goes on and appears to he due to incomplete and irregular attempts at amitotic division. The discussions, alluded to above, have centred round the mode of origin of these highly characteristic giant nuclei. Uilfour. \vh«> first described them (1874), did not express any opinion as to their origin. JJiiekert in his first paper (1885) on Elasmobranch develop- ment looked on them simply as specialized embryonic, nuclei and gave the masses of protoplasm in which they are embedded the name "merocytes." Latterly however Riickert, after his discovery of polyspermy in Eiasmobranchs, has taken the view that the yolk- nuclei are really the accessory sperm-nuclei before alluded to which have altered their character in correlation with the altered environ- ment in which they find themselves after leaving the germinal disc. In spite of Kiickert's more recent observations and conclusions, and in spite of their being supported by Sainassa, Beard and others, FKI. 10. —Views of the segmenting germinal disc of Bdt'llnxtmmi. sh.nti. (After Bashford Dean, 1899.) it must, I think, be admitted that the sperm-origin of the yolk-nuclei is by no means demonstrated. And all general considerations are in favour of Riickert's earlier view being the correct one, namely that the nuclei of the yolk-syncytium are genetically of the same order as the ordinary embryonic nuclei. Such general considerations render it extremely improbable that accessory gamete nuclei should really play an important physiological part in the developing embryo : :,ir more probable that such nuclei simply persist for a time. undergo mitosis a few times and then degenerate and disappear. The variations in the process of seamen tat imi arc well illustrated by the three cases just described and it will be con\.'iiient now ;iimaii/c th«- general characteristics of the process in the various remaining groups. LUCFBKI -In the Lamprey the phenomena of segment it inn agree rlo.se I y \\ith those nbsened in t,he 1'n.g ami need nut,' be further described MvxiNoms. In the Myxinoids the some\\hat sausage shaped egg is heavily yolked and p.- I germinal diw situated 01086 to one pole. A le\\ segmentation stages of /////V/o-s-A.///// Fig, ID SEGMENTATION 19 have been y I'.asljlnni Dean (1899) and as might be exported ihe segmentation is meroblastic. Apparently the first two furrows (a and /?) have the normal meridional arrangement the specimen figured by Dean (Fig. 10, A) showing a displacement at the intersection of the two furrows. These latter do not reach the edge of the germinal disc. The third set of furrows (Fig. 10, B) appear to be vertical and in the next stage figured (Fig. 10, C) the furrows have become joined up to form an irregular network \\liicli still barely reaches the edge of the blastoderm. CUOSSOPTERYGIANS. — Our knowledge rests entirely on the observations of Budgett (Graham Kerr, 1907). These, necessarily fragmentary, observations suffice to show that the process of segmentation is of great interest. In the earliest stage observed, but not figured, by Budgett the egg was " segmenting in four equal FlO. 11. — Segmentation and gastrulatiou in Polypterus. (Drawings by Budgett. Graham Kerr, 1907.) A, represents a view of the apical pole : the remaining figures an- side views. portions, the constrictions being deeper than in the frog." A second egg (Fig. 11, A and B) is in the eight-blastomere stage. The blastomeres are practically equal in size and it may be inferred with considerable probability that in Polypterus two meridional furrows are succeeded by a latitudinal one which is very nearly equatorial. The nearness of the latitudinal furrow to the equator is remarkable in view of the fact that the egg of Polypterus, as shown by the study of sections (Fig. 1, B, p. 3), is not by any means nearly isolecithal. ACTINOPTERYGIANS. — The typical Teleost is characterized by the fact that its richly yolked eggs show a more complete segregation of protoplasm and yolk than do those of any other Vertebrata. In • < Delation with this the segmentation is here the most markedly meroblastic in character. These featuresisuggest that in the ancestral Teleost the yolk was large in quantity and that the egg as a whole was of great size. Amongst present-day Teleosts however it is only, comparatively speaking, a few forms mostly inhabiting fresh water, 20 EMBRYOLOGY OF THE LOWKl; VERTEBRATES pit which pruduee eggs of very lar-e size e.g. Gywnarchns niloticus { Budgett, 1901; Assheton, 1907) where they im-asuiv about 10 nun. in diameter, or the Salmon or Trout where they measure from 4 to 5 mm. Tin1 majority of fishes produce eggs in enormous numbers, amounting in some cases to several millions, and correlated with this the size of the individual egg has become much reduced. The average diameter of a Teleostean egg may be taken as about 1 mm. In an eu;u <»f this size segmentation of so markedly meroblastic a character would be puzzling except on the hypothesis that the meroblastic condition had arisen in ancestral forms in which the eggs were much larger. The larger part of the egg consists of a spherical mass of practically pure yolk. On the surface of this is a thin layer of protoplasm containing droplets of oil, and this layer of protoplasm is more or less distinctly thickened in the region of the apical pole to form a germinal disc in which is contained the nucleus. Irregular prolongations of the superficial protoplasm may sometimes, especially in immature eggs, be traced inwards into the substance of the yolk. A characteristic feature of many teleosts is the tendency for the yolk to assume a liquid form. This is particularly marked in many pelagic eggs where it is not merely liquefied but runs together at the time of spawning or of fertilization to form a sphere of glassy transparency. There may further be, interspersed amongst the ordinary yolk, droplets of oily looking fluid often with a distinctive colour. These may unite into a few droplets or into a single larger drop forming a conspicuous, often coloured, sphere in the midst of the ordinary yolk. The colour and size of such drops frequently afford an easy means of recognizing the species to which a particular egg belongs. They may also have a characteristic position and may be surrounded by a special condensation of protoplasm or, on the other hand, they may simply float freely in the main mass of fluid yolk. Although these droplets may, as already indicated, exhibit peculiarities characteristic of particular species they do not seem to give indications of genetic affinity in regard to genera or I groups: nor do they show any definite relation to the conditions. pelagic or otherwise, under which the egg develops (Prince, 1886). The yolk of teleosts is also characterized by a diminution <>r its specific gravity which causes the egg to assume a reversed position with the apical pole below, and which further, in the case ol' a vast numher of marine lishes, causes the egg as a whole to float freely suspended in the sea water. Sing that 1 lie Teleostri as a uri'"Up isahoyeall eharart eri/ed by ~p.-ei;ili/alion for a swimming existence, independent of a solid substratum, we are perhaps justified in a — inning that the freeh float. in-_r pelagic- modi- of development above mentioned was originally present throughout the -jronp. Tin- demersal type of development. \\lieiv the egg! are deposit, -d on the solid substratum, \\ould then SEGMENTATION 21 l»c regarded as a secondary reversion to, rather than a persistence of, ji pre-teleosteao habit. Possibly tin- reversed position of the egg is to be regarded as a means of protecting its more sensitive apical portion from injury by contact with the surface film of the water ill which it float s. When fertilization takes place the most conspicuous immediate result is t he onset of a gradual concentration of the protoplasm in the uvrminal disc — the disc becoming at .the same time more, heaped up, its vertical diameter in- creasing and its horizontal diminishing. The segmentation of the germinal disc in teleostean fishes is usually of ;t very regular and characteristic kind. It is illustrated as seen in surface view by Fig. 12. Trje germinal disc lengthens out into an elliptical shape. The first farrow to appear (A) is meridional and occupies the shorter diameter of the ellipse. The second furrow is also meridional and in a plane perpendicular to that of the first. The third and fourth sets of furrows (B, C, D) are vertical and they become arranged so as to be practically parallel to the first and second, with the result that the blastoderm as seen in surface view assumes a very characteristic arrange- ment of sixteen segments arranged in four rows (Fig. 12, D). The internal phenomena of segmentation may be described from what occurs in the Trout (Kopsch, 1911). In the first place it has to be noted that the early furrows do not extend Flo 12. —segmentation right through the substance of. the germinal disc but leave a continuous basal stratum of protoplasm next the yolk. The blastoderm assumes a two-layered condition by the 3rd and 4th furrows curving round in their deeper portions so as to intersect the preceding division-planes which were throughout perpendicular to the surface (Fig. 13, B). Up to the 16- cell stage all the segments remain connected by broad protoplasmic bridges apart from the continuous basal layer of protoplasm which connects the deepest cells together. In the 32-cell stage (Fig. 13, C) the cells of the superficial layer have become completely isolated while the deep cells are still connected together. With the next division the blastoderm becomes t hive layered, the cells of the intermediate layer being derived some from the superficial, some from the deep layer, as is shown by the evidence of broad bridges of protoplasm which persist here and th'-re bet \\een sister cells. With the next division (128-cell stage, Fig. 13, D) the four-layer in the blastoderm of a teleostean fish (Ser- ranus airarins) as seen in surface view. (After Wilson, 1891.) KM BRYOLOGY OF THE LOWER VERTEBRATES CH, condition is reached, tin- cells of the basal layer being still connected by a thick continuous stratum of protoplasm. By this time it is found that the nuclear divisions of the basal layer are clearly lagging behind those of the other layers. As segmentation proceeds further the continuous basal sheet of protoplasm decreases relatively in thickness. For a time (Fig. 13, E) bulgings of its upper surface indicate that it is giving off cells into the overlying layer but as the thinning process goes on these become less and less numerous. H. Virchow distinguished three zones in the basal sheet of protoplasm — marginal, intermediate and central, although the latter Fi<;. 13. — Vertical sections through the blastoderm of a Teleost (£/////« ft(rit>) illustrating the process of segmentation. (After Kopsch, 1911.) A, end of second division. Section perpendicular to plane of first furrow which is therefore seen cut across. B, commencement of fourth division. Plane of section as in A. The division surfaces of the third division are seen to curvt- inwards so as to meet the first division surface. As a result the latter has become distorted and no longer forms a plane. C, middle of sixth division. D, be- ginning of eighth division. K, beginning of tenth division. F, «>2-honr blastoderm. (The dark portion at the top of Fig. B represents the free surface bounding the second furrow : the dark tone at thf lower edge of each ligure represents yolk.) is not quite central but situated rather towards the posterior or embryonic edge of the blastoderm. The intermediate zone is marked oll'iVom the others by tin- fact that the thinning process lias there progressed farther. !']» to about the twelfth division the nuelei all through the •derm divide practically synchronously except those of the basal layer \\hich EL£ already indicated la^ behind. Soon alter this however from about the -list, hour Kopsdr the divisions become ilar. The basal layer becomes the yolk svncyi mm : the cell limits >le on its upper >ide become obliterated and it becomes more and more Ilattem-d out. Although Ltfl nuclei undergo repeated mitOftifl i SEGMENTATION 23 then- is no long»-r any budding off of cells, the nuclei simply lying within the substance of the syncytiuin. they increase in number nuclei from the central and marginal regions spread into the intermediate zone, which up to now contained very few nuclei, while others pass outwards into the peripheral protoplasm (Periblast — Agassi/, and Whitman, 1885) ivini: outside the limits of the blastoderm. Towards the end of the second day the syncytial nuclei begin to increase markedly in size and they begin t<> undergo abnormal multipolar mitoses. During the third day they complete the assumption of these peculiarities which are characteristic of the nuclei of a yolk-syncytium — enormous size,curiously lobed appearance, and the tendency for the lobes to become nipped off irregularly so as I.. -i\e rise to groups of small nuclei. During these later stages of segmentation the blastoderm becomes Battened somewhat and instead of bulging out over its attached base all round, its surface passes into the extrablastodermic surface by a slope very much as it did before segmentation began (Fig. 13, F). ACTINOITKUYCIAN GANOIDS. — The ganoid fishes are of special embryological importance because, so far as actinopterygians are concerned, they appear to be the least modified descendants of those ancestral forms from which the Teleostean fishes have been evolved. Study of their developmental phenomena is desirable in order to see to what extent they throw li^ht upon the peculiarities of development which characterize tr« Teleostean fishes. It will be necessary therefore to review the segmentation processes so far as they are known in each of the three types — the Sturgeon, Amia and Lepidosteus. The only Sturgeons of which anything is known regarding their early development are the common sturgeons of the genus Acipenser. Polyodon, Psepliurus and Scapliirhynclius are so far completely unknown, though it is highly desirable that their development should be investigated. In both Acipenser ruthenus (Salensky, 1878) and A. sturio (Bashford Dean, 1895) the segmentation (Fig. 14, A) is of the same general type. The unsegrnented egg measures about 2 mm. in di . i meter in the Sterlet (A. ruthenus), about 2*8 mm. in the Sturgeon (A. sturio). The lower part of the egg contains coarse yolk granules while in the region of the apical pole it is richer in protoplasm and the yolk is more finely granular. The first furrow (a) is meridional, appearing first at the apical pole and gradually spreading downwards and at t he same lime cutting more deeply into the yolk. The second furrow (/2) is similar and at right angles to the first. The third set of furrows seem to be typically vertical (A 2) but they show much variation and may be practically meridional or may show a t endency to be latitudinal. The next set of furrows again vary between vertical and latitudinal and from now onwards there is no apparent regularity in the segmenting of the various blast omeres. There 24 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. eventually results a blastula (A 5) the upper portion of \vliich. forming rather less than a hemisphere, is composed of micromeres while the lower part is composed of large, richly yolked, macromeres. In Ainia the " oval"-sha prd egg measures about 2'5-3 mm. by Ki<;. 14. — Segmentation in Acipenser (A), Ainia (B), and Lepidotfau (0). (After Baslifnnl I », liynn-r. ;ui. I Whitman.) A 1, 2 and 3, 15 I ami •_', C 1 and '.'are \ii-ws .if th.- :i|.u-:il |...l-: the remaining lignn'* an- M.lr \n-\\>. al.nut L^-2'5 mm. In tin- iv^ion of the aj.ical pole which li.-s at lln- «ml of lln- luii-j .1 hajH-il |Mi|-(.i<>ii riclirr in |»n>l oplasin ,ni ;i].|.K.ach in lact to a ^'Tininal disc; — while lln- rest of lh- [| i ich in dark g»1 i-h l>n»\\ n y«»lk. The segmentation i-'i-_r. 11 I', begins, about ao IK.UI- and a hair i SEGMENTATION 25 after Irrlili/alimi, with the successive appearance, at the apical pole, of two meridional furrows (a and /i) \vhi<-h gradually sweep down- wards to the opposite pole <»f the egg. I'.eiore th»-y ivarh it, four \vnii-al I'm mws make iheir appearance, commencing at a point on furrow ./ not. far fr«>m tlie pole and gradually extending downwards over I In- lower pail <>!' tin- t--^ (Fig. 14, B 1). I'ld'niv thfs.r \rrtical furrows reach the lower pole a new furrow —latitudinal — develops a short distance from the apical pole, mark in- oil' a polar group of eight inicrorneres (Fig. 14, B 2). At the next division these divide into a superficial and a deep segment (the former being separate — the latter continuous with the yolky mass beneath) while the macromeres divide by vertical furrows. Next an irregular latitudinal furrow develops below the previously existing one, by which a new micromere is segmented off from the upper end of each macromere. Lepidosteus. — The ellipsoidal or " oval " egg measures about 3'5 mm. by 3*2 mm. and has a cap of protoplasm with fine grained yolk at its apical pole. Segmentation (Fig. 14, C) in its early stage is like that of Amia except that the furrows are more sluggish in spreading downwards over the egg-surface. They never in fact reach much beyond the equator; in other words, in the case of Lepidosteus, the lower hemisphere of the egg does not normally segment at all. The egg therefore has advanced beyond the condition seen in Amia and has become meroblastic. In the later stages of segmentation the region of the upper pole is occupied by a lenticular mass of blastomeres which may be termed the blastoderm, and this is bounded at its lower edge and over its lower surface by a set of elements which remain in continuity with the yolk. Later on the divisions between these elements tend to disappear and their place becomes occupied by a " yolk-syncytium " containing numerous nuclei. To summarize then, we have exemplified by the three ganoids Acipenser, Amia and Lepidosteus, three steps in evolutionary change, associated with an increasing degree of telolecithality, from the holoblastic type of egg met with in Lampreys or Amphibians or Crossopterygians to the meroblastic type as it exists in modern Teleosts. LUNG-FISHES. — The early stages of segmentation have been observed in two out of the three still existing lung-fishes — Ceratodus (Semon, 1893) and Lepidosiren (Graham Kerr, 1900). In the case of Ceratodus the egg measures about 3 mm. in diameter and is pigmented in the neighbourhood of the apical pole. The first two furrows (Fig. 15, A 2 and 3) are meridional and at right angles to one another. Each appears first at the apical pole and extends downwards with varying rapidity. The third set of furrows are vertical and make their appearance usually before the second meridional furrow (/3) has reached the lower pole. The egg 26 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. Uti.,1. ill (A) Ceratodua and (B) / - dosiren. (A after Si-mon, 1893.) Figi. 2-6 are vieu pole : PigK. 1, 6 an> I 7 I becomes thus divided into eight practically equal blas- tomeres. A latitudinal furrow then develops about 45° above the equator, so that the egg now consists of eight micromeres round the apical pole and eight macromeres. After this stage seg- mentation usually becomes irregular although some- times two additional latitu- dinal furrows make their appearance in succession so that the egg consists of four tiers - each of eight blasto- meres. Eventually, as seg- mentation proceeds, a bias- tula is formed of the type shown in Fig. 15, A 7. The segmentation cavity first appears about the time of the fourth cleavage as a small chink. It rapidly expands and in the blastula figured (Fig. 15, A 7) it is of large size. In Lepidosiren (Fig. 15, B) the egg measures usually between 6 '5 and 7 mm. in diameter. It is free from pigment in correlation with the fact that it de- velops in a burrow shaded from the action of light. In the region of the apical pole is a whitish cap in which the yolk is in very minute particles while else- where it is in laruf cnarse granules, The first two furrows Ki-. !;"», 1'. 'J.-unl :> are meri- dional and at ri.irlit angles to one uimlher. The third set nf furrows ( I-'i-. 1 5, I- -I ) are vertical though occa- i SEGMENTATION 27 sionally one or other of them may become Latitudinal Tin- various meridional and vertical furrows gradually extend downwards towards tin- lower [)oh- of the egg in the order of their appearance, arid during the earlier stages the lower hemisphere of the egg possesses only such furrows (Fig. 15, B 6). As the hlastomeres go on segmenting there is produced eventually a Itlastula with an upper hemisphere of small cells which appear white because of the h'nely subdivided condition of their yolk and a' lower hemisphere of larger more yolky elements (Fig. 15, B 7). Already at the stage when the egg is divided into four segment.- a space develops between the blastomeres. As segmentation goes on the micromeres tend to round themselves off, leaving wide chinks between containing fluid. By the blastula stage the fluid has collected together into a spacious segmentation cavity which is visible in the whole egg as a dark shadow in its upper hemisphere. At first the cavity is rounded and is roofed in by a single layer of cells but later it spreads out, takes a planoconvex form and its roof comes to be composed of two layers of closely apposed cells. AMPHIBIA. — The Amphibia are in the matter of segmentation the most interesting and important group of the vertebrata, for in no other group does there exist so much variety in the proportional amount of yolk present in the egg. Much work still remains to be done in regard to this group in the way of detailed study of the process of segmentation in its relation to the amount and concentration of the yolk. As already indicated the extent of the influence which the yolk exerts in retarding the living activities of the protoplasm, such as growth and division, bears a rough relation to its proportional amount. As regards the majority of cases this may be said to vary directly with the size of the egg. The largest eggs are as a general rule the most richly yolked. But the rule is by no means an invari- able one that the influence on the segmentation is directly pro- portional to the total amount of yolk in the egg as a whole. For a smaller egg, containing a smaller amount of yolk, may yet have that yolk more concentrated in 'one region so as to produce there a more intense retarding influence — as is the case naturally in many small Teleostean eggs or as may be demonstrated experimentally by con- centrating the yolk artificially through the action of centrifugal force. 0. Hertwig was able by centrifugalizing frogs' eggs and so causing the yolk to become concentrated in the abapical hemisphere, to bring about a complete cessation of cleavage in that hemisphere so that the egg thus assumed a meroblastic character. The variety in the size of the egg within the limits of the group Amphibia has already been indicated by the table on page 2. The process of segmentation agrees in the main with what has been described for the frog but there is much variation in detail. The variations have to do both with the position of the furrows and with their appearance in point of time. One gets a good idea of the 28 EMBEYOLOGY OF THE LOWER VERTEBRATES CH. general tendency of variation by studying numerous eggs of a single species, for example in the case of Eana palustris Jordan and Eycleshymer (1894) found amongst other variations in the mode of appearance of the first furrows, those illustrated in Fig. 6 (p. 12). And similar ditteivncrs oivur between the eggs of different species. KM.. l»j. — Variations in topographical relations of early segmentation furrows in the egg of Rana temporaria. (A, B after Morgan, 1897. ; C after Jenkinson, 1913.) The figure in each case represents a view of the apical pole of the egg. As regards difference in position of the furrows two of the commonest variations are the following. At the four-blastomere stage two blastomeres may be pressed outwards from the apical pole as in Fig. 16, A. Again meridional furrows may be replaced by vertical furrows as in Fig. 16, B and C. As regards variations in time these are chiefly associated with the retarding of segmentation in the lower yolk-laden segments. This reaches its maximum, so far as Amphibians are concerned, in the Gymnophiona, where segmentation spreads so slowly into the lower KM.. 17. Vertical n stages however 'hat the yolk does eventually segment although lily, I']."" tin- whole il leemfl 1» he the case that the I'mdele i SEGMENTATION 29 Segments more slowly during JiL lra>t the first stages — than does thai, <»!' Lin- A n lira. It may be said also that, on tin; whole, eggs with a lar^c mass of yolk show a tendency for the first latitudinal furrows to IK- nearer t he apical pole, so that the inicromeres which they cut oil' are relatively smaller. Also it seems to he the case that in the lower, more yolky. parts of the egg the latitudinal furrows are retarded to a particularly great extent, so that in such heavily yolked then- is trt'quently visible a preponderance of vertical and meridional furrows in the lower parts of the egg. Ki.As.MniiiiANviui. — Of the more typically nieroblastic vertebrates the Elasmobranchs call for little in the way of further remarks. The general features of their segmentation have already been described (p. 12). The eggs of all Elasmobranchs hitherto investigated are of large si/.e and undergo a nieroblastic segmentation. Up to the present time no Elasniobranch has been discovered in which the eggs are small and holoblastic, though it is quite possible that such forms exist. It need hardly be said that if they do the study of their embryology will be of extraordinary importance as it will be of the greatest help in enabling us to disentangle those developmental phenomena of Elasmobranchs which are primitive from those which are merely secondary modifications due to the accumulation of yolk. SAUROPSIDA. — The Sauropsida, like the Elasmobranchs, possess large and richly yolked eggs with a meroblastic segmentation, but the process of segregation of yolk and protoplasm has not been carried to such an extreme as in Elasmobranchs, not to mention Teleosts. A germinal disc is present but this still contains a con- siderable amount of yolk and at its periphery passes by much more gradual transitions into the main mass of yolk. Further in the more primitive Eeptiles the blastoderm frequently occupies a much larger proportion of the whole egg than it does in the Elasmobranch. The general features of segmentation resemble those of Elasmo- branchs though the earliest phases depart in many cases less than they do in Elasmobranchs from what is seen in holoblastic eggs. Thus the process may commence with the appearance of a meridional furrow followed by a second at right angles to it and then by two pairs of vertical furrows very much as in an actinopterygian ganoid (Fig. 14, B and C). This is seen most clearly in the less specialized egg of Eeptiles. Even in the Reptile however the process is liable to become irregular at an early stage by the reduction of particular furrows or their irregular orientation. In the Birds (Patterson, 1910) the irregularity is still more marked and even the third set of furrows may no longer be clearly recognizable. As in the case of other bulky and heavily yolked eggs poly- spermy appears to occur normally and an abortive accessory segmen- tation may make its appearance round the accessory sperm-nuclei. As in the Elasmobranch (Fig. 8, B*) this is only a transient 30 EMBEYOLOGY OF THE LOWER VERTEBRATES CH. phenomenon the accessory furrows flattening out and disappearing as the accessory sperm -nuclei degenerate. Again as in the Elasmobranch a yolk-syncytium is developed beneath and around the segmented portion of the blastoderm. A marked difference between the Sauropsidan 1 and the Elasmo- branch type of egg at a fairly advanced stage of segmentation :nes apparent on comparing them with the corresponding stages of eggs of a less markedly telolecithal character (e.g. Fig. 14). It is seen that the blastoderm in an Elasmobranch such as that shown in Fig. 8 D, E corresponds to the mass of jnicromeres of the holoblastic egg, while in the Sauropsidan it corresponds to the mass of inicromeres together with the apical ends of the large macromeres. This is really an expression of the fact that in ,the Sauropsidan the germinal disc extends outwards into the main yolk, and shades o'f gradually into it. The result is that the segmentation process in the outer portion of the blastoderm is delayed by the presence of yolk precisely in the same way as in the lower portion of the holo- blastic egg. GASTRULATION The segmentation process is in the more primitive Vertebrates, as in many other groups of the Metazoa, succeeded by a process of gastrulation, in which the blastula becomes converted into a gastrula i.e. a type of embryo consisting of the two primary cell-layers, ectoderm and endoderm, enclosing a cavity, the archenteron, which corresponds morphologically with the coeleriteron of the Coelenterate and which opens freely to the exterior. While the process of gastrulation is fairly clear in the most primitive vertebrates it becomes less and less so in the more highly modified members of the group until finally in the Amniota it becomes completely obscured. To facilitate the understanding of the modifications which the process of gastrulation undergoes it will be well to study it first as it occurs in three of the more. primitive Vertebrates namely Amphioxits, Polypterus, and Lepidosiren. (1) AMPHIOXUS. — The blastula of Amphioxus is composed of a single layer of cells, those towards the apical pole being smaller, those on the opposite side being larger and containing in their cyto- plasm larger and more numerous granules of yolk. The process of nlation is ushered in by the large-celled portion of the blast ula- wall becoming flattened as shown in Fig. 18 A. The portion of the flattened area which, as shown by later Stages, is anterior in position develops a slight depression (Fi///«-./ •//.%• and gradually becomes extended at its two ends until complete. Tin- i.- of importance as brl raying a tendency for the in\a^inat ive activity to be accent uuted in this portion of the -astrular lip and diminished elsewhere. GASTKULATION 33 The fact that the yolk portion of the blast u la. consists not of a single layer of cells as in Amphioxus but of a solid bulky mass forming a large proportion of the whole blastula, renders it physically impossible for the yolk hemisphere to be involuted bodily into the interior of the apical hemisphere. As a A F consequence \vc find in Polypterus that the process of in- volution is replaced to a -ivatt-r extent than in Amphioxus by overgrowth, the gastrular lip grow- iiiL1, over the mass of yolk-cells as seen in Fig. 19,C,D and E. As this process of overgrowth con- tinues the project- ing yolk-plug — the mass of yolk - cells not yet enclosed —gradually dimin- ishes in size and eventually disap- pears completely in the now narrow blastopore. As yet material is not avail- able to show defin- itely whether the overgrowth is more* active in what corre- sponds to the an- terior portion of the; gastrular lip of Amphioxus but the probability is in favour of this being the case anc^ the figures y.p. Km. T.». Illustrating jivore.ss of gastruliitiou ill 7W/////V/-/'.-,-. (Ki-s. A, C, E, F after drawings by Budgett.) are onen- '/./, gutrttlax lip; .)>liin the left: it is becoming extended by Hi-- addition, to its lower ed«ef of cells split oH' from the yolk-cells. The latter are recognizable by their larger sixe and by the larger size of the yolk -granules \\ith \\liich their r\ toplasm is laden. B and C showing involution of the surface along the inva.dnation-xroov- -. processes are at work (1) Involution of 1 1n- surface — this is conspicu- ous in the first stages (see Fig. 22, I'. ;mlk cells. During subsequent stages the ^move eontinues to mark the boundary between the two types of cell, so that in the last Ma^e mentioned \\ h<-n tin- -move forms a enni]>lete eirele t he mass of almost whit- yolk-eell< \sitlun it. (yolk j.ln^) stands out in striking contrast with the coal-black < Bring th«- n-st of the egg surt'aee. GASTBULATION 39 As will be gathered from an inspection of Fig. 24 the gradual covering in of the yolk-cells takes place in an eccentric fashion. On Mir side opposite to that <»n which the original involution groove makes its appearaner Mien- is comparatively little displacement of the boundary betwt en large cells and small, while on the side where thr groove is the displacement is relatively great — from a tod in the diagram. Intermediate points of the boundary between large and small cells are displaced more or less according to their greater or less proximity to the point of original involution. As regards the method by which the yolk -cells become covered in, it would appear that the "dorsal" lip of the groove advances over the yolk by a process of overgrowth, while at those parts of the boundary where there is no in vagination -groove the process is one of delamination. The growth of the dorsal lip is clearly indicated by the outline of the yolk-plug in sagittal sections which indicates distortion by pressure from the dorsal lip. It will be realized from what has already been said that the outer lip of the circular groove (Fig. 24, d) is simply the rim of the gastrula-mouth or proto- stoma and that the preceding stages are above all characterized by this rim being incomplete. In other words the activity concerned in the involution of the gas- trular rim is accentuated at one point (a) while it is suppressed to such an extent elsewhere as only to become apparent at a comparatively late stage when the edge of the small- celled region has already spread to a great extent over the yolk-cells by a process of delamination. It will also be realized that it is not strictly accurate to speak of the circular area bounded by small cells as representing the gastrula- mouth until it is completely enclosed by the gastrular rim. The internal changes which accompany the phenomena just described are illustrated by the sagittal sections shown in Fig. 25. In C the portion of the involution groove which first appeared has become much deepened and runs for some distance parallel to the surface as the archenteric cavity. It is bounded superficially by a completed portion of gastrular wall showing the two primary cell layers, ectoderm and endoderm. Some of the yolk-cells round the margin of the segmentation cavity are frequently to be seen, though not in the section figured, to be spreading along the inner surface of its roof, towards the point which was the apical pole of the blastula. the overgrowth by the gastrular lip, Fi«. 24.— Diagram to illustrate overgrowth by dorsal lip of blastopore in the Frog. (After Morgan, 1897.) The lines u, 6, <; d represent the involution groove at successive sta->-.i of development. In the later stages 40 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. accompanied, no doubt, by a certain amount of involution though this is difficult to determine with certainty, has proceeded much ent J-'i'.. 25. — Sagittal sections through the egg of llmm tem/Hn'nrin illustrating tin- ]>r< trulation. (After Jenkinson, 1913.) ect, ectod'-nii; tnt, .•nrli.-jili-i i<- <-a\ity; ;/./, ^astnilar lip; *.<; Kf^inentjition cavity; ;/./>. yulk-iilug. 'J')n- arrow indicates the original apical pnlr. liirther so that the archenteron is much deeper. The spreading of tin- yolk-cells ;ili>u;j tin- rnol' of (he sr^nnMit.-i 1 i«m ravity. alrcadv I GASTKULATION 41 alluded t<>, has in these later stages sometimes proceeded so tar t hat that cavity is nearly completely walled in by yolk-cells. While the archenteric cavity increases in volume the segmenta- tion cavity UMMMHUS gradually reduced. Normally the latter cavity goes on shrinking until it is finally obliterated but according to 0. Schultxe (1887) a certain proportion of eggs show a variation from the normal which appears to be of importance for the inter- pretation of what happens regularly in certain other groups. As seen in Kig. _."», Iv the layer of yolk-cells which separates the archenteron from the segmentation cavity is liable to become extremely thin, and Schultze believes that in certain cases this thin partition breaks down and disappears, so that the archenteric and segmentation cavities are thrown into one. What appears at first sight to be the archenteric cavity of subsequent stages would in such cases be really a complex consisting of the true archenteron fused with the remains of the segmentation cavity. If these observations are to be depended upon, they are of very special interest. For, if the confluence of archenteric and segmenta- tion cavity really occurs as an occasional variation in such Amphibians as the Frog, this may be taken as a foreshadowing of the similar phenomenon which has become a normal characteristic of the development of many of the higher Vertebrates. It must however be borne in mind that there exists a dangerous source of possible errors of observation, which it is difficult to guard against, namely that when an egg of the stage of Fig. 25, E, is trans- ferred from one fluid to another, as in the ordinary technical pro- cesses which precede section-cutting, violent diffusion currents are set up between the fluid in the segmentation cavity on the one hand and that in the archenteron on the other, and these currents must be very liable to cause rupture in the intervening partition, even when in life this is quite continuous. As gastrulation nears its end the circle formed by the gastrular lip becomes gradually smaller. Finally its lateral edges come together so that it takes the form of a short longitudinally placed slit, the remains of the yolk-plug at the same time passing out of sight. The gastrula is now complete. As regards the subsequent fate of the slit-like blastopore it may be mentioned that, for the most part, it becomes obliterated by fusion of its two lips. The portions at its two ends, however, remain open as two pores of which the more anterior becomes the neurenteric canal while the posterior becomes, either directly or after temporary obliteration, the anus. The process of gastrulation in the majority of Anurous and Urodele Amphibians pursues a course similar in its main features to that of the frog. Detailed studies of the process are, however, urgently needed in those Amphibians which have particularly small eggs and in which therefore gastrulation is less modified by the presence of yolk. 42 KMBRYOLOGY OF THE LOWER VERTEBRATES CH. ( • VMNOPHIONA. — There are certain Amphibians in which the telolecithal condition of the egg is so pronounced as to lead to a condition nearly approaching the meroblastic. Any such forms occurring either amongst the Dipnoi or the Amphibia must necessarily be of great importance owing to the fact that these groups are less far removed than are any other Vertebrates, from the line of descent of the Amniota and that, in consequence, the study of their develop- ment may be expected to throw light upon features occurring in the meroblastic eggs of Amniotes. Amongst Amphibians of this type the Gymnophiona alone have been subjected to careful study (P. and F. Sarasin, / 1887-1893 ; Brauer, 1897). The following sum- '•-...^— ^ / mary of the main features in their gastrulation processes is based on Brauer's description. The egg of Hypogeophis shows at the period ••' preceding gastrulation a round patch of micro- meres, or blastoderm, covering ! - J of the surface of the egg in the neighbourhood of its .••*' apical pole. Gastrulation commences with the posterior edge of the blastoderm losing its forward curvature and becoming curved backwards (Fig. 26), the curved part of the edge becoming sharply demarcated by the formation of a slight A cleft-like invagination of the egg-surface — which is deepest in its centre and shallower towards its extremities. In front of this invagination the Fi<,. -26. — Successive superficial (ectodermal) cells of the blastoderm stage^ofgastruiarHp take Qn ft distinctiy columnar form. The edges seen i/pianT (After of the blastoderm apart from the line of invagina- Brauer, 1897.) tion are in the meanwhile gradually spreading outwards over the yolk. As shown in Fig. 27, A, the cells (g.l) forming the anterior wall of the invagination are columnar in form, and the fine-grained character of their yolk makes their general appearance resemble that of the ectoderm cells. This is, however, to be taken, not as meaning that they really are of ectodermal nature but rather merely as an indication of active metabolism associated with active growth. The invagination -groove gradually, by backgrowth and ingrowth of its lateral portions, assumes a more pronounced backward curvature (Fig. 26) taking first the shape of a crescent, later of a horseshoe and finally of a closed riiiL,r- The central part of the groove almost from the U-^ inning increases rapidly in depth so as to form a narrow cavity — the archenteron — which extends forwards. The roof of this cavil \ is formed of cells agreeing in their fini'-gniincd protoplasm with those of thu ectoderm, while its floor on the other hand is composed of cells which in bheir coarse-grained character resemble rather tin- yolk-cells. In front, of the archenterou are tin- irregular remains of the -••••Jim-nut i'"i cavity and a communication Incomes established CASTIM LATION 43 between the two cavities so that they form a continuous space — the broader front part of which is derived from the segmentation cavity, the narrower posterior part from the true archenteron (Fig. 27, (- FIG. 27. — Sagittal sections illustrating the process of gastrulatiou in Hypogeophit. (After Brauer, 1897.) eft, ectoderm ; i-nt, au-lienteiic cavity ; ;/./, gastrular lip; s.c, seKmentation cavity. and D). The two sections of the cavity remain for a time clearly distinguishable by the character of the cells which form the roof — those of the archenteric portion being composed of fine-grained protoplasm like that of the ectoderm while those of the portion derived from the segmentation cavity are typical yolk-cells. 44 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. At a stage when the involution groove forms a nearly complete circle a sagittal section presents the appearance shown in Fig. 27, C. The ectoderm is thick and columnar posteriorly, but in front and laterally it thins out into a cubical epithelium which has extended o\vr the whole surface of the egg with the exception of a small area behind the gastrular rim. In the roof of the enteric cavity the boundary between the archenteric portion formed by overgrowth (and probably involution) and that formed from yolk-cells is marked by an abrupt change in the character of the cells which at once become less tall and less columnar. Farther in still the yolk-section of the roof shows marked irregularities of its inner surface and its cells assume a more rounded form. The anterior limit of the blastocoelic portion of the enteric cavity is not, as yet, clearly defined. The last section figured (Fig. 27, D) is taken from an egg in which the gastrulation lip forms a complete ring. Consequently the section shows a conspicuous yolk-plug (y.p} within the gastrular lip which, it will be noted, has developed a covering of small fine-grained cells over its surface. The inrollmg of the gastrular lip visible in the section indicates that the enteric roof is growing actively in length though Brauer does not make it clear to what extent the formation of the archenteron is due to this and to what extent to actual involu- tion. Naturally it would be very difficult if not impossible to decide this point definitely without experiments on the living egg. The gastrular opening gradually decreases in diameter (the yolk-plug disappearing from view as it does so) and eventually it closes from before backwards by its lateral lips coming together (Fig. 26) ; its posterior part however remains open as the anus. In the foregoing description is given merely a summary of those features in the gastrulation of Hypogeophis which appear to be of importance in relation to the corresponding phenomena of the Amniota : amongst these may be specially mentioned the process of constriction of the gastrular opening, and the double origin of the enteric cavity from archenteron and blastocoele, only its hinder portion being derived from archenteron. Another important feature not specifically alluded to in the text but which is indicated clearly by Fig. 26 is that during the process of gastrulation the boundary of the small-celled area is sweeping onwards over the egg's surface. It does this probably by a process of delamination as in Lepidosiren. The important point to notice, however, is that the small-celled boundary is not blocked in its ex- tension onwards by the gastrular lip. The yolk-plug becomes covered wii li small cells and after the ends of the rim have met so as to form ;i complete circle the small-celled region still spreads onwards, so that the slit-like hlaslnj..'n: of later stages lies well within tin- margin of ili> .-mall-celled area. Thus were development modified by the slurring over of the early stages of the inva-iiution Lrroovr so that i his '.nly became apparent at the period when il had assumed the 1'nnn of a longitudinal slit, it would at the time of its first appearance GASTEULATTON 45 be situated well within the small-celled area instead of at its hinder margin. The importance of this consideration will become manifest later on in connexion with the interpretation of the developmental phenomena of the Amniota. ELASMOBRANCHII. — The egg of the Elasmobranch at the time immediately preceding gastrulation differs from the blastula of the ordinary Amphibian or Lung-fish in its much greater size. The small- celled or micromeric apical portion of the blastula is represented here by a relatively small mass of cells — the blastoderm — in the region of B frn. Fui. 28. — Sagittal sections through Elasmobranch blastoderms (Torpedo] illustrating the process of gastrulation. (After Ziegler, 1902.) (I.I, ^astruhir lip ; *.c, segmentation cavity; y.n, yolk nuclei. the apical pole while the large-celled portion is represented by the yolk. This latter is composed, practically, of a mass of yolk granules, the protoplasmic matrix being reduced almost to vanishing-point. As in the eggs previously described, the micromeric portion gradually spreads round and encloses the yolk and here again we find the same three factors at work — involution, overgrowth and delamination. The first step in the gastrulation process consists in the involu- tion of the surface along the posterior edge of the blastoderm. This involution groove spreads outwards on each side until it may ex- tend along J to J the circumference of the blastoderm. The blasto- derm is meanwhile spreading outwards all round and, as it does so, the central part of the groove becomes deepened to form a tubular 46 EMBRYOLOGY 01 Till; LOWER VERTEBRATES en. cavity, the archenteron, which runs forwards from the mid- posterior margin. In the roofing in of this archenteron it is apparently a process of overgrowth which plays the main part — but along the rest of the blastoderm margin the process of overgrowth appears to die away and its place is taken by delamination very much as was the case in Lepidosiren. This is shown by the fact that the invagination- groove, which, as already remarked, extends outwards on each side for some distance, never deepens to any considerable extent except in its middle part. In the region in front of the archenteron the deeper or lower layer cells of the blastoderm increase greatly in number and spread forwards so as gradually to fill up the segmentation cavity. The remains of the latter persist longest near the anterior margin and the ectoderm covering the last remnant of the segmentation cavity commonly projects as a small but conspicuous elevation above the general surface of the blastoderm. These lower cells eventually take on a mesenchymatous character for the most part. Those lying next the yolk-syncytium however give rise to a definite epithelium, known* as the yolk epithelium. Some of them are said to penetrate actually into the yolk where their nuclei assume the characters of the nuclei of the yolk-syncytium. The floor of the archenteron is formed by the yolk epithelium which is continuous round the inner, or anterior, end of the archenteric cavity with the endoderm of its roof. ACTINOPTERYGII. — It is unfortunate that in the more familiar Actinopterygians belonging to the group Teleostei — of which it is so easy to obtain developmental material — the phenomena of gastru- lation are obscure and their investigation is impeded by technical difficulties in the way of making satisfactory sections. We shall therefore confine ourselves to indicating in a few words the more conspicuous features of the process. On the whole the features of gastrulation closely resemble those met with in Elasmobranchs — a resemblance which however we are not justified in regarding otherwise than as a phenomenon of con- vergence, seeing that the general evidence of morphology poiuts to the ancestors of the Teleosts being much more closely related to the holoblastic Ganoids than to the existing Elasmobranchs. A characteristic feature to be noted is that here, as will be found t<> he ill-- case in various mammals, the superficial cells of the blastoderm become much flattened and form a thin protective covering layer which lakes no part in the development of the embryo. When gastrulation is commencing the posterior margin of the hlastoderm presents in longitudinal vertical sections the appearance of beiu ur turned inwards to form the two primary layers. There is no actual patent archenteric cavity th«»u-h the inflected portion el'-arly represents the arehenteric root', the floor hem- apparently represented l.y the underlying syncytial layer. Tin* irrowih m len<_'th of the arohenteric roof seems to he brought i GASTRULATION 47 about mainly by a process of overgrowth similar to that met with in other forms. A point of special interest is that the posterior portion of the arch- enteric roof, in the neighbourhood of what will become later the mesial plane, is without the distinct demarcation between ectoderm and LMnlnilrrm which is pn -sent elsewhere. This continuity of the two primary cell layers apparently represents what is known in the Amniota as the primitive streak — a structure of great morphological interest which will be discussed later on (Goronowitsch, 1885 ; Jablonowski, 1898). While these processes are in progress the margin of the blasto- derm elsewhere is also advancing over the surface of the yolk so as gradually to enclose it. This enclosure of the yolk clearly corre- sponds to what we have seen in other cases but it is difficult to be quite certain as to how far it takes place by actual delamination and how far this has been replaced by a secondary independent growth. It is only when the exposed surface of yolk becomes reduced to a small round patch that the cell-margin bounding it shows inflection all round so as to justify us in speaking of a blastopore. In the surviving Ganoid members of the group Actinopterygii we find that the process of gastrulation, as is the case with other characteristics, repeats conditions which are probably to be looked on as ancestral. The gastrulation clearly belongs to the same general type as that of Lampreys, Amphibians, and Lung-fishes. That of Acipenser (Salensky, Bashford Dean, 1895) seems more nearly to resemble that of Polypterus, and that of Amia (Bashford Dean, 1896) and more especially Lepidosteus (Bashford Dean, 1895) to point towards the mode of gastrulation found in the modern' Teleosts. GASTRULATION IN AMNIOTA In comparing the process of gastrulation in the Amphibians and Lung-fishes with that in Amphioxus or Polypterus we have seen that there is a tendency for the greater part of the gastrular rim either to become completely obsolete or to be, at least, greatly delayed in its appearance, for example in the frog the greater part of the gastrular rim makes its appearance only in a comparatively late stage in the process of gastrulation. In the Amniota we find that this tendency has gone further. It is only in the lowest group — the Reptilia — that an undoubted gastrular lip is clearly recognizable. In the two remaining groups, the Birds and Mammals, there is no convincing evidence that it has not completely disappeared from development. REPTILES. — In a Reptilian egg before the commencement of gastrulation the apical portion is covered by a blastoderm consisting of a superficial layer of flattened ectoderm cells and, underneath this, rounded lower layer cells which are separated by interstices containing fluid. In the centre of the blastoderm (Fig. 29, A) an area, circular 48 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. or elliptical or pear-shaped with its narrow end posterior, becomes distinguishable from the rest of the blastoderm by its slightly greater opacity. The area in question is known as the embryonic shield (e.s), and its opacity is due to its ectoderm being thickened, the individual cells having taken on a columnar form. Either enclosed within or projecting beyond the posterior outline of the embryonic shield (Fig. 29, B) is a small area in which there is B e.s. , e.s. Tries I/,,;. 29.— Illustrating gastrulation in the Gecko (Platydactylus). (After Will, 1892.) A, showing complete blastoderm with the embryonic shield in the centre. This is hounded behind by the gastrular rim, precociously developed in this specimen. B, embryonic shield of specimen at the stage in which the archenteric floor is breaking down. C, embryonic shield at later su-e where gas- trular rim is bent back into a A-shape bounding the yolk-plug: the outline of the mesoderm sheet is M-i-n on each side. |), embryonic shield showing stage at which the -astrul.-ii lips hav ( i,- to-rthei BO as to bound a longitudinal slit, h.ti, edge of blastoderm; • ..,. embryonic shield; ././, gastrnlai lip ; HIKS, limit of mesoderm. no layer of columnar ectoderm sharply marked nil' f'n>in the lower This forms the primitive plate (Fi#. :'>!, p.p). The boundary of the embryonic shield gradually spreads outwards and the primitive plate comes to be, if it is not already, enclosed within it. Within the limits of the primitive plate the surface of the e-,u now becomes inv..lut.i-(| to form a groove bounded anteriorly l>y a lip which from its correspondence with \\hat \\e have seen in lower forms, more especially in the ( lymnophiona, is clearly to he n \ / I- <;. \STKULATION 1'.) nized as the gastrular lip. This lip gradually shifts backwards and, as it does so, underg"«'s alterations in shape, which differ some- what in different species and even in different individuals of the same species but which in their main features are illustrated by Fig. .'50. In its later .stages the lip becomes Lent or curved back- wards so as to have the shape of a A or a A (Figs. 29, C, and 30). Considerable variation occurs between different individuals in the time of the first appearance of the gastrular lip, and in the Gecko Platydactylus Will (1892) observed a correlated variation in shape. Where it appeared relatively early, the involution had the form of an elongated crescentic groove, while in cases where its appearance was delayed the involution formed a shorter and more rounded opening. ^ B As in other cases the central part of the ^^ invagination groove becomes deepened to form a cavity which is clearly homologous with the main part of the archenteric cavity in, say, a frog. This cavity starts by passing directly inwards, perpendicular to the egg-surface, but it soon bends forwards and runs parallel to the surface (Fig. 31, C and D). The cavity just mentioned (Fig. 31, 1), enf.) being an archenteron the layer of cells lining it corresponds to that which in the lower forms is called endoderm. It is therefore misleading to replace this by any FlG- 30. -- Successive ,, 1-1 fit j i -I • stages of srastrular run other name : to emphasize the fact that they line or iip as seen in sur. the true archenteric cavity it may be advisable face view. A,cheionia, to speak of the cells in question as the archenteric ^itsuJkuri'71896A17.S' j ! . P ~l T . P . , Platydactylus. (Will, endoderm in spite of the clumsiness of the ex- 1892). pression. In the meantime the lower layer cells immediately underlying the ectoderm assume a flattened form and become joined together by their edges to form a definite epithelium which may conveniently be termed the secondary endoderm (Fig. 31, C and D, end'). The more deeply situated cells underlying the secondary endoderm remain spherical and are separated by wide spaces forming a seg- mentation or subgerminal cavity. These deeper spherical cells have their numbers constantly reinforced by additional cells which are apparently budded off from the underlying yolk-mass. The floor of the arehenteron becomes closely apposed to the secondary endoderm immediately beneath it (Fig. 31, D). The two cell-layers fuse, irregular perforations develop in the membrane formed by their fusion, and the result is that the archenteron is thrown into communication with the " subgerminal " cavity (Fig. 31, E). Shreds of the partition persist for some time but eventually the two spaces form a perfectly continuous cavity just as happened with archenteric and segmentation cavity in the Gymnophiona. The portion of the primitive plate which is embraced by the VOL. II E EMBRYOLOGY OF THE LOWER VERTEBRATES CH. shaped Lfastrular lip corresponds to the yolk -plug of A * *•''•• "•'•''•'- ""••• B end' ent y? 31.— Sagittal .sections tlnougli early st;i-v> o!' /'/,//i//,/,,,-//////.s-. (Altrr Will, 1892.) A is of the Kta-i* shown in Kin. -"•'- A ; E is of (),.• sta.^-- of Fig. l_'9, B; B, C, ami Dan- ; ///./<, t hickiMUM] ••ctodiTin wliicli will ;ji\c i :.-.• lali-r i<> tin- mitral m-rvons sxstt'iii Oii«-ilnllaiy plat*-); p.p, \>i iinit ivc |>lai«- : //.//. \-olk-jilnn. ani])liil»i;ins ami in s«»un- cases ton ( l.,i,-t rh( — Will) it become- <-nclosed, the tips of the, horsr-shoe c-urvin-j- inwards and i GASTKULATION 51 meeting to form a closed ellipse. .The yolk-plug dill'crs from that of amphibians merely in its being elliptical in outline instead of During these changes tin- anterior or dorsal part of the gastrular lip grows actively backwards over the surface of the yolk-plug, the portion of yolk-plug which is covered over in this way becoming added to the floor of the archenteron and its superficial cells becoming converted into archenteric eudoderm. The last phase in the closing of the gastrular opening consists in its lateral walls approaching the mesial plane so that the opening assumes the form of a longitudinal slit (Fig. 29, D). Part of this slit persists for some time as a neurenteric canal — a communication between the enteric cavity and the cavity of the neural groove or tube (Fig. 32, C) — while a portion farther back seems to be repre- sented by the anus although in this case the patent opening disappears temporarily so that no absolute continuity can be traced. In the region where the lips have undergone fusion there persists for a time complete continuity between the different cell- layers. The study of sections shows this continuity to be precisely the same as that which occurs in the primitive streak of Birds and Mammals (Fig. 32, B, D, E), and we have thus suggested a clue to the meaning of that otherwise enigmatical structure. It will have been gathered that the archenteric cavity has become greatly reduced in importance in the Eeptile as compared with the more primitive vertebrates. It has become much reduced in relative size,1 and it soon loses its individuality, becoming merged with the irregular segmentation spaces lying beneath the blastoderm. Correlated with this we can 110 longer speak of direct conversion of the archenteric cavity into the enteron or alimentary canal, except to a trifling extent. The latter arises, for the most part, as will be shown later, in a quite different manner from the secondary endoderm. BIRDS. — In the Keptile, as compared with one of the more primitive ananinia, the main peculiarity of the gastrulation process lies in the fact that the cavity which opens to the exterior by the blastopore is normally of double origin, only its posterior portion being derived from archenteron. Consequently the layer of endoderm which lines it is only to a comparatively small extent derived from the archenteric lining, the much greater anterior part being formed from elements of independent origin. In the Amniota above Eeptiles the replacement of archenteric by secondary endoderm has gone still further, inasmuch as the formation of an archenteron has in them either completely dis- appeared from development, or at the least is reduced to a faint vestige, and the endoderm is therefore entirely secondary. 1 In some forms, such as Lacerta, the archenteric portion of the enteron appears to In- for u time much shorter relatively than in others (e.g. Platijdnctiilu^ hut this is corrected later on by active overgrowth on the part of the dorsal lip (Will, 1895). 52 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. As regards the Birds, which of the higher Aumiotes alone concern us in this volume, there is com- plete agreement that they are to be looked on as highly-special- ized descendants of Reptilian ancestors. It follows therefore that their develop- mental phenomena should be considered in relation to the cor- responding pheno- mena in Reptiles. Leaving out of account certain vague phenomena which have been inter- preted, in the present writer's opinion un- justifiably, as remin- iscences of gastr ula- tion - processes (see Chapter X.), the for- mation of a gastrular lip seems to have FIG. 32. — Transverse sec- tions through region of iifurenteric canal of (1h<-- I n nia embryo with about 16 segments. (Alter Mitsukuri, 1896.) The mid -dorsal ect"deim has become covered in to form the neural tube (*.<•.) as will be de- scribed in Chap. II. Ki.u'. C shows the lieu rent eric canal opening upwards ihroiiiih this, while Fi-s. II. 1) ami K. taken from section-, anterior and pos- lei jiir to the neiirentei ie open- in-, show the continuity of tissue from ectoderm to endo- der in \\hieh is a cliaiactei i>t ie feature of a primitive streak. •iddei 111 ; . n,l, I'lidodemi ; natodarn ; N , notochoni; ne.C, Mem-enteric caiia: spinal coid. i GASTRULATION 53 been eliminated entirely from ontogenetic development in Hinls. What is conspicuous is a well-marked primitive streak which makes iis appearance in the posterior half of the blastoderm along \vliat will be the axial line of the body of the embryo (see Chap. X.). A groove develops along the surface of the streak — the primitive groove. Histologically the primitive streak is, in its early stages, a line of proliferation from the inner surface of the ectoderm, the blasto- derm being composed only of the two primary layers at the time of its appearance. That the ectoderm alone is responsible for the first appearance of the primitive streak, a point difficult to make absolutely certain by ordinary observation, appears to be demonstrated by the study of an abnormal 36 -hour embryo Peawit (Vanellus cristatus) described by Rothig (1907) in which the endoderrn was completely absent while ectoderm and primitive streak were quite normal. An inspection of blastoderms at successive periods in development shows the primitive streak lying always behind the medullary folds (cf. Fig. 227, Chap. X.), and it might therefore be readily assumed that the embryonic body develops entirely in front of the primitive streak. That this is not so is clearly shown by experiments (Kopsch, 1902) in which a, scar is made with a hot needle about the front end of the primitive streak during an early stage in its development. If the egg is carefully sealed up again it may go on with its development, and in such a case the scar is found later on to be situated not near the hind end of the embryo but well forward in the head region. What apparently happens is that the primitive streak grows actively in length with the general growth of the blastoderm but that all the while it is becoming correspondingly shortened at its headward end. As a matter of fact its anterior end becomes gradually converted from before backwards into notochord and the adjoining parts of the mesoderm. The front part, which is under- going this change, loses its connexion with the ectoderm while it becomes on the other hand continuous with the endoderm and is reinforced by proliferation from it : it then forms what is known as the Head process. The point of special morphological importance to notice about the primitive streak is its continuity with the two primary cell- layers. Throughout the greater part of its length it is continuous with ectoderm, in its front half with both ectoderm and endoderm, and in its forward prolongation — the head process — with endo- derm. Correlated with this is the further fact that in some cases (Tern, Goose, Duck, Wagtail, Melopsittac'us) the tissue of the primitive streak is traversed by a typical neurenteric canal. Taking these various features into consideration it is impossible to avoid the conclusion that the primitive streak represents the line of coalescence of the gastrular lips just as it actually is in 54 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. Reptiles, and that the neurenteric canal represents a persisting portion of a once slit - like gastrula mouth which is otherwise obliterated. ORIGIN OF THE MESODEIIM GENERAL REMARKS. — Already during the process of segmentation the differentiation of the two primary cell-layers commences — the superficial cells towards the apical pole dividing more actively, being smaller, and being less laden with food-yolk and thus establishing a character of their own as ectodermal cells. The full establishment of the primary layers however is only consummated during the process of gastrulation when the ectoderm comes by the various processes already described to enclose the remaining cells the (archenteric) endoderm. The establishment of the two primary layers is followed immediately (indeed the two processes frequently overlap) by the development of the intermediate cell-layer — the mesoderm — which will in the adult form the great mass of the body — all in fact except the epidermis and its derivatives on the one hand and the enteric epithelium and its derivatives on the other. The problem of the evolutionary history of the mesoderm of Vertebrates is one upon which there is little agreement. Anything of the nature of elaborate and detailed treatment of the subject would be out of place in a textbook of moderate size and a short sketch such as the following is necessarily coloured by the general morphological views of the writer. While the views set forth in the following paragraphs seem to the author to fit most satisfactorily the facts so far as these are established beyond reasonable doubt there are other embryologists who would give an account differing consider- ably from that given here. To the present writer it seems of importance in endeavouring to arrive at reliable general conclusions from the facts of observation to bear in mind particularly' the risk of reaching erroneous conclusions through basing arguments upon phenomena observed in the head region or tail region of the embryo. Intense cephalization, i.e. intense structural modification of the anterior region of the body, to form a head, is admittedly one of the fundamental characters of the phylum Yerte.iintta. In this modification the mesoderm has been deeply involved so that there is always a considerable weight of probability against conditions observed in the head region iM-in- ])rimiti\ e. A'jain the tail region is also intensely nmdilied. as is in 'I mated «.//. by the transient appearance within it of a vesti-ia! portion of alimentary canal \vith surrounding hody-cavit v. Here aiMin thru. ihuiiL-h not to the saint' extent as in tin- head-region, su-picion rests upon tin- pnmit m-nrss «»!' all phenomena of develop- ment peculiar to this region «•!' the body. It is advisable t h.-n I or these reasons, to exercise great caution in i OKIGIN OF THE MESODEltM 55 making use of any developmental phenomena except those observed in typical trunk segments as a basis for speculations upon the evolu- tionary origin <•!' tin: mesoderin. It has, further, to be borne in mind that observations upon the development of the mesoderm in its early stages have to be made by the method of serial sections, and that in the interpretation of such sections the liability to error becomes greatly increased if the sections are not exactly in one of the three following sets of planes — (1) transverse to the morphological axis, (2) " horizontal," and (3) parallel to the sagittal plane. This type of technical difficulty is in many Vertebrate embryos most marked in the head and tail regions. For these reasons it seems safest, in considering generally the ontogenetic development and the probable evolutionary history of the mesoderm, to ignore all observations except those made on typical trunk segments between the level of the otocyst in front and of the anus behind. This will accordingly be done in what follows. It is agreed by the majority of students of Vertebrate embryology that the most nearly primitive condition of the mesoderm known to occur in the embryos of Vertebrates is that seen in Amphioxus, where it consists for a time of a series of endodermal pockets, converted later into closed sacs, upon each side of the body (Fig. 34, B). It appears fully justifiable to conclude that both of the stages mentioned represent ancestral conditions in the evolution of the Vertebrate inesoderm. The excretory organs of the Vertebrate, in the form of paired segmentally arranged tubes, afford in themselves strong evidence that at one time the Vertebrate coelome was in the form of isolated segmentally arranged chambers. In the case of Amphioxus the segmented character of the ineso- derui persists only dorsally, the ventral portions of the successive segments becoming fused together so as to give rise to a continuous unsegmented splanchnocoele or peritoneal cavity. In the Craniata the smallest departure from the condition in Amphioxus is seen in such comparatively primitive forms as Lam- preys, Crossopterygians and Lung-fishes. In these a solid continuous mesoderm rudiment becomes split off from the endoderm on each side, remaining for some time continuous laterally with the endoderm (Fig. 40, B, C, p. 65). In the outer or lateral part of this mesoderm rudiment the segmentation, which even in Amphioxus was only temporary, never makes its appearance. The dorsal portion does segment but the segment is a solid block of cells in which a cavity only appears later on. It is fairly clear that these mesoderm seg- ments, except for the fact that they are continuous in their ventral portions and that they are at first solid (a modification of develop- ment which is very common in hollow organs), agree closely with the s» 'Laments of Amphioxus and that they are homologous structures merely somewhat modified from the primitive condition met with in Amphioxus. In endeavouring to institute a more precise comparison of the 56 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. niesoderui segment in its earliest stage, in the typical Vertebrate, with that of Amphioxus, the way is found to be blocked by a second- ary adhesion (or absence of separation!) having coine about between the mesoderm segment and the endoderm from which it has arisen. The young mesoderm pouch of Amphioxus is attached to the endoderm at its base — i.e. its ventral end. Its honiologue in the more typical Vertebrate, on the other hand, is continuous with the endoderm in two different regions, one ventral and one dorsal. This is illustrated by such a diagrammatic section as that shown in Fig. 33, B, in which the solid mass of mesoderm on each side, indi- cated by the medium tone, is continuous with the mass of endoderm or yolk-cells at the points a and I. The question is, which of these two points is to be interpreted as representing the root of the meso- derm pocket in Amphioxus ? Clearly only one of them can represent FIG. 33.— Diagram illustrating (B) the origin of mesoderm from endoderm in an Amphibian, and (A and C) the two methods of correlating it with the mode of mesoderm formation in A mphioxus. a, b, see text ; eci, «-rtoe shown to exist between mesoderm and endoderm at the point a would In- regarded as secondary and without evolutionary si^niln •,-ince. If however due weight be accorded to \\hat is observed in the development of the lower holohlastic vertebrates it seems more reasonable to the present writer to e. ,n. dude that the m.,re vent rally sit u a ted connexion, that marked nt, untt-ric cavity; //*./<. im-dullaiy plate", /;/?>•. mesodrrm ; N, notochordal rudiment. 58 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. itself becomes nipped off from the remainder of the archenteron (Fig. 34, C). The original archenteron is now replaced by a main portion, the ect ent. if». — Transverse sections through embryos of /,»7//W",s-//v/< to illuMniU- the origin of the mesocliTin. A, Ktegf 12 ; H and C, hta^c ] J. ect, ectoderm ; cml, riiiloilcnii ; » nt, cnti'iic cavity ; nn-s. N, iii.to<-hur«l;il i inlirncnt . enteron, t.h«; \\j«ll <•!' whir.h — the definitive endoderm— will ally IM-C.OIIU- 1:1 H- liniiiM- c] lit I id in in of lln' ulinn-iilary c;inal, ami, lyiiiLC dorsal to this on , Bfl ofddscd sacs, <»j- j.ract ically solid i ORIGIN OF THE MESODERM 59 blocks (their outer and inner walls being in contact). These sacs or blocks are the mesoderm segments and their cavities are the seg- im -ii tally arranged rudiments of the coelome. The subsequent fate of the mesoderm segments will be traced later ((hap. IV.). Of the lower holoblastic forms amongst the Vertebrata in the stricter sense we will consider first Lepidosiren, in which, owing to the large size of the cell-elements, the details of mesoderm formation arc particularly clear and unmistakable. The mode of origin of the mesoderm which occurs in Lepidosiren is illustrated by Fig. 35. The section shown in Fig. 35, A is taken from an egg of the same age as that figured on p. 35 (Fig. 21, C) in illustration of the disappearance of the segmentation cavity. Im- mediately below the ectoderm is a mass of rounded blastomeres with intervening chinks — remnants of the segmentation cavity : towards the mesial plane the blastomeres are more closely packed together. The small blastomeres in question are clearly distinguished by their finely-grained yolk from the large yolk-cells with their coarsely- grained yolk which form the hulk of the egg. The mass of small blastomeres is destined to give rise laterally to the mesoderin and mesially to the notochord. It must clearly be borne in mind that the mass is composed simply of small blastomeres and that it passes at its outer margin without any break into the ordinary yolk-cells. As development goes on, the mass of small elements becomes compacted together (Fig. 35, B), the chinks between the cells disap- pearing. At the same time the boundary between them and the yolk -cells becomes more definite, so as to delimit more clearly the mesoderm rudiment (mes) from the definitive endoderm. Fig. 35, C is taken from an egg of the same age but here the mesoderm rudiment has become limited also on its mesial side by a split which marks it oft' from the notochord (N). At a somewhat later stage, the mesoderm mass on each side becomes divided into segments by splits, transverse to the axis of the body, which make their appearance at regular intervals from before backwards, but it is to be noted that in Lepidosiren (as in all Verte- brates except Amphioxus) this splitting of the mesoderm is confined to its dorsal portions. There is thus produced along each side of the body a series of incomplete mesoderm segments1 which pass at their lower or ventral ends into an unsegmented sheet of " lateral " 1 Such incomplete mesoderm segments as are described above occur in all the typical vertebrates and are known by various names such as mesoblastic somites, protovertebrae, myotomes. These names are in various degrees erroneous or mis- leading. The word somite means a complete body segment and it is not allowable to apply it to a single organ. The name protovertebra dates from the days in which these structures w<-re supposed to be the embryonic vertebrae, which they are now known not to be. Of the three terms mentioned myotome is the least objectionable U it least the greater part of the segmented portions of mesoderm become definite myotomes later on. On the whole however it seems most convenient to retain the ex- pression mesoderm segment, the word segment not being necessarily used in the precisely defined way in which such a purely technical morphological term as " somite " must be used. 60 EMBEYOLOGY OF THE LOWER VERTEBRATES CH. mesoderm. This latter gradually spreads ventralwards by delamina- tioii from the large yolk-cells and eventually the mesoderm sheets on the two sides become continued into one another ventrally. As will be noticed there are no coeloinic spaces within the derm rudiments at these early stages: they arise secondarily later on. If we review the above-described stages in the early development of the mesoderm segment in Lepidosiren, in which, as already indi- cated, the large size of the cell-elements ensures unusual freedom from the danger of errors of observation, we see that the last described stage is clearly in agreement with the hypothesis that it is a repetition of the stage in Amphioxus when the mesoderm existed in the form of a series of enterocoelic pouches on each side. The only conspicuous difference is that, whereas in Amphioxus these were actual pouches, here they are solid blocks of cells in which a cavity only makes its appearance at a later stage of development. That this difference is in no way a serious one will become apparent to the reader as he realizes that it is one of the commonest modifica- tions of developmental phenomena, when yolk is ^abundant, that primitively hollow organs develop in the embryo from solid rudi- ments and only form their cavity secondarily. It may be accepted then with confidence that the solid mesoderm segments of Lepidosiren at the stage indicated, continuous ventrally with the'endoderm, represent the enterocoelic pouches of Amphioxus modified in correlation with the abundance of yolk. The first stages in the development of the mesoderm of Lepido- siren are obviously very different from what are found in Amphioxus and the differences here also we may justifiably attribute to the immense thickening of the endodermal wall of the archenteron corre- lated with the storing up of a large amount of yolk in its cells. In the other groups of holoblastic vertebrates the main features in the early development of the mesoderm agree with those just described for Lepidosiren. In all of them the archenteron is pro- vided with a thick wall of heavily yolked endoderni cells, those forming the roof or dorsal part of the wall being smaller and pro- vided with finer yolk-granules. Out of this smaller-celled mass the mesoderm segments become carved by the development of splits very much in the same way as in Lepidosiren (cf. Fig. 40, B — Petromyzon}. Amongst these groups the Amphibia call for a little further consideration. In the frog a split develops <»n cadi side which separates the mol ot tin- ardienterie, cavity into two layers, an inner layer, one cell thick, «»f definitive endoderm and an outer, two cells thick for the most part, the mesoderni. This split is seen in V\^. .">»'» \\hidi repre- sents a section, transverse to the axis <>1 the archenteron, through an egg with lar^e, yolk-plug. The split in t his section terminates UdoNv •he level nf tin- ll<><>r nf the ardienteric cavil \ \\liile ahnve ps short of the level of the notochord. OKIGIISr OF THE MESODKKM 61 N. ect. end A little later a split at its dorsal end denian -;iie< ihe rudiment from Uie notochord. The mesoderm rudiment, funning now a broad hand on each side of the embryo, Incomes divided into segments hy splits which cut it across and a condition is reached corresponding closely with that already described for Lepidosiren where the mesoderin consists of a series of solid segments on each side continuous ventrally with the mass of yolk cells forming the main part of the endoderm. As in Lepidosiren the ventral unsegmented part of the mesoderin becomes prolonged ventrally by the extension downwards of the split between it and the endo- derni. In other words the mesoderin extends ven- trally by a process of de- lamination from the endo- derm. In the anterior part of the body the sheet of meso- derm becomes split off completely from the yolky endoderm before it quite reaches the mid - ventral line so that the sheets be- longing to the two sides are discontinuous ventrally hut in the hinder region the two splits meet ven- trally so as to give rise to a sheet of mesoderm con- tinuous across the middle line. Under ordinary cir- cumstances the mesoderm sheets in the anterior region grow ventrally and eventually fuse with one another (as will be described later) while in the posterior region this fusion is anticipated by the two lateral rudiments being continuous from the beginning. So far everything seems fairly simple, but it now remains to allude to certain peculiarities which have done much to obscure the clear understanding of the method of mesoderm formation and which are especially important for the proper comprehension of the first forma- tion of mesoderm in the me'roblastic vertebrates. The peculiarities in question are to be seen in the hinder part of the trunk region. In this region the split which separates off meso- derm from endoderm remains for a time incomplete at a point just external to the notochord. Each segment therefore remains for a t ime cont inuous with the endoderm at this point. The level of these junctions of mesoderm and endoderm is marked by a longitudinal FIG. 36. — Transverse section through an embryo of I'm HI illustrating the origin ot the mesoderm. (After Schwink, 1839.) ect, ectoderm ; end, enioderm ; mes, mesoderin ; X, notochord. G_> EMBRYOLOGY OF THE LOWER VERTEBRATES CH. ifioove of the inner surface of the wall of the archenteron so that where the junction exists the archenteric cavity may be said to pro- ject slightly into it. The cells at tin's point develop pigment in their protoplasm (Fig. 37, B) ; they frequently show mitotic figures and appear to be actively proliferating, cells being added at this point to the mesoderm. The peculiarities which have just been described, and which occur in various amphibians, have important bearings in two different directions. In the first place they form an important part of the basis for O. Hertwig's hypothesis of mesoderm formation in the Vertebrata, the junctions, which have just been described, between endoderm and mesoderm being interpreted by him as representing the original stalks of the mesoderm segments as they occur in Am- phioxus. As already indicated there do not appear to the writer to B mes. ?nd. N. Fie. 37. — Transverse sections through embryos of (A) Triton and (B) Rana teinp«r«ri.//. medullary plate : mes, mesoderm ; A", notochordal rudiment. be sufficient reasons for regarding these connexions as primitive rather than those more ventrally situated. The balance of probability appears rather to favour the view that of the two connexions it is the ventral one which is the persistent original one, and that it is the dorsal which is to be interpreted as due to secondary fusion. The second bearing is at least equally important. It rests on the occurrence of active cell-proliferation on each side of the noto- chordal rudiment. For in some of the meroblastic vertebrates Amniotaj — correlated with the more and more complet , it ion of yolk from protoplasm — this /one of proliferation becomes ap- iily tin- main source of the mesodenn. Kl.ASMOMKANCHII. — Tn the Elasmol. ranch, \\hile there are si ill traces of formation of mesodenn by a pi ocess of delamination from the main mass of endoderni or yolk ( Ki--. :'.s, A), a mme oon- .spieuous mode «.f formation is provided l>\ active proliferation of or T1IK MKSnDKltM 63 Mic ciidndcnu cells along tin- inner and outer Hie sheet "I mesoderm. In early stages and iu the anterior part of the embryo this pro- liferation pror.css may alone be in evidence, so that in place of a broad continuous sheet of mesoderm there are found two narrow strips- one (Fig. 38, C, mes') arising from the endoderm just external to the notochordal region and the other (mes") arising from the endoderm mes. mes l-'i< . 38. — Three transverse sections through an embryo of Pristiurus (Stage B, Balfour), illustrating the origin of the mesoderm. (After C. Rabl, 1889.) Section A, through the posterior halt' of (lie embryo ; li, through the middle ; C, through the anterior halt'. >-<•(, ectoderm : r primitive Streak region l>ei\ve«-n the t\v«> primary »-ell layers. r,u:i»s. In th«- Ilirds also the method <>f first mesnderm ORIGIN OF THE MESODERM 65 appears to be closely comparable with that of Keptiles and Amphib- ians. Here, at the time when the mrsuderm begins to rnak«- ii> appearance, the position of the notochord is occupied by the primitive stn-ak. The mes.iilfrm forms a loose sheet of irregularly shaped cells spreading out on each side and added to from two distinct sou* on its inner side by proliferation from the primitive streak and on its outer side by dela initiation from the endodenn of the germ wall. It will facilitate comprehension of the evolutionary changes which 8 ent n.r: tries. TM: mes. ent ect mes. end. Fi«.. 10. — Semi-diagrammatic transverse sections tli rough the embryos of various vertebrates to illustrate the origin of the mesoderni. A, A in i>li i<» us ; 15, J'etrumyztni ; C, Lepidosiren ; D, Amphibian ; E, Elasmubranch. ect, ectoderm ; .•/('/. endoderm; ,ent, enteric, cavity; mes, mesoderm ; 2V, notochord; n.r, neural rudiments. The small crosses indicate regions in which active extension of the mesoderm is taking place. the writer believes to have taken place in the mode of development of the mesoderm within the phylum Vertebrata if the main steps are summarized in a diagram. In Fig. 40, A shows the primitive condi- tion where the mesoderm segments are in the form of enterocoelic pockets (Ampliioxus}. In B, with increasing amount of yolk, the hollow pocket is represented by a solid block in which the cavity will develop secondarily (Petromyzori). In C the condition is similar but the dorsal portion of the embryonic body is more flattened out, the bulk of the yolky endoderm over which it is spread having VOL. II F 66 EMBRYOLOGY OF THE LOWER VERTEBEATES en. become greater (Lung- fish). In D the secondary continuity of the niesoderm with the endoderm just outside the notochord is present and proliferation of uiesoderm cells has commenced in this region (Amphibia). Finally, in E, with the very great increase in the bulk of the yolk, the dorsal part of the embryo is still more flattened out, and the addition to the mesoderm by proliferation of endoderm cells into it close to the notochord has now become conspicuous (Elasmo- branch). THE MESENCHYME The fate of the mesoderm whose origin has just been traced is to give rise directly to the peritoneal epithelium which lines the body cavity and covers the organs lying within it, and also to the muscular system. Indirectly it, however, also plays a great part in the forma- tion of what is known as the mesenchyme. Whereas for a time the Vertebrate body is composed of compact masses or layers of cells, it is a general characteristic that, as develop- ment goes on, individual cells detach themselves and wander away through the body, multiplying by fission accompanied by mitosis, and behaving in fact very much as if they were independent organisms. In the course of the many generations of these cells which arise during the process of individual development, they become divided into various strains which show marked differentiation for the per- formance of different functions. Some retain a relatively primitive amoeboid form and undertake such functions as the transport of food material, the absorption of moribund tissues in regions where shrinkage in volume or atrophy is taking place, and the ingestion and destruction of attacking organ- isms such as disease germs. Some, their protoplasm laden with insoluble excretory products as a result of their active metabolism, wander towards the light and settle down near the surface of the body as pigment cells or chromatophores which serve on the one hand to protect the underlying tissues from the light and upon the other to give distinctive coloration to the animal. Others again settle down in an abundant jelly-like intercellular matrix to form connect- ive or packing tissue, which iu turn shows evolution in various directions in accordance more particularly with different developments of the intercellular matrix. Of special importance are these types in which the matrix becomes hard and rigid so as to form skeletal tissues such as bone and cartilage. Another important strain of these cells is characterized by the fluidity of tin- matrix and the independence of the individual cells which float in it. This liquid connective tissue forms the blood which, pumped through an elaborate system of vessels, serves on the OIK- hand for tin- transport of food and oxy^vn tn the tissues ami on the otli'-r for carrying away the waste products of mctaholism t«» the special excretory o! be 'lnl,\ <>f which is finally t<> remove these harmful i MESENCHYMK 67 The sum of these amoeboid cells, which proceed along the various evolutionary paths al>uv<; indicated, were, by 0. Hertwig, given the luiiiic Mesenchyme — to distinguish them from the mesothelium, or mesoderm in the restricted sense, in which the cells remain in the form of continuous layers or epithclia. The original mesenchyine cells arise by emigration from the pre-existing cell layers. Possibly all three layers give rise to mesenchyme cells. It is the mesoderm however which does so most conspicuously. In an Elasmobranch embryo, for example, active budding off of mesenchyme cells is seen over large areas of the somatic mesoderm and similarly from the inner surface of the splanchnic mesoderm. Most active of all is the production of mesen- chyme cells from the splanchnic mesoderm near the lower end of the rnesoderm segment, where the proliferating mesenchyine cells may form a conspicuous mass projecting towards the mesial plane and termed the sclerotome.1 The special consideration of the sclerotome and of the mesenchyme in general will come most conveniently after the other derivatives of the mesoderm (Chaps. IV., V., VI.). 1 The use of the word sclerotome in this restricted sense has come to be practically universal in embryological literature and is therefore followed in this volume. The word was invented by Goodsir and denned by him, at the British Association meeting in 1856, as meaning a segment of the supporting tissue or framework (whether "fibrous" or cartilaginous or osseous) in a segmented animal. LITERATURE Agassiz and Whitman. Mem. Mus. Comp. Zool., xiv, 1885 and 1889. Assheton. The Work of John Samuel Budge tt. Cambridge, 1907. Balfour. Quart. Journ. Micr. Sci., N.S. xiv, 1874. Brachet. Arch. Biol., xix, 1903. Brauer. Zool. Jahrbucher (Anat. Abt.), x, 1897. Budgett. Trans. Zool. Soc. London, xv, 1901. Cerfontaine. Arch, de Biol.,»xxii, 1906. Dean, Bashford. Journ. Morph., xi, 1895. Dean, Bashford. Quart. Journ. Micr. Sci., xxxviii, 1896. Dean, Bashford. Kupllers Festschrift, 1899. Goette. Abhandl. zur Entwickelungsgesch. d. Tiere, v, 1890. Goodsir. Edinburgh New Philosophical Journal, v, 1857. Goronowitsch. Morph. Jahrb. x, 1885. Hatschek. Arb. zool. Inst. Univ. Wien, iv, 1881. Hertwig, 0. Jenaische Zeitschrift, xv and xvi, 1882 and 1883. Hertwig, 0. Handbuch d. Entwicklungslehre, Jena, 1906. Jablonowski. Anat. Anzeiger, xiv, 1898. Jenkinson. Vertebrate Embryology. Oxford, 1913. Jordan and Eycleshymer. Journ. Morph., ix, 1894. Kerr, Graham. Phil. Trans., B, cxcii, 1900. Kerr, Graham. The Work of John Samuel Budgett. Cambridge, 1907. Kopsch. Internat. Monatsschrift f. Anat. u. Phys., xix, 1902. Kopsch. Arch. mikr. Anat., Ixxviii, 1911. Kowalevsky. Mem. Acad. Sci. St. Petersbourg, Ser. 7, xi, 1867. Mitsukuri. Journ. Coll. Sci. Tokyo, x, 1896. Morgan. The Development of the Frog's Egg. New York, 1897. Patterson. Journ. Morph., xxi, 1910. Prince. Ann. Mag. Nat. Hist.. Fifth Scries, xviii, 1886. Rabl, C. Morph. Jahrb., xv, 1889. Rb'thig. Arch. mikr. Anat., Ixx, 1907. 68 EMBRYOLOGY OF THK LOWER VERTEBRATES CH. i Ruckert. Sll. <:.-. Mm-ph. Tliys. Mmu-h.. 1885. Ruckert. SI',. c;«-s. Morph. Phys. Miin.-li., 1890. Ruckert. Kuptfers Fatteohrift, 1899. Salensky. Memoirs of the Society of Natural ist* »f Kasan. \ii. pt. :}, 1878. (In Russian. A French translation of tiic more important parts will lie fomul in Arch, de Biologic, ii, 1881.) Samassa. Aivli. Bntwiok^langsineohanik, vii, 1898. Sarasin, P. and F. ErgebniftM nuturwiss. Forsch. auf Ceylon. WiesbaiU-n. 1887 1893. Schultze. O. K-.llikors Fostsclirift. Lcipxig, 1887. Schultze. O. Arch, inikr. Anat., h, 1899. Schwink. Die Entwicklung dr> niitth-n-ii l\.'iml>lattcs und der Chorda dorsalis lit -i- Aniphihien. Munich, 1889. Semon. Zoologist-he Forschungsreisen, i. Jena, 1893. Will. Zoolog. Jahrbiicher (Anat.), vi, 1892. Will. SB. Bt-rl. Akad., 1895. Wilson. E. B. .lourn. Morph., viii, 1893. Wilson, H. V. Bull. U.S. Fish-Commission, i\, 1891. Ziegler. Entvvickehmgsgesch. d. niederen Wirbeltiere, Jena, 1902. CHAPTER II THE SKIN AND ITS DERIVATIVES THE skin of the vertebrate consists of the epidermis — the persistent and less or more modified ectoderm — resting upon the superficial layer of niesenchyme — the dermis — which in the higher forms becomes strengthened by the formation of numerous tough inter- lacing fibres. In studying the development of the skin in the various types of vertebrate we find that the ectoderm undergoes characteristic modifications to fit it for the carrying out of special functions. In the fishes it becomes converted into a highly glandular mechanism concerned with the production of slippery mucus for the diminution of what the naval architect calls " skin-friction/' in other words the friction between the surface of the body and the water in contact with it. Local or general specializations of this glandular apparatus lead to the development of cement organs by the secretion of which the young animal is able to attach itself to solid supports, to the production of digestive ferments by. which the eggshell is softened or, in the case of the portion of ectoderm which lines the buccal cavity, the digestion of the food initiated, or to the production of poisonous defensive or offensive secretions. In the case of the terrestrial amphibians the glandular apparatus serves to keep the skin moist, while in the Birds it develops arrangements for oiling the feathers. Again the ectoderm develops important protective functions. It U'comes hardened and toughened to give mechanical protection: it becomes more or less loaded with opaque pigment to prevent the penetration of light rays, while in those highest vertebrates, in which, correlated with intensely active metabolism, the body is kept at a higher temperature than its surroundings, the superficial horny layer becomes as it were frayed out into a fluffy coating of feathers or hair which with its entangled air retards loss of heat from the surface of the body. Finally the ectoderm forms the great mechanism for the reception of impressions from the external world. It develops sensory cells which may become crowded together to form organs of special sense 69 70 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. while from its deeper layers arise the main portions of the central nervous system. THE EPIDERMIS The ectoderm covering the surface of the embryo becomes converted, normally, into the epidermis of the fully developed individual. Very usually the embryonic ectoderm consists of two layers of cells, the lower layer composed of actively living cells, the superficial of flattened plate-like protective cells. This outer layer has been termed by Krause the periderm : its superficial protoplasm is commonly hardened to form a cuticle in the strict sense of the term. Normally it "plays no active part in development and is shed at an early period. The deep layer of the ectoderm on the other hand is active. Its cells multiply so that it becomes several layers thick : the outer layers become cornified to form the horny stratum of the epidermis while the deeper cells, composed of active living protoplasm, form ttie stratum of Malpighi. The outer layer of ectoderm cells may be for* a time ciliated. This is well seen in young Amphibian embryos (Assheton, 1896). In Rana temporaria the 6 -mm. embryo possesses ciliated cells scattered thickly over its surface, the movement of the cilia being such as to drive a current of water tailwards over the surface of the embryo. "When the external gills develop, a specially strong ciliary current sweeps backwards over them and it is noteworthy that this current passes over the olfactory organ en route to the external gills so that the olfactory organ possibly plays an important part in testing the quality of the water going to the respiratory organs. The ciliary apparatus is sufficiently powerful at the stage in question to cause an embryo of this stage when laid on the bottom of a flat glass vessel to slide along at the rate of a millimetre in from four to seven seconds. As development proceeds the ciliation becomes less and less prominent and in a 20-mni. tadpole it has almost disappeared except on the surface of the tail which remains richly ciliated until the time of metamorphosis. This persistence of the tail cilia is doubtless correlated with the fact that the skin of the tail plays an important part in the process of respiration. HORNY DEVELOPMENTS OF THE EPIDERMIS Scales. — In many terrestrial Vertebrates the horny layer of the epidermis Incomes so thickened and hardened as to become practically nuid. In such eases the flexibility of the skin as a \\hole is retained by the thickened areas of epidermis being separated from one another I , v lines along which thickening dues not take place. The tliiekened portions nou i<>mi epidermal >eales of the type seen in lleplilcjs. They may take the i'<.rm nl' simple rounded prnjeetin^ bosses or tu: n Chameleons, or they may be llattened ii THE SKIN AND ITS DERIVATIVES 71 homy plates arranged edge to edge — as in Cheloniana «»r as <>n the ventral side of the body in Crocodiles or the dorsal surface of the head in Snakes and Lizards — or, finally, they may overlap like slates on a roof as is the case on the bodies of Lizards and Snakes. Occasionally, as in certain Lizards, individual scales may become greatly thickened and assume a conical spike-like form. The individual scale arises in development (Fig. 41) as a slight elevation of the surface beneath which the dermal connective tissue is somewhat concentrated. The epidermis covering the projection develops a well-marked cuticle. As development goes on the epidermis increases much in thickness and the cells of the outer layers become entirely cornified so as to form a horny plate or scale — supported by the underlying tough condensed portion of the dermis. It will be borne in mind that such typical reptilian scales have to be sharply distinguished from the morphologically quite different Fi<;. 41. — Early stage in the development of the scale of a snake as .seen in a longitudinal section perpendicular to the surface <>f tin- skin. scales developed in the -dermis in fishes. The ordinary reptilian scales serve mainly to protect the body from mechanical violence and from desiccation. Feathers. — In the homoiothermic Birds, where the body is kept at a constant temperature usually higher than that of the surround- ing atmosphere, the scales have become for the most part replaced by fluffy feathers which with the air entangled in their interstices form an admirable non-conducting envelope to retard the loss of heat by radiation, or convection, from the surface of the body. The rudiment of the feather begins (Fig. 42, A) as a slight thickening of the epidermis resting upon somewhat condensed dermis. The rudiment in fact differs little from that of a normal scale. The rudiment comes to project backwards (B) and then in- creases in length (C), projecting freely tailwards while its now relatively narrow base of attachment becomes sunk below the general surface into a pit or follicle. The rudiment now consists of a core of dermis surrounded by thick epidermis. The epidermis becomes incised along its axial surface by deep longitudinal grooves which divide its deeper portions into longitudinally arranged masses (Fig. 42, D, 6), the rudimentary barbs, while leaving the superficial portion as a continuous sheath (s/t.). The grooves in question do not reach to the base of the rudi- 7_> EMBRYOLOGY OF THE LOWER VERTEBRATES CH. inent — the unincised basal portion forming the quill of the feather. The horny sheath becomes strongly cornitied and then breaks open and the longitudinal thickenings of the epidermis, now also strongly cornified, break away from the sparse cornified dermal tissue of the axis and form the fluffy barbs of the down feather. In the basal quill portion of the feather the epidermis immedi- Ki.;. -12. Illustrating the development of feathers. (After Da vies, 1889.) A, H, (', longitudinal sections ; I), E, F, transverse SIT! ions (I). K, down leather ; F, tli^ht feather) : rudiment showing developing barlmles : H. Ion-it n\\\]> ', '/, quill ; •/•, lachis ; sii. sheath. ately covering the outer end of the axial dermal tissue m- pulp terms a thin Strongly cnniiiied superficial LiyiM* \\liicli st'paralt-s nil' as a septum cutting across the cavity of the quill. This pn»rv>s being repeated |MTio over the genej-.il smiace and as l;rmi:j(;s and Ke«-irices in ihe wings ii TMK SKIN AND ITS DERIVATIVES 73 and tail, originate IVum Liu- basal portions of down feathers which undergo a -mat increase in length. The basal part of the rudiment in this case inn-rases much in diameter. The epidermis here again becomes incised on its inner surface to form barb rudiments. These however are mucli more numerous (Fig. 42, F) than in the typical down feather and, further, instead of being arranged strictly longi- tudinally they are arranged somewhat spirally, starting from a con- tinuous epidermal thickening (?•) which runs along the outer side of the feather rudiment. This thickening is the rudimentary rachis or shaft and the barb rudiments run from it spirally round the feather rudiment until their tips meet along its inner side. The feather is thus in early stages curled into a cylindrical form round the central dermis or pulp — the whole being enclosed in a continuous sheath which disintegrates sooner or later setting free the elastic barbs and allowing them to flatten out to form the vexillum or vane. As is well known the barbs are united together in the fully- developed feather into a functionally continuous web, through the agency of the barbules which project from the two sides of the barbs much as the barbs do from the rachis. The mode of origin of the barbules is seen in a longitudinal radial section through a barb such as that shown in Fig. 42, G, where the outer portion of the barb rudiment is seen to be splitting up into barbules (&&) while its inner portion remains continuous to form the definitive barb (b). Traced downwards, towards the base of the feather, the rachis increases in width so as to extend round the whole periphery of the feather rudiment. Its outer layer assumes a translucent character and forms the cylindrical quill (calamus), the basal end of which becomes somewhat narrowed, bounding the umbilicus, the opening through which the dermal pulp extends up into the interior of the quill. The pulp of the feather undergoes a gradual shrinkage leaving behind it the series of cornified caps (H, c) formed on its apical surface as already mentioned and which eventually lie loose within the quill. The lips of the umbilicus are continued (Fig. 42, H) into a deep rim of uncornified epidermis (t of tin- claw. C, section similar to A, but from a chick 12 hours after hatching. c,p, claw-].; sole Of Haw ; /,, Haw-]>ad (neonychiuin). from its study is that the successive sets of feathers — the down feathers of the nestling, and the annual or other sets of feathers in the adult — are not to be looked on, as has been customary, as suc- cessive series of independent individual feathers. On the contrary the down leather and the definitive feathers, which succeed it in the series of moults, are all simply portions of a single Lrivatly elongated and basally growing structure — the first down feather hem- its tip. and the succeeding leathers being successive portions of it. The moult consists not, in the shedding of the whole feather hut merely in t.he hreakimj oil' of its proj,-ri m^ portion. Claws, \\hieJi make thrir lust appearance in A niira .\' ii THE SKIN AND ITS DEEIVATIVES 75 arise as >).<•( ial developments of the horny layer ensheathin^ the tip of the digit. To produce and retain a sharp edge or point by differ- ential wear the claw is normally of denser consistency and harder on the dorsal side and laterally, forming the " claw-plate " (Boas) (Fr_ C, c.p), while on the ventral side it forms the softer " sole " of the claw (Boas) (Fig. 43, C, c.s). Neonychia or Claw-pads.— To the embryo of an Amniotic Verte- brate, enclosed within its delicate membranes, the possession of sharp claws on the digits would obviously be a source of considerable danger during the later stages of development when the embryo moves its limbs, because of the liability of such structures to tear the foetal membranes. This danger is obviated by a beautiful adaptive arrangement which has been described by Agar (1909). In the embryo, the concavity on the lower side of the claw is completely filled up by a soft rounded pad or cushion (Fig. 43, A, n) formed by a thickening of the horny layer of the epidermis superficial to the sole of the claw. Agar has given the name Neonychium to this structure. In addition to mammals, which do not concern us here, Agar has studied these claw-pads in the Fowl and in the Lizard Tejus and there can be no doubt that the expanded claw-tips observed by Kathke (1866), Voeltzkow Fl(;: 44.— Right pectoral /-inrvrvx jn i j • /-i r\ A^N • n TT /T-- IA\ limb of an embryo Cro- (1899) and Goeldi (1900) in Crocodilian (Fig. 44) codile about two months embryos are the same structures and it seems after oviposition, show- probable that they will be found to occur in ing the hoof-like neouy- i , J. . . -v-r . , ,, ehia. (After Voeltzkow, claw-bearing Amniotic Vertebrates generally. 1399.) The neonychia are purely foetal structures which become detached soon after hatching (Fig. 43, B and C) leaving behind the functional claw. Jaws and Oral Combs of Anuran larvae. — Amongst the most interesting developments of the horny layer are the jaws and oral combs of frog tadpoles. The buccal opening is bounded by an upper and lower horny jaw, and external to and roughly parallel with these are rows of little horny denticles which form the oral combs and are used for fraying out the food. The number and arrange- ment of these rows of denticles — "upper labial" and "lower labial" —differs in different Anura and they afford useful characters for the identification of tadpoles (see Boulenger, 1897). The horny jaw is composed simply of a row of denticles so closely apposed as to be in contact. The terminal functional portion of each denticle is seen in longitudinal section (Fig. 45, A and B) to be com- posed of a series of hollow cones of hard horny material which closely ensheath one another. The terminal cone as it undergoes wear and tear eventually drops off, its function being taken over by the cone which it previously ensheathed. OF THE LOWER VERTEBRATES < MI. These cones form simply the terminal members of a series which extends inwards in the form of a curved column nearly to tin- inner surface of the epidermis. Only the terminal members are strongly cornified, the other members showing less and less cornification until at a little distance down the series the cone is seen to be composed of unmodified protoplasm containing at one side, near its base, a A. Fi'.. lf>. -Illustrating the devclopiiient of tin- larval t.-eth of Tadpoles. A. J'.. ' ...nntliil'i : l>, /,'<(/,,.p, mrsodrrmul papilla ; .-•, functional spine; >•!. spinr rudiment. of being regularly conical in shape it is claw-shaped with serrated edges (Fig. 45, D) the tip being recurved, and (2) the hollow base of the cornified cell is not entirely occupied by its successor in the series : it also accommodates an indifferent cell of the epidermis (supporting cell of Gutzeit) which bulges into it. I have described the development of these interesting structures 78 EMBRYOLOGY OF THE LOWER VERTEBRATES en. as they occur in a South American tadpole (Paludicola}1 but the description fits quite well the mode of development as it occurs in Tadpoles generally (Keiffer, Gutzeit),the differences between different species and genera, though of systematic import- ance, being differences in detail such as shape and arrangement of the individual teeth of the comb. " Teeth " of Cyclostomes.— The horny teeth of cyclosto- rnatous fishes, though they would naturally fall to be treated in the next chapter, situated as they are within the buccal cavity, may con- veniently be considered now owing to their resemblance— on a much larger scale and with multicellular structure — to the horny denticles of the tadpole. The tooth-like spines of the cyclostome are cones of highly cornified epidermal cells. Each tooth develops in the substance of the epidermis (Fig. 46, A) being strikingly like a hair- rudirnent during early stages. Successional spines develop be- neath the bases of the func- tional ones as shown in Fig. 46. GLANDULAR DEVELOPMKNTS OF THE EPIDERMIS. — In the Anamnia it is usually the case that scattered cells of the epi- dermis take on a glandular function and serve to form a slimy secretion which amongst other functions serves to dimin- ish the " skin-friction " which is tne main resistance to movement Fii.. 17. -Illustrating the development of the ri-meut - organ of Lepidosiren as seen in 1 ion*. A, staK« 23; B, HI.-. In A tin- r< i< I inn-iil »t tin- 1-i-nK'iii i : to IM- a tliick-niiix; <>t tli- «l'-.-|. lay-,- .,1 tl <-to.i.-i in ; in 1} anil (' tin- sii|x-i-licial layi-r lia-. ilisa|i|M-an-il MM-I- tin- Uiicki-iH-'l -lamlnlai • an-a ; in l> t In- m.-an i> i-oninn-n- ar« cullci-ti-d :;,::;;; •';;;;."" " '" through water. Such ,.nioell..I.-..- glands ina.y l>ecome collected together In I'nrin mull ic«-lliilar glands. ( M lln-sr Ilic most rnnspirimus examj'l"-- BM found, OUteide 'the Maniinalia, in tin- Lim^-lisli<>s and Aiujiliibians \\ ln-iv Mn-y I'MI -in 1 In- llask-'4lands and the cniuMit -, during the process of development., commonly 1M different directions so that in the fully developed 1 Th'- in: D <•!' tip- l>r:iii'-hr.l <'lir<>iii:ito|.li(iivs as nn'Sfiicli\ niat<>us i appears to the antlior 1o ac<-onl best \\itliob-cr\atimi but it slmiiM !>.• mm! mncd thai n-^.inl tli-in a> mo/>ticiii,i. 1,11. Hi-., cianial : • I .•-. A ami H an- more lii^lilv ma.nnilied Iliaii (', and Fin. (' Ilian 1>. | wards — tin- IMI]-III:J in ihi> case Uung the rudimcni «>!' i In- cerebral hemisphere A • .// The jiorlimi «.!' ihc jiriinii ivc lin-r-lirjiiii Ivin^ .just in I'mnt u\' tin- transverse fold <>l tin- l.iain-floor is the infundibulum. l-'.-n-tlu-r forwards the inner surface <•! tlie brain-llotn- fnnns a transverse BKAIN 87 groove bounded behind and in front by a slightly projecting ridge— the rudinii'iits <>r MM- optic chiasma and of the anterior commissure respectively (Fig. 53, ch, a.c). About stage 31 a little pocket-like diverticulum of the roof of the primitive fore-brain makes its appearance (Fig. 53, D, pin). This is the pineal body and its appearance is of topographical importance as serving to demarcate the primitive fore-brain roof into thalamencephalic and mesencephalic portions. thai. to. B c.H. thai FIG. 52. — Brain of young Lepidosiren as seen from the left side. A, stage 2»> ; Ji, stage 31 ; C, stage 35- ; D, stage 39. c.H, cerebral hemisphere ; /, primitive fold nt' l>min- tlnor ; inf, infiiiulibulum ; o.fe, olfactory bulb; o.t, olfactory tubercle; pin, pineal body; th}>im'ut. I .in- iliawn unil'Tllir NJIIIII- iii:i;jiiilir:itiuii. I-'JL;. ( i under :i lc.\\ .T m;i.mii!ir;it ion. A 1, • IM-HIHII ; -.//. r.-|.-l,|;il li.-||lis|»lirl-r ; C/i, nplir ch i;i-,Ili:i | ./, prilll!' Hi-linn ii;iii.-iiui:u- ; hi, liabenular (taperior) oommlMun ; L^, iofUndibnUr inf, infiiii'libiiliuii ; /«».-, |i:n;i|ih\ -is : ,,'tii, |.in.-;il li(H|\ ; //.<-, |M^t i-i-im- ruiiiNiisMii ••• ; /./>, dun < n<\ p ,ll\ :iiili-iinr i-inl nt ln.-iin-II.Hir. Stiu. -iiu. -s Mfciniiii- nut |.l;nii- lint in M-ctiuns |h-tr.-illfl tu I. ut some ili-,t:mr.- I'l-'in it, ITB -|i;i.|.-.| \\ith uliliijin- lni.-,. ii NERVOUS SYSTEM 89 on the constriction between thalamencephalon and mesencephalou becomes more marked. The roof of the former remains thin and membranous, forming the cushion-like dorsal sac upon which the pineal body rests. The roof of the meseucephalon becomes slightly thickened on each side of the mesial plane forming the tectum opticum l)ii t correlated with the small size of the eyes in Lepidosiren the thickening never becomes so great as to produce projecting optic lobes such as are formed in most Vertebrates. In the hind-brain region the greater part of the roof, covering in the fourth ventricle, becomes thin and membranous. Across the anterior boundary of the hind- brain the roof does not undergo this secondary process of thinning but persists as a transverse thickened band — the rudiment of the cerebellum. SUBSEQUENT DEVELOPMENT OF THE BRAIN REGIONS RHOMBENCEPHALON or Hind-Brain. — The hind -brain, correlated perhaps with the fact that it contains nerve-centres of supreme importance to life, develops precociously and reaches a relatively enormous size during early stages (Fig. 51, A, rk). The bulging inwards which marks its anterior limit is doubtless to be regarded as an expression of the active growth in length of its floor during these early stages. During later stages of development it forms a conspicuous pro- jecting restiforrn body on each side reaching forwards nearly to the anterior limit of the mesencephalon but this becomes again less and less prominent as the adult condition is approached. The cerebellum retains through life its primitive condition as a simple transverse thickening of the hind-brain roof. MESENCEPHALON. — The roof, as already indicated, becomes thick- ened somewhat on each side (tectum opticum) but not to such an extent as to bulge outwards and form optic lobes. Close to its anterior limit a conspicuous bridge of transversely -running nerve- fibres makes its appearance at a late stage of development. This is the posterior commissure — an important brain landmark (Fig. 53, G, p.c). THALAMENCEPHALON. — The side wall of the thalamencephalon becomes greatly thickened to form the optic thalamus which bounds on each side the slit-like third ventricle. The roof becomes for the most part thin and membranous forming the dorsal sac. On either side of the pineal body however it becomes greatly thickened to form the habenular ganglion. As these ganglia develop a bridge of transverse nerve -fibres makes its appearance uniting them — the superior or, better, habenular commissure. The pineal body as development goes on enlarges somewhat and assumes a carrot shape. Its lumen becomes obliterated posteriorly so that it no longer opens into the third ventricle. The anterior isolated part of the cavity becomes eventually almost filled with 90 EMBKYOLOGY OF THE LOWEK VERTEBKATES CH. granular matt-rial produced by the breaking down of the epithelial fining. The paraphysis forms for a time a conspicuous tube passing upwards and forwards in the space between the two hemispheres and ending blindly. In later stages of development it undergoes a relative reduction in size, and heenmes irregularly twisted and mixed up with the choroid plexus of the ventricles. On either side of the para.pl iysis and just dorsal and posterior to its base, the wall of the brain becomes involuted into the third ventricle, the involuted portion being thin and membranous and enclosing an ingrowth of blood-vessels. This vascular ingrowth represents a structure which in most Vertebrates is continuous across the mesial plane with its fellow so as to form an unpaired structure the velum transversum. This is regarded by most writers on the lira in as an important landmark in brain topography. On the floor of the thalamencephalon the optic chiasma and the anterior commissure form prominent bulgings into the ventricle. Each develops nerve-fibres in its substance, connected in the one case with the organs of vision and in the other with the cerebral hemi- spheres, especially those portions devoted to the sense of smell. In front of the optic chiasma lies a deep optic recess which is prolonged outwards by an outgrowth of the side wall of the brain, the optic outgrowth, which gives rise to a great part of the eye and will be described later. Behind the chiasma is the infundibulum, the tip of which at a late stage in development (about stage 38) sprouts out into narrow tubular diverticula. These increase in length, wind hither and thither, and partially penetrate into the substance of the pituitary body which lies immediately beneath. The epithelium of these tubular diverticula assumes a glandular appearance and together they constitute the " infundibular gland "- often called the " nervous portion of the pituitary body." The series of sagittal sections in Fig. 53 is of interest from its bearing upon a question which has excited some discussion, namely as to what point in the fully developed brain of the vertebrate < "in-sponds to the morphologically anterior end of the brain rudi- ment in earlier stages of development. It has been held by many morpholounsts, such as von Baer, His, Sedgwick, that the tip of the infundibulum represents the anterior end of the primitive brain, the nt condition having been brought about by the anterior portion of the brain becoming bent upon itself into a retort shape. As will ill »y an inspection of the figures the brain of Lepidosiren lends no support, to this idea. On the contrary the tip of the infundibulum clearlj ponds to a point close, to the. letter A of l-'i.u. r»3, A. On tin- otln-r hand, iMjnally clearly the anterior tip (*) of the hrain- lloor of an i-arly stage sueh as that shown in Fi-- 53, l> is represented in the adull by a point well up on the anterior wall of the t halameii- e.-phalon lamina terminalis) and just, ventral to the PM.I. of the paraphj ii HEMISPHERES 91 CEREBRAL HEMISPHERES. Tin- hemispheres arise as bulgings of the side, wall of the fore-l.rain. As development goes on they increase in si/e jiml grow first dorsalwards and later on forwards until in the adult they are relatively very large. This increase in si/e is associ- ated witli a corresponding growth in the thickness of the wall of the hemisphere — except at its hinder end next the thalamencephalon. Here the inner wall of the hemisphere facing the thalamencephalon remains relatively thin. About stage 35 a small rounded portion of this thin part of the hemisphere wall l.ulges into its cavity — the lateral ventricle. This ingrowth contains a vascular loop and is the rudiment of the choroid plexus of the hemisphere <»r lateral plexus. The plexus grows rapidly into the ventricular cavity, forming an irregular crumpled lamina which in the adult attains to great size and complexity traversing the whole lateral ventricle (Fig. 53, F and G, l.p). No doubt this, by diffusion between the blood in its vessels and" the fluid in the lateral ventricle, helps to provide for the nutritive and respiratory needs of the hemisphere wall. During the later development of the hemisphere its walls become differentiated into regions in the manner described by Elliot Smith (1908). Most important from the point of view of general vertebrate morphology is the fact that a distinct cortex is developed in the form of a layer of ganglion-cells traversing the roof of the hemisphere parallel to its surface, and at about one-third of the distance from the surface to the ventricular cavity. This cortex extends on the one hand just on to the mesial face of the hemisphere and on the other to a point rather more than one-third of the distance from dorsal to ventral edge on the outer face of the hemisphere. Of this cortical formation, which constitutes the archipallium, the mesial portion corresponds to the hippocampus of higher verte- brates, and the outer portion to the pyriform lobe. The neo- pallium which in the higher forms becomes interposed between these does not appear yet to have become distinctly recognizable in Lepidosiren. Less important from the point of view of general morphology but more conspicuous in their structural expression are certain changes which take place in relation to the olfactory apparatus. The portion of hemisphere wall to which the first cranial nerve is attached — the olfactory bulb — is at first simply part of the lateral wall of the hemisphere but as development proceeds it is found to take the form of a sort of cap lying on the dorsal side or roof of the hemisphere at about the middle of its length as viewed from above. This change in position is brought about by an enormous hyper- trophy of the portion of the ventral wall of the hemisphere which lies in front of the optic chiasma — the olfactory tubercle. Later on, from stage 38, the portion of hemisphere roof lying posterior to the olfactory bulb undergoes active growth in length with the result that the bulb is gradually carried forwards and 92 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. eventually comes to lie right at the anterior end of the hemisphere (Fig. 52, D, o.Z>). At the same time the bulb comes to. form a definite hollow projection of the brain surface immediately dorsal to the still greatly enlarged olfactory tubercle (o.t). DIFFERENTIATION OF THE BRAIN REGIONS IN ACANTHIAS. — The development of the brain of Elasmobranchs has been worked out by Kupffer (1906) for Acanthias and his account has been made use of in writing the following short summary. Figures of the early stages of the medullary plate as seen in surface view are given in Chap. XI. The medullary plate projects forwards from the posterior boundary of the blastoderm and is raised well above the general surface. As it increases in length its lateral edges become raised up so that the portion on each side slopes inwards and downwards into a kind of valley. Each half of the medullary plate extends back into one of the " caudal lobes " which with growth in length come to project freely beyond the edge of the blastoderm. Another result of the increase in length is that the anterior end of the medullary plate comes to project freely forwards over the blastoderm forming a head-fold. Each side of the* medullary plate arches inwards towards the mesial plane and the whole becomes converted into a neural tube in a perfectly normal fashion. As in the case of Lepidosiren, the first sign of differentiation of the brain into its parts is a division into primitive fore-brain (Arch- encephalon) and hind-brain. The demarcation is again most distinct ventrally where the brain-floor bulges into the ventricle (Fig. 54, B) as a prominent fold. Later on this fold spreads upwards on each side to the dorsal surface forming the rhombo-mesencephalic fissure which marks off the mid-brain from the hind-brain. It is only at a later stage in development that the mesencephalon becomes marked off by a constriction from the anterior portion of the archencephalou which forms the thalamencephalon. It is of interest to compare sagittal sections through the 1 train of the Elasinobranch with the corresponding sections already described for the holoblastic lung-fish. Neglecting small differences in detail there is seen to be a striking difference between the two brains — most marked in the middle stages figured — in relation to the longitudinal axis. In Fig. 54, C the Elasmobranch brain is seen to be as a \vhole strongly curved in a ventral direction: it shows a hi^h decree of •l»nil flexure." Tin- corresponding stage of the Dipiman brain is on the other liand almost straight, the superficial appearance of curvature beiii1^ due mainly to the prominent Ibid of its floor which projects up into the cavity at the level of the mid-brain. This cerehral llexure, which is especially conspicuous not only in tin; hrain of the Klasmohranch hut also in the other types of lira in 'Mammalian and AvianJ that wen- first investigated developnient- a.lly, has p laved ;i lar^e part in discussions on hrain morphology. Thus the idea, already alluded to. that the tip ol1 the inlundihulum is th«- morpholo'jic;i||v anterior end of the hrain rests upon it. BRAIN 93 That this idea is unsound seems clearly t«. In- indicated \>y such series of stations ;is that shown in l-'i^. f>.">. AJB 8 mai t.i of fact it f>4. — Sagittal sections through the brain o! ch. (After Kupffer.) A. :;-3 mm., embryo; B, 7-8 nun. ; C, 10 mm. ; 1), 27 mm. ; E, 70 mm. tvc, ci>ivl>.-llum ; ••/>, optic chiasma ; «•/. rxtrinal rcUnlrrm ; li.c, lialirnulai- cnmmissiu-c ; inf. int'iindiliuliim : .V, cavity d' mi'srii- cfj>h;ili>n : .V. imtuchonl ; n.ji, auti-rinr ncuropore ; p.c, iwstt-rior (-(immissurc ; i>in, piiifal or.^iii ; Rh, cavity dt' ilmnibi'iicrplialiiii ; t.". tectinn ujitii-iiiii ; r, rclum trunsrerxinii ; r.\\. fourth vi-utriclf. seems probable that very pronounced flexure of the brain, such as is seen in the developing Elasmobranch, is to be regarded as a secondary 94 EMBRYOLOGY OF THE LOWER VERTEBRATES Oft result of the heavily yolkcd condition of th- As a result of the concentration of yolk towards tin- vrntral side in such heavily yolked Vertebrates the processes of growth ,-nv retarded upon that side. But it is clear that retardation <>t growth in length on the ventral side as compared with the dorsal would bring about a flexure towards the ventral side. That the cerebral flexure is due rather to such a general cause than to any inherent peculiarity in the brain itself is supported by the fact that the notochord is also strongly flexed (see Fig. 54, C). As a consequence of these considerations, we are inclined to take the view that the phenomenon of cerebral flexure is of much less fundamental morphological significance than is commonly supposed. Comparing the later stages figured for Acanthias with those of Lepidosircn, it will be seen that the brain shows the same elements although these differ in their relative size and other features in the two cases. Thus the cerebellum of the sharks — correlated with the active and complex movements of these fishes — becomes much more developed. It grows greatly in anteroposterior extent and that causes it to bulge outwards as shown in Fig. 54, E (cer). The pineal body is slender and elongated in form : the velum forms a conspicuous infolding of the thalamencephalic roof continuous across the mesial plane. The wall of the anterior portion of the primitive fore-brain under- goes a fairly uniform increase in thickness throughout with the excep- tion of a transverse band just in front of the velum which becomes thin and membranous. This portion of the brain increases somewhat in transverse diameter so that it is broad in shape as seen from above, but there is no definite bulging in its side wall to form a distinct hemisphere. The material that would ordinarily go to form the hemispheres remains here in the thickness of the wall. The olfactory bulb arises as a slight projection from the side wall of the fore-brain, but as development proceeds and the olfactory organ becomes removed from the brain by the interposition of mesenchyme the olfactory bulb remains in contact with the olfactory organ, its attachment to the brain becoming drawn out into a nn»iv or Lesa elongated stalk the olfactory peduncle or olfactory tract. The salient features in the establishment of the topography ol' tin- Vertebrate brain have been illustrated in outline in the sketch which has just been given. It would be beyond the scope of this work to make any at tempt to fill in the picture in detail but it is necessary to j-eeall a lew points which are of interest to morphologists apart from •uli-ts in neurology. It should in the first place In- borne clearly in mind I hat the hrain like indrcd the whole of the nervous system (see below, p. 1 IS'i [fl to he looked upon as a fundamentally con tiniious struetmv. The which compose the adult brain medulla, ceivhellum, meseii- c.-plialon and so on are not to he regarded as constituent units which iro to build up the compl.-te brain, bill rather as speciali/ed portions of a once homogeneous \\hole. The process of specialization ,, I1KAIN has proliahly hern linked up ni<>iv particularly with the process' Inrali/ation or centralization «>r particular functions iii particular brain regions. When this has conn- about, increase in tin* number dl' Lra,n--li( in-crlls devoted to tin- particular function will cause an increase in bulk of that portion of the brain in which they are situated ami it will assume definite characteristics of its own. The first step in the development of such a brain region consists in the mere thickening of the brain wall but with still greater increase in the number of cellular elements involved mere increase in thickness becomes insufficient for their accommodation and an increase in area comes about in addition. This necessarily causes a bulging of the particular part of the brain wall and some of the most characteristic differences between the brains of different types of Vertebrate depend upon whether the bulging takes place outwards or inwards. Thus in the majority of Vertebrates the cerebellum bulges out- wards as has been indicated in the case of jlcanthias. In Teleostean fishes on the other hand this is the case with only the hinder part of the cerebellum : its anterior portion in these fishes bulges downwards and forwards underneath the roof of the mesencephalon forming the well-known valvula cerebelli. In the more primitive ganoid fishes on the other hand such as Polypterus (Graham Kerr, 1907) the hind portion of the cerebellum also grows inwards, so as to form a structure projecting back into the fourth ventricle in just the same fashion as the valvula cerebelli projects forwards. A somewhat similar difference appears to be present in the case of the hemispheres. These originate in most subdivisions of the Vertebra ta as paired bulgings of the wall of the primitive fore-brain, and the present writer agrees with Studnicka (1896) in feeling com- pelled to accept on this ground the view taught by many of the older morphologists such as von Baer, Eeichert and Goethe that the hemi- spheres are to be looked on as fundamentally paired structures, rather than the view, more fashionable of recent years, which regards the portion of the primitive brain lying in front of the velum and optic recess as forming together with the hemisphere region an impaired complex (Telencephalon — His). The more complete knowledge that we now possess regarding the develop- ment of the brain in the more primitive Vertebrates with holo- hlastic eggs, seems to the writer to make it clear that the reasons which have led to a departure from the older view can no longer be regarded as adequate. We take it then that the hemispheres are fundamentally paired projections of the side wall of the primitive fore-brain. Physiologically they are probably to be regarded as portions of the brain wall which have become specially enlarged in relation with the sense of smell, just as are the optic outgrowths in relation with the sense of sight. Now whereas in the majority of Vertebrates the hemispheres bulge outwards, in the more primitive Teleostoines (e.g. Polypterus, 96 EMBEYOLOGY OF THE LOWER .VERTEBRATES CH. Graham Kerr, 1907) they bulge inwards. lu the typical Teleosts what apparently corresponds to the hemisphere forms simply a solid mass projecting into the cavity of the fore-brain, the structure which is usually and probably erroneously spoken <>f ;is the corpus striatum in these fish- A part of the brain which is of very special morphological inteivsi is the thalamencephalon — which persists with singularly little change throughout the series of Vertebrates. Even in Amphioxus sagittal sections through the front end of the neural tube present appearances which vividly suggest the thalamencephalon of the more typical Vertebrates (Kupffer) and raise the question whether — as is probable enough on other grounds — the head region in Amphioxus is degener- ate and once possessed a brain. Amongst the structures connected with the thalamencephalon special interest attaches to the pineal body.1 So far this has been alluded to merely as a comparatively simple diverticulum of the thalamencephalic roof. In the majority of Vertebrates it remains comparatively uncomplicated and its main function appears to be that of forming a peculiar internal secretion. In two sets of Vertebrates — the Lampreys on the one hand, and Sphenodon and many Lizards on the other — there becomes developed in relation to it an organ possessing a close resemblance to an eye, of the " camera " type, possessing a retina and in some cases a lens. The organ appears to be functional as the tissues overlying it are commonly free from pigment and its retinal cells on exposure to light show a change of position in their pigment granules similar to what is commonly found in visual organs. Though functional it does not follow that the organ serves for the detection of what we call light : it may be that its sensitiveness is rather towards radiant energy of other wave-lengths than that included within the range of the visible spectrum. There is a general tendency amongst those who have carried out researches upon the pineal eye to regard the eyelike condition as a relatively archaic condition of the pineal organ — a tendency which is encouraged by the evidence of palaeontology that certain ancient Tetrapods of the palaeozoic and mesozoic periods possessed a highly developed pineal organ — the skulls of these animals possessing a relatively huge parietal foramen, corresponding with the* foramen in the roof of the skull of modern lizards in which tin- pineal eye lie- embedded. The evidence of embryology indicates that the most archaic con- dition «»!' the pineal organ was a simple diverticnlnni of the brain projecting towards the skin on the dorsal surface nf the head. TlnTc is no clue whatever .as to the original meaning of this diverti- culum. But we do know from the study of invertebrates that \\ln-re tissue rich in ner\«'('lcni.-nt,s comes to he exposed to light, ihere is 1 An adiiiiral.li- arrount of tin- .structure ami ilc\ rlujmn'iit of this r.-^ioii of tin- braili l.y Slu.lnicka will b<- fouii'l in Oppd l'.»05). ii PINEAL OKGAN 97 frequently shown a well-marked tendency to the evolution of eye-like structure. In Molluscs for example we find eyes developing «.u the edge of tin- mantle /v/-//:/i), round the tips of tin- siphons (Oardium sp.), on tin1 rsul surface of the body (independently in Ch-ttnn and Oncidium) — and similar instances might be quoted from other groups. Hearing such facts in mind one is compelled to acknowledge the possibility, if imi probability, of such a projecting piece of nervous tissue as t.-hi- piii'-al diverticulum, lying close under the surface of the head on its dorsal side, in the position where light stimulus would be most pronounced, developing secondarily in some cases into an organ of the nature of an eye. Discovered by Leydig (1872), its structure investigated by Spencer (1886) and other workers, the development- of the pineal eye has formed the subject of a number of excellent researches. It will be convenient to take as an example that of the common lizards of the genus Lacerta (Novikol'f, 1910). The first indication of the organ appears in embryos of about .') mm. in length in the form of a thickening of the thalamencephalic roof, in the region of the mesial plane, and divided by a transverse furrow on its outer surface into a smaller anterior and a larger posterior portion. This thickened part of the brain roof comes to bulge outwards and forms a prominent projection (Fig. 55, A) the groove dividing it externally into anterior and posterior portions being still visible though less distinct. The projecting pocket now grows forwards parallel to, and in close contact with, the brain roof (Fig. 55, B), its forwardly projecting portion becoming constricted off from the rest. The constriction in question deepens and the anterior portion (parapineal body) becomes nipped off to form a completely closed vesicle (Fig. 55, C) — the rudi- ment of the eye. As the external ectoderm recedes from the brain roof, with the increase in the amount of mesenchyme between the two, the parapineal vesicle remains close to the ectoderm and con- sequently retreats from the brain surface (Fig. 55, D). The eye is now seen to be connected with the brain wall by a distinct optic nerve which, in full accordance with the view taken in this book with regard to nerve-trunks in general, is merely a primary bridge which already existed (Novikoff) at a time when the eye vesicle ;ind the brain roof were still in immediate contact and which simply be. -a. me extended in length as the gap between eye and brain became greater and greater (Fig. 56, p.n). Nerve-fibres develop in this optic nerve which pass at their cerebral end into the habenular commissure. Transverse sections through a 9-inm. embryo show that the fibres on entering tin- commissure bend away to the right, passing eventually to the right hahenular ganglion. In this connexion with the right hahenular ganglion Lacerta resembles the other lizard Iguaii" /'(to (lvlineko\\ strom, 1894) but curiously differs from Sphenodon wbere according to I )endy (1899) the connexion is with the left hahenular ganglion. VOL. II H 98 EMBKYOLOGY OF THE LOWEK VEETEBKATES CH. Meanwhile the wall of the vesicle lying next tin- outer skin pin. eel. thai A I I'.. 55. — Sagitt:il MM lions tln-mi^li tin- jiinr;il nr-.'iu id riiilnyns (.1 I.«<; rln. (Aft. -i Ni. \iknlV. I'.MO.) A,/.. rum.; H. .lilln. I nun.; ( tl nun.; h. • \irin.il "ct^l.-iin ; /. l.-ns ; /..«. |.m.-;il .>••; p.n, |.im-;il II.TV; /(.s. |,in,-;il >lalk : pC«, |'iiii-;il <.nl -i n\vt h ; • !,;il;nn«-ni-i-jili:ili.n. PINEAL OEGAN 99 assumes a lenticular form, its cells becoming much eltMiLrutrd thougli remaining in ;i single layer. This lenticular thickening "nM>i"!ially hecoines lost during de\ elopinent a fact which may be taken as formiii;_r a. piece «»!' evidence in favour of the view tliat the eye, at the present time, is in a retrogressive phase of evolution. Tlmsc pails ol ihc vesicle wail which do not take part in the format ion oi' i h«- lens undergo histogenetio changes into retinal tissue. The cells undergo differentiation in two directions. The one set heroine pi-ineiit cells — tali columnar cells which traverse the whole thickness of the retina and have their nuclei towards the basal or outer end and which develop dark melanin granules in their proto- plasm. Interspersed with these are the percipient cells, shorter in form, their basal ends not reaching the outer surface, and carrying at their inner ends cilium-like structures which project into the cavity of the vesicle. The idea that these projec- tions correspond physiologically to rods appears to be negatived by the fact that they occur also on the inner ends of the cells forming the lens. At their basal ends these cells are continued into nerve-fibres, which form a distinct layer internal to the nuclei of the pigment cells and are eventually continuous physiologi- cally with the fibres of the pineal nerve. Scattered amongst, and in the neighbourhood of, this fibrous layer 37s, ganglion -cells are present: they are about tlie first definite elements to become recognizable, during the histogenesis of the retina and appear first close to the point of attach- ment of the optic nerve. The. cavity of the pineal eye is kept distended by a clear substance, the vitn-ous body, and this is colonized by a certain number of cells Fig. 55, C) which are most probably to be regarded as immi- grant mesen chyme cells. In Sphenodon, the sole survivor of the only other existing group of Eeptiles in which a pineal eye is present, the development of the or-aii according to 1 )endy (1899), who has worked it out in detail, - with that of l,itcerta in its main features. In the Lampreys, also, somewhat eye-like developments occur in i he pineal re-ion. In the adult two vesicles — a dorsal ("pineal") and a veniial (" parapineal ") are found overlying the roof of the FK;. 56. — Lacerta muralis, 25 mm., sagit- tal section through roof of Thalainen- cephalon. (After Novikoff. ) h.c, habenular commissure ; p.c, commissure ; /<.«•, pineal eye ; p.n, pineal nerve ; stalk ; /xii; 100 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. thalamenoephaloiL In each of these the lower wall shows histological characteristics of retinal tissue ami each is in continuity with the iiraiu in the case of the parapineal or^an directly and in the case of the pineal by an elongated stalk containing nerve -fibres. The parapineal or^an lies in some cases (Geotria — Dendy, 1907) slightly to the left of the pineal and its nerve-fibres have been traced into the left liuhenular ^an^linn while those of the pineal organ have been traced to the right ha hen u la r ganglion. In neither case does the outer wall of the vesicle show any signs of thickening t«> form a lens — so that neither organ can form an image — but the overlying tissue is comparatively transparent so that diffuse light stimulus can reach it. According to Studnicka the two organs develop as evaginatimis of the brain roof one (parapineal) in front of the other. The para- pineal evaginatioD soon loses its lumen and becomes solid and it is noteworthy that at first it is continuous on each side with the habenu- lar ganglion of that side. Later on it becomes by differential growth shifted far fur wards, away from the region of the habenular ganglia, and it loses its connexion with the right ganglion while it remains connected by nerve-fibres with the left. The two questions of special interest which present themselves in regard to the pineal and parapineal organs are (1) were they originally ocular in structure and function and (2) were they paired or unpaired ? (1) It is obvious that the presence of an eye-like pineal or para- pineal organ in certain Reptiles and in Lampreys, and of a large parietal foramen in the skull of various extinct Vertebrates suggests the possibility of these organs having had the form of visual sense organs in the ancestral Vertebrate. Against this however must be set the fact that in all other Vertebrates than those mentioned, including such relatively archaic forms as Elasmobranchs, Cross- opt erygians, Dipnoans and Urodeles, there is no trace whatever of eye structure. It seems highly improbable that a well-developed visual organ once present on the dorsal side of the head in the ancestors of Vertebrates should have disappeared without leaving a trace in all the varied Croups, with their very different modes of life, outside ihe limits of the Lampreys and Reptiles. To the present writer it does not appear that the evidence, so far as it. exists at present. :iythiiiir like convincim: that the pineal eye is an ancestral of Vertebrates in general rather than a mere secondarv development. (2) Various ivc.-iil investigators <>l the pineal Organs are inclined to look upon them as beiii^ < ,i i- 1 na 11 \ paiivd struct ures, the pineal Organ in the sti<< bein^ the ii'jht hand member of the pair and the parapme.il organ I In- I'-l't. This is perhaps most clearlv : by the Lampreys in which the para pineal or^aii is con- d by Oerve ftbrea with the left habenular -an--li«'ii and the II NEUROMERY 101 pineal oi'Liiiii \vitli the right; (thougli also with tin- posterio missure). Again in various Vertebrat.es (Teleostomatoufl lislies — Hill, 1894: Amphibians, Birds — Cameron, 1903, 1904) the parapineal nr^an is in early stages slightly to the left of the pineal oi-^.-m. On the whole it does not appear to the pivM-nt \\i-iti-r that the evidence is sufficient to make the view probable. In the Lampreys the connexion of the parapineal hody with only the left hahenular ganglion appears, as indicated above, to be secondary: it is originally connected with both right and left. Again, to turn to the Kept ilia. the eye is in Sphenodon connected with the left habenular ganglion and in the Lacertilia with the right, although it seems perfectly clear from the figures given by Dendy and Novikoff respectively that the eye is morphologically the same organ in the two types mentioned. Were it not so we should lie driven to the position that of a, pair of pineal eyes originally present one has disappeared entirety in Sphenodon and the other has disappeared equally completely in the lizards. The improbability from a physiological point of view of this having happened need not be accentuated. Here again, then, there seems to be up to the present no sufficient reason for departing from the view that pineal and parapineal organs were primitively median in position, one in front of the other. As to their original significance we have no obvious clue : the absence of convincing evidence that they were originally eyes does not of course preclude the possibility of their having been originally some kind of sense organ. NEUROMERY. — It has been noticed in various Vertebrates, particu- larly Elasmobranchs, Amphibians and Birds, that the neural rudi- ment while still in the form of an open plate is sometimes divided up by numerous and regular transverse markings. Whether this appear- ance of segmentation is caused simply by the active growth in length of the medullary plate or whether on the other hand it has some deeper significance has not been conclusively determined. The name neuromere has been given to the apparent segments. That these are really primitive morphological segments as is believed by many and as is implied in the termination "-mere" seems improbable. The existence of a clearly marked segmentation of the nervous system where it occurs — in the phyla Annelida and Arthropoda — is brought about by the concentration of ganglion-cells in serially repeated masses, in correlation with the presence of serially repeated appendages (parapodia in Polychaeta), and there is no sufficient evidence to show that such were ever present in the ancestral Vertebrate. The fact that the longitudinal muscles are divided into myotomes would not be sufficient by itself to account for the external form of the central nervous system being segmented, for in that case the segmentation would be still clearly marked in the many tishes where the myotomes remain practically unmodified. During later stages, after the neural tube has become closed in, " neuromeres" are particularly conspicuous in the brain region. 102 EMBRYOLOGY OF THK I.nWKl; \ KLTEBRATES en. They are sometimes very distinct in the hind-brain of fowl em- bryos of about the fourth day (see Fii:- -•"><'» in chap. X.). It is however an outstanding characteristic of tin* head region as compared with the trunk that the segmentation of its mesoderm lias become Idurred and in great part obliterated. It is under those ciroumstancefl improbable that a primitive segmentation of the central nervous system, which is in its nature linked up to a segment a thm of mesodermal structures, should have remained particularly distinct in a region where the Diesodermal segmentation itself has become particularly obscure. The appearances mentioned seem to be adequately explained by the active growth of the developing brain within its confined space, combined with the presence round it of mesodermal tissue with vestigial segmentation. It will be noticed in the figure referred to above that the dividing lines between the " neuromeres " are spaced out at exactly the levels where we should expect to see boundaries of mesoderm segments were the existing series prolonged forwards. Segments are no longer visible in this region but there is, as will appear later, convincing evidence that the series of segments did formerly extend through this region now occupied by continuous mesenchyme. It may well be that the individuality of the segments, no longer visible as such, is still expressed by a difference in consistency of the mesenchyme, sufficient to mould by its varying resistance the actively growing hind-brain as it presses against it. DEVELOPMENT OF THE PERIPHERAL NERVES. — The development of the peripheral nerves of Vertebrates has been the subject of a large amount of investigation, partly on account of its intrinsic interest and partly on account of its bearing upon physiology and pathology. In spite of the labours of numerous investigators tl it- problem — for we may take it that the mode of development is //'?&' /"- mentally the same throughout — has not yet by any means been satisfactorily decided. While bearing in mind the undesirability of making use of modern facts merely to support, or to undermine, old hypotheses, it will l>e convenient to approach the question by stating shortly the t hive prevalent views as to the main features of the development- of peripheral nerve-trunks neglecting differences in detail. Kor shortness these three views may be termed alter their nmst prominent supporters (1) the His view, (2) the Balfour view and (3) the Hensen view. THK His VIKW (Outgrowth theory). — This hypothesis may In- said to have been found. -d by KupfVer in the rmbry<>l»»Lrieal portion of Hiddcr and Kuplfer's work (lSf>7 «»n the spinal cord. As however KupH'.-r lat«T nn LMve up the view, in favour of that of Uall'mir. t he bvpOtheflU now under Consideration is eumniouly associated with the name of His. who pl.m-d the main part in building up i he theory and who full its principal founder. ii NERVE DEVELOPMENT 103 It is to be iioted in passing that Kupfifdr'fl origin;! 1 observations were made upon M.-nnmals and those of His (1868) upon the Fowl. In other words, in Ix.th cases the embryos were such, in regard both to the minute size of their cell elements and to their high position in the Vertebrate scale, as to be unsuited to provide a reliable basis for the generalization that has been built upon them. The His view as expounded by one of its most distinguished supporters S. Eamon y Cajal (1909) may be summarized as follows, the case of the motor nerves being taken for the sake of simplicity. Each motor nerve-fibre arises as an outgrowth from a neuroblast, or young ganglion -cell, lying within the spinal cord. The fibre sprouts out from the neuroblast, makes its way to the surface of the spinal cord, perforates that surface, and proceeds to grow freely through the mesenchyme. The free end of the fibre forms a " cone of growth," Commonly shaped somewhat like a grain of barley and with a pointed end. This "cone of growth" shows an active amoeboid movement, by which it insinuates itself through the interstices of the mesenchyme. Sometimes it may be seen to flatten or mould itself against obstacles in its path. In the Fowl these processes take place during the third and fourth days. Eventually (about the fifth day, in the Fowl, in most cases) the growing nerve -fibres reach their destination and become joined up to the muscle cells which form their definitive end-organs. The essential feature of the His view is that the nerve-fibre (which already shows the characteristic specific reactions of a nerve- fibre, e.g. on impregnation with silver salts, and is therefore not merely a strand of undifferentiated protoplasm) sprouts out from the central nervous system and grows through the intervening mesenchyme with a free end until it becomes joined up secondarily with the end-organ. The His view at the present day rests upon a large body of observed facts. In studying the embryology of almost any Verte- brate it is easy to find in sections what appear to be freely ending nerve-fibres sprouting out from the spinal cord. Some of the most beautiful preparations of this kind have been made by Eamon y Cajal and others by the use of silver impregnation methods. Perhaps the most striking evidence, which has recently been adduced in favour of the His view, has been obtained by experi- mental methods, especially by Harrison (1908, 1910). In one set of experiments which have been regarded as particularly convincing Harrison removed small pieces of embryonic spinal cord from Frog embryos at a period just before that at which the motor nerves became visible, and was able, by using ordinary antiseptic pre- cautions, to keep them in a living condition for relatively prolonged periods mounted in a drop of sterile lymph under a coverslip upon a slide. The lymph soon clotted and held the piece of spinal cord in position. Harrison now observed in many cases little projections 104 EMBRYOLOGY OK TIIK LoWKi; \ KIMTJ'.L'ATKS making their appearance from the pieces nf spinal cord which he identified as rudimentary motor nerves. Any possible doubt as to the correctness of this identification \\as removed by Burrows (1911) repeating the work on the chick and obtaining the specific staining reaction of neuro-fibrils in the structures in question. These nerve -rudiments when kept alive under the conditions mentioned were observed to increase rapidly in length, the rate of growth being in one case as high as 56// per hour. The end of the rudiment (Fig. 57) was somewhat enlarged and projected into fine protoplasmic tags which showed aeti\e uim>el>oid movement. It is this amoeboid protoplasm at the free end of the fibre which, in Harrison's belief, is the active agent in the extension of the nerve-fibre. As to the method by which it is, in the actual body, guided along the proper path to its destination, Harrison does not commit himself, but he appears to have a leaning towards the view held by Ramon y Cajal that it is mainly a matter of chemiotaxy. In the words of their author (1908) these " experiments place the outgrowth theory of His upon the firmest possible basis, — that of direct observation. The attractive idea of Hensen must be abandoned as un- tenable." It should be added that the His theory fits in very well with current views in physiology and pathology — in particular with the fashionable neurone doctrine, according to which the cellular units which compose the nervous system are not in organic continuity with one another. Ob- viously this hypothesis and the ;. view of outgrowing nerve- outgrowth hypothesis, according to .,!„,. Hotl, figUM are, drawn from whj h th nerve.fibre fe f()r a \ mu. tin- ^ame live preparation, B twenty- me minntei later ti,.,n A. (After separated by a gap from its end- Harris.,n. !•• organ, lend one another mutual support.1 1 It must In- Imnif iii mind, h<>\\c\ »-r, that the historical l.a.sjs of tin- ncuroiif :..t univiTsally admit ted i<> hr ln-ymnl suspicion. \\^ main foimdat ion | tin- (iol^i and .similar met hods of metallic impregnation. In preparations mad.- in tln> \\ay single e.dlular units aiv l'n-«|in'iitly picked «uit witliont the i-«M«-ti«n taking pla«-,- in n< •i^lil.«nirin^ unit> arrant'1'! in ^erie> \\ith them. A ^an^lion-eell A \villi ilsa\<«n and ti-rminal luanehes stands out deep lilack ill the pp-paiatioll, while tin- 11 li, ln-\t hi it ill the sellev, sl,o\\s Ho 11. Sueh .1 ' - diM-ont innity. '!'}). |i..s-ilil«- fallacy in I ID lh'- I'a.-l that the stain used is not in the old "I tin- \\oid I, ut merely a precipitation of metal upon NERVK DEVELOPMENT 105 ,1. r. nc- nty. —9 The His view is concerned primarily with tin- actual lunctinnal nerve-fihres. As regards the primithe sheath (Gray Sheath uf Schwann . in which I hi lihres are enclosed, l lu- ll is view regards it as hi MULT formed by mesen- clivme cells which apply t hemselves t<»,and spread • Mil over, the surface of the originally naked nerve- fibre. (2) THE BALFOUR ViEW(Cell -chain theory). -While Schwann(1839) Inn^r ;t^<> described the multicellular structure of nerve-trunks in the foe- tuses of mammals, it was F. M. Balfour (1876) who really founded the view that the nerve - trunk arises in development from a chain of cells. Balfour found in Elasmo- branch embryos that the nerve -trunk was repre- sented by a chain of (-ells in early stages (Fig. 58, v.r), and similar observations have been made by subsequent observers. According to this view the whole nerve- trunk is multicellular in origin, the cells not only forming the sheath of the nerve-trunk but also giving rise to the nerve-fibrils which come into existence traversing the cellular strand from end to end. On the question of the origin of the cells which constitute the nerve -rudiment opinions vary. Most supporters of this view have regarded them as having emigrated from the spinal cord (e.g. Balfour, van Wijhe, Dohrn) : while others (Kolliker) have looked on them as mesenchymatous in nature. Sedgwick took this latter view and as he regarded the mesenchyme as a continuous syncytium, the bridges connecting the cells being primitive — persisting from the .v. FIG. 58. — Section through the dorsal part of the trunk of a Torpedo embryo. (From Balfour's Embryology.) d.r, dorsal root; ;/, spinal -'uiglion ; my, myotome ; N, noto- n, nerve-trunk ; nc, cavity of spinal cord; v.r, ventral the surface of the cell and its processes. We know from the recognized unreliability of the method that the occurrence, or not, of this precipitation is liable to be decided by extremely delicate chemical differences. We know further that the axis cylinder, however it arose in development, is morphologically and physiologically a prolonga- tion of the cell-body (ganglion-cell), and therefore that its metabolism is under the control of tin- nucleus of that cell-body. The individuality of the cell and its pro- longation, due to the metabolic control by its own special nucleus, is probably quite enough, in itself, to account for a chemical character of its surface sufficiently different from that of its neighbours to influence the precipitation Avithout there being, as the neurone theory assumes, any absolute discontinuity. 106 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. incomplete separation of tin- cells during tin- processes of segmenta- tion and cell division — the view in his hands came to approach tin- next view to be mentioned thai of Ilensen. (3) THE HENSEN VIEW (Primitive continuity theory).— This view has found its strength in general physiological considerations rather than in convincing facts of observation. According to Hensen (1864, 1868, 1903) the nerve which connects centre with end-organ is a primary connexion which has been there from the beginning. It existed first as a simple bridge of protoplasm, such bridges being present between the various cells of the body owing to the fact that the processes of segmentation and cell division are not complete so as to lead to absolute isolation of the cells or segments from one another. According to this view the growth in length of a nerve -trunk is simply the extension of a pre - existing bridge, as the organs at its two extremities — centre and end- organ — are pushed apart from one an- other during the course of development. FIG. 59.— Illustrating Hensen's view of the origin of peri- Hensen figures in pheral nerves. The section is taken from a 9-day Rabbit J^g papers fFi0" 59) embryo, and passes through the trunk region. (After , f , Hensen, 1903.) what he takes to be such nerve-rudiments, in the form of numerous fine filamentous structures passing across the space between spinal cord and myotome. There is however no evidence to show that these filaments have anything to do with nerve-trunks. Although for this reason it is impossible to accept the main observational basis of Hensen's view, that does not neces- sarily invalidate the physiological considerations which may be held to give an a priori probability to the correctness of his general theory. The three views which have been outlined above were fashioned by their respective authors long ago as embryological science goes. Since then new facts have become known which have to be taken into account when considering their acceptability as working hypotheses at th«- present time. Some of these facts will now he touched upon. DKVKLOl'MKM OF 1'KIM I'll KKAL NERVE-TRUNKS IN LEPIDOS1REN It ifi Obvious enough from the diversity of statements by skilled that the investigation of the method of development nf the peripheral m-i'M-s in Vert ebrales is beset, hy technical ditlieullies and resulting liability to error. In such a ease it is of special importance In choose for investigation types of animal in which this liability Do error u reduced i«> its narrow. -si limits. Such an animal should ii NEBVE DEVELOPMENT 107 1m, mi ilit- iiin> hand, comparatively archaic it slmuM belong to one nf ihe relatively more primitive groups of Vertebrates — and, on the other hand, its histological texture should be as coarse as possible, its <;ell elements In-ill^ «»f larjjv si/e. Amongst Vertebrates Investigated up to tin- present time in regard to nerve -development Lepidosiren (Graham Kerr, 1904) is unrivalled in its combination <•!' these qualifications and ;i summary will nn\v IK- ^iven of the ma.in features which h.-ive been made out from the study of the development of the motor nerves in this animal. It will be emivenient to commence with the fully formed nerve-trunk and then work backwards towards the earlier and more obscure stages. Fig. 61 represents a portion of nerve-trunk from a fully developed larva of stage 34. The nerve-trunk consists of a cylindrical bundle of nerve-fibrils, dotted over the surface of which are the numerous large nuclei of the protoplasmic sheath. The sheath itself is so thin as to be. practically invisible even under a high-power immersion objective except in the angle close to a nucleus where it is distinctly visible. Fig. 60, D is taken from a larva ten days after hatching. At this stage the nerve-trunk, when examined superficially, has the appear- ance of a thick strand of protoplasm containing numerous nuclei or a chain of cells. Careful examination of well-fixed and well- stained specimens shows however that this conspicuous mass of protoplasm is really only the sheath, the true nerve-trunk (ri) being visible traversing it from end to end. Scattered about in the thick sheath of this stage there are still to be seen granules of yolk (black in the figure) which have not yet been used up. Fig. 60, C is taken from a larva at the time of hatching. At this stage the nerve-trunk is a well-developed bundle of nerve-fibrils, just as in the later stages, but throughout the greater part of its Length it is devoid of a sheath of protoplasm. In the section figured the sheath is visible as a mass of nucleated and heavily yolked protoplasm enclosing a portion of the nerve-trunk towards its outer end. This mass of protoplasm is obviously just a condensed part of the general mesenchyme which is scattered about in the form of irregular heavily yolked masses throughout the spaces between the main organ systems. The mass in question is identical with the rest of this mesenchyme in its various features and every here and there it is continued into it without a break. The section figured shows the whole length of the motor nerve-trunk from the ventral root to the myoblasts or muscle cells which form the myotome. Towards its outer end the trunk is seen to break up into numerous diverging strands which are directly continuous with the protoplasm of the myoblasts (see below, p. 204). Fig. 60, B is taken from an embryo about three days before hatching. At this stage the myotome has barely commenced to 108 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. : i- s.-ctimis tin- ilcvi-lnpiii.-nt iif ill,- spinal in-i\, \mlial ..... I' ni\..ti.iin- ; -.. ii. •! M-II unk ; >... ^pmal «-nnl ; sh, to illu-t rate II NKUVK DEVELOPMENT 109 Fie. 60A. — Portions of transverse sections through young Lt'piiloshvns to illustrate the development of the spinal nerves (ventral roots). B. si.-t-f -_':. ; ( ', si. -I-.- -J7. in, myotome ; n, nerve-trunk ; >-.c,|rspiiial con! ; s/i, sheath 110 EMBEYOLOGY OF THE LOWER VERTEBRATES • 11. recede from the spinal cord, but yet each motor nerve is already present as a distinctly tibri Hated trunk bridging across the narrow gap between spinal cord and myotome. A few mesenchyme cells have wandered into the gap but they have not yet begun to con- centrate round the nerve-trunk. l-'i.u- 60, A is taken from an embryo of stage 24 at a time when myotome and spinal cord are still in close contact with one another. In specimens which were extended in one plane under normal salt solution while still alive and subjected to the action of the fix in- agent in that position, it is found that the myotome is frequently pulled slightly away from the spinal cord (as in the specimen figured) and in such cases it is found that the nerve- trunk already exists in the form of a bridge of soft granular protoplasm (?i) with- out any trace of fibrillation, connecting spinal cord and myotome. That these bridges are really the nerve -trunks is indicated by their occur- rence one to each myotome, apart from the fact that a con- tinuous series of stages have been observed between them and the fully developed nerve-trunks. In summing up we may FIG. 61. — Part of transverse section of Lepidosiren take the Various stages in (stage 34), showing a portion of nerve-trunk. ^eir proper chronological /////, myotome; N, notochoi Hint impulse- are actually t ra nsmit t ed acro>> tin- protoplasmic l.rid^c at a very early stage in tin- cast- of Elsunoonnoha. - Tin- hypothesis lien- out lim-d in connexion with the ciulirvonic di-vrlopmeiu of ills in wi-11 also with ci-rtain of the phenomena olisn \ , d in tin- n--, -m rat ion of which have Ix-en severed and joined \>,^t-\ her a^ain [see Trim xli, p. 1'Jti. uls.. Molt. II illiluirtoii and Kdmunds in /'/•.-•. /,' vol. 78]. •alii of n,i\,- flbrei il mm oellulu and appears to In- produced hy the -••cn-tor\ art i\ ily of t he protophum of t.lic a\on. NERVE DEVELOPMENT 113 • • xist.s tin: MIMIC divergence of opinion as in the case of the motor nerves, and in end< -iivmiring to decide which view has upon its side the balance of probability, it is well to bear in mind similar conditions to those alluded to on p. 106. Bearing these in mind, it is of interest tn notice that in Lepidosiren (Elliot Smith, 1908) the process of development of the olfactory nerve takes place along exactly similar lines to that <>l the motor trunks. And it is significant that, in the opinion o! tlmse well qualiluMl to judge (Retzius, Golgi, Ramon y < 'a jal, van Gehuchten, Kolliker, Elliot Smith), this nerve has advanced less from the primitive condition than has any other nerve, and in its general arrangements has undergone extraordin- arily little complication during ontogeny. Already at a time when the olfactory organ has not yet com- menced to recede from the wall of the hemisphere the olfactory nerve exists as a stout protoplas- mic bridge (Fig. 63, /) which gradually increases in length as the olfactory organ recedes from the hemisphere. This observation seems to indicate clearly that the mode of development of the sen- sory nerve-trunk is fundamentally the same as that of the motor : that it develops out of a pre- existing protoplasmic bridge be- tween centre and end-organ. cH. , I. FIG. 63. — An early stage of the olfactory nerve of Lepidosiren. (From Elliot Smith, 1908.) REMARKS UPON THE GENERAL PROBLEM OF NERVE DEVELOPMENT <:II, lateral wall of hemisphere ; df, olfactory organ ; 7, olfactory nerve. The nuclei seen in the region where the olfactory nerve enters the hemi- It Will be admitted by mOSt sphere belong to the olfactory bulb. Zoologists that we are justified in believing that the process of nerve -development is probably fundamentally the same throughout the Animal Kingdom. It will also be clear, even from the short and imperfect statement which has been given here, that the detailed study of the phenomena of nerve -development has led, in the minds of different observers, to widely divergent conclusions as to the exact nature of the process. The subject is one to the discussion of which we may devote with advantage some further space. It is in itself of great embryological and physiological interest. It presents many problems still unsolved. And it may be taken as a type of biological controversy with which it will be. to the student's advantage to become acquainted. In approaching the question from the present-day standpoint it ap pears impossible to get round the fact that in two of the most VOL. II I 114 EMBRYOLOGY OF THE LOWEII VEKTEBRATES CH. archaic groups of Vertebrates (Elasmobrancliii and J )ipm>i , tin- motor nerve-trunk is already present as a protoplasmic bridge at a time when rayotoine and spinal cord have not yet commenced to recede from one another. It does not seem possible to explain the appear- ances recorded in these cases by any conceivable errors of observa- tion. But if sucb bridges exist in these relatively archaic groups, tlu' balance of probability is entirely on the side of their represent- ing the primitive1 mode of development of nerve-trunks in general. and of a fundamentally similar mode of development occurring in other Vertebrates though possibly in a modified and less distinct form. On the other hand appearances of the kind which led to the original formulation of the His view, and which are still adduced in its support, and which are easily observed in series of sections through almost any type of Vertebrate embryo — nerve -trunks passing out from the spinal cord and ending freely amongst the mesenchyme — are peculiarly apt to be misleading. Such a misleading appearance is produced sometimes by compara- tively simple causes — by breakage of the nerve-trunk or by the nerve-trunk passing away out of the plane of a section and being unrecognizable when cut transversely in a neighbouring section. In other cases the appearance of a freely ending nerve-trunk is due to the portion of nerve-trunk which has received its protoplasmic sheath being distinctly visible in a stained section, while the delicate peri- pheral portion which is still naked is practically unrecognizable. On account of such liability to misinterpretation a very large proportion of the observational evidence which supports the His view is open to suspicion. A physiological difficulty which has been raised against accepting the His view is that involved in the idea that the free end of the growing fibre tracks down and finds its appropriate end-organ. It is pointed out that it never makes a mistake — never becomes joined up to a wrong end cell. And yet, if it be the case that nerve-fibres do grow outwards with free ends in the way involved in the His theory, certain experimental results show that such fibres do p.. a very decided power of making mistakes. This is brought out clearly by the beautiful experiments of Braus (1905). In the experiments in question Braus made use of the method, invented by Horn and developed by Harrison, Spemann and others, in which portions of our amphibian embryo are Drafted upon the body of another, when tin- Drafted portion ("parasite" I'.raus pro- . develop as part of the individual ("autOSlte" — Braus) upon which it has been grafted. In the experiments which are most important, in their hearing on the point now under discussion the early rudiment of the pectoral limb ifled upon ;t Imst in the region of the head. In this fcion tin- rudiment \vent on developing into a perfectly normal limb eolltallilli'j a normal ;i rra ll'jemelit .of the lililb nerves. \..\\ the ii NERVE DEVELOPMENT 115 implanted liml> in such a . jr. 04) is situated in a region inner- vated by the facial aerve and the study of sections showed that the nerves in tin- implanted limb were continuous centrally witli 1. randies of the facial nerve. If we attempt !,<• interpret, this experiment on the outgrowth view we find ourselves compelled to admit that the facial iihres concerned made the serious mistake of Crowing into a limb rudiment and thru continued on their mistaken course until finally they established the muscular connexions normal for the nerves of such a limb. Braus repeated this type of experiment in a number of cases and there appears to be no question as to the accuracy of his observations. If accurate, however, they provide a formidable, if not unsurmountable, difficulty for the outgrowth view — a diffi- culty which is by no means got rid of by the suggestion (Harrison, 1908) that after arriving in the limb the nerves are " merely guided in their growth by the structures present in the transplanted part." A similar difficulty is seen in post- embryonic nerve -development in the fact well known to surgeons that functional continuity can be established between the cut central stump of one nerve (e.g. spinal accessory) and the severed peripheral portion FIG. 64.— Young Toad (Bom- of another (e Q. facial). Unator) on which an addi- And so again in the development of anastomoses between peripheral nerves such Braus, 1905.) as the well-known " dialyneury " of Gastero- pod molluscs, or the short circuiting of the left pulmonary nerve over the dorsal side of the oesophagus which has come about in the evolu- tion of the Crossopterygians and Lung-fishes. All such cases present great if not insuperable difficulties to the His view. Again much of the evidence which is brought to the support of the His view is seen when looked at critically to be less convincing than it appears to be at first sight. Thus for example with the experiments of Harrison already described, which are regarded by their author as settling the whole question. Their true value will become more apparent if we bring Harrison's results into correlation with the results described above for Lepidosiren. As has already been shown, in this animal the motor nerve-trunk is represented at an early stage by a bridge of soft fragile protoplasm. These bridges require a very favourable object and very careful technique for their detection, and it is clear that one could not. expect to see them in comparatively coarse preparations made by • •xcisiiiM- a piece of living unfixed spinal cord. There is therefore no guarantee that such protoplasmic nerve-rudiments were not already present iii the pieces of spinal cord investigated by Harrison. 116 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. Let it be assumed that such an experiment is repeated, upon Lepidosiren with a small piece of spinal cord rudiment with the protoplasmic bridge attached to it (Fig. 65, A). The piece of spinal cord is well supplied with food material in the form of yolk and, if kept under suitable conditions, it would go on developing. So also niiuht the protoplasmic bridge, for every one agrees that the metabolic control of the motor nerve is exercised by the central ganglion -cell nuclei within the spinal cord. If this happened and the process \\viit on quite normally we should get in succession stages such as those shown in B and C of Fig. 65. Now these would be inter- preted by Harrison presumably as demonstrating the outgrowth view, whereas all that they really show is that, given suitable con- ditions, the motor nerve increases in length — a fact which of course is obvious. What is needed as a demonstration of the His view is not merely to show that a nerve -trunk increases in length but to show (1) that it normally has a free end and (2) that it grows- within the body at a greater rate than the tissues in which it is embedded, so that there is brought about a differ- ential movement in which the free end pushes its way through the tissues surrounding it. This shewing a piece of spinal cord with the has not been sllOWll by Harri- developing motor nerve but ignoring the son'g experiments nor COllld it niyotome which is in the actual embryo ., , \ , ., . continuous with the outer end of the nerve. pOBBlbly be shown by this type of experiment. In Lepidosiren the study of sections shows as has already been pointed out that. although the motor nerve-trunk grows actively in length with the i i MM vase in bulk of the body, at no period from the earliest stage li.L'uml has it a free end; it is throughout connected with its end- organ.1 In ;i word, it appears to the present \\riter thai what are eoinnionly regarded as the most convincing pieees of evidenee in favour of the His view are by no means eon\ ineiii-. Views ivMMuhling that of His in thai they also involve a.n out- 1 Tin; actively moving pMudopodium-Ukc fcAffl \\hidi Harri^m ohser\ed ;it the end id' his out^rou in^ nerve trunk .-ire l>elie\ed by (he promt \\riier i«> lir nirx-n '•hymatoiis in their nature possibly shreds of sheath protoplasm. li i feature of eiubryonie meseiichynie ihat its protoplasm slm\\ .s act i \ «• aim n-bo id ' ni,»\ e men). FIG. 65. — Drawings taken from the same preparations as those illustrated in Fig. 60, ii NEEVE DEVELOPMENT 117 growth of I hf motor trunk from the spinal cord, but differing from it in tin- essential feature th;it ili«- out growth is simply protoplasmic ;tiid not fibrillar, have been enunciated by some modern \\orkers such as Doli rn ;ind Held. Dohrn (1888) describes the motor nerve-rudi- nn-iit as arising by ;i " plasmatic outflow from the neural tube " but I'a.ton later on linds that at the stage referred to by Dohrn the ncrvr- rudiment is already continuous at its outer end with the protoplasm of the myotome. Held ( I DOM) also regards the motor trunk as arising by outgrowth from the spinal cord at a time when the myotome is still com- paratively close to it. It has to be borne in mind in interpreting such sections as Held figures that there is more liability to error in demonstrating the abseu.ee of continuity than in demonstrating its presence, owing to the extremely fragile character of the nerve- trunks during early stages in development and their consequent liability to rupture during the ordinary processes of preparation which precede section -cutting. It is sometimes said that the difficulty attaching to the His view involved in the idea of the nerve-fibre tracking down its own particular end-organ disappears if the view is taken that the out- growth takes place at a stage so early as that indicated by Dohrn and Held. "But as a matter of fact this involves, as indicated, a distinct departure from the view enunciated by His according to which not merely undifferentiated protoplasm but definite fibrillated trunks grow out from the spinal cord. Further if, as Held believes, the individual fibrils grow out in the substance of the protoplasmic outgrowth each one has still to seek out the particular portion of the myotome which will eventually be converted into its own proper muscle-cell— a view which, looking to the comparatively undiffer- entiated condition of the myotome cells at these early stages, is even more difficult to comprehend physiologically than the outgrowth towards a specialized muscle. The embryological evidence upon which the His view rests is seen, when submitted to critical examination, to be unconvincing. Tlie same is the case with the observational evidence upon which the Balfour view rests. The nuclei and cell-bodies which commonly give a multicellular appearance to the nerve -rudiment are quite reasonably interpretable as sheath -cells, i.e. mesenchyme elements which have collected round and it may be migrated into the, at first noneellular, nerve-trunk. In Lepidosiren, with its coarse and heavily yolk-laden mesen- ehyme, it is comparatively easy to distinguish such elements from the actual nerve-trunk embedded in them, but in most Vertebrates this criterion is not available and there is no certain means of distinguishing in ordinary microscopic preparations the protoplasm of the nerve-trunk from that of the sheath-cells. The primitive protoplasniie. bridge described in 1902 for Lepidosiren as representing the motor trunk at a time when 118 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. myotome and spinal cord have not yet commenced to move apart, confirmed later in the case of the motor trunks of Elasmobranchs by Paton, and in the case of the olfactory nerve of various Vertebrates by Elliot Smith and others, seems to rest upon a secure basis of observation. It is difficult, therefore, to avoid the expectation that the progress of future research will show such a primitive protoplasmic bridge between centre and end-organ to be the normal forerunner of nerves in general. But, if this be so, we are faced by the question as to the actual mode of origin of such bridges and here we pass into a region where direct observation is either impossible or unreliable. Those who accept Hensen's views in their entirety would look upon them as representing intercellular connexions persisting from the earliest segmentation stages. Reasons have already been given (p. 37) for disbelieving in the persistence of such bridges between the cells of the segmenting egg. The connexion appears certainly to arise at some later period — but exactly when seems to be a question incapable of answer by direct observation. When considering these general problems regarding the nervous system it should be borne in mind that the nervous system has for its main purpose the keeping of the various parts of the body linked together into an organic whole, in spite of their increasing differentia- tion and specialization. It has for. its function the providing of exquisitely specialized pathways by which the living impulses can traverse the whole length of a relatively immense body at least as readily as they originally did the minute blob of ancestral protoplasm. Bearing in mind this primary consideration will cause one to reflect that the evidence must be overwhelming before one is justified in believing that this organ system, whose most striking functional feature is continuity, has come in the course of evolution to be characterized by the structural discontinuity involved in the neurone theory of adult structure, or in the outgrowth theory of ontogenetic development. Again it is important to bear in mind the high degree of probability attached to the view, originated long ago by 0. and R. Hertwig (1878), that the nervous system of the higher metazoa, including Vertebrates, has been evolved out of a sub- epiihelial nervous network of the kind still seen in sonic of the more lowly organized groups such as Coelenterata and Kchinodermata, We may suppose that such a plexus was present in the far hack .mc.-stors of Vertebrates over the hasal suri';i! free-ending fibrillated outgrowth as non-proven and for various reasons improbable, (3) to believe that the nerve- trunk already exists as a proto- plasmic bridge between centre and end-organ at a period when these are still in immediate contact, even although this has up to the present been definitely shown by actual ob- servation only in a few peculiarly favourable instances, (4) to leave the exact period at which the protoplasmic bridges come into existence an absolutely open question as being beyond the limit of reliable observation, (5) as regards the sheath of Schwann, to accept the view that it is derived from mesenchyme. It will be noticed that little has been said so far regarding the mode of development of the actual neuro- fibrillae. Their origin is indeed u n verifiable by direct observation, with any certainty, owing to their minute size. They appear to spread outwards from the centre, and II -Id interprets this appearance by a kind of His theory on a minute scale, holding that each fibril grows out with a free end through the protoplasmic bridge. On the other hand if it be the ease as suggested on p. 112 that the fibrils simply represent the specialized paths of nerve impulses there would be nothing surprising in their becoming visible first in the neighbourhood of the ganglion- cell from which the impulses start and from which also is exercised over the metabolism of the nerve-trunk. Were this the FIG. 66. — Illustrating the structure of a hypothetical primitive Vertebrate at a time when the protostoma was still open. In the lower figure an entero- coelic pocket, the rudiment of a mesoderm segment, is becoming de- marcated from the rest of the endo- derm by tlu- downward spreading of a split between the points a and b. In the earlier stage shown in the upper figure this split has not yet lie^uii to develop, and the points a and b are seen in close proximity to one another on the outer surface of the endoderm. in. 11, medullary plate; p.s, pnitnstoma. 120 EMBKYOLOGY OF THE LOWEE VEETEBEATES CH. case we should get appearances which would closely simulate growth of freely ending fibrils — centrifugal in the case of motor nerves and centripetal in the case of sensory nerves. Tliis view as to the meaning of the fibrils bridges over a good many of the difficulties in the way of accepting the outgrowth view, either as regards the individual fibrils or the nerve-trunk as a whole. Thus the secondary establishment of anastomoses between peripheral nerves becomes less surprising if it be the case that undifferentiated protoplasm is liable to develop nerve-fibrils as a reaction to the passage of nerve impulses through it, for wherever there are nerves there must be a certain amount of leakage of the particular form of energy which constitutes the nerve impulse. So also with the joining up of the central and peripheral ends of a severed nerve or of the central stump of one nerve with the peri- pheral portion of another. In such cases we should assume that indifferent protoplasm accumulating between the cut ends gradually becomes fibrillated in response to the passage of more or less imperfect impulses through it, the newly developed portions of fibril being necessarily, from their mode of formation, continu- ous with both central and peripheral fibrils, leading respectively to the "high -potential" and the " low-potential " end of the nerve- fibre. Again it is known that a mass of embryonic ganglionic tissue implanted into some abnormal portion of another individual may establish nervous connexions with the surrounding tissue. On the outgrowth hypothesis this demonstrates, "error" on the part of the outgrowing fibres : on the functional view it simply involves the gradual differentiation of paths along which impulses spread out- wards from the high potential ganglion-cells into the low potential surrounding tissue. On the whole, the present writer believes that this view, that the formation of nerve-fibrils is a response to functional activity, is at the present time the most plausible working hypothesis and also the one which is most likely to lead to fruitful research. Before leaving the subject it may be well to emphasize the fact that the solution of this general problem of nerve-development is to be sought in the study of Vertebrates of large-celled coarse histological texture, com- bined with a low degree of specialization of general structure. No amount of observations upon small-celled highly spcciali/cd Verte- brates will ever lead to a really convincing solution with the mrtliods now at our disposal. Finally we would once more emphasize the fact (hat the kernel of the problem seems to centre round the origin of tin- tibrillae. Do they or do they not develop in a pre-existing bridge of protoplasm ' Assuming that they do, the possibility of such bridges dating hack to the period of segmentation seems to be definitely excluded. The question at. \vh;il precise moment lhe\ do become rst a hlished seems to !,<• of minor impoi t.-mce. ii NERVOUS SYSTEM 121 While the present writer is inclined to believe that the junction is already in existence while end-organ is still in close appnsiiion to the central nervous system there is n<> difficulty in principle in the way of admitting that the bridge may in certain cases be formed somewhat, later, as I)<»lini describes, provided always that the gap to he bridged over is small and the bridge itself protoplasmic and not librillar. It is probably along such lines that we may look for a reconciliation between the supporters of His (the outgrowth view) and those who believe in the protoplasmic bridge view but it will involve dropping what are essential features in the outgrowth view as enunci- ated by His himself — (1) that the outgrowth arises at a time when the end-organ has already retreated to a considerable distance from the nerve-centre and (2) that the outgrowth is already fibrillated dur- ing the outgrowing process and before it is united to its end-organs. SPINAL GANGLIA AND DORSAL ROOTS. — As has already been in- dicated, the central nervous system of the Vertebrate consists in its most primitive condition of a specialized area of the ectoderm of the dorsal surface. It is .further very characteristic of the Vertebrate that those ganglion -cells which belong especially to the sensory fibres have become concentrated into segmentally arranged clumps towards the margin of the nervous plate and have eventually come to lie out- side the limits of the actual plate, or tube into which the plate be- comes converted. These little detached pieces of the central nervous system are the ganglia of the dorsal roots or the spinal ganglia.1 During actual ontogeny the ganglion rudiments in some cases (e.g. Birds, Fig. 67, A) become distinctly apparent while the spinal i-ord is still in the form of an open medullary plate. They appear in the form of a continuous proliferation from the inner surface of the ectoderm in the angle between the medullary plate and the external ectoderm. In such cases the two rudiments become carried in towards one another, as the edges of the medullary plate curve inwards to form the neural tube, and undergo fusion across the mesial plane. There is thus formed a median unpaired plate or tract of cells lying just over the roof of the neural tube and between it and the external ectoderm. This is known as the neural crest (Marshall). More usually the ganglionic rudiment makes its first appearance after the closure of the neural tube and in such cases the paired stage of the rudiment is slurred over, the neural crest being formed by proliferation of the roof of the neural tube. This is well seen in the case of Elasmobranchs (Fig. 67, B, C). However it originated, the plate -like neural crest splits into two 1 There is reason to believe that this is an instance of a widespread tendency in evolution for groups of ganglion-cells to undergo gradual shifting towards the direction from which their most frequent afferent impulses come. In other words there is a tendency to shorten the afferent path by shifting the cell-body. This principle of neurobiotaxis has been developed by Aru-ns-Kappers in his various papers (e.g. 1913). 1 1 ifl particularly conspicuous in the changes which have come about in the position of the ganglionic centres of the- various cranial nerves within the brain in the ditlVivnt groups of Vertebrates. 122 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. lateral halves and then grows outwards on each side opposite each myotome, each outgrowth representing a single spinal ganglion. Eventually these break apart but in some of the more primitive Vertebrates the intervening portions of neural crest persist for a time in the form of a distinct longitudinal commissure (Fig. 68) linking up tin- scries of spinal ganglia to one another (Elasruobranchii — 1 lalfour : Dipnoi). The mode of development of the fibres forming the dorsal root, whether by outgrowth from the ganglion-cells of the spinal ganglion or by differentiation of an already existing protoplasmic bridge, comes eel FIG. 67. — Illustrating the mode of origin of the spinal ganglia. A, fowl embryo with four mi-sodi-rni s.-mi-nts (aft IT Ni-uinavr, I'.'Dti); \\ and (', 'l',>r/»;l,> -1 mm. i-mbryo (after Dohrn, 1902); ect, ectoderm ; g, rudiment of ganglion ; .s-.c, spinal cord. under the general controversy as to nerve-development and need not be specially discussed. >s^'v/\ CRANIAL NERVES. — The development of the cranial nerves has iiriMi investigated by many workers and an immense amount of detailed observation has been accumulated. There is however pvat "paney in detail between the results obtained by different workers, ;«nd much «.f the observation seems t<» be perilously near the limit of probable error. Consequently the material seems hardly ripe for treat incut in a text-book of a gcnei-al kind and not liin^ «'f • rt will br atlriiiplcd hdv beyond nolin^ one or tw<> points of parti(-nlar importance.' 1 A modern account of the development of cranial nerves \\ill In- round in .. yr (1906). ii CEANIAL NEEVES 123 In the first place we find in the head region as in the trunk a tendency for the nervr-librcs to cnino oil' from the central nervous system in segmentally arranged clumps, and for the motor fibres to be situated more ventrally and the sensory more dorsally. In the head region however the dorsal root becomes reinforced by a large mass of motor fibres which have become shifted dorsalwards and incorporated with it. A neural crest develops resembling that of the trunk and in the Birds it can be seen similarly to have a paired origin, arising before the complete closure of the medullary tube. This neural crest of the brain region forms an anterior prolongation of that in the trunk : it is quite continuous with the latter, it develops outgrowths similarly, and the intervening portions here also persist for a time as a longi- tudinal commissure. A number of the most important cranial nerves pan B? FIG. 68. — Acanthias, stage 23, 9 mm. long, showing ganglia of cranial and spinal nerves. (After Scammon, 1911.) int, intestine ; I, lens ; li, liver ; pan, pancreas ; sp.g, ganglia of spinal nerves ; Th, thyroid ; V, ventricle ; v.c, visceral cleft ; y.st, yolk-stalk ; IV, V, etc., ganglia of cranial nerves. are simply prolongations of the outgrowths in question — V, VII, VIII, IX and X. A conspicuous feature in the development of the cranial nerves is that in portions of their length they receive components directly from localized thickenings (placodes) of the ectoderm (Kupffer, Beard) a possible reminiscence of the time when nerve-trunks became evolved out of n i this simplest mode of origin of the external and internal nares as seen in Protopterus and Ceratodus. In the Actinopterygian fishes thr phenomena are quite similar to those described, only here differential growth leads to the gradual shifting of the olfactory oru'.-in and its openings from the ventral side of the snout up to its dorsal side. The result is a topographical reversal of the positions of 126 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. anterior and posterior naris : tin- morphologically posterior naris eoniini: t<> lie in front of that which \\as originally anterior. In the Amphibian and Aiimiote tin- upper tip, which compl< the huccal cavity in front, is situated between the anterior and posterior narial openings, so that the latter opens into the huccal cavity c.o •. N'oiii.i] vi.-u- ..I ln-;i«l region of larva of Protopterus at stages 31 (A),' 32 (B), 34 i I)), 35 (E), and 36 - (F). to illustrate the devi-lopim-nt .>f tin- oliai-tm-v organ. '•••inent or^an : t.'j, external ^ill ; <•//, olfactory organ ; o*/1, anterior ("external ") naris; «//-', ) n.-uis. In C the cut \ eil line running across tin? ventral side of the head is the : margin ul' the moutli: th<' darkly sh.-nled -ii)o\rs |i;issin^ imvunls ;ttid ror\\;ird> I'mni its :- an- the dltact' ii'\ i ud iiii''iit s. ^internal naris;, \vhile tin- innm-r ivmains outside (external naris). In ill.- d«-\«-i<,],in;j Anininti- embryo (cf. Fowl, Chap. X.) the -vneral arraii^'fiii'-ni -. \\liil'- ••ssi-niially the same as those of /'/Wf/>/f/v.s. are -niin-wliat ol.scmvd l-v ill'- inod.-llin^ of the face region. The rid^e which loi-ins i In- iij. pci- lip, or anterior Uumdary of the buccal oavity, IB cut across l.y the olfactory *lil; ''(-re a wide and dee]» elet't. into a OLFACTOEY OKGAN 127 nnMli}i.ii portion (median nasal process) and a lateral portion (maxillary process). Tin- ridge hounding the olfactory involution on its outer side iv 1 1 1, ( ins for a time separated by a distinct groove from the maxillary process hut as the hittn- grows i or wan Is it obliterates this groove as well~as the superficial portion of the cleft wliidi srp;irates it from tin- iiii'di.in nasal process, the deep portion of the cleft remaining as ,i definitive canal le.-iding from olfactory organ to huccal cavity. In Amphibians, as in Lepidosiren among the lung-fishes, the posterior naris is frequently formed as a secondary perforation which Thai. FIG. 70. — Horizontal sections through the olfactory organs of Polypterus of stages 25 (A), 26 (B), and 27 (C). c.o, cement organ ; olf, olfactory rudiment ; opt, optic stalk ; Thai, cavity of thalamencephalon. hreaks through from the posterior portion of the olfactory organ into the part of the huccal cavity lined by "endoderm." This is a secondary modification of a type which will be discussed in the next chapter in the description of the development of the huccal cavity in these forms. The first rudiment of the olfactory organ has been described as a thickening of the ectoderm. As in the case of other nervous or glandular developments of the ectoderm the superficial layer (Fig. 70) takes no part in its formation. Commonly it degenerates and disappears over the actual olfactory epithelium. Again, as frequently happens in the development of primitively hollow organs, the rudi- ment may be for a time solid, forming a simple solid downgrowth in 128 EMBKYOLOGY OF THE LOWKli VERTEBRATES CH. which a cavity makes its appearance secondarily, the actual involu- tion of the surface being delayed or reduced or absent (Fig. 71). Sometimes, as is well shown in the case of Polypterus (Fig. 70, A), the olfactory thickenings are at first continuous across the mesial plane ami this fact, taken in consideration with the fact that in the Lampreys the olfactory organ of th,- adult is unpaired, obviously suggests the possibility that the olfactory organ of Vertebrates in general was originally unpaired (Kupffer). Though this must be admitted as a possibility the evidence does not appear to be sufficient to give the idea probability in view of the fundamentally paired character of the portions of the brain associated with the olfactory organ, even in the case of the cyclo- stomes where the organ as a whole has an unpaired appearance. After the olfactory involution has become definitely established it under- goes various complications of form, differing in detail in the various groups but consisting for the most part of bulgings outwards on "the part of the lining epithelium so as to bring about an increase in its area. In the Elasmo- branch these outgrowths take the form of parallel grooves which gradually become converted into deep slits separated by thin partitions — the Schneiderian folds. In Crossopterygians instead of numerous folds with free edges complete septa are formed which radiate out from an axis formed by the olfactory nerve and divide the cavity as seen in transverse section into distinct chambers, the lining of which in turn forms deep folds. In the higher forms the outgrowths of the olfactory lining are fewer in n umber and the projections left between them form the turbinals which have characteristic arrangements in the different groups. Amongst the Reptiles a conspicuous development of the olfactory apparatus is the Organ of Jacobson. This arises as a pocket-like oiit'jTowth of the lining epithelium, on its mesial side and near its ventral edge, which becomes gradually constricted oft' from the olfactory organ and opens into the buccal cavity in the region of the po-t.'i-ior nan-;. In < 'helonians, Crocodiles and Minis this organ has di-,tppe;tivd except for a possible vestige in the form of a transient bulging of t he olfactory lining. A divert iciilum which may correspond to Jacobsoii's organ makes pearancein Lung-fishes anil Crodeles hut in this case it becomes gradually dispkieed oiit\\anls until it lie- external to t he olfact nrv Ltjr, i : Fi«;. 71. — Longitudinal vertical sec- tion through Polypterus (stages 28-29), showing the olfactory rudi- ment as a thickening of the deep layer of the ectoderm in which a cavity has developed secondarily. ii OTOCYST 129 A curious, possibly adaptive, arrangement has been noticed in late developmental stages of certain Saunipsida, \vln-iv for a lime 1 he exiernal naresare plumed by a proliferation of ectoderm (Aj)teryx — T. J. Parker, ^filn-ii'iilnn- •hendy.i. Such temporary obliteration of a channel al a period oi' dc\vli ipmeiit \\here it is unnecessary or harmful is a phenomenon which occurs fairly frequently: examples of it will lie nici with later in connexion with the alimentary canal and the excretory or^an-. ( JTOCYST. — The Vertebrate possesses a pair of otnrysts situated one on each side of the hind-brain. Each arises in the typical fashion, familiar in the invertebrates, by a sensory portion of ecto- derm becoming depressed below the surface of the skin and eventu- ally isolated as a closed vesicle. As in the invertebrates certain of the lining cells of the otocyst secrete otolithic masses of Calcium carbonate. The otocyst of the Vertebrate however shows two developments which do not occur amongst the invertebrates. Firstly, in connexion with the primitive function of the organ, that of balancing, the wall of the growing otocyst becomes moulded into the three semi- circular canals which are arranged in planes at right angles to one another. These canals have for their function the analysis of any rotatory movement into its components in these three planes. And secondly a special region of the otocyst wall becomes specialized in connexion with a new sense — that of hearing — and grows out into a curved horn -like pocket, the lagena, which may become greatly enlarged and spirally coiled, in Vertebrates in which the sense of hearing is very acute, forming the organ known as the cochlea. The development of the otocyst may be described as it occurs in the Fowl (Rothig and Brugsch, 1902). The otocyst begins to make its appearance during the second day of incubation as a thickened area of ectoderm on each side of the hind-brain. This thickened area becomes depressed below the general surface, forming a saucer- shaped depression which gradually deepens till it forms a deep pit. The lips of this pit, especially dorsally, grow inwards so as to constrict the opening which is finally completely obliterated, the original open depression being thus converted into a closed, somewhat pear-shaped, sac the otocyst. The wall of the otocyst remains for a time con- tinuous through a solid bridge with the outer ectoderm (Fig. 72, A) but as a rule this bridge persists merely for a very short time and only a small cellular tag attached to the otocyst remains to mark its position. As development goes on the otocyst increases in size by growth of its wall and this growth is especially marked ventrally and later- ally with the result that the point which was originally connected to the ectoderm becomes displaced so as to be situated on the mesial side of the otocyst. This portion of the otocyst wall now conies to project upwards as a distinct pocket-like outgrowth the recess (Fig. 72, JB, r). External to this a wider bulging of the wall fore- VOL. II K 130 EMBRYOLOGY OF TTIK LOWER VERTEBRATES CH. s.e se. shadows the anterior vertical semicircular canal (a.v) and a little later a similar bulging more ventral in position — the horizontal canal (h.c). The lagena also is foreshadowed by a slight downward bulging of the floor of the otocyst. \Vith further development 'the posterior vertical canal rudiment appears also as a bulg- ing of the otocyst wall continuous with that which will form the anterior canal (Fiir. 7_. C-F, p.v). The three canal rudiments come .to project more and more prominently, the recess assumes the tubular shape of the endolymphatic duct and the lower portion of the otocyst (saccule) with * its projecting lagenar pocket and en- dolymphatic duct be- comes more sharply marked off from the rest of the otocyst (utricle). The pouch -like rudi- ments of the semicir- cular canals, as they V" ^^ come to project more freely from the utricle, assume a flattened form and finally the central FJ«;. 72.— Illustrating the development of the otocyst in the portion of the wall on Koul. (A-G after Rothig and Brugsch, 1902 ; H after Q^Q^ side billies in- Retzius.) , i «• '"" wards and fuses with that on the other. In this way the central portion of the cavity of each pouch becomes obliterated while the persisting peripheral part takes the form of a, curved tube — the definitive canal. At lirst the space sublended by the canal is traversed bv a continu- ous Heptuni formed out of the fused walls but this soon disappears leaving the canal Bfl a freely projecting arch which opens into the utriel- h end. The ampulla appears at an early stage as a dilatation of the canal rudiment at one end. will already have been gathered, the three canal rudiments do A and H, early slaves; C. towards '-nd ot si-vcntli day; D, to- wards cud ot fi-litli day; E, fivo days ; F, tmvants cud of ninth !n\\;iids end of twelfth day; H, adult. A is a view CM mi in front, 15 from In-hind, while C-H represent the left otocyst as seen from the left side. «.c, antci ior vertical canal ; It.c, horizontal canal; li' the deep layer of the ectoderm (Fig. 73, B) but the examination of earlier stages (Fig. 73, A) shows that here also then- is an actual involution of the deep layer although there is at first no patent cavity. In the Elasmobranchs the otocyst retains throughout lite its con- iii-xion with the exterior, the connexion becoming drawn out into a slender tube. In Birds the recess, and therefore the endolyniphatic duct, represents the remains of this original connexion, but curiously enough in certain other Vertebrates e.g. Lung-fishes (Fig. 73, C) the last connexion of otocyst with external ectoderm lies lateral of and somewhat anterior to the eudolymphatic duct which latter here develops as an independent outgrowth of the otocyst wall. This is to be looked on as a secondary modification of the more primitive arrangement seen in Elasmobranchs. The structure- named endo- lymphatic duct in Teleosts also arises as a secondary outgrowth of the otocyst wall. The endolyniphatic duct or recess commonly persists in the adult as a conspicuous blindly ending diverticulum of the otocyst wall. In Lung-fishes and Amphibians its wall proliferates actively giving rise to projections which in the Lung-fishes and some Urodeles, e.g. the Axolotl, meet to form an irregular sac over the roof of the fourth ventricle. In the Anura the irregular thin-walled sac formed in this way spreads forwards and also laterally until it becomes continuous ventrally so as completely to surround the hind-brain. An unpaired prolongation of this sac extends tailwards immediately dorsal to the spinal cord within the vertebral canal. Paired outgrowths of this extend outwards along witli each spinal nerve and expand at their ends round the spinal ganglia to form the calcareous bodies so con- spicuous in the adult frog. The whole system of outgrowths is con- spicuous in the adult from the white otolithic particles in its interior. Tin- vertebral portions become eventually broken up into a network of irregular tubes which is interpenetrated by a network of capillaries & 1889). Somewhat similar outgrowths of the endolyniphatic duct make their appearance in Sauropsida although in this case they do not undergo the wide extension that they do in the Anura. In the however they do heroine extended so as to form a large superficially plaeed irregularly lobed sac which covers over a great part of tip- neck region close under the skin (Wiedersheim). LATKI;AI. LINK ORQAN8.— These sense-buds (neuromasts), which are found arranged in rows on the head and body of lishes and aquatic, amphibians, take their origin in linear thickenings of the dec], layer of the ectoderm which spread along the surface .if the head and bndvand eventually b»'eome segmented up into separate pieces. In correlation with the function of these organs, which ii XKK'Vnrs SYSTEM 133 apparently is t<> deiee.t slow vihmtions in the water and is t lnTft'ore closely allied to hearing, it is of interest to notice that the eotodermal rudiment from which they arise appears to be in some cases continuous at lirst with that which gives rise to the otocyst. The sense organs are in correlation with their origin at first placed superficially but as development goes on they in most cases become depressed beneath the surface either in isolated pits or in continuous grooves. The latter may in turn remain open or may become covered in to form tubes except where at intervals openings remain leading to the exterior. This is the condition which is reached in the adults of the majority of fishes. The lateral line sense organs being correlated with an aquatic habit commonly degenerate on the assumption of a terrestrial existence. Various Anura however which remain purely aquatic after metamorphosis retain their full equipment of lateral line organs, ORGAN OF PINKUS. — In Lepidosiren and Protopterus a peculiar organ of special sense lies deeply embedded in the tissue on each side of the head in close contact with the wall of -the auditory capsule. This organ, discovered in Protopterus by Pinkus, has been shown by Agar (1906) to be developed from the ectodermal ingrowth which forms the outer end of the spiracular rudiment. EYE. — As the eye develops in the same general manner, differing only in detail, in the different subdivisions of the Vertebrata it will be convenient to describe first its development in the Fowl — the Vertebrate of which it is easiest to obtain material for practical study. The first obvious rudiment of the eye consists of a projection of the side wall of the thalamencephalon which juts out at right angles to the axis of the body and gives a characteristic hammer- shape to the fore-brain region (see Fig. 231, Chap. X.). A transverse section across the head near its front end in a chick about the middle of the second day of incubation shows (Fig. 234, D) the thalameucephalon extending out on each side as the optic outgrowth. As development goes on and mesenchyme accumulates between the brain -wall and the ectoderm the proximal part of each optic outgrowth becomes constricted, from above downwards, to form a relatively narrow optic stalk (Fig. 74, A, B, D, o.s). The optic outgrowth is closely apposed against the inner surface of the external ectoderm and a slight thickening of the latter soon becomes app.nent just where it is in contact with the surface of the optic outgrowth in (Fig. 74, B, /). This thickening is the first rudiment of the lens. The lens- rudiment gradually becomes sunk below the general surface to form a saucer- and later a oup- shaped depression. As the rudiment becomes involuted in this way, the outer wall of the optic outgrowth also becomes 134 EMBRYOLOGY OF THE LOWER VERTEBRATES MI invaginated to form a cup-like structure — the optic cup (Fig. 74, thai pi ect \\m\\\\\\ A. lhal. limliiiil liiiilinil n|' Kn\vl t-mliryos. .•.jiiiH-m «.r iii,. eye as MTU in A, latter half Of *«<-\\ KR VERTEBRATES CH. Kerr, 1902). Similar observations have heen subsequently made in the case of Amphibians. The first obvious step in the specialization of tin- visual cell is the appearance of a fatty globule in its protoplasm. The end of the (.vll turned away from the lens now grows out into a projection and pushes back the fine cuticular limiting membrane (external limiting membrane) which has developed over this surface of the retina, into a little pocket. The oil globule which gradually increases in si/e passes into this pocket (Fig. 76, A, f.g) and lies in it ensheathed in protoplasm. The protoplasm now becomes heaped up into a little conical protuberance (Fig. 76, B, r) which is the rudiment of the rod. At first the limiting membrane is distinct over the surface of the rod but gradually, as the latter assumes a cylindrical shape, its protoplasm takes on a clear structureless appearance throughout : it apparently becomes in fact converted into cuticular material. This cylinder of cuticular material increases in size, assumes a character- istic appearance with alternating discs of dimmer and more trans- parent material as seen in the fixed specimen and the rod is complete (Fig. 76, E and E*). The rods complete their development sooner or later according as they are nearer or farther away from the optical axis of the eye and their time of development shows great variation in different individuals. The cones in those Vertebrates in which cones are present are merely specialized rods. LENS. — The lens shows in its early stages, in various groups of Vertebrates, departures from the normal condition as described for the Bird, of exactly similar kinds to those seen in the development of the otocyst. In particular, the lens tends to develop out of a solid downgrowth of the deep layer of the ectoderm. This is well seen in Elasmobranchs (Fig. 77, A-E) where a rounded solid lens-rudi- ment is formed by proliferation of the ectoderm, this rudiment becoming isolated and developing a cavity secondarily. It is of interest to notice that even here a slight dipping down of the external surface into the lens rudiment is apparent for a time (Fig. 77, B). In Amphibians, Lung-fishes and Teleostomatous fishes the lens arises in a manner intermediate between what occurs in Elasmo- branchs and what occurs in Sauropsida. In the forms mentioned the lens arises as a downgrowth of the deep layer of the ectoderm I-'i'j. 77. V-\j and iii some cases this downgrowth is simply an inva-i nation of this layer, the only difference from the Sauropsidan condition hein^r that here the opening of the invagination is closed by the superficial layer b.-in^ continued across it (Fig. 77, J, K As regards the later stages in the development of the lens all that need be said is that it. undergoes an enormous increase in size — l>y absorbin^ nourishment IV-.m its surroundings, fur it has no Mood- is tin- e.-lls of the deep wall bc.cnmm^ -really elongated and taking on a cl< \ appearance, while the Superficial frail remains as a l:i\.-r "I cubical epith.-lial celU over th«- outer surface of the lens. II EYE 139 SCLEROTIC, CORNEA, CHOROID. — These portions of the eyeball are gradually diH'rrcntiati'd out of mesenchyme which becomes concentrated round the primary parts of the eye. In the case of the cornea the first stage in the developmental process consists in the accumulation between 1ms and ectoderm of a clear jelly-like secretion FIG. 77. — Variations in the early stages of the development of the Lens. A-K, PrMfemc; K-l, >/m/o/< (after Habl, 1808); J, K Phyttomednsa (after Hudirctl. is-.".'). the continuous, and identical in character, with that which nils optic cup. As development goes on (Knape, 1909), a thin layer of this jelly-like material, about midway between the lens and the ectoderm, becomes condensed to form the rudiment of Descemet's membrane. Amoeboid cells from the mesenchyme round the optic cup creep 140 EMBRYOLOGY OF THE LOWEK VERTEBRATES CH. along the deep surface of Descemet's membrane and there settle down to form a single layer of flattened cells. On the deep side of this corneal endothelium a split gradually develops in the jelly-like matrix : this contains a watery secretion (aqueous humor) and becomes the anterior chamber of the eye. The portion of matrix lying superficial to Descemet's membrane becomes colonized by cells from all round its margin. It forms the main portion of the cornea, while a thin layer lying next the ectoderm remains uncolonized and gives rise to Bowman's membrane. VITREOUS BODY. — The cavity of the optic cup is from the beginning filled with clear fluid which keeps it distended and there is no ap- parent reason to assume that this arises otherwise than by the same method as holds with the eyes of many invertebrates i.e. as a secre- tion of the surrounding retinal cells. The fluid gradually acquires the jelly-like consistency characteristic of the fully-formed vitreous body. Amoebocytes wander' at a com- paratively early period into the vitreous rudiment — in the Fowl embryo about the third day — and at a later period a continuous mass of ,/ ^^^^^ mesenchyme tissue projects into it S&S through the choroid fissure. This mass of mesenchyme develops a Fn;. 78. — Semidiagrammatic figure of the network of blood-VCSSels Continuous bisected eye of a Vertebrate embryo with those of the surrounding tissue. (liana o mm.) to show the course ot T ., ••-• \r j v the optic nerve-fibres (after Asshetou, In the more primitive \ ertebrates 1892). this mesenchymatous mass reaches c.f, Wall of choroid fissure ; ,,, ganglion-cell ; 110 great development but ill the indifferent, supporting, <-.-n ; /, lens; «./, Teleostei and the Sauropsida, the amongst the non-mammalian Verte- brates, it does so and persists throughout life, as the falciform process with its muscle-fibres for the purpose of accommodation in the one case ( Teleost ei ), and the highly vascular, and probably mainly nutritive, pecten in the other (Sauropsida), OPTIC NERVE. — As already indicated the optic nerve is not strictly speaking a peripheral nerve ai all. It is a slender drawn-out portion of the brain analogous with the olfactory tract of a teleostean tish. con- necting the main portion of the brain with the small highly spccial- i/ed portion which h;i> become converted into the optic cup. Its function bem;j a conducting one the main mass of this stalk-like portion df the brain is composed of white substance or ner\ 'e-libres. Th< lead of parsing oiit\\ard< over the rim of the cup EYE 141 all round, as they possibly did originally, have heeome crowded together during tin- (MUM-SI- of evolution into ;t single large bundle on the ventral side of tin- nip. In accordance with the general principle of economy of tissue this bundle .»t nerve-fibres has become sunk into ;i deep notch in the wall of the cup — the choroid fissure — so that it passes directly to the optic stalk. While this is probably a correct statement as regards phylogenetic evolution, matters an- somewhat simplified in the development of the individual, inasmuch as the choroid fissure is brought about not by the notching of the already formed cup rim but by the rim ceasing to develop at the site of the choroid fissure while it grows actively everywhere else. As regards the development of the actual nerve-fibres, all that need be said is that they first make their appearance in the wall of the optic stalk ventrally and that they increase rapidly in number, passing between the epithelial cells of the stalk, loosening them out, and causing them in great part, if not entirely, to degener- ate. The individual fibres certainly for the most part become differentiated in a centripetal direction i.e. from the retina to- wards the brain, but whether this means that they are actually sprouting out frqm ganglion- cells of the retina as is generally believed, or on the other hand that their fibrils are simply becoming differentiated centripetally within a continuous pre-existing protoplasmic connexion, remains to be demonstrated. EMBRYOLOGY AND THE EVOLUTIONARY ORIGIN OF THE EYE. — The peculiar reversed position of the Vertebrate retina may perhaps be taken as an indication that that organ had already come into exist- ence, though no doubt in a very simple form, at a stage in Vertebrate phylogeny when the central nervous system had not yet sunk down below the surface. It is therefore of interest that in certain Verte- brates the rudiment of the retina does actually become apparent during embryonic development at a period when the medullary plate is not yet closed in. Thus Eycleshymer (1895) has described in Eana palustris and in Amblystomn how a patch of pigment appears lor a time on the surface of the medullary plate (see Fig. 79) in the position which will later on form the optic outgrowth. Although we are perhaps justified in believing that the eye of existing Vertebrates was already present as a patch of epithelium sensitive to light in the far back evolutionary period when the fore- FIG. 79. — Transverse section through the still open neural plate of Rana palustris near its anterior end, showing the position of the optic rudiments (E) already marked out by the formation of pigment (after Eycleshymer, 1895). 142 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. of tlu* central nervous system was still a portion of the outer surface of the body, we do not. in the present writer's opinion, appear to be justilird in connect inur up the eye of typical Verte- brates with the "eyes" of Amphioxus or of Tunicates. It seems more probable that tli highly specialized creatures are ifl which have developed independently within their own groups. PITUITARY BODY. — To be included amongst the derivatives of the ectoderm is that enigmatical organ the pituitary body (" Hypophysis cerebri"). This — the "anterior lobe" of the pituitary body in mammalian anatomy — arises normally as an ingrowing pocket of ectoderm on the ventral side of the head, situated as a rule close to the hinder limit of the stomodaeum but in the case of the Cyclo- stomes just outside its anterior boundary. This pituitary involution extends inwards beneath the infundibulum. In the Cyclostoniata it retains its original form of a tube communicating with the exterior, but in the gnathostomatous Vertebrates its outer end becomes gradu- ally constricted into a narrow duct which in the great majority of cases becomes finally obliterated, so that the organ now forms a closed sac lying immediately underneath the infundibulum. The wall of the sac sprouts out into numerous tubular projections between which develops highly vascular mesenchyme, providing the rich blood supply necessary to the definitive function of the organ as a ductless gland. As regards variations in the development of the pituitary involu- tion : it may arise as a solid ingrowth of ectoderm (Lung-fishes, Am- phibians) ; it may be two-lobed (Teleosts) or three-lobed (Lacertilia) in its early stages ; its external opening may become secondarily dis- placed up on to the dorsal side of the head (Lampreys); its inner end may come to open secondarily into the pharynx (Myxinoids). As already indicated the wall of the infundibulum in the Gnatho- stomata comes into intimate relation with the pituitary body in the restricted sense, forming the so-called posterior lobe, or cerebral portion, or nervous portion, of the pituitary body. LITERATURE Agar. An:it. An/eiger, xxviii, 1906.. Agar. Anat. Anzeiger, xxxv, 1910. Assheton. u;irt. .lourn. Micr. Sci., xxxiv, 1892. Assheton. ^Mmrt. .loiiru. Micr. Sci.. \.\\viii. 1896. Balfour. Phil. Tr.-ms. K..V. Sor., dxvi, 1876. Balfour. Monograph on fcne development of Elaamobranoh Fi>hrs. London, 1878. Bidder und Kupffer. ITMti-r>ii'-himj,'»'n iibi-r ;iii.n lii.m (,f Kurope. Ray Society, 1897. Braus. A jer, cxvi, 1905. Budgett. i.'tMit. .luiim. Mi.-r. Sk5L, \Iii, 1899. Burrows, .lomn. lv\|M-iim. /.M.I.. \. 1911. Cameron . Sue-. Ivlinl... xxiv, 1903. Cameron. \'i< > • Ivlinl-.. \.\v, 1904. Coggi. Alti Aooad.1 HMO 286, Ser. iv, M.-ni. Cla^. Sri. lis.. \ i. 1889. ii NERVOUS SYSTEM 143 Cunningham and MacMunn. Phil. Trans. Ko\. So... I',, clxxxiv, 1893. Davies. Mm-ph. .lahrh., xv, 1889. Dendy. Quart, .lotini. Mii-r. Set, xlii. 1899. Dendy. Quart. .Jotirn. Miol. Sta. Ncapi-1. xv, 1902. Eycleshymer. .Imini. Morpb., x, 1895. Goeldi. Zoolog. An/e.iger, xxiii, 1900. Gutzeit. Zritsebr. wiss. Zoo]., xlix, 1889. Harrison. Anal. Record, ii, 1908. Harrison. .Journ. Kxperim. /ool., ix, 1910. Havet. La Cellule, xviii, 1901. Held. Die Entwioklung a«-k\\.n«l shilling of the anus, the yolkv mass 144 CH. in Till- AUMKNTAUY CANAL 145 becomes extended into the form of u thick-walled tube— the rudiment of the intestine (cut). Krom the stage of Fig. 80, E, onwards active growth of the true tail or postanal region i< taking plaee, and it is noteworthy that, during this process, the endoderm retains for a considerable time its continuity with the mass of actively growing undiU'erentiat ed tissue at the, tip <'f the tail and becomes drawn out into a cylindrical postanal gut (/"«.#). This remains conspicuous for a time hut eventually disintegrates and disappears completely. The main mass of yolk -cells, forming the ventral wall of the middle part of the enteron, gradually shrinks in volume, as the yolk is ahsorlied and carried off by the circulating blood for distribu- tion to the growing and developing tissues of the body, and eventually the gut wall is no thicker in this region than it is elsewhere. I.UCCAL CAVITY. — The alimentary canal of the adult Vertebrate commences with the buccal cavity which is in part at least — as shown by the presence within it of placoid and glandular elements corresponding with those of the skin — stoniodaeal in its nature. The stomodaeum however is not as a rule developed, as is so usually the case in the Invertebrates, from a simple involution of the ectoderm forming a depression of the surface below the general level. It arises rather by the walling in of an area on the ventral side of the head through the development of ridge-shaped outgrowths. These ridges may be termed respectively the maxillary ridge and the mandibular ridge accordingly as they give rise later on to the upper or to the lower jaw. The roof of the buccal cavity, or at least its anterior portion, is simply to be looked on as part of the primitive ventral surface of the head, delimited by the maxillary ridge on either side. The floor, on the other hand, represents the mandibular ridge (Fig. 80, H, m.r) which has grown forwards in a direction parallel to the roof. The inner wall of the buccal cavity is in close contact with the anterior extremity of the endodermal alimentary tube but for a time the two cavities remain separated by a thin membranous diaphragm made up of the apposed layers of ectoderm and endo- derm. This may conveniently be termed the velar membrane as the organ known as the velum in A-mphioxus or Petromyzon consists simply of the remains of this membrane. The formation of the anterior, stomodaeal, portion of the buccal cavity is seen in its simplest form in some of the lower holoblastic Vertebrates such as (Jrossopterygians, Lung - fishes or Urodele amphibians. In Fig. 69, D (p. 126) in the case of Protopterus, or in Fig. 100 i p. 178) in the case of Polypterus (see also Fig. 80, H), what will became later the anterior part of the buccal roof is seen to be simply a portion of the ventral surface of the head, bounded behind by the transverse mandihular ridge — the rudiment of the lower jaw — and on each side by the longitudinal maxillary ridge. As is well shown in the figure of Polypterus, and as is also the VOL. II L 146 EMBRYOLOGY OF THE LOWER VERTEBRATKS en. case with Protuj'tcri's, the luiccal roof in front, that is to say in the neighbourhood of the mesial plant', passes without interruption into the external skin: in other words the maxillary ridge is not con- tinued to the mesial plain- BO as i<> m< , i its fellow. In later stages ar end. N E FIG. 80.— Sagittal sections through Polypi. A,stage]4; H, sta-c 17 ; C, stage 20 ; I), stajje 2S; E, stage 24 +. ct, anu» ; or , archenteroj endoderm; *nt, enteric cavity ; /./-. cavity ,,f fnn-liniiri : /.;/, fm.-ut ; N, notochord ; t,.f, |.rim:ir\ Inl.l nr liraiii I1r; pin, pineal ru.lirn.-nt ; .v.c, cavity of .sj.inal r..r,| ; //. yolk. thereof of the mouth would be hidden in ,i view fnun the ventral side owing to the forward growth of the lower jaw. The anterior portion of the huecal cavity in l'i-<,,l,-le Aiiij.hiliians arises in a iinnm-i- essentially similar in that deserihed above. In the Gjmnophiona and the Amninta a e.haraeter^tie modiliea- tion nf the mouth margin is hroiio-ht aii.mt l.\ tin; tact thai, as Ill urCCAL CAVITY 147 already mentioned, the maxillary rid^e is cut across by the olfactory groove and so divided into the outer in axillary process and the inner median nasal process, the latter of which is continuous with its fellow across the mesial plane, forming with it the so-called fronto-nasal process (see Chap. X.). ac. pan. FIG. 80A. — Sagittal sections through Polypterus. \'\ stage 2t> ; G, stage L'H ; II, sta.ue :{•_'. «. anus; a.c, anterior commissure; cA, optic chiasnia ; I, cloaca ; ent, enteric cavity ; H, heart ; h.c. habenular commissure ; Zi, liver ; ///./•, mandibular ridge ; p, pituitary involution ; /'.'•. posterior COmmttstire ; /»'.«, pani-reatic rudiment : pin, pineal rudiment.; .«t, stumacli ; y, yolk. It is of interest to notice that in various Vertebrates the buccal opening is at first elongated in an antero-posterior direction instead of from side to side. Such is the case with Scyllium (Sedgwk-k, see Fig. 81) and Torpedo amongst Elasmobranchs. In these cases the slit-like mouth is bounded on each side by a longitudinal ridge. 148 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. Later oil each ridge becomes sharply bent, about the middle of its length, in such a way as to give the buccal opening a rhomboidal shape and at the same time to mark off the ridge into a maxillary portion in front and a mandibular portion behind. In Anura a somewhat similar arrangement is found. "ENDODERMAL" SECTION OF BUCCAL CAVITY. — The fully developed buccal cavity has incorporated in it a posterior portion— varying in relative extent in different A^ertebrates — which is de- rived not from the ectoderm but from the anterior portion of the " endodermal " enteric rudiment. The simplest way in which this portion becomes added to the anterior portion is seen in those Vertebrates in which the anterior part of the enteric cavity is patent throughout develop- ment. In this case the velar membrane simply ruptures — its remnants soon be- coming absorbed — and the stomodaeal cavity is thrown into open communication with the enteric cavity. This is the case in certain Anura (Rana) and in Amniota. In many Verte- brates no velar mem- brane is present, owing to the fact that the foregut either becomes solid for a time (Polypterus, Fig. 80, D-G) or is so at the beginning (Teleostei, Urodela, Lepidosiren and Protopterus). In such cases the peripheral layer of the yolky foregut rudiment gradually assumes an epithelial character and the yolk along its middle breaks down, so that a cavity arises — continuous with the stomodaeal cavity and forming the hinder section of the definitive buccal cavity. The pro- portion which this posterior portion bears to the anterior section derived directly from the outer surface is very different in different groups. It apparently attains its maximum in Teleosts where it forms practically the whole of the buccal cavity. Points of critical importance to the germ-layer theory are raised in this connexion by tin- fact that teeth, organs belonging originally to the outer surface, are developed in this posterior region of the buccal cavity from yolky " endoderm." This is well seen in a Urodele, or a lun^-iish such a< /x///V".s//r//. or I'r»f<>/>/< -ni* ( Kig. S2). The attempt is made to got round this dillicnlty by assuming thai tin- layer of • •pit helium which makes its appearance over the surface of the bin -ody ; par, paraphysis ; Pit, pituitary body ; Th, thyroid rudiment; t.o, tertian i>j>ticiiut. The position of dental rudiments is indicated by the two upward projections of the dorsal wall of the buccal cavity. hut examination of carefully prepared celloidin sections (Fig. 83) shows that at its inner end the epithelium passes by imperceptible 1 \\ecf. ect. l-'i'i. .V!.- -Sagittal sections through the region of the buccal cavity of (A) Lepidosifcn. stage 30, and (B) AmWystoma, 7 '5 mm. in length. b.e, buccal epithelium ; re/, ectoderm ; //. solid mass of yolk-cells in position of bnccal cavity. gradations into the ordinary yolky endoderm, with no trace of the sharply defined edge which it would possess were it a layer of 150 EMBRYOLOGY OF THE LOWER VERTEBRATES OIL ectoderm pushing its way in wan Is. It extends inwards simply by a process of delamination from the yolky " endoderm." The real lesson to be learnt from these cases is that the characters of one germ-layer are liable to spread over its boundary into territory belong n.ur to another layer or, in other words, that the territories of the various layers are liable to be separated by an indefinite debatable zone rather than by a mathematically sharp line. It follows that the apparent position of an organ-rudiment in relation to such a boundary is not necessarily to be taken as C. 1 i . 84. —Sagittal sections illustrating the development of the tongue in UroiU-li->. A and B, Triton ; C, Salmnandra (alter Kullius, 11*01) ; .t, primary tonj;-"'1. giving any definitive proof as to which of the two cell-layers that organ belongs to. THK TONGUE.— The tonkin- is a portion of the buccal floor which becomes demarcated oil' 1mm the iv>l by a. split formed by a down- growlh of the, lining epithelium of the mouth. Its mode of develop- ment is well illustrated by what happens in Urodele Amphibians as described by Kallius. Her.- then- develops first a primary tongue, ensheathing the anterior and ventral portion of the hyoid arch 84, ;;.£), \\hieh becomes marked off, except at its hinder end. by a deep 'jroove in the Hour of the mouth. A hoi aped thiekenini: o| the buci-.nl epithelium no\\ in BUCCAL CAVITY lol develops external to, and parallel with, the groove bounding the prinuiry tongue. JIIH! consequently lying on the floor of the mouth between the primary tongue and the lower jaw. The thyroid involution is situated between this thickening and the tip of the tongue. The ectodermal thickening develops numerous glands, each originating as a solid ectodennal down-growth, and is known as the gland-field. Externally it is bounded by a shallow groove. Later on the cleft or groove separating the gland-field from the primary tongue becomes obliterated by fusion of its walls, and the gland-field becomes raised up in a dorsal direction (Fig. 84, B) the tongue-tip shrinking backwards so that eventually the demarcation between primary tongue and gland-field disappears (Fig. 84, C). Meanwhile the groove bounding the gland-field externally becomes deepened. It forms the outer limit of the definitive tongue which is thus a compound structure, its tip and edges developed from the original gland-field, its postero-median part from the primary tongue. In the fishes the tongue remains non-muscular and non-glandular: it is simply the primary tongue. In the Axolotl the tongue appears also to be a primary tongue, the gland-field making a transient appearance as a rudiment but eventually undergoing atrophy (Kallius). In the Amniota the tongue is, as in the terrestrial Urodeles, a compound structure, the primary tongue rudiment becoming fused with an elevation of the floor of the mouth lying in front of the Thyroid rudiment. This elevation, called by His the tuberculum impar, represents morphologically the gland-field of the Urodeles. The tongue of Cyclostomes is remarkable for its complexity : it has complex muscular and skeletal arrangements and on its surface it develops the horny spines which function as teeth and simulate teeth in their appearance. In Bdellostoma the tongue develops as a cushion-like swelling of the floor of the mouth at an early period while the velar membrane is still intact. In Petromyzon, on the other hand, it does not develop until the time of metamorphosis. It lias already been shown how the olfactory organs come to communicate with the buccal cavity by the posterior nares. In the Amniota these become sunk into a recess in the roof of the mouth and in the higher Reptiles, as in the Mammals, this recess becomes shut off from the buccal cavity by a horizontal shelf which grows in from the side and meets its fellow to form the palate. How this has come about in evolution is illustrated by the three Lizards shown in Fig. 85. In ontogeny the mode of origin may be similar, the palatine out- growths meeting and fusing with one another in the middle line (Crocodiles) or, as happens more usually, a median ridge or -septum «'\ tends backwards from between the primitive posterior nares and llif palatine processes meet and fuse with its ventral edge. In the two cases the physiological result is the same — the shunting back- 152 EMBRYOLOGY OF THE LOWER VERTEBRATES « n wards of tin- communication hetween olfactory and buccal cavities, a process which reaches its extreme in the Crocodilia \vhere the palate extends hack to about the level of tin- glottis. STOMODAEAL GLANDS. — AYhereas in the majority of Fishes the stomodaeal lining po- .inly isolated gland-cells, in the air- breathers on the other hand there are developed definite multicellular glands. These originate as a rule from solid down-growths of the ch. B FIG. 8:>. View of tin; roof of the mouth in three species of Lizard (A, Kr the communication between nose ;uid mouth. (After Voeltzkow, 1899.) ••I,, recess into which primit ivt- postrrioi naivs open ; /«//, palatine; /,t, ptei-y^oid ; tr, 1i;insviM>r hone; i;>. VOllHT. Lining epithelium which develop a cavity secondarily. In I'rodeles then- is, as already mentioned, a special a-uiVLrat inn <>!' these glands forming tin- ^land-Held in IVoiit «»!' lh«- tongue, \\hilc ;t single -'land of coiisidcralilc si/c dcVL-lojis IVoni the runl' of the mouth in the region l'«-l \\rcii the olfactory sacs (I nl ei-ma xillai'y or inlernasal ^land). In tern-stria! Ilept iles glands an- present in nnnihers mi the roof of the mouth ' 1'alaline;, hrn.-at h I he loii-jiie on each side of the middle; line Suhlin l>e,-n different. The outer surface of the viscer.il arch m the region when- the external ^ill will de\elop is covered by a la yer of cells thicker than the neighbouring ectoderm, and in some cases this thickened portion of the ectoderm sho\\s in its end. FlG. 86. — Diagrammatic longitudinal section through the early rudiments of the external gills of Gt (Stage 25). e.g, external trill ; end, emloderm ; v.a, visceral arch : visceral cleft rudiment. Ill EXTERNAL GILLS 155 deeper portions a rich deposit of yolk, so as to look exac.tly like the yolk-laden endoderm. (ireil explains this appearance by supposing that true endoderm o-lls actually spread out \\ards and replace the deep layer of the ectoderm, so that the external gill-rudiment would be partly endodernial in its nature. There is however no definite evi- dence of any such process taking place and the present writer would interpret the appearances as mean- ing simply that the ectoderm cover- ing the external gill - rudiment becomes thickened, and stores up a supply of yolk in its deeper layers, as a physiological preparation for the active processes of growth whie.h are about to take place as the external gill rapidly increases in length. In this he agrees with Marcus (1908). The general appearance of the developing external gills is well seen in Hypogeophis (Fig. 87) or in Lepidosiren (Fig. 200). In Lepidosiren there are present four FIG. 87.— Hypogeophis embryos showing upon each side of the body. At first the four are quite independent of one another but as development goes 011 they become raised Upon a projecting in B from the hyoid arch, and also common base so as to give the /» * 1 •.! LMjaoiMiv C.VUGJ. lien Km AIAVAIIIH.* appearance of a single organ with go on with their development four branches (Fig. 200, B-E). The distribution of true external gills amongst the main groups of Vertebrates is shown in the following table : development of the (After Brauer, 1899.) external gills. xternal gill; 77, hyoid arch; olf, olfactory organ. The rounded kim! from the mandibular arch in front of it, are possibly external -ill rudiments which do not I I. II. in. IV. V. VI. Vis,,ral Arch, .Mandibular. Hyoid First Hiunehial. Second Branchial. Third Biiim-hial. Fourth Hrani-hial. KluMiiobraiirhii Crossopterygii X Dipnoi. X X X X Amphibia . . TO.* V, X X X Amuiota In those animals in which they are well developed the external gills are, for a time the main functional breathing organs. They are 156 EMBRYOLOGY OF THE LOAYKi; VERTEBRATES CH. richly vascular and the renewal of the water in contact with their surface is provided Tor l>y a well-developed muscular mechanism by which they are sharply liicked from time to time, or, in early stages, liy rich cifiation of their surface as in the Frog (Assheton, 1896) or fobranchus (Smith, 1912). They are as a rule merely temporary organs. As the respiratory function comes to be sufficiently per- formed by other organs their circulation becomes sluggish, their tissues moribund. They become invaded by leucocytes and eventually undergo complete atrophy. In Protopteras distinct vestiges persist t«>r a prolonged period while in various Urodeles they remain func- tional throughout life. The external gills, highly vascular and projecting freely into the surrounding medium, present tempting objects for attack by other organisms. They are therefore extremely liable to injury, and cor- related with this they present a high power of regeneration. In correlation also with the same fact we find that they tend to be eliminated from development in certain members of groups which are as a whole characterized by their presence. Such is the case in the Amphibia where they are characteristic of the group in general but where in particular cases they are reduced (Hyla arbor ea) or com- pletely absent (Boiribinator) although we must believe they were present in the ancestors of these forms. This tendency for the external gills to become eliminated from development in the process of evolution raises the interesting morpho- logical question : were External Gills at any period more widely dis- tributed amongst Vertebrates than they are at present ? And, if so, are their vestigial representatives still to be found in any cases where they no longer develop as functional respiratory organs ? This interesting problem, which offers an inviting field for research, has not yet had sufficient attention devoted to it. Even if it were the case that external gills once existed in the ancestors of forms in which they are no longer present as functional organs there is always the possibility if not probability that their disappearance has been so complete as to leave no observable trace. Nevertheless such vestiges might persist and are worth looking for. Under these circumstances it is of interest to note that already certain structures are known which are interpretable as vestiges of once-present external gills. Thus in Gymnophiona what appear to be transient rudiments of mandibulsir and hyoidenn external gills make their appearance during development (Fig. 87, B). Again in the case, of the Mandihular and Hyoid arches of Urodeles, on which no functional external -ills develop, Driiner (1901) has found what appear to be vestiges «>f the muscles of external gills. A-'ain in the larvae of various Urodeles there occurs in connexion \\ith each mamlil.iilar arch a curious styliform projection known as the balancer, from the fact that the 1,-irva ha lanc.es itself upon them as upon a pair of limits (V\^. HS, //). Kach of these has a vascular loop within it and it in l:n-i appears to be the modified external ^ill of Ill EXTERNAL GILLS 157 * the mandibular arch which has lost its respiratory and taken on a supporting function. While (i.\t.i-i mil -ills occur within three main subdivisions of the Vertebrates, namely Teleostomatous fishes (Crossopt- —the most archaic of existing Teleostomes), Lung-fishes, and Amphibians, there are two main groups — Elasmobranchs and Ainniotes — in which they are conspicuous by their absence. Having regard to the tendency of the organs in question to disappear (as in the cases already alluded to amongst the Amphibia) their absence in a special group would not in any case constitute strong evidence that they were never present in the ancestors of that group. As it happens however there is in the two groups mentioned a definite cause which seems quite competent to account for the dis- appearance of external gills, namely the de- velopment of a new organ — the yolk - sac with its highly developed vitelline network of blood-vessels — which in addition to its primitive function must neces- sarily also function as a very efficient organ of respiratory exchange and so render any pre-exist- ing respiratory organ no only their basal stumps. longer necessary. Taking into consideration the presence of external gills in three archaic groups of Vertebrates it seems to the present writer to be clearly indicated that these organs are a very ancient characteristic of the Vertebrate phylum. The only alternative indeed is to regard them as having become evolved independently in the three groups in which they occur. It is difficult to accept this as in any way probable having regard to the similar morphological relations of the organs in question. It might be suggested that somewhere on the course of a large blood-vessel, such as an aortic arch, would be a most natural place for the development of a new respiratory organ. Such a suggestion however is entirely fallacious for simple physical reasons : for new breathing organs will tend to become evolved not on the course of a B. C. FIG. 88. — Three stages in larval development of a newt (Triton fui'nfitfi(s) as seen from above. (After Egert 1913.) ft, balancer ; e.(j, external gill of first branchial arch. In i what looks like a posterior external gill is the pectoral limb. In Figs. B and C the external gills have been cut away leaving 158 EMBRYOLOGY OF THE LOWKK VERTEBRATES « n. large vessel where the quantitative relation of surface to volume in the blood-vessel is at its minimum hut r; it her where there is present a rich superficial net work of capillaries, in which the ratio in question is at its maximum. (II.) VISCERAL CLEFTS. — The visceral clefts develop in what appears to be the most archaic method in Lampreys and Elasrno- branchs where each arises as a lateral pocket (visceral pouch) of the pharyngeal wall which meets and fuses with a, much shallower, ingrowth of the ectoderm, the apposed portion of endoderm and ectoderm breaking down so as to bring about a free communication between pharynx and exterior. Each cleft thus consists of a, usually much larger, inner portion lined with endoderm and an outer portion lined with ectoderm. The most frequent type of modification of this probably primitive mode of cleft development is that so usually met with in the develop- ment of hollow organs, namely that the cleft-rudiment, instead of being a hollow pouch from the beginning, is for a time in the form of a solid lamina of endoderm, which only at a later period develops a cavity in its interior and becomes an open cleft. This modification is found in Teleostomatous fishes, Lung-fishes and Amphibians. In the young Elasmobranch the gill-clefts are at first long slits traversing the whole dorsi-ventral extent of the lateral wall of the pharynx. Each septum or arch grows back at its outer edge to form a valvular flap overlapping the cleft next behind it. In most cases this backgrowth fuses with the next septum at its dorsal and ventral ends so as to reduce the external opening of the cleft to a compara- tively small dorsi-ventral extent. In all Gnathostoines, excepting the typical Elasmobranchs but including the Holocephali, the hyoidean backgrowth becomes greatly enlarged to form the operculum which overlaps the whole series of clefts behind it. Correlated with this the outer portions of the suit- sequent septa with their backgrowths become reduced. In these cases we frequently find a marked tendency for the edge of the opercular backgrowth to become fused with the body so as to restrict the size of the opening behind it. Thus in the Eel the opercular opening becomes reduced to a small persistent ventral portion, while in Symbranchus the same holds but in this case the two openings have fused together to form a small ventrally placed median pore. A similar condition to this occurs in the tadpole of Discogh* while in other Anuni the persisting opening is displaced to the left side, Finally in Aniiiiot.-s the, fusion of opercular margin with body- wall takes pla •••• along its whole extent so that the branehial region Omefl completely enclosed (see Chap. X.). Sl'lU.vi.K. Tin1 spira -le m- livomaiidiluilar cleft aluays sh<>\\> a considerable amount of modification. In Klasmohranchs its dorsal portion ;iloiie hiM-umes perforate, although fusion of the pouch with the ectoderm take- j.la«-<- throughout its whole dorsi-ventral extent. Ill GILLS 159 Ar an. act 1 ; expiratory lamellae develop only on its anterior wall and these, as development proceeds, become vestigial forming the pseudobranch. In Teleostean fishes the spiraeular pouch (Fig. 89 A, vc. I) flattens out and disappears • duetto) so that the pseudobranch (ps.~) on its anterior wall comes to lie on the inner face of tin; base; of the operculum and appears to belong to the second cleft (Fig. 89, B). In Lung-fishes the solid endodermal rudiment never becomes perforate. It becomes gradually reduced during development while its outer ectodermal portion becomes, as already indicated, converted into a special sense-organ. In Anurous Amphibians and in the Amuiota the distal portion of the cleft rudiment becomes greatly dilated to form the tympanic cavity, while the proximal part forms the relatively narrow Eustachian tube. •Just as the varying condition of the spiracle indicates a tendency for this cleft to undergo reduction so a similar but still more marked tendency exists for the gill clefts to become reduced at the other (posterior) end of the series. This is illustrated in the first place by the reduction in the number of functional clefts seen in passing from the lower Vertebrates to the higher. It is also frequently manifested in developmental stages. Thus amongst the Amphibia we find FlG. 89. -Horizontal sections through that in the Gyuinophiona (Hypoge- opJds, Marcus) a rudimentary 7th cleft makes its appearance though it never reaches the ectoderm, while the 6th JS^^'^^StlSL^ is open for a time. In Urodeles a 6th rudiment appears and is for a time connected with the ectoderm but does not become perforate, while in Anura this cleft appears only as a small and transient rudiment which never reaches the ectoderm. (III.) INTERNAL GILLS. — The internal gills or respiratory lamellae arise as ridge-like or, at first, finger-like projections of the cleft lining. The chief matter of dispute regarding their development has been the question whether they belong to the endodermal or the ectodermal portion of the cleft lining. In cases where, as frequently happens, the lamellae begin to develop after the cleft is completely formed, the appearances are sometimes in favour of the one sometimes in favour of the other interpretation. Goette (1901) in fact goes the length of regarding the lamellae as being of endo- origin in the case of the spiracle and ectodermal in the case Salmon embryos explaining position of pseudobranch on inner surface of operculum. (After Goette, 1901.) I and II; Hy, hyoid arch; op, operculum; Ph, cavity of pharynx : /is, vc, visceral cleft. 160 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. of the succeeding clefts, so that the spinicular pseudobrauch would on a strict interpretation of the germ*layer theory not bo serially homologous with the other gills. In the present writer's opinion, as already indicated, such obser- vations upon the first origin of organs which develop in the region of the blurred boundary between two layers are not to be taken as affording evidence of any serious importance in regard to the morphological nature of such organs. Greater weight however seems due to evidence obtained from cases where the hrst traces of gill lamellae are visible at a period before the bounding membrane of the cleft is ruptured, when the cleft consists still of two distinct pouches — one ectodermal, the other endodernial — separated by a still complete partition. Such is the case in Acipenser and Goette shows that in this case the lamella-rudiments arise outside the partition from what is undoubtedly an ectodermal surface (see Fig. 90, g.l). The same discussion extends to the general lining of the cleft — as to how much of the lining of the adult cleft is ectodermal and how much endodernial. Goette and Moroff (1902) hold that only the portion of the cleft in the inime- diate neighbourhood of its pharyngeal penser showing the ectodermal opening is to be regarded as endodernial, origin of the gill lamellae. (After all the rest being ectodermal. But here again in view of the blurred character aa, aortic- a,ch ; ,.i, raiment »f Kiii of the boundary between the two layers lamella ; /fy, byoia arch ; op. operculura : . i -M ,., i i % rity of pharynx, it seems hardly profitable to speculate on the matter. In certain fishes the gill-lamellae are for a time prolonged out- wards into long threads which project through the cleft opening into the surrounding fluid. Such is the case in the embryos of Elasino- branchs, in which it is only the lamellae upon the posterior face of each arch that become prolonged, those on the anterior face not projecting beyond the edge of the septum. Eventually the pro- jecting part of the filament disappears while its attached basal purtinn becomes the definitive lamella. In a few Teleosts a similar temporary modification of the lamellae takes place — perhaps the best example bein^ Gytnnarchus (Budgett, 1901; Assheton, 1907. See Fig. 1 BVOLUTIONABY HISTORY or 'IIIK BRANCHIAL RKSIMKATMKY < M:«;ANS. — As regards the early evolutionary history of these branchial rc.-j.iratory organs one very generally accepted view looks upon the visceral elefts as In-ill^ the most primitive, the internal ^ills as «],ed next, and the external uills a.s belli!;- due to •second an «n of respiratory tissue out \\ards frnm the clefts. in mind what \\e now ku<»\\ n-^irdin^ the • l-menl and .list rilmt.inii nt' external -ills, at least e«|iially if imt. in LUNG 161 more probable that the evolution of these organs has been in the opposite direction. On this latter hypothesis the external gills would be regarded as the primitive respiratory organs, inherited probably from pre- vertebrate ancestral forms. The evolution of clefts between their bases would be explicable as an arrangement for pumping water over the surface of the external gills, while it could be readily understood that the respiratory tissue would then tend to spread inwards along the lining of the clefts, where it would be both advantageously situated for carrying out its breathing function and, at the same time, protected from the dangers to which external gills are exposed. The development of respiratory lamellae to increase the area of this respiratory tissue on the wall of the cleft would be a further and natural development. The chief difficulty in the way of accepting this as a working hypothesis lies in the existence of animals admittedly near the base of the Vertebrate scale — such as AmpJiioxus and the Cyclosto- mata — in which there are no external gills and no vascular yolk-sac to account for their disappearance. This difficulty is undoubtedly a serious one but on the whole the present writer is inclined to think the difficulty is not so great as to justify the immediate rejection of the hypothesis: it becomes less formidable when it is borne in mind that the forms mentioned although evidently archaic in some of their characteristics bear in others equally convincing evidence of high specialization. LUNG. — In all the groups of Gnathostomata excepting the Elasmo- branch fishes the pharyngeal wall develops a great outgrowth which, as will become apparent later, is to be looked upon as homo- logous throughout the series and as primarily respiratory in its function — the lung. The lung appears in its most familiar and typical form in the tetrapod Vertebrates and its development in these will accordingly be considered first. Here in an early stage of its development the lung is in the form of a pocket of the pharyngeal floor projecting downwards in the mid - ventral line. This pocket commonly makes its first appearance as a longitudinal groove or gutter in the floor of the pharynx at about the level of the last visceral cleft. The groove becomes constricted off from behind forwards, so as to form a blindly ending pocket communicating in front with the pharyngeal cavity by a narrow opening — the glottis — and extending back immediately ventral to the pharynx. The blind end of the pocket grows actively tail wards and becomes deeply bilobed — the two lobes becoming respectively the right and left lung, while the unpaired portion connecting them with the glottis becomes the trachea or pneumatic duct. While the lung passes in its early history through stages corre- sponding on the whole with those described there are differences in detail in different groups- -the most conspicuous of these variations VOL. II M 162 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. being, as is so often the case in the development of hollow organs, that the rudiment is at first solid and the cavity appears secondarily in its interior. This is the case in various anurous amphibians and in Lepidosiren and Protopterus, It has been indicated that the lung is primarily a ventrally placed pocket of the pharyngeal wall, that is to say its wall is a portion of splanchnopleure. It follows that the cavity of the lung is lined by endoderm while its outer layers (connective tissue, blood- vessels, muscles, etc.) are composed of splanchnic mesoderm. As regards the further development of the lung, the main steps are concerned with its respiratory function and have to do with the increase of the respiratory surface. In such an animal as the Newt, where the lung retains a relatively primitive condition, the endo- dermal lining grows equally as the organ increases in size, so that even in the adult the lung has the form of a simple sac with smooth endodermal lining. In a Frog or a Lizard, however, growth activity is specially marked at particular spots so that at these spots the endoderm forms outward bulgings into the covering of splanchnic mesoderm. In these Sauropsida in which the pulmonary apparatus reaches its highest degree of evolution (Tortoises, Turtles, Crocodiles and Birds, in an ascending series) these pockets of the endodermal lining become more and more extensive, and more and more com- plicated, so as to give rise to a thick spongy mass, which forms the bulk of the lung, surrounding the now relatively small clear central space. The latter, forming as it does an apparent continuation of the bronchus or paired portion of the trachea, is spoken of as the intrapulmonary bronchus. Further the respiratory function becomes concentrated towards the terminal portions of the pockets, their proximal portions forming simply conducting channels — branches of the intrapulmonary bronchus. In the Chameleons, towards the end of development, a number of the endoderm outgrowths bulge out beyond the general level of the surface of the lung upon its ventral side. These persist in the adult as large diverticula which when the animal blows itself out are inflated with air. In the embryos of Birds similar outgrowths make their appearance, four from each lung, but in this case as development goes on the outgrowths continue to increase in si/e and form the characteristic air-sacs of the adult bird. THE LUNG OF BIRDS. — As the Birds, in correlation with the intensely active metabolism as indicated e.g. by their hi^h body t.-injM rat nit-, stand preeminent amongst Vertebrates in the high stage of evolution which lias brim reached by their Inn^. tin1 onto- genetic development of this organ will be followed out in a little more detail Moser, L902; Juillet, 1012). In tin- l''owl lln- pulmonary divert iriiliini of the pliarviiLival floor makeH its appearance about, tin- Ix'Liiiiiiiii.u "I" th<« third day. By the end of this (lay the rudiment is hil'iin-aird at its hind end. cadi loin- Ill LUNG 163 eel mes. In -ing the rudiment of a lung in the restricted sense and containing a prolongation of the enteric cavity lined by tall columnar endoderm cells. Outside tin- mdoderm is a thick layer of mesenchyme and lliis in turn is covivd l>y columnar coelomic epithelium. The eiidoi It-nil -lined cavity is destined to become the main int rapulmonary bronchus — the mesobronchus. This remains un- branched until the fifth day when its endoderrn begins to Iml^e out, near the point where it enters the lung, to form the first ento- bronchus. During further development a series of three other entobronchial outgrowths sprout out from the external surface of the mesobronchus close behind the first outgrowth. The four ento- bronchi so arising are closely contiguous and form a longitudinal row (Fig. 92, E1-E4). A set of similar outgrowths make their appearance spaced out along the mesial side of the mesobronchus posterior to the ento- brouchi : these are the rudiments of the ectobronchi. A third set of outgrowths on the lateral side of the mesobronchus are the rudiments of the small secondary lateral bronchi (Campana). Of these sets of outgrowths the first and second are the most important and they are arranged in a slightly spiral row along the wall of the meso- bronchus. The mesobronchus as if- r secondary diaphragm (ornit hit- diaphragm of r.rrl.dli, pulmonary aponcurosis nf Huxley). Ill AIR-SACS 165 cer / The air -sac rudiments sprout out (Fig. 92) from the main pulmonary cavities — the cervical from the first entobronchus, the interolavioular and anterior thoracic jointly from the third ento- bronchus, the posterior thoracic and the abdominal from the meso- bronchus. Later on additional secondary communications between the air-sac cavity and the pulmonary cavities are established (except in the case of the cervical air -sac) by means of the recurrent bronchi of Juillet. These arise in the ordinary fowl about the tenth day of incubation in the form of outgrowths of the wall of the air-sac either near its tip (interclavicular and anterior thoracic) or just before it emerges through the general surface of the lung (posterior thoracic and abdo- minal) as shown in Fig. 92. These outgrowths burrow into the superficial layer of the lung, branch and become joined up, in a manner the details of which have not yet been worked out, with the system of parabronchi. The communications are visible in suitable preparations of the adult lung as groups of openings, each group leading into the lung from the appro- priate air -sac — those of the interclavicular and anterior thoracic lying towards the FlQ 92.— Diagrammatic view of the right lung of lateral edge of the ventral a Fowl embryo of the tenth day as seen from Surface of the lung, about the the ventral side> illustrating the origin of the e> ,i /• air-sacs. (After Juillet, 1912.) level Of the attachment Of entobronchi are shaded. The four ab, abdominal air -sac; at, anterior thoracic air-sac; the bronchus, and those of the posterior thoracic .and cer, cervical air-sac; Eland E4, first and fourth abdominal SaCS lyino- near bronchi ; ic, interclavicular air-sac ; mts, mesobronchus ; the hind end of the lung, f*W^«W^^J**« close to the direct opening between it and the corresponding air-sac. It would appear that the function of these recurrent channels is to conduct the air forced out of the air-sacs in the expiratory effort through the system of air -capillaries, the muscular coat of the parabronchi doubtless playing an important part in directing the passage of the air through the system of air-capillaries rather than through the parabronchi themselves. The formation of the air-sacs does not exhaust the remarkable proliferative powers of the wall of the lung in Birds. Further out- 166 EMBRYOLOGY OF THE LOWER VERTEBRATES en. growths arise from the walls of the air-sacs which burrow through the neighbouring tissue, even through bone, taking the place of the marrow and rendering the bones pneumatic. Such outgrowths may extend even into the terminal phalanges of the digits. They may also extend in amongst the connective tissue of the skin or between the muscles.1 THK LUNG IN FISHES In the typical fishes or Teleos'tei, which of all Vertebrates are the most highly specialized in adaptation to a purely swimming habit, one of the most characteristic organs is the swim-bladder or air-bladder. In its most highly developed form (in the physoclistic Teleosts) this consists of a closed sac, lying above — dorsal to — the splanchnocoele and filled with gas containing a large proportion of oxygen. Special developments of the lining epithelium provide a mechanism whereby the amount of gas in the organ can be increased by a process of secretion or diminished by a process of absorption. This mechanism, which is under the control of the nervous system, has for its main function the keeping the body of the fish at the same constant specific gravity as the water in which it is swimming — counteracting changes in its specific gravity which would otherwise result from variations of pressure due to change of depth, or from variations in volume of gas produced by fermentative processes in the alimentary canal. The air-bladder with its com- pensating mechanism keeps the fish precisely at the specific gravity of the surrounding medium so as to obviate the expenditure of muscular effort in order to keep at one depth such as is necessary in the case of a shark or other fish unprovided with an air-bladder. The air-bladder arises in development as an outgrowth of the wall of the alimentary canal behind the region of the gill-clefts. Tliis burrows its way backwards dorsal to the splanchnocoele and eventually attains to a large size. Its tubular connexion with the alimentary canal (pneumatic duct) becomes constricted across and severed so that the organ is completely isolated from the alimentary canal. In a good many cases however — namely, in the physosto- matous Teleosts — the duct persists and remains patent throughout life. In many fishes the dorsal wall of the air-bladder bulges out in a headward direction (Fig. 93) forming a diverticuhnn which may reach a great size so that in the adult tin- oruan has the appearance of ln-ing composed of two se^menls marked nff Irmn OIK; aimthcr by a constriction Fig. 93, !*>;, the pneumatic duct communicating with the hinder one of tin- The constriction may become tin aliovi- a'-roll II t \\:i> \\littrn ;i lull ;il|.| \\i-l 1 - i 1 1 ||.st r;i tc( I • II-M-J i ] it iiili ..!' I In- development «,: been published by Locy and Lu ../.•///•//. Ana' i HC, !!»!•; ;. These :uit Imi.s' ivsults amplify and in tin- main (-(infirm those ol'.luillct. Ill 167 FIG. 93.— Development of the air-bladder of a Teleost. (After Moser, 1904.) A. Hlnitli'.ns, 5 mm., longitudinal section; B, Rhodeus, 6 mm., longitudinal section; C, Rhodeus, 7 nun., transverse section, showing small pourh-liko outgrowth of pneumatic duct; end, endoderm; ent, enteric cavity; I, air-bladder; //. liver; .V, notochonl : /«•. pionephric chamber; /».(/, pneumatic duct >/, yolk. 168 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. to form a kind of diaphragm perforated iu its centre and capable of being thrown into vibration by air being forced from one chamber into the other so as to function as a sound-producing organ (e.g. Gurnards). Other outgrowths may develop : thus for example in many Siluroids numerous branched projections are formed along each side of the air-bladder. The air-bladder rudiment is at its first appearance in some cases approximately dorsal in position (Salmo). In Rhodeus Moser (1904) has shown that the diverticulum is at first on the right side of the alimentary canal. The same observer found that during the early stages of development of the air-bladder the portion of alimentary canal from which it springs undergoes a process of rotation about its long axis in such a direction that a point on its dorsal surface is carried towards the left side. Although the actual development has been worked out only in a few cases, we may infer safely from the adult relations (Rowntree, 1903) that the amount of this rotation differs greatly in different members of the group Teleostei. Thus in Siluroids and Cyprinodonts the glottis or pharyngeal opening is in the adult still' to the right of the mesial plane ; in others such as t^he genera Osmerus, Clupea, Chirocentrus it is practically median ; in still others such as Mormyrids, Characinids, Gymnotids and Cyprinids it has passed the mesial plane so as to lie upon its left side, while in the case of the Characinids Macrodon, Erythrinus and Lebiasina the glottis has come to l»e completely lateral on the left side. This rotation of the gut in the region of the glottis is of much morphological importance as will be shown later. In the young Rhodeus, 7 mm. in length, Moser finds that a well- marked diverticulum from the pneumatic duct is present (Fig. 93, C). Later on it gradually disappears. A similar diverticulum occurs in Salmo and in the Carp, and in all probability in numerous other Teleosts : its morphological significance will be discussed later. ACTINOPTERYGIAN GANOIDS. — In these fishes the development of the air-bladder takes place on similar lines to that described for Teleosts. In Amia the additional detail has been made out that the rudiment is at first in the form of a longitudinally placed groove which becomes constricted off from the alimentary canal from behind forwards just as frequently happens in the case of the typical lung- riidiment of air- breathing Yertehrates (Bashford Dean, 1896; Piper, 1902). A rotation of the section of alimentary canal in the n-j/iun of the glottis takes place similar to that which occurs in the Teleost. LUNG-FISH KS. — In the adult Ceratodus an organ occurs which is equally lumj ami air-bladder. It forms an unpaired sac lying dorsal to the splanchnocoele just like a typical air Madder, but the pneu- matic duct, instead nf ..prmn;_: dineily into the alimentary canal • urn! the riuht side and op.-ns by a Neutrally placed is. In Lepidosiren and /'/Wr//.s the -jvneral arraii-emeiit is Ill LUNG 169 the same except that line the organ is deeply bilobed: a right and a left lung or air-bladder occupying the place of the single organ of Ceratodus. The meaning of the ventral position of the glottis in these Lung- fishes, and, in fact, the morphological nature of the whole organ, is B FIG. 94. — Transverse sections through the endoderm of the pharynx showing an early stage in the development of the lung. A, Polyptei-ns, B, Ceratodus, and C, BomUnator (C after Goette, 1875). I, lung-rudiment ; ph, pharynx. demonstrated by the examination of early stages in development. In these the organ is found to be a perfectly typical /%7z#-rudiment (Fig. 94, B) — a mid- ventral projection from the pharyngeal floor of precisely the same kind as that found in tetrapodous vertebrates (C).1 Th r/cIT e-ff FIG. 95. — Views showing early stages of the lung-rudiment of Protopterus as seen from the ventral side (stages xxxii, xxxiv, xxxv). .ternal pll ; 7, luiis,' ; oes, oesophagus ; part, dorsal pancreas ; p.f, pectoral limb ; Th, thyroid ; '-.<•. visceral cleft rudiment. (Cut surfaces are indicated by uniform light tone.) Subsequent stages are illustrated by Figs. 95 and 96. The lung rudiment at first a rounded knob (Fig. 95, A) grows backwards and soon becomes bilobed (B). The figure does not bring out one im- portant fact namely that the lung-rudiment as it grows backwards 1 The projection is at first solid in the case of Lepidosiren and Protoptcnis. 170 EMBEYOLOGY OF THE LOWER VERTEBRATES CH. twists upon itself, in such a way that points upon its ventral surface would move towards the eiiihryn's right side. (In other words the lung -rudiment rotates about its long axis in a counter-clockwise direction as seen from behind, its front end remaining fixed.) The two lobes are the right and the left lung-rudiment but on account of the rotation just mentioned which extends through more than 180° the left lobe at this stagr represents what was originally the right side of the rudiment. The two lungs of Lepidosiren or Protopterus are thus reversed in FIG. 96.— Dissections of mid-gut of Lepidosiren at stages 32 (A), 35 (B), 36 (C), and 37 (D), showing the modelling of the intestine and also the later stages in the development of the lungs. S«-cii from the dorsal side. c.c, cloacal caecum ; int, intestiiif ; /./, IH't Inn/,: //, li\cr; /// ;<.n, p;mr: I'h. pharynx; /./. ri-lit liiii^ ; »/', splc.-n. position — the right lung of these I mi us l.rin^ homologous with tl it- left of other Vertebrates. An important detail is that in early stages tin- original right lung, i.e. the definitive left, is decidedly larger than its fellow (\''\x. 95, B). In later stages this ine([ualitv o8ition .simply hy ^i'd\\in- directly tail wards, the oesoplia^ns heinu pushed (nil nf the \\ay |u\\ards the left side (Graham Kerr. I'.'Ki . ilain sjii which doubtless in this respect retain Ill LUNG 171 the archaic mode of development, the lung-rudiment (Fig. 97, /) describes a spiral curve round the oesophagus so that the bifurcated hnbtiil - - oes. linililiil liiiiliinl FIG. 97. — Portions of transverse sections through a Lepidosireu larva (stage 34) to illustrate the changing relations of lung to gut from a short distance behind the glottis tail wards. In A the lung is ventral to the alimentary canal ; in B it is directly to the right ; in C it has become displaced dorsally ; while in D (where it is commencing to 1>ii'mrate) it has come to be mid-dorsal in position. A, aorti ; ijJ, gloinerulus of pronophros ; I, lung; N, notochord ; oes, oesoplia.mis. hinder end of the rudiment, which will give rise to the lungs in the restricted sense, comes to lie dorsal to the alimentary canal. - The lungs continue their tailward growth in the substance of the 172 EMBRYOLOGY OF THE LOWER VERTEBRATES < H. dorsal mesentery (Fig. 97, D) but eventually the portion of this mesentery containing the lung and dorsal to it becomes greatly thickened from side to side and finally merges completely in the roof of the splanclmocoele, so that in the adult condition the lungs lie completely outside the body-cavity — between it and the vertebral column. In Ceratodus (Gregg Wilson, 1901 ; Neuinayr, 1904) the lung is at first, as in the other two lung-fishes, ventral in position (Fig. 94, B) but in this case the originally left lung, which in Lepidosiren and Protopterus is for a time during development reduced in size, seems to have disappeared almost entirely, being represented only by a small and transient rudiment. Further detailed studies of the early stages in the development of the lung of Ceratodus are much needed to make clear the origin and fate of this vestigial left lung. But it seems clear from what is already known that the monopneumatic condition of Ceratodus has come about in evolution through the suppression of the originally left lung. As the lung completes its development, its cavity becomes en- croached upon by two median longitudinal ridge-like* ingrowths, one dorsal and the other ventral. It used to be supposed that these marked an incipient division of the lung into a right and a left half so as to bring about the condition seen in Lepidosiren or Protopterus — the monopneumatic condition being supposed to be the more nearly primitive. It will have been gathered from what has been said that this point of view is no longer tenable and that the mono- pneumatic condition of Ceratodus is to be looked on as secondary and not primary. CROSSOPTERYGIANS. — Of the two surviving examples of the Crdssopterygian ganoids — the most archaic existing members of the Ganoid-Teleostean stem — a few stages in the development of the lung have been investigated in Polypterus (Graham Kerr, 1907). In the earliest stage observed the lung-rudiment was in the form of a mid- ventral groove formed by an outgrowth of the pharyugeal lining (Fig. 94, A, /). This groove becomes deeper and towards its posterior end widens out ventrally so as to have a J_ -shape in transverse section. l'"steriorly the lung-rudiment grows back into a pair of horn-like pi -ojections — the rudiments of the right and left lung. These exit -IK I backwards in the connective tissue of the splanchnopleure and they very soon show a marked inequality in their rate of growth the left lagging behind the right. As growth goes <»n this inequality becomes more and more marked, so that in a larva of about, .">() mm. in Im^th the ri^ht luiitf extended right back to the cloaca \\liilc the Id'i pro jected back only about 3 mm. behind the glottis. In these later stages another important feature is to be noticed, one which is correlated with the fact that the air-lill»'ii.i«.n symmetrical ahoiit the median plane. Thus in the anterior jv^ion win-re h-.th Imej-s BT6 present ihe\ are Ill AIR-BLADDEK 173 situated laterally, balancing one another, while farther back where only the right lung is present this shifts towards the mesial plane until it is symmetrical about that plane, lying in the dorsal mesentery (Fig. 98, A and B). EVOLUTION OF THE AIR-BLADDER. — The facts that have been • -n undated above, with regard to the development of the lung in Dipnoan and Crossopterygian fishes, are of much morphological intrmst. When pieced together with what has been said regarding the development of the air-bladder of Teleostean fishes they afford • la l; i. from which the evolutionary history of the Teleostean air- Mjulder can be traced out with a high degree of probability. That history may be stated in a few words to have probably been as follows : 1. The primitive condition was that of a lung, communi- cating with the pharynx by a ven- trally placed glottis —for we have seen that the embryonic rudiment of the organ in the most archaic forms pos- sessing it is a typical lung-rudiment. 2. The organ became bilobed, growing back into a right lung and a left lung. 3. In the forms which took to a purely swimming existence, and became specialized in the direction of adaptation to this, there came about an asymmetry of the lungs, the right lung increas- ing and the left lung diminishing. Why this should have happened is not yet absolutely certain : it may probably have been in adapta- tion to active movements of lateral flexure, for we see the same thing taking place in Gymnophiona, Snakes and Snake-like Lizards. That it has been the right rather than the left lung which has increased in size, is probably correlated with the rotation of this region of the alimentary canal in a counter-clockwise direction as seen from behind (see p. 168) which would tend to interfere more with the circulation through the left lung than with that through the right, by lengthening the course of the left pulmonary artery. Steps Fio. 98. -Sections through the lungs of a larva of Polypterus 30 mm. in length. A, more anterior; B, more posterior; A, aorta; ent, enteron ; 1.1, left lung ; N, notochord ; opn, opisthonephi os ; p.v, pulmonary veins ; r.l, right lung ; v, interrenal vein. 174 EMBKYOLOGY OF THE LOWER VERTEBKATES CH. in the development of this asymmetry are seen in Polypterus and in the Lung-fishes. 4. In purely aquatic creatures the dictates of adaptation would naturally cause the air-filled lung to assume a dorsal position. An initial phase of this is repeated in Polypterus where the right lung has become dorsal and median in its hinder portion. In the Lung- fishes a further step is taken — the whole of the lung becoming dorsal except the pneumatic duct which still remains to mark out the path by which the lung moved dorsalwards round the right side of the alimentary canal. That the movement dorsalwards was round the right side was no doubt due to the right lung being predominant and the left reduced in size. In the case of Ceratodus the predominance of the original right lung has been retained, the other being completely obsolete except for a short period during development. In Lepidosiren and Protopterus, on the other hand, the lopsidedness disappears, the original left lung regaining during ontogeny its primitive equality in size with its fellow. 5. In the Actinopterygians — those fishes which show the highest degree of evolution in adaptation to a swimming mode of life — the lung has in the course of its evolution passed through similar stages to those exemplified by Polypterus and Ceratodus. Here again only the original right lung persists as the air-bladder, the vestige of the left lung being possibly represented by the little diverticulum found by Moser upon the pneumatic duct in early stages of development.1 In the Actinopterygians a further step onwards has been made in that the glottis has assumed a dorsal position. This is fully ex- plicable by the rotation which this part of the gut has undergone, aided no doubt by the principle of economy of tissue which would tend to bring about a shortening of the unnecessarily long pneumatic duct. In some cases there still persist vestiges of the ancient cellular respiratory lining of the swim-bladder (e.g. Z/ebiasina, Erythrinv*}. 6. Finally in the Physoclistic forms — the most highly specialized of all — the swim-bladder has become completely isolated from the gut, its respiratory function has gone and it subserves a mainly hydro- static function. The outline given above represents a scheme of evolution which in the light of modern research has a high degree of probability. Of course as in all such evolutionary speculations there exist details wh ifh are still difficult to explain. While most of the facts of com- parative aii.ihuiiy lit in well with it, some do not — such as, for example, the nerve-supply and the blood-supply <>!' tin- air-hladdcr of Amia — but it may be anticipated with considerable confidence that these diiliculiics will be lessened or disappear with tin- ]>r..-ress of research. , ],. l»iS. Tin- mait'-i- jillunls ;ui interesting sul'j'-. i for I'lirllu-r r.-M-an-li. Ill THYROID 175 DERIVATIVES. OF PHARYNGEAL WALL OTHER THAN THE RESPIRATORY' ORGANS THYROID. — The Thyroid gland arises as a mid-ventral outgrowth of the pharyngeal or buccal floor about the level of the Hyoid liiiiiiinl FIG. 99. — Sagittal sections through anterior portion of alimentary canal of Lepidosir< •// illustrating the development of the Thyroid. A, B, C from spffiim-iis of stage 30 ; D, stage 31 ; Th, thyroid ; t, ton.uur. arch. In those Vertebrates in which the pharyngeal rudiment is solid at this stage the thyroid outgrowth is also solid at its first appearance (Fig. 99, A, Th) and develops its cavity secondarily by cytolysis. 176 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. The Thyroid becomes gradually constricted off from the pharynx (Fig. 99, B and C) remaining for a time connected by a narrow stalk or duct with the pharyngeal or rather buccal floor just in front of the primary tongue (see Fig. 82, p. 149). This stalk of attachment becomes nipped across and the thyroid forms a mass (Fig. 99, D) or vesicle rounded in form or somewhat elongated in an antero-posterior direction lying in the mid-ventral line beneath the pharynx and just in front of the ventral aorta. The originally simple vesicle undergoes a process of sprouting and division by which it becomes converted into a mass of rounded vesicles, each possessing a wall composed of a single layer of cubical epithelial cells and separated from its neighbours by highly vascular mesenchyme which penetrates in between the vesicles to form the stroma of the organ. During later development the Thyroid undergoes characteristic changes of form in different subdivisions of the Vertebrata. Thus in Teleosts it frequently assumes a more or less diffuse character, the follicles being distributed in the neighbourhood of the ventral aorta and roots of the afferent branchial vessels. In the- Amphibia and Amniota the organ becomes deeply constricted into two laterally placed lobes which may remain connected or may become separated, so that it assumes a paired character as happens in Amphibians and Birds. With the processes of differential growth involved in the develop- ment of the neck, the thyroid may undergo considerable displace- ment from its point of origin. Thus in adult Lizards it lies across the trachea well forwards from its hind end while in other reptiles and in birds it lies farther back close to the roots of the great arteries. It is riow generally accepted that the clue to the phylogenetic history of the Thyroid is afforded by its development in Petromyzon (W. Miiller, 1871). Here there develops a mid-ventral outgrowth of the pharyngeal floor, forming a short gutter in the branchial region, the lining of which is composed partly of glandular cells which secrete a sticky mucus and partly of cells which bear powerful flagella. Morphologically this gutter is the same as the endostyle of Amphioxus and during larval life its function is also similar : it appears to be in fact simply a shortened up endostyle. The slit-like pharyngeal opening becomes gradually reduced in length till it forms merely a small pore. At the time of metamorphosis the pore becomes obliterated so that the organ becomes a closed vesicle underlying the pharynx. Tln> vesicle divides up into a number of small vesicles and its mucous secretion accumulates in th«-ir interior as a colloid subst;in< ••• lik<; that of th<- Thyroid vesicles of the Gnathostomata. In a word, the endostyle of tin- Ammocoetes stage becomes the Thyroid of the adult, and ther<- no n-ason to douht that the same has in |ihyl"^«-iiy and that the thyroid of the YiTtvhruti' is in BBANOHIAL BUDS 177 .simply (lit- modern representative of the endostyle of the proto- chordate ancestor. An interesting feature is that while the physiological importance of the thyroid in the modern Vertebrate is that of a ductless gland for the production of internal secretion to be absorbed by the blood, it still goes on producing the mucous material used by the far back protochordate ancestor for entangling food particles, though that substance is no longer, owing to the disappearance .of the duct, (lisc.luirm'd into the pharyngeal cavity. BRANCHIAL BUDS. — There make their appearance in the develop- ing Vertebrate a series of bud-like proliferations of the endodermal epithelium of the branchial clefts which may be known as branchial buds. They appear at the upper and lower angles of the clefts and the series shows its fullest development in the Lampreys, where buds develop at the dorsal and ventral angles of all the clefts. In the majority of fishes investigated they have been found to appear at the dorsal angles of all the clefts except the first; in Urodele Amphibians at the dorsal angle of all clefts and at the ventral angle of II., III. and IV. ; in Anura at the dorsal ends of I. and II. and at the ventral ends of II.-V. ; in Lacerta at the dorsal ends of I.-IIL and the ventral ends of III. and IV.; in Gallus at dorsal and ventral ends of III. and IV. The morphological significance of these organs is still completely obscure. Physiologically some of them appear to be of importance during the later stages of development preceding sexual maturity inasmuch as they give rise to that often bulky organ the Thymus. This arises by the fusion together of more or fewer of the dorsal buds, the others undergoing no further development. Thus in Lepidosiren (Bryce, 1906) dorsal buds III. and IV. develop into thymus while II. and V. undergo no further development : in Ceratodus (Greil, 1913) II., III. and IV. give rise to Thymus while V. and VI. do not develop further : in Hypogeopliis II., III., IV. and V. give rise to Thymus while rudiments on I. and VI. atrophy. In regard to the much discussed histogenesis of the thymus all that need be said here is that the originally solid epithelial rudiment becomes in the course of development loosened out into a sparse reticulum interpenetrated by mesenchyme richly traversed by blood- vessels and crowded with leucocytes. The ventral buds, where they occur, become constricted off from the branchial epithelium forming simple rounded masses of epithelial '•ells (Amphibians) or they may be subdivided up by intrusive con- nective tissue into solid portions (Eeptiles) or hollow vesicles (Birds). The small organs so formed are termed by their discoverer Maurer epithelial bodies : their physiological significance is quite unknown. There normally develops in the Vertebrate either on both sides or only on the left side a small pouch-like diverticulum of the pharyngeal wall close to the ventral edge of the last gill cleft, what- ever the number of this be in the morphological series. The VOL. II N 178 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. di vertical u m bee.omes separated from the pharynx and commonly gives rise to numerous rounded vesicles somewhat resembling those of the thyroid in appearance. The organ thus formed was named by van Bemmelen who discovered it in Elasmobranchs — suprapericardial body — while Maurer lias termed it the postbranchial body. Nothing is definitely known regarding either its function or its evolutionary history, though it is sometimes regarded as representing a vestigial last gill-pouch. A curious point is the tendency of the organ to unilateral development as it makes its appearance only upon the left side in a large number of cases (Acantkias, Lepidosiren and Protopterus — see Fig. 109, B — most Urodeles, some Lizards). CEMENT ORGANS OF TELEOSTOMATOUS FISHES. — It has long been c.o B FIG. 100. — Ventral views of Polypterus larva to show the cemeiit-orgaiis. A, Stage 30 ; B, Stage 33 ; c.o, cement-organ ; e.n, olfactory organ ; /«, mouth ; V, ventricle of In-art. known that the. larvae of Aetinopterygian ganoids possess cement- organs on the head in front of the mouth. Balfour (1881) \\n>te "I this as "a very primitive Vertebrate organ, which lias disappeared in the adult stair of almost all the Vertebrata; but it is probable that further investigations will show that the Teleostei, and especially the Siluroids, are not without traces of a similar structure." The, organs in question were Ljvnrralh regarded as being developed from a thickening «,f the ret nderm. Miss 1'helps (IHD'.I lii>t stated that they "i i^mated from endoderm (Ainirk, was greatly surprised to tind himself' forced to this same eonelusinn by the examination of I'.ud. iierial of 1'nlypteru*. The c.-iiient OTgaO "I /'<>! ///>/«' /• x s ( ( \ ra 1 ia in Keri\ I'.MMjand 15)07), uh'-ii at the height of its develcpnient.. InNns a stnin cylindrical -tiueture with a deep hollow at its free end. prcjeetin^ fmm the Ill CEMENT-ORGANS 179 head on each side as shown in Figs. 100, A, and 197, C, c.o. A longi- tudinal section through the centre of tlie organ at about this stage (stage 26, Fig. 101, E) shows that the organ is covered by the ordinary ^-layered ectoderm. Round the lip of the opening at its I I c.o: ^B Ki<;. 101. — Illustrating the development of the cement-organ of Polypterus. B represents p;irt of a transverse section, the other figures portions of horizontal sections. A and B, sta^e 'JO ; C, stage 23 ; D, stage 24 ; E, stage 2C>. r.o, cement-organ. The darker tune indicates ectoderm. live end, the superficial layer of ectoderm stops, while the deep layer set ins to dip down as a dee]) involution to form the secretory epithelium (c.o) which lines the cavity. All the appearances seem to point to the secretory epithelium being ectodermal in its nature. How deceptive these appearances are will be gathered from an inspection of Fig. 101, A-E. 180 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. The tii si rudiment of the organ is seen to be a simple pocket-like outgrowth of the gut -wall (A, c.o) : this becomes more and more prominent (B, C) : it becomes gradually constricted off at its base from the gut-wall, its cavity becoming isolated first (D). Finally it separates completely from the main endoderm and its outer end undergoes fusion with the deep layer of the ectoderm. Its cavity then opens to the exterior and the fully functional condition is reached — the endodermal origin of the secretory lining being for a time betrayed by the conspicuous persistent yolk granules in its cells. It will be noted that the exposed side of the secretory epithelium, that on which the secretion is extruded, is that which originally faced inwards towards the lumen of the alimentary canal. In other words the direction in which the extrusion takes place is morpho- logically the same as that of any other part of the glandular lining of the gut- wall. As is the case in other forms the cement-organ is a transient, purely larval, structure. About stage 31 (Fig. 197, D) degeneration commences : the gland shrivels up, the gland-cells becoming more slender and dark pigment making its appearance in their interior, the epithelium becomes penetrated by ingrowing blood-vessels, its cell-boundaries become indistinct. The process of atrophy goes on rapidly and by stage 36 (Fig. 197, F) the organ has completely disappeared. An interesting variation from the normal course of development is found in specimens in which the cement -organ rudiments are more or less approximated to one another. This variation reaches its maximum in occasional individuals in which they are completely fused and form an unpaired structure, continuous across the mesial plane. In the actinopterygian Ganoids the cement-organ develops along the same general lines as those just indicated. In the Sturgeons the development has been worked out recently by Sawadsky (1911) in Acipenser ruthenus. Here the organ forms a rounded projection, very much in the same position as that of Polypterus, but in this case each becomes divided by a groove so as to form two rounded knobs. These knobs eventually grow out to form the tactile barbels of the adult, the secretory epithelium being carried out on the surface of the barbel as it grows. The secretory epithelium is here also endodermal, its rudiment being the gut- Wall immediately dorsal to the position in which the mouth will develop later and bein^ continuous across the mesial plane. The unpaired condition which occurs in /'i>/i/-/iterus as a variation is thus normal in the ease «»f the sturgeon. As the head increases in length the BeCFOtOIJ epithelium becomes carried out on . ntral surface, looking just as if it were the thickened ectoderm of this surface. Finally the paired condition comes about, the lateral part- ol' the BecretOTy epithelium coining to be supported by the knob like projections already mentioned. in CEMENT-OKGANS 181 Amia is of special interest in regard to its cement-organs as it was in this form that their endodermal origin was first announced.1 The organs arc for a time in the form of a pair of rounded knobs, one on each side, hni these take on a crescentic shape so that together they form a circular wall, interrupted anteriorly and posteriorly. Kaeh organ contains a pocket-like projection of the gut-wall which lakes on a somewhat sausage-like form in correlation with the curved shape of the organ as a whole. This endodermal sac separates from the main endoderm and becomes constricted across, so as to form a curved row of closed vesicles from six to ten in number. Each vesicle fuses with the ectoderm and develops an opening to the exterior so that it takes on the appearance of a cup at first deep and narrow, later shallow and wider, its Lining continuous with the deep layer of the ectoderm. When the larva reaches a length of 13-14 mm. it makes less use of its cement-organ and the latter commences to degenerate, sinking , beneath the surface with which, however, it remains connected by a narrow tubular channel. By about the 20 mm. stage this has disappeared and soon there is no trace of the organ to be found even in sections. In Lepidosteus the organ appears to be similar while in the other ganoids its development still remains to be worked out. These cement-organs are of special interest and importance for more than one reason. In the first place they are of importance in revealing a quite unexpected pitfall in the way of the investigator trained to have implicit faith in the germ-layer theory, for they show how a particular organ may become transferred from one germ-layer to another even though not belonging to the transitional zone where the two layers are continuous. A very common modification of ontogenetic development consists in the slurring over or even omission of particular -stages in early development. "Were this to happen in the case of the early stages in the development of the cement -organ say of Polypterus, it is easy to see that the organ might have every appearance of being purely ectodermal in its nature, although it is, as a matter of fact, endodermal. It appears to the present writer quite possible, if not probable, that this modification has actually come about in the Dipnoi and Amphibians, and that the cement-organs of these groups, although they develop from the ectoderm in those forms which have been investigated (p. 79), are really homologous with the cement-organs of the Teleostomi, their endodermal stage having been eliminated from ontogenetic development. Further investigations are needed in the Amphibia — to see whether no trace exists, in any member of the ,nTOup, of an original connexion with the endoderm. As regards the original nature of these organs it is impossible to arrive at any certain conclusion. Arising as they do in the form ps (1899). The actual discovery seems to have been made by (.'I'. ItYigliard and IMu-lps (1908). 182 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. FIG. 102.— Larva of Sarcodaces odoe. (After Budgett, 1901. inent-organ. of endodermal pockets, they obviously recall gill pouches on the • •MI- hand and cnrl.-ntn -ie pouches on the other. Their position suggests a pair of premandibolar Lrill pouches: their function, that of forming an excretion (cement), perl laps indicates rather coelomic affinities and the present writer suggested (1906) their possible correspondence with premandibulai head cavities of other Vertebrates. Reighard and Phelps (1908) homologize c.o. — rr' ' " i~7 - - --- x them with the anterior .pair of head -cavities of Elasmobranchs while van Wijhe (1914) sup- ports a homology with the ciliated organ of Amphioxus. Altogether these cement-organs are very interesting and puzzling structures and would well repay further investigation. A thorough comparative study should be made of their development in the archaic Crossopterygians and of their possible, homologues in Elasmobranchs. Little is known regarding cement-organs in Teleosts, though it is probable they will be found to occur in various tropical fresh- water fishes. Budgett (1901) found a large cement - organ on the head of the larva of the Characinid Sarcodaces odoe (Fig. 102, c.o). In a larva believed to be that of the Mormyrid Hypero- pisus bebe he found six well-marked cement glands on the head which in this case secrete fine threads by which the larva hangs suspended in thp water until tin- F"- 103.— Teleosti-mi larva.-. Mi|M...M-.l to W tln»st- of HyptropiwbJK, suspended from the rootk-ts in UN V'.lk 18 Used up (1<1-. nes£ (From Budgett, 1901.) 103). Il» lerotis and Gymnarch.il* aN«. |><>--rss similar organs — very small in tin- latt« i • •tun, 1907). Tin; orjjans in tlir-M- \;iri iufonnataon as t<» whfl h,-r. as may \»- siisprdcti, tlicy really originate IVom the DICKSTIVK TKACT. — Tin- ivs].iral«.ry region of the alimentary canal seeded by ihe true «li^i-sti\r trae.t and this shows "t leSfl pniiuMincmd (lillnviii i;it KMI intn siieeessixe |n»niniis Ill THE ALTMENTAKY CANAL 183 oesophagus, stomach, intestine and its subdivisions, cloaca. In correlation with the digestive and assimilative function of the intestinal endoilrrm this serves during early stages }IH the favourite storehouse of food -yolk, and the concentration of yolk in the abapical portion of the unsi'gmented egg is to be looked on as a foreshadowing of the fact that this portion of the egg will later become the endoderm. In the holoblastic Vertebrates the mass of heavily yolked endo- derm cells becomes. ,is it \vnv, modelled into a tubular shape by the Kn;. 104.— -Illustrating the modelling of the yolk in Icht/iyophis. (After Sarasins, 1889.) A and B illustrate the same stage, B representing a view from the dorsal side.. The small-celled epithelial portion of the gut-wall is seen passing down the centre of Fig. B. C, D, and E represent lain- stages drawn from the vent nil side ; F (7 cm. embryo) ventro-lateral view from the right side. reciprocal activity of endoderm and splanchnic mesoderm; the rudiment so formed undergoing active growth in length and differentiation of structure while the yolk is being assimilated. In the two most archaic groups of holoblastic gnathostomes, the Crossopterygians and the Lung-fishes, a feature of special interest is the development of the spiral valve. In Lepidosiren, as is indicated by Figs. 105 and 106, this takes its origin by the solid mass of yolk-laden endoderm becoming modelled into a right-handed spiral coil — the deep incision which separates successive turns of the spiral being filled up by ingrowing mesenchyme belonging to the splanchnic mesoderm. There can be little doubt that this is a secondarily modified mode of development, but nevertheless it is probable that the spiral coiling of the endodermal rudiment is to be 184 EMBEYOLOGY OF THE LOWER VEETEBRATES CH. explained as a repetition of an ancestral condition in which the intestine as a whole was long and spirally coiled. An important feature of such ;i spiral coiling of the gut rudiment is that it would ii"cess,n ily tend to bring about a twisting of the ..-V \'i'.. ]();,. hi-,-, ti<,i,> ,,f yoiiii- L.-pi.Iosiivns «•! lUges -TJ A , :;f, ( H), M (C), ami 37 Irom tin- ventral side to show tin- modelling of the inti-stiiir. g.b, jjall-I'lu.l.lei-: //, liv.-r: I', v.-nti i.-l,-. .iliiii.-ritai y i-aiial just, in front of the spirally coiled port,i«»n in a counter-clockwise direction as seen I'mm behind, *.«. a movement in which points «»n 1 he ventral side nf t.he aliinentai-y canal would [lifted towards I In- ri^lil side. A- already indiealed such atwistm-j"! thU region o|' |h<- alini.-iilary canal ad ually docs lake Ill THE ALIMENTAKY CANAL 185 place in development causing the lung rudiment to shift dorsally round the right side of the alimentary canal. In the more richly yolkrd Vertebrates the ventral portions of the gut-wall are more and more clogged uj» with yolk and this results in a greater and greater concentration of de-^ veloprnental activity in the dorsal wall. This is cfearly in- dicated by transverse sections through the developing gut ot Vertebrates which though rich in yolk are still holoblastic. Such sections (Fig. 107) show the dorsal wall of the gut to consist of small active cells ar- r.-mged as a columnar epithelium, while the side walls and floor consist of large comparatively inert yolk -laden elements. It is only as development goes on, and as the yolk is consumed, that the epithelial small-celled character gradually spreads ventrally. In the actually meroblastic Vertebrates, the heavily yolked portions of the primitive gut- wall never undergo segmenta- tion at all, unless possibly as regards a thin superficial layer. They remain as a continuous mass of yolk, round which the epithelium gradually spreads. In this case the formation of all the important organs of the ali- mentary canal is concentrated in the dorsal portion which be- comes gradually folded off from the main mass of the yolk. This folding -off process takes place most actively in the anterior region, so as to form the tubular fore -gut, and also posteriorly, the intermediate portion re- 1- 1 5 I 5 6 || rH .^T 186 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. maining for a time as a longitudinal groove opening ventrally towards the yolk. As the lips of this groove gradually coalesce at each end the communication between the gut cavity and the yolk becomes gradually narrowed down to the tubular cavity of the yolk- stalk situated at first behind the liver but later becoming shifted forwards by differential growth. Eventually this becomes obliterated and the definitive alimentary canal becomes completely isolated from what remains of the yolk. In many Teleostean fishes this isolation takes place at a very early stage in development. The alimentary canal is, in correlation with its digestive function, necessarily a highly glandular organ. Primitively the secretory functions are carried out by unicellular glands, scattered about amongst the other epithelial cells of the endoderin, but in the Verte- brates, as in all the more complex Metazoa, special concentrations of gland cells and of secretory activity take place in localized portions of the enteric wall. Each of these specially glandular patches under- goes a great increase in its area, which causes it to bulge outwards as a simple or much subdivided Fi,,107.-Transversesertion through hind an(J Complicated pocket, forming portion of intestine of a larva of ich- a distinct glandular appendage of //- siren and /'/w»/>/o"w,s) as a solid knob of yolk-laden cells (Fig. 105, li). Tin- lapiilly in M /.e, as it usus up its food-yolk, and becomefl COHStricted <>lf from tin; main mass of yolk by inuTowin^ 'ichyme, until its at lachment becomes narrowed do\\n to a slender itolk— -the rudiment of tin- bile-duct. The pouch-like rudiment of the li\ei undergoes an active pr<>< of sprouting into numerous secondary pockets, each of \\hich becomes 111 LIVER 187 greatly elongated and branched, and gives the gland a tubular character. This character may be retained throughout life (Lampreys) but normally the tubules undergo anastomosis so as to form a net- work of trabeculae. While this is to be regarded as the primitive mode of development of the tubules it is to be noted that they more usually in actual fact show the modification of development which we have learned to associate with the presence of yolk, being at first solid and taking their origin not by a process of outgrowth but rather by a process of modelling by ingrowing mesenchyine. In the meroblastic Vertebrates also the liver may be described as originating from a mid-ventral outpushing of the enteric wall. Variations occur in detail, in correlation with the varying relations of the hepatic portion of enteric wall to the fore-gut and yolk-sac. If this part of the gut-wall has already been folded off from the yolk-sac and incorporated in the fore-gut, then the early stages of development of the liver diver ticulum pursue their normal course. If, on the other hand, it still forms part of the yolk-sac wall, the hepatic rudiment makes its appearance as a projection from this, and it may be in its first beginnings paired, its two halves separated by the longitudinal slit by which the cavities of the definitive gut and the yolk-sac are still continuous. ELASMOBKANCHII. — The hepatic diverticulum at an early stage bulges out to form a conspicuous outgrowth on each side anteriorly —the rudiments of the right and left lobes of the liver. The median portion between these becomes in its anterior region converted into secretory tissue while its posterior part becomes the bile-duct, with its dilatation the gall-bladder. In Acanthias (Scammon, 1913) the first rudiment of the liver, which makes its appearance at a time when this region of the enter on is not yet floored in but opens freely into the subjacent yolk-sac, is distinctly paired. In view of the unpaired condition in Amphioxus and the holoblastic Craniates there can be little doubt that this condition in Acanthias is a secondary modification as indicated above. Secondary pockets soon make their appearance on the wall of the secretory portion of the rudiment, and grow actively into elongated and much -branched tubules. These fuse together secondarily to form the network characteristic of the fully developed liver. This network is bathed by the blood of the vitelline veins (see Chap. VI.). After the embryo (Acanthias) has reached a length of 25-28 mm. the walls of the tubules, or trabeculae of the network, increase greatly in thickness so that both their own cavities and the intervening blood-spaces become relatively reduced and the organ assumes the compact definitive condition. Whereas the tubules become throughout the greater part of their extent secretory in function the proximal portions, each common to a group of tubular branches, function merely as ducts. These communi- cate with the main bile-duct formed from the posterior and median 188 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. cnt. b.d.1 bd.2- td.2 ent Ki-,. 10*. Illi; !y l tin- livi-r in Minis. A, 47-lionr Hih-k : H. .V_- hour chirk : < '. .Mi-h our chick (after Hr-.m. D I'.iiith-luy rln.-k ; K, 7 nun. ••mbry, of Din KoM-at.- I Hamiu - '. iii'liiii.-nt ..! ani.-imi , ilnct ; ' \it\ ..i 1 ; '/'-, niiliiiioit ul'^'ill-l. ni. -i n >r -I rii'liini-nt of pancreas. portion of the rudiment. The Lcall Madder originates as a bulging of the floor of the bile-duct towards its anterior end. The formation of the pos- terior and longer section of the bile-duct, which will be extrahepatic in the adult, lags in its development behind the anterior portions of the rudiment. Such differences in the time of appearance of different parts of the hepatic apparatus — liver, gall-bladder, bile-duct — are to be looked on as mere secondary modifica- tions of development, — the primitive condition being that of a simple pocket of the gut- wall such as persists in Am- pliioxus. SAUROPSIDA. — The hepatic apparatus here again makes its appearance as a longi- tudinally situated pocket of the morphologically ventral wall of the gut. In birds this is situated at first on the anterior wall of the yolk-stalk (Fig. 108, A). The diverti- culum grows actively into an anterior (dorsal) and a pos- terior (ventral) pocket (Fig. 108, C, li. 1 and li. 2) while the intervening portion In- comes flattened out and incor- porated in the gut- wall. There thus come to be two ilistinct liver -rudiments an anterior and a posterior. Of t.hrse each sprouts out at its • •IK! into invii-nlar projcrt ions \vliic.li r\vntually fuse and form a sj.oimy mass, surnumd- inu tli«' cavity of tin- duct us venosus. and having in it's MM' IH-S Mood-spares which 111 1'ANCKEAS 189 commiiiiiejite with the just - mentioned vessel. This spongy mass, the trabeculae of \\hirh ;nc ;it lirst solid and only secondarily develop a lumen, forms the secretory portion of the liver, while the proximal portions of the outgrowths persist as the two con- spicuous bile-ducts of tin- adult bird (Fig. 108, D, E, Id. I and Id. 2). In such birds as possess a gall-bladder this is formed by a dilatation close to the point of junction of the posterior bile-duct with the gut-wall (Fig. 108, 1), E, gV). PANCREAS. — The pancreas, though in the adult a single structure, arises typically from three distinct rudiments, each of which is at first a simple pocket-like outgrowth of the splanchnopleure. One of the rudiments (cf. Fig. 80, H) is situated dorsaily a little posterior to the stomach, the other two, which appear somewhat later, are ventral and arise as outpushings of the hepatic diverticulum in the region of the bile-duct. The ventral pancreatic rudiments are commonly paired, arising one on the right and one on the left of the bile-duct. The three rudiments increase in size, secretory tubules sprout out from them and the two ventral rudiments become carried in a dorsalward direction, up the right side, by the rotation which the gut undergoes in this region (see p. 168). -The right ventral rudiment comes in contact with the dorsal rudiment and fusion takes place — all three rudiments forming a single organ the three-fold origin of which is indicated by its three communications with the alimentary canal. Such may be considered the typical mode of development of the pancreas, but important variations in detail occur in the different groups. In Cyclostomes and Elasniobranchs only the dorsal pancreas is known to occur. Its development in the former group requires further investigation. In Elasmobranchs it arises as a longitudinal groove of the enteric wall dorsaily and a little posterior to the open- ing of the bile-duct. It becomes constricted off from before backwards and in accordance with the rotation of the alimentary canal it becomes shifted to the left side and ends up by being ventral. In Crossopterygians the three typical rudiments appear (Fig. 80, H) but their development has not been followed in detail. Eventually the pancreatic complex extends forwards beneath the liver and com- pletely fuses with it forming a thick layer over its ventral surface in the region near the opening of the bile-duct. In Actinopterygian Ganoids also (Piper, 1902 ; Nicolas, 1904), the pancreatic complex derived from the original three rudiments becomes fused with the substance of the liver, only its posterior dorsal portion remaining extrahepatic. The main duct of the pancreas is 1 he persistent stalk of the right ventral rudiment which opens into the gall-bladder formed by the dilated terminal part of the bile-duct. Of the two other pancreatic ducts the left ventral apparently atrophies entirely, while the dorsal is said in the case of Amia to disappear but in the Sterlet (Acipenser ruthenus) to persist. 190 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. In Teleosts the early development agrees closely with that of the ganoids, only a doubt exists whether the definitive pancreatic duct (Duct of Wirsung) may not be formed by a fusion of the two ventral ducts rather than by the persistent right duct alone. During later at differences arise between different members of the group. In >ome (Silurus, Esox) the complex forms a single compact gland, in others (Scomber, Cyprinus) it becomes divided into a number of independent lobes, in others, including the majority of the more familiar Teleosts, it becomes greatly branched and is diffused in the substance of the dorsal mesentery while in still others (Labridae, Syngnathus) the condition resembles that of the ganoids a large part of tiie organ being intrahepatic (Laguesse, 1894). B Fi<.. 109. — Dorsal view showing indiments of dorsal pancreas and lung in larvae of Protopterus (stages 32 and 34). /, lung; , open-ilium ; ]*i.d, dorsal pancreas ; p.b, postbranchial body; p.f, pectoral limb; ,il deft rudiment. I n I. u HIT- fishes the three typical rudiments make their appearance. In Protopterus tin- dorsal rudiment (see Fig. 109, A,pa.d) is a solid oin growth .'hollow in Lepidosiren) from the gut- wall, usually rounded in form but occasionally elongated in an antero-posterior direction the specimen figured ( Kig. 109, A). The attachment to the gut beenmes rapidly constricted to a narrow stalk and a cavity ops in the interim- of the rudiment. The ventral rudiments appear a littl.- later, M .-olid projections one on each side of the aitachment. of the bile-dnel to the ^uf. The two ventral rudiments. ;is th« \ UKireBM m si/e, meet and fuse dor.sil to the bile-duet, and later on the dorsal surface of the ri^ht \entral rudiment mines in COBtecl and fuses \\ith the y.-ntra! M:I face of the dorsal rudiment. The stalks of the three rudiments remain as three ducts, the l\\o \ent r, il opemirj JUSI |". -i. -nor (original ri^hl i iidmieiit ) ami antei ior 111 PANCKEAS 191 (original left) respectively to the opening of the bile-duct, while the dorsal opening is situated at the extremity of the spout-like pyloric valve. The general course of development in Lepidosiren is similar and in both it is characteristic that the pancreas never bul^-s ln-yond the mesodermal coating of the splanchnopleure. It remains embedded throughout life in the gut-wall and is consequently not noticeable in an ordinary dissect inn. In Ceratodus (Neumayr, 1904) the development of the pancreas is similar though here the left ventral rudiment, which in Protopterus is smaller in size than the right, remains rudimentary. The Amphibia are of special interest from the fact that it was a member of this group (JBombinator) in which Goette (1875) first observed the origin of the pancreas from three separate rudiments. Goeppert (1891) was able to extend the observation to various other Amphibians, both Urodele and Anuran, and to show that in Urodeles the dorsal rudiment retains its duct, opening just behind the pylorus, while in the Anura this duct disappears. In both cases the ducts of the two ventral rudiments undergo fusion to form a duct of Wirsung which opens into the bile-duct. In Keptiles (Lacerta — Brachet, 1896) the right ventral and the dorsal rudiments fuse to form the definitive pancreas, the left ventral atrophying (cf. Lung-fishes). According to Brachet the duct of the dorsal rudiment does not disappear but fuses with that of the right ventral to form the definitive pancreatic duct. Birds show three rudiments which undergo fusion into a complex in the normal fashion, all three ducts remaining functional and conspicuous in the adult. Suppression of the left ventral rudiment occurs as an occasional variation. The observed facts of development of the Pancreas clearly justify the conclusion that this organ of the modern Vertebrate has arisen in the course of evolution from three originally separate diverticula of the glandular enteric wall — a pair arising from the hepatic pouch and the third from the dorsal wall. The precise localization of the rudiments at comparatively distant points of the enteric wall point to the probability that the nature of the secretion was originally different in the case of the ventral pancreas from that of the dorsal. PYLORIC CAECA. — The caeca which are present in the pyloric region in many actinopterygian fishes arise as simple outgrowths of the gut-wall. The interesting suggestion has been made (Taylor, 1913) that the simple circle of these caeca, which is apparently their most primitive arrangement, corresponds morphologically with the curious valve found in various fishes (Amia, Lung- fishes, Symbranchus, Anguilla, etc.) in which the pyloric end of the stomach is prolonged back into a kind of spout which is ensheathed by the anterior end of the intestine. The circular prolongation forward of the intestinal cavity round the gastric spout might clearly give rise to a circle of pyloric caeca simply by subdivision into a number of separate 192 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. portions each of which continued to open into the gut cavity at its hinder end. RECTAL GLAND. — This organ, which occurs in Elasmobranchs, arises as a simple pocket-like outgrowth of the gut-wall. The super- ficially similar caecum of Lung-fishes will be dealt with in con- nexion with the renal organs. CLOACA. — In the more archaic Vertebrates the ducts of the excretory organs open into the terminal part of the intestine which is thus a cloaca. It is believed by many that the excretory ducts originally opened at the hind end of the trunk independently of the alimentary canal and it is natural to suppose that the openings of the ducts have become gradually shifted first into close proximity to the anus and finally on to the lining wall of the ali- mentary canal. This again suggests that the cloaca may really be a proctodaeum — that the skin has been involuted to form its lining and that with this involution the renal openings have also been carried inwards. Unfortunately the facts of ontogenetic development do not so far as can be seen at present fit this simple and attractive hypothesis. The cloaca is, except for a small portion close to its opening, of purely endodermal origin — the renal ducts open on what is part of the primary enteric wall. A suggested explanation of this fact differing from that mentioned above will be found in the chapter dealing with the renal organs. A cloaca seems always to be developed though in some cases (e.g. Teleostean fishes) it flattens out and disappears later so that the renal organs and the gut come to have independent external openings. The bursa Fabricii, a conspicuous glandular appendage of the dorsal wall of the cloaca in young birds, has usually been regarded as proctodaeal in its origin but it is now known to arise in ontogeny from vacuolar spaces in a solid projection from the cloacal rudiment, dorsal to the stalk of the allantois (Wenckebach, 1888) and would therefore appear to belong to the mesenteron rather than to the proctodaeum. The anal opening of the Vertebrate, as may have been gathered from Chap. II., is to be regarded as representing morphologically a portion of the gastrular mouth or protostoma. In a large number of Vertebrates however the opening arises in ontogeny not in this way but rather as a secondary perforation, although even in such t IK- perforation arises in the line of the closed protostoma. TEMI'OKAKY OCCLUSION OF T1IK ALIMENTARY CANAL.— The ali- jiientary canal is, in correlation with its function, a hollow tube. In a large number of Vertebrates, ho\\rver. then- ;nv more or less extended periods of development dunn- which the cavity is com- plete 1 i. either throughout tin; length of the canal or in c.-ruin porti< In its Hinple.-i condition this ...-curs as a special case of the in THE ALIMENTAEY CANAL 193 temporary absence of luinen so frequently found in the development of eventually hollow m^ans from a richly yolk-laden rudiment. An idea of how it has come about will be got from an inspection of the various stages of the development of the alimentary canal of Polypterus as shown in Fig. 80 on p. 146. During early stages the archenteric cavity is seen to be widely patent throughout, except that there is no mouth opening. During the later stages of develop- ment, inn i KM I lately prior to the canal becoming functional, its walls throughout the region between the fore-gut and the cloaca become closely apposed. so as almost entirely to obliterate the cavity. Later on the walls recede from one another and the lumen becomes again patent. It would obviously be merely a slight accentuation of this modification of development for the cavity to be completely obliter- ated for a time. A still further modification would be brought about by the omission altogether of the original hollow stage from the ontogenetic record. This actually occurs in the case of the fore-gut in those Vertebrates in which this region of the enteric rudiment is yolk -laden : where, on the other hand, the yolk is practically completely concentrated in the mid-gut region as in meroblastic Vertebrates it does not occur as a rule. The most striking temporary occlusions of the alimentary canal during development have to do with its terminal apertures. Thus there is not a single existing Vertebrate, so far as is known, in which the mouth opening persists from the gastrular stage, or in which even any connexion has so far been traced between the definitive mouth opening and the protostoma. In every case, even in Amphioxus, the mouth opening develops comparatively late as a secondary perforation. This modification of development is in the present writer's opinion to be attributed to the entire dependence of members of the Vertebrate phylum upon food-yolk during early stages of their development, the need for a functional mouth having thus disappeared. The auteroposterior extent of this occlusion of the alimentary canal in the region of the oral opening differs in different sub- divisions of the phylum. It may include a large part of the stomodaeal as well as the endodermal portion of the buccal cavity as in the Lung-fishes (p. 148) but more usually it is confined to the boundary between the two, i.e. to the site of the original mouth opening the closely apposed ectoderm and endoderm being at this level continuous across the site of the future opening as the velar mi'inbrane (p. 145). The secondary perforation by which the alimentary canal comes to communicate with the exterior at its front end is in the case of some larval Vertebrates (e.g. Lepi- dosiren} closely correlated with the commencement of pharyngeal respiration but where the development is embryonic it commonly still takes place long before the existence of any obvious functional need (e.g. Chick, fourth day). At its hinder end the archenteron is, VOL. II 0 194 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. as has been shown in Chap. I., widely open to the exterior in all the lower Vertebrates during rarly stages and in various cases this opening can be traced either into direct continuity, or into less direct but still clear relationship, with the anal opening. The explanation of this lesser degree of modification of the development of the anal opening as compared with the mouth may probably be associated with the less accentuated delay in the functional need for this opening. At stages long before iugestion or inspiration takes place by the mouth, the formation of waste products during the digestion of the yolk necessitates an outlet from the enteric -canal at its hinder end. Where obliteration does take place during still earlier stages this is probably correlated with the fact that the need of the opening is still non-existent. It is of interest to notice that obliteration of the anal opening which is of a directly adaptive significance may take place at a later stage. Thus in Lepidosiren during about the first two weeks of larval life, when large numbers of practically motionless larvae are lying crowded together in the nest, the anal opening, which had been continuously patent in earlier stages, is closed, so as to prevent the poisonous excretory products from finding their way out. So also in the case of the Elasmobranch embryo enclosed within its egg-shell. In the Amniota the perforation of the anus is delayed to a relatively late period doubtless for a similar reason. It is characteristic of the phylum Vertebrata that the anal opening no longer occupies its primitive position at the extreme end of the body but has become shifted forwards along the ventral side. This shifting has probably come about with increased specialization for swimming by lateral flexure of the body, the withdrawal of -the alimentary canal with its surrounding splanchnocoelic cavity from the hinder portion of the body, leaving the space they occupied free for increased development of the lateral muscles. This shifting forwards of the anus, leading to the differentiation of a distinct postanal or tail region, has occurred in all Vertebrates, least markedly in the more archaic groups. It reaches its maximum in some members of that group of Vertebrates which is above all others highly specialized for active swimming, the Teleostei, in some families of which the anus has actually assumed a jugular position. During i he actual ontogeny of the Vertebrate the process by which the anus comes to occupy a position more or less distant from the tip of the tail region is smnewhat modified fnun that which probably occurred during phyletic evolution. We do imt find that the anus remains at tin- tip of tin- tail •luring the growth in length and that it then inadually shifts forwards along the ventral side. What happens is that the ujH-riin^ al an early stauv assun ventral position and that the tad iv-imi proceeds to sprout <>ut dorsal to it. The pi- » «•_ will he understood from an inspect inn nf SO <|>. 140). In 15 the anus is at the hinder end. in (' it has 111 THE ALIMENTARY CANAL 195 assumed a ventral position being overhung by the bulging tail rudiment, in D, E, F, G the tail rudiment is seen to be extending actively past the position of the anus, the specially actively growing tissues bein^ indicated by the darker shading. In Fig. 80, G, a feature is well shown which occurs in the embryos of most Vertebrates — the postanal gut (pa.g). It was shown in Chap. I. how a connexion — the neureuteric canal — existed in some Vertebrates between the cavity of the enteron and that of the neural rudiment at their posterior ends. Here, in the postanal gut, we have such a connexion still persisting in a drawn-out form though, as in the present case, it may be a solid strand of yolky cells and not a hollow tube. The postanal gut is a purely transitory structure which at a relatively early period of development dis- integrates completely. In endeavouring to determine the morphological significance of the postanal gut it is necessary to bear in mind that the Vertebrate in early stages develops from before backwards and that the growth in length by the addition of new segments takes place at its hinder end where there is a mass of actively growing embryonic tissue forming a kind of "growing point." The tissue of this, although to the eye quite undifferentiated, contains the elements which form all the various tissues such as nerve cord, notochord, myotomes, alimentary canal, etc. As growth goes on these gradually become differentiated out, the differentiation always proceeding from before backwards. If we now look at such a young Vertebrate as that shown in Fig. 80, G, we see the typical Vertebrate structure, includ- ing alimentary canal (pa>.g) extending right back practically to the tip of the tail : it is only at the extreme tip that the various organs, merge together into undifferentiated embryonic tissue. The only striking peculiarity is that the communication of the alimentary canal with the exterior, the anus, is not in the midst of the growing tissue of the tip, as it would be, for example, in a young Chaetopod worm, but well forwards on the ventral side. This peculiarity, in the writer's opinion, finds its explanation in the development from before backwards already alluded to. The appearance of the anus at a point relatively far forwards means that it and the organs related to it such as the excretory ducts complete their development at an earlier period of time. As it is of functional importance that the organs in question should do so, in contradistinction to the purely motor arrangements farther back, we see a physiological reason why evolution should have brought about a development of the anal opening in its anterior position from the beginning, and the elimination of those stages in which it was situated farther back. As regards the phyletic evolution of this part of the enteron, we may sum up probabilities as follows : that the alimentary canal with its surrounding splanchnocoele originally extended to the hind end of the body : that the anal opening came to be shifted on to the 196 EMBRYOLOGY OF THE LOWER VERTEBRATES HI ventral wall of the canal : that it then underwent a gradual shifting forwards along the ventral side: that as it did so the now postanal portion with its splanchnocoele gradually atrophied the position they occupied becoming filled mainly with muscle. LITERATURE Assheton. Quart. Journ. Micr. Sci., xxxviii, 1896. Assheton. The \Vork of J. S. Hudgett. Cambridge. 1907. Balfour. Comparative Embryology, ,ii, 1881. Brachet. Journ. de I'Anat. et de la Physiologic, xxxii, 1896. Brauer. Zool. .lahrb. (Anat.), xii, 1899. Brouha. Journ. de I'Anat. et de la Phys.. xxxiv, 1898. Bryce. Journ. Anat. and Phys., xl, 1906. Budgett. Trans, Zool. Soc. Lond.. xvi. 1901. Dean, Bashford. Zool. Jahrb. (Syst), ix. 1896. Dniner. Zool. Jahrb. (Anat,), xv, 1901. Egert. Zool. Anzeiger, xlii, 1913. Goeppert. Morph. Jahrb., xvii, 1891. Goeppert. Morph. Jahrb., xx. 1893. Goette. Entwicklungsgeschichte der Unke. Leipzig, 1875. Goette. Zeitschr. wiss. Zool., Ixix, 1901. Greil. Semons Forschungsreisen in Australien, i. Jena, 1913. Hammar. Arch. f. Anat. u. Entwicklungsgesch., 1893. Hammar Anat. Anzeiger, xiii, 1897. Juillet. Arch. zool. exper. [5], ix, 1912. Kallius. Anat. Hefte (Arb.), xvi, 1901. Kerr, Graham. Proc, Roy. Phys. Soc. Edin., xvi, 1906. Kerr, Graham. The Work of J. S. Budgett. Cambridge, 1907. Kerr. Graham. Quart. Journ. Micr. Sci., liv, 1910. Laguesse. Journ. de I'Anat. et de la Phys., xxx, 1894. Lankester, E. Bay. Quart. Journ. Micr. Sci., xvi, 1876. Marcus. Arch. i'. mikr. Anat., Ixxi, 1908. . Moroff. Arch. f. mikr. Anat., Ix, 1902. Moser. Arch. i. mikr. Anat., Ix, 1902. Moser. Arch. f. mikr. Anat., Ixiii, 1904. Miiller, W. Jenaische Zeitschrift, vi, 1871. Neumayr. Semons Forschungsreisen in Australien, i, 1904. Nicolas. Arch. Biol., xx, 1904. Phelps. Science, N.S. ix, 1899. Piper. Arch. f. Anat. uml Kntwicklungsgesch., Suppl. Bd., 1902. Piper. Verli. Anat. Ges., Halle, 1902. Beighard and Phelps. Journ. Morph., xix, 1908. Bowntree. Trans. Linn. Soc. Lond., (2) ix. 1903. Sarasin, P. and F. Krgclmissr natunviss. ForsphongOt) ;iuf (Yylon. ii, o. \Vii--badru, 1889. Sawadsky. Anat. An/ciger, xl, 1911. Scammon. AIIHT. Journ. A nil.. \iv, 1913. Sedgwick. Quart. Journ. Micr. Sci., xxxiii. 1892. Smith. Juiiin. Moiph.. xxiii. 1912. Taylor. Quart. Journ. Micr. Sci., lix, 1913. Voeltzkow. Abh. Senck. Ges.. xxvi. 1899. Wenckebach. Ontwikkeling en de bouw .cli. Anttttzdain, Tuccde \\iii. 1914. Wilson. Gregg. I'n-.-. Hoy. I'liys. Soc. 1-Min.. \i\. 1901. CHAPTER IV THE COELOMIC OKGANS INTRODUCTION. — The mesoderm of Amphioxus consists in an early stage, as already indicated (p. 57), of a row of closed sacs arranged serially one behind the other upon each side of the body. At this time the coelome of Amphioxus is in the extremely archaic condition of a series of metamerically arranged paired compartments — a condition resembling that of the less modified forms of Annelids. The coelomic sacs gradually spread in a ventral direction until they meet. For a time after this happens the sacs of opposite sides of the body remain separated by a longitudinal partition the ventral mesentery. Similarly the apposed posterior and anterior walls of neighbouring sacs belonging to the same side of the body, form thin membranous septa like those of Annelids. A highly characteristic difference from the Annelid arrangement begins to show itself a little before hatching in the ventral portion of the body, in as much as the transverse septa break down and disappear thus converting what was hitherto a chambered coelome in this region into a continuous space. There is no obvious reason why this loss of segmentation of the ventral portion of the meso- derm has come about in evolution. A general characteristic, however, of the phylum Vertebrata is the loading up of the ventral part of the endoderm with yolk and it may well have been that the loss of the mesoderm septa ventrally arose in correlation with the presence of a greater amount of yolk in the ancestral condition than exists in the present-day Amphioxus. A further striking difference between the Vertebrate and the Annelid is expressed in the extent to which the coelomic wall gives rise to muscular tissue. In the Annelid practically the whole extent both of the somatic layer lining the body-wall and the splanchnic layer covering the gut gives rise to muscular tissue. In Amphioxus however, and the same holds for Vertebrates in general, the ventral portion of the somatic mesoderm, the portion which loses its segments! character — loses also its capacity for producing muscle. On the other hand the dorsal portion of the mesoderm, which retains its segmentation, retains also, and to an accentuated degree, its muscle-forming capacity. It separates off from the ventral or 197 198 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. splanchnocoelic portion of the mesoderm in the form of a series of segmentally arranged sacs — the myotomes — and the wall of these gives rise to almost the whole of the muscular system. The myotomes are at first, from their mode of origin, restricted to the dorsal side of the body, but as development goes on active growth of their ventral portions takes place and they extend downwards, overlapping and covering in the splanchnocoelic mesoderm right down to the mid- ventral line. In this way a muscular body-wall is provided for the ventral region of the body in which the original muscle-producing capacity of the somatic mesoderm had disappeared. The evolutionary origin of this curious secondary muscular ization of the ventral body-wall of the Vertebrate is unexplained but the suggestion may be hazarded that it was associated with the loss of segmentation of the ventral splanchnocoelic mesoderm, the primitive mode of movement of the Vertebrate — by waves of lateral flexure — being only able to utilize longitudinal muscles divided into segments. We may take it that the splanchnocoelic muscular layer, as it lost its segmentation, would become less efficient for purposes of move- ment, and that, correlated with this, its territory would then tend to be encroached on by the still segmented, and therefore more efficient, dorsal portion of the muscular layer until eventually it came to be replaced completely by it. As a result of the developmental processes which have just been indicated the mesoderm of Amphioxus, which for a time consisted of a metameric series of paired sacs, is now represented by (1) the segmentally arranged myotomes and (2) the unsegrnented splanchno- coelic lining. To these a third element becomes added in the form of a pocket-like outgrowth from the myotome wall close to its lower end (Fig. 144, A, scl, p. 285). This grows first towards the mesial plane and then dorsally, insinuating itself into the space between myotome on the one hand and notochord and spinal cord on the other, until it occupies practically the whole of that space right up to the mid-dorsal line. This pocket-like diverticulum is the sclerotome (p. 286). In the typical Vertebrate a fourth derivative of the mesoderm segment is of importance : it takes the form of a connexion which persists for some time between the myotome and the splanchno- coelic mesoderm as a narrow stalk or isthmus. This — the proto- vertebral stalk or nephrotome (Riickert, 1888) with its cavity the nephrocoele is of in-eat importance from its relation to the nephridial orpins but its existence has not up to the present been demonstrated in Amphioxus. We will now proceed to trace out the subsequent late of these various derivatives of the. primitive mesoderm segments. .;i< CAVITIES, Tin- only portinns <•!' the eoelomie cavitiefl which remain patent are the nephrocoeles (which \\ill he dealt with later on) and I he splanchnocoele or peritoneal cavity. It mai SaSD as probable that the 1 \ ity ol' the iv COELOME 199 ancestral Vertebrate was divided up into segmentally arranged compartments by transverse septa, and into a right and left half by a sagittally placed partition supporting the alimentary canal and forming the dorsal and ventral mesentery; in other words that the general arrangement was like that of a primitive Annelid worm. This seems to be indicated by the mode of development of the mesoderin in Amphioxus. In Vertebrates above Amphioxus the segmented condition of the splanchnocoele has disappeared even from development.1 The sagittally placed mesentery on the other hand still appears in ontogeny in the form of the partition remaining between the edges of the lateral mesoderm as they approach one another on the ventral and on the dorsal sides of the alimentary canal respectively. In correlation with the great increase in length, and consequent coiling, of the alimentary canal of the Vertebrates — a condition which probably existed even in the ancestors of those gnathostomes in which the alimentary canal is now short (p. 184) — the ventral mesentery disappears at an early stage of development throughout that portion of its extent which lies on the tailward side of the liver. The dorsal mesentery on the other hand persists throughout life, serving as a bridge to carry the complicated connexions of the gut wall with the vascular and nervous systems, although perforations may appear in it, more or less extensive in different groups of Vertebrates. The complicated foldings and frillings which the dorsal "mesentery undergoes, owing to its enteric edge having to keep pace with the increase in length of the gut, are of interest mainly to specialists in the anatomy of particular groups and need not be dealt with here. In the fishes, in which the lung performs an important hydro- static function, that organ grows back in the substance of the dorsal mesentery, and in accordance with its tendency to assume a more and more dorsal position, the portion of mesentery lying above it may become incorporated in the dorsal wall of the splanchnocoele, the result being that the lung in the adult now lies entirely dorsal to and beyond the limits of the body-cavity (Dipnoi,2 Actinopterygii). Apart from its primary segmentation, the splanchnocoele shows a tendency for special portions to become secondarily separated off from the main cavity. The most important case of this occurs at 1 While it has to bo granted that the splanchnocoele of the Vertebrates represents the ventral portion of the coelome which has lost its segmentation, care must be taken not to assume that this loss of segmentation has necessarily extended dorsal- wards to precisely the same level in all Vertebrates. Like other anatomical boundaries the dorsal limit of the splanchnocoele i§ doubtless fluctuating and vague. It is therefore wise not to attach too great importance to the exact position of the first rudiment of an organ which develops in one case on the dorsal and in another on the ventral side of the boundary between segmented and unsegmented mesoderm such as for example the gonad (p. 270). 8 Cf. Graham Kerr, 1910. 200 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. the hinder end of the heart where there exists on each side a broad bridge by which the duct of Cuvier passes from the somatopleure to the sinus venosus. This bridge becomes extended headwords and dorsally on each side of the oesophagus until it meets the dorsal wall of the splanchuocoele thus forming with the oesophagus a floor separating the anterior portion of the splanchnocoele into two cavities, one dorsal and one ventral, each opening posteriorly into the main splanchnocoele. Of these two cavities the dorsal becomes completely obliterated by fusion of its splanchnic (oesophageal) and somatic walls from before tailwards. The ventral one on the other hand roofed in by the oesophagus persists as the pericardiac cavity. The communication of this posteriorly with the main splanchno- coele is obstructed in the middle by the flattened headward surface of the liver which is embedded in the distended ventral mesentery, while laterally the communication is for a time open. As develop- ment goes on however the opening on each side becomes obliterated by an ingrowth from the somatopleure which spreads downwards from the bridge of tissue containing the duct of Cuvier and the free edge of which meets and fuses with the mesoderm covering the headward surface of the liver. The pericardiac cavity comes in this way to be bounded posteriorly by a complete wall of tissue a large part of which consists simply of the mesodermal sheath of the liver. As the body of the embryo increases in diameter this wall of tissue keeps pace with it as does also the liver. The latter organ however in subsequent growth of its anterior or headward surface does not keep growing in continuity with the substance of the septum but becomes separated from it by a deep cleft, the region of continuity between liver and septum becoming thus restricted to a small area dorsal and close to the mesial plane. Similarly the region of continuity between the headward face of the septum and the wall of the sinus venosus which is at first of relatively considerable dorsiventral extent becomes reduced to a narrow bridge of tissue. In the Elasmobranchs the isolation of pericardiac cavity from the main splanchnocoele is only temporary. A median pocket-like extension of the pericardiac cavity spreads tailwards immediately dorsal to the sinus venosus in the substance of the mesodennal sheath covering the ventral surface of the oesophagus. This develops on each side a communication with the main cavity of the splanchno- coele which persists throughout life as a crescentic slit on the ventral sin-fan- of tin; oesophagus fl lochstetter, 1900). This secondary com- munication hetween pericardiac coelome and splanclmocoele is known as the pericardioperitoneal canal. In Myxinoids, 1 liroii^liont life, and in /V//-//, during the larval period, the rudiment of the wall scjiarat in- jierieardiae from splanohnoooelk cavity remains in the innu of a simple bridge enclosing the duct of divier so that the two cavities air in wide communication with one another. IV COELOME 201 In the Am- phibia and Aniiii- ota the pericardiac cavity becomes telescoped back A. into the general peritoneal cavity, its hinder Wall be- coming extended so -as to form a thin membranous bag enclosing the heart and separat- ing it from the other viscera. Apart from the walling off of the pericardiac from the main periton- eal cavity there is found in the case of the Amniota a well-marked tend- ency for the latter cavity to undergo further subdivi- sion, special por- tions becoming more or less com- pletely walled in by secondary fu- sions taking place between apposed portions of the peritoneal lining. FIG. 110. — Differentia- tion of the ruyotome as seen in transverse sections of Lepidosircn larvae. A, st age 30 ;B, stage 31 + ; 0, sta-r 83; I), stage 35-; E, dividing ntyoblasts of iniM-r wall from stage 3<>. 'ii>>', niyoblasts ()f imirr wall: „/>/'. inyohlasts of "ut.T wall ; ,nf, contractile tihrils; cm:, vacimlc. The contractile fibrils cut across an- shov. n as distinct black • lots. 772 1 vac. B. 202 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. For example in Birds l the mesodermal coating of the lung upon its ventral side becomes continuous (1) with that lining the body-wall laterally so as to enclose the portion of splanchnocoele dorsal to the lung as. a pleural cavity, and (2) with that covering the surface of the liver, forming a ventral pulmonary ligament which serves to wall in a pulnio-hepatic recess lying between it and the mesen- tery. A third connexion, the origin of which is associated with the development of the abdominal air-sacs, forms the thin post-hepatic septum which stretches from the ventral surface of the lungs obliquely downwards and backwards to the ventral body-wall. Amongst Reptiles somewhat similar arrangements exist, differing in detail in the different groups. THE MYOTOMES. — The developmental changes by which, in a gnathostomatous Vertebrate, the myotomes become converted into masses of muscle-fibres are excellently shown by Lepidosiren in which the cellular elements are particularly large and distinct. In this animal the myotonie is at first solid, but later on develops a small cavity or myocoele by the breaking down of its central cells. This myocoele soon becomes obliterated by its inner and outer walls coming together. The cells of the inner wall assume a more regular shape, taking the form of large parallelepipedal cells (Fig. 110, A, mb'), flattened dorsiventrally and stretching in an anteroposterior direction throughout the whole length of the myotome. The nuclei of these large cells — myoblasts or myoepithelial cells- divide, mitotically, so that they assume a syncytial character. Their protoplasm develops a longitudinally fibrillated appearance and presently distinct cross-striped contractile fibrils (mf) make their appearance in the protoplasm — each fibril running through the whole length of the myoblast or in other words from end to end of the myotome. The contractile fibrils, which as seen in a trans- verse section are arranged in a ^ -shaped pattern (Fig. 110, A, mf), become more and more numerous and soon fill up the inner two- thirds of the myoblast almost entirely, there remaining only a relatively small amount of perifibrillar protoplasm between them (Figs. 110, B, and 111). The outer end of the myoblast does not for some time develop any contractile fibrils but there appear in its protoplasm large \ -i< -uoles (vac) which form a broad clear band in horizontal sections — of much use as a landmark to indicate the outer limit of the inner wall of the myotome. The cells of the outer wall of the myotome take the form of elongated cylinders stretching throughout the I'-irjUi of the myotome and in their protoplasm longitudinal 10 make their appi-aram-.' as in the case <>!' tin- inner wall myoblastH (Figs. 110, C; 111, B). The longitudinal fibrils Income fused at tln-n ends \\iih <-<,n native-tissue septa formed by mesen- << II which wander in In 'tween consecutive myotomes. 1 For a wcll-illii-ii .nt of the rimij'lir.-il.-.l .irrin^-im-nts in «l«-tail sec (1909). IV MYOTOMES 203 Some such mesenchyme cells also penetrate into the substance of the myotome and settle down then1 to form connective tissue. The cylindrical myoMasts of the outer wall undergo active multiplication (Fig. 110, C) so that it conies to be greatly thickened, composed of many layers <>!' muscle-cylinders — those towards the outer surface going on dividing actively while those further in towards the mesial v-ac. mb mb mf- FIG. 111. — Differentiation of the myotome as seen in horizontal sections of , Lepidosiren larvae. . <• :31 ; H, stage :?1 -}-. mb', myoblast of inner wall ; mb", myoblasts of outer wall : ;///, contractile fibrils ; me, vacuoles ; y, yolk. plane increase much in size as they develop more and more fibrils in their interior. As the outer wall of the myotome continues to increase in thickness the myoblasts of the inner wall become relatively more and more insignificant. Eventually they divide up into muscle- cylinders like those of the outer wall so that it is no longer possible to distinguish the inner wall portion of the myotome from the outer wall part. The muscle-cylinders become the muscle-fibres of the adult, the unditferentiated protoplasm between^the fibrils persisting as the sarcoplasm the supi-rlicial layer of which may be somewhat condensed to form the sarcolemma. 204 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 771. A point to be noticed, of much morphologic; il interest, is that the inner wall myoblasts of Lepidosiren are for a time (Fig. 110, A) in the form of typical myoepithelial cells such as are familiar in some of the lowest invertebrates. They are, as indicated in Chap. II., in continuity with the central nervous system by a protoplasmic tail-like extension of the cell-body closely resembling that which occurs in Nematode worms (Fig. 112). The peripheral portion of this remains as a muss of granular protoplasm on the surface of the muscle-fibre — the motor end-plate. The latter is therefore to be regarded as a portion of the muscle-cell which retains its proto- plasmic condition rather than as a portion of the nerve-fibre. The mode of conversion of the embryonic myotome into the muscle-segment has been described as it occurs in Lepidosiren because of the two special safeguards against error which exist in that animal, (1) the large size of the histological units and (2) the 'fact that the boundary between outer and inner walls of the myotoiiies is marked by a clear and unmistakable land- mark in the form of the vacuolar zone constituted by the outer portions of the inner wall myo- blasts. It now remains to indicate shortly the more important differ- ences in detail which are to be FIG. 112. — Diagram of a motor ganglion- ,. j • j • *_• x> I.-L cell in the" spinal cord continuous f°Und m descriptions ot the prOCCSS through the substance of a nerve-fibre as observed in Others of the lower Vertebrates. The chief of these concerns the fate of the outer wall of the embryonic myotome. In Lepido- siren as has been stated the outer wall gives rise to muscle. In the case of Elasmobranchs and Ganoids, Balfour stated explicitly that the outer wall of the myotome similarly takes part in the development of muscle. Many authorities (Hertwig, Rabl, Maurer), however, deny that this is the case : according to them the outer wall plays no part in muscle-formation: it simply breaks up into amoeboid cells which contribute to the dermal mesenchyme. Hence these investigators term the outer wall of the myotome the " CutiH-layer." In the cast- of the Sturgeon, Maurer corroborates I i;i 1 four's statement that the myotome is composed of two layers of inuscle-elemriits but ;iecnnliniLr i,, Mm {,h,- outer layer is simply budded off from the inner and does not represent tin- nn-in.il out IT wall di ihr myotome as ?>all<>ur supposed. In the . \miiini i the myniuiiif in early stages is almost s« JUM i seen in a transverse section pruetieally tin- \\lmle <»f toe wall with a muscle-cell in the myotome. c./, contractile tibrils in myoepithelial cell ; ?«, myoepithelial cell; ?t.r, ganglion - cell ; «./, motor nerve-fibre. iv MYOTOMES 205 next the endoderm representing the sclerotoine. Cells proliferat- ing from this invade the myocoele and completely fill it up. It is only in later stages that the myotome becomes extended into the normal plate-like form by active growth at its inner (dorsal) and outer edges. Of the two walls of this stage the inner admittedly becomes converted into muscle -cylinders. The outer becomes loosened out into a mass of irregularly shaped cells and these are commonly believed to give rise to dermis. In view of what happens in Lepidosiren, where accuracy of observation is so much more easily attained to, it seems advisable not to accept this as absolutely certain. At the same time it may be allowed that there is no a priori difficulty in the way of admitting tnat portions of myotome which in one type of Vertebrate give rise to muscle, may in another have ceased to do so, for, as already indicated, a quite similar process of concentration of muscle -development in a localized portion of somatic mesoderm is a fundamental characteristic of the whole Vertebrate phylum. The series of paired uiyotomes, each composed of a mass of longitudinal muscle-fibres traversing it from end to end, forms the material out of which is formed the, often extremely complicated, system of voluntary muscles of the adult Vertebrate. The various myotomes as they increase in size become divided up into it may be numerous pieces and these are pushed hither and thither by processes of differential growth until the arrangement of the numerous adult muscles contrasts greatly with the simple longitudinal arrangement of the original myotomes. During the various displacements which it undergoes the individual muscle or fragment of myotome remains in organic connexion with its nerve-centre by means of its motor nerve and the course of these nerves in the adult frequently gives an important clue to the developmental migrations of the particular muscles. No attempt will be made here to follow out the evolution of the complicated muscular arrangements of the adult beyond a short sketch of the method in which the muscles of the fins or limbs originate. The median fin is simply the extension of the body in the median plane and we should therefore naturally expect it to be muscularized by prolongations of the myotomes growing into it. The actual process is clearly illustrated in Fig. 113. In A a muscle-bud is seen to be projecting from the end of each myotome where a median fin is developing — the upper group of buds belonging to the dorsal fin, the lower to the anal. • The buds diminish in size towards each end of the series and in the case of the dorsal fin, towards its anterior end, there are a considerable number of abortive buds which never come to anything. The muscle-buds grow into the fin fold and then become cut off from the main part of the myotome to form the muscles of the fin as is shown in B. 206 EMBKYOLOGY OF THE LOWER VERTEBKATES CH. B The paired fins or limbs become rnuscularized by very similar, segmentally arranged, buds and it is necessary from the outset to bear in mind that this similarity need have no deeper signifi- cance than that the paired fins also necessarily obtain their muscular- ization from the segmentally arranged myotumes. The process as it occurs in the pelvic fin of the shark Spinax is illustrated by Fig. 114. In the 20 mm. embryo (A) the fin rudiment is seen as a longitudinal ridge and a series of myotomes in the neighbour- hood of this ridge are seen each to be forming at its lower edge two projecting muscle - buds. These sprout out into the limb rudiment, assume an elongated form (B) and then become separated off from the myotome (C). Each bud now splits into two layers a dorsal and a ventral and. each of these undergoes histological differentiation and be- comes a bundle of muscle- fibres — one of the radial muscles of the fin : so that four radial muscles are derived from each myotome, a dorsal and a ventral from each of the two original buds. Such is the process in its main outlines. The existence of a disturbing complication of this simple scheme is indicated by the adult arrangements, in as much as it can be shown that a single motor spinal nerve (i.e. the nerve belonging to a single myo- tome) is related to more than the four radial muscles to which alone we should expect it to be related were the account which has just been given complete. This discrepancy is brought out particularly clearly by I »li ysiological experiments. Careful stimulation of a single spinal nerve very commonly causes three consecutive (dorsal or ventral) radial muscles to contract instead of only two, and in some cases apparently a -till greater number. This seems clearly to indicate that the end- organs, in other words the muscle-fibres, belonging to a particular motor nerve or myotome are in the adult not strictly confined within tin- limits of the two pairs nf radial muscles corresponding to that motor nerve or myotome. To those who believ- in the organic continuity of muscle-cell and nerve-iihre from an extivmeh eirlj stage of development, the idea obviously suggests itself thai .1 -hil'iin^ of some of the :'uents from one um-elr .hud int.. its nri^lilmiirs lake< place Pi-.. 1 13. — Muscularization of median fin in Lepulosteus (After Schmalhausen, 1912.) A, I:; iinii. ; 15. •_'! mm. Tlic muscl>--buds and, in the lower figure, the nerv.-s c<>iiii>Tt<- nun. (74 m.s.); C. 2(i mm. ; l>, :<-J mm. The myotomes an- inilit-ated by Arabic numerals. The muscle-buds are shown in black, those within the fin rudiment briii- iiimilifivd with Roman numerals. The nerve-trunks are shown with double contour. with their neighbours near their proximal ends. These connecting bridges persist for a short time and then disappear. According to the authors mentioned they are the expression of a cellular interchange taking place between neighbouring muscle rudiments. 208 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. E. Miiller (1911) believes the connecting bridges in the case of Acanthias to be special developments of a syncytial network which lies between the buds from the commencement : he fails to find in this animal any evidence of shifting of muscle-cells along the bridges. The matter appears to stand in need of further investigation. Already within the group of Elasmobranchs we find modification of the typical mode of muscularization of the fins outlined above. In the case of- the most anteriorly placed muscle-bud of the pectoral fin of Spinax the bud resolves itself into its constituent cells which separate before giving rise to muscle-cells. Again at the anterior and posterior limits of the pectoral limb musculature in Pristiurus and Torpedo the compact stage of the muscle-bud is eliminated entirely and the cells which muscularize the fin are budded off ot FIG. 115. — Diagram to illustrate the arrangement of mesoderm segments in the head- regiou of a young Elasmobranch embryo. (From a drawing by Agar.) •>t, otoc-yst ; *jil'-, splanchnocoele ; t, u, v, w, occipital myotomes ; 1, 2, 3, anterior myotomes ; I, II, etc., visceral clefts; *, "Fourth" myotomt- of van Wi.jhc. separately from the inyotome, wandering from their place of origin into the limb rudiment and there settling down (Braus). Amongst Vertebrates outside the group of Elasmobranchs such modification appears to be the rule. Thus in Acipenser and apparently in Lacerta typical muscle-buds arise singly from the myotomes concerned. In Teleosts Harrison finds muscle-buds in the pelvic fin but a diffused origin in the pectoral. In Lung-fishes and Amphibians the origin seems to be again diffuse and the same appears to be the case in Birds. MESODERM OF THE HEAD-REGION. — There are two important. characteristics "f tlm head -region of the Vertebrate ultimately connected with the muscular system: (1) loss of Ilex i I >ility, associated with the evolution of brain and skull and ('_') special muscularity of the wall of tin; alimentary canal, associated \\iih the presence of important movable skeletal >i met ures enclosed in the substance of the visceral arches. These peculiarities find their expression (1) in the tendency in Mippre.s>ion of the myotomes of the head- iv MKSODKK'.M OJF HEAD 209 region and (2) in the retention, to a greater extent than in thr trunk, of the iiMisrl^-l'orminx «-;ip;icit.y «>|' that part. «f th«- ni'-soderrn which lies ventral to the myotoines. The niesoderm of the head-region shows the least amount of modification posteriorly where its relation to the mesoderm of the trunk is still clear. In the occipital region — the region between the otocyst and the occipital arch, which may be taken as the hinder limit of the skull — we find a series of typical (" occipital "- Fiirbringer) myotoines, the mesoderm ventral to which takes part in the lining of the splanchriocoele just as in the trunk-region. This series of occipital myotoines seems clearly to be undergoing a process of reduction. It is largest in such, comparatively primitive forms as Elasmobranchs. Again during ontogenetic development the series commonly shows progressive reduction. In Spinax for example seven occipital myotoines make their appearance, but as development goes on the anterior three (t, u, v) l break up and dis- appear, the fourth (iv) does so incompletely, while the last three (x, y, z) develop into definite muscle-segments though of small size. As each anterior my o tome disappears those behind it become shifted forwards so that its place becomes occupied by the myotome originally behind it in the series. It will be realized that there is thus introduced a serious source of possible error which has to be carefully borne in mind in observations on the development of the occipital region where the identification and correct reference of individual myotoines to their place in the series is of importance. Anteriorly the series of occipital niyotomes is prolonged forwards past the otocyst by a mass of mesoderm (* in Fig. 115) which was regarded by van Wijhe (1883) — who may be said to have laid the foundations of modern work upon the segmentation of the mesoderm of the head — as the equivalent of a single (" fourth ") rnyotome. It has already been indicated that the series of occipital myotoines is undergoing reduction from its front end backwards and it seems on the whole more probable that van Wijhe's " fourth " myotome in the Gnathostomata is to be interpreted not as a single myotome but rather as the degenerate remnant of a series of myotomes. The number of myotomes originally present in this series does not appear to be capable of decision with any degree of certainty. Possibly it was very considerable and Froriep finds even in ontogeny (Torpedo) that during early stages (Stage "D" of Balfour) as many as six distinct segments are recognizable in the region in question — in other words that the series of myotomes commences not with t but with n, the anterior members of the series disappearing in turn as development proceeds. A point of 1 The hind end of the series — the occipital arch — being taken as a iixrd point while the front end varies, Fiirbringer has introduced the convenient method of designating the individual occipital myotomes (or their nerves) by the terminal letters of the alphabet — the last being c, the one next in front y, and so on. The myotomes behind the occipital arch are counted as belonging to the trunk and arc- designated by numerals 1, 2 and so on (cf. Fig. 220). VOL. IT P 210 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. interest is that the anterior limit of this series of recognizable segments agrees approximately ' with the anterior end of the definitive notochord. In front of the " fourth " myotome of van Wijhe we find what appear to be fairly typical third and second myotomes, each con- tinuous ventrally with the wall of the pericardiac portion of the splanchnocoele. Of these myotome III gives rise to the External Rectus muscle and II to the Superior Oblique. At the front end of the series we have the first or premandibular or oculomotor myotome, peculiar in that it is fused with its fellow across the mesial plane and that it no longer shows any connexion with the splanchnocoelic mesoderm. It gives rise to the four eye-muscles supplied by the Third cranial nerve — the Superior, Internal, and Inferior Rectus, and the Inferior Oblique. We have so far dealt only with the myotomes but the lateral or splanchnocoelic mesoderm is also continued well forwards into the head-region. Its more ventral portion forms the lining of the pericardiac cavity, while its more dorsal portion becomes traversed by . the visceral pouches or clefts. The splanchnocoelic mesoderm ventral to myotomes II and III comes to form a stalk -like connexion between the myotome and the pericardiac wall (Fig. 115). This stalk is hollow in the case of myotome II and lies in the mandibular arch: in the case of myotome III it is solid and lies in the hyoid arch. In both cases the wall of the stalk gives rise to the muscular apparatus of the particular arch — in the one case the masticatory muscles and in the other the hyoidean musculature which is destined to attain to such a development in the mammals as the musculature of the face. The splanchnocoelic mesoderm corresponding to the myotomic mass behind myotome III (* in Fig. 115) is said to give rise to the musculature of the branchial arches. As the myotomic mass in question shrivels up during development, and the occipital myotomes move forwards to take its place, these myotomes come to overlie the splanchnocoelic mesoderm which gives rise to the branchial muscles. Consequently as will be realized the position of myotomes t, u, and v in relation to clefts III, IV, and V as shown in Fig. 115 is secondary, the myotomes having moved forwards before the formation of these clefts. The above sketch has dealt with the cephalic mesoderm of Miobranchs but a similar scheme of development with minor variations in detail holds for other Vertebrates. Upon tin- wlmlr it niav be said that with upward progress in the evolution of the V. -rtcl.rjLta the segmentation of the mesoderm in the hinder part of tin- head becomes more and more obscured. Ri^ht up to the highest forms liou.-ver traces of it persist. In Fowl embryos of aUmi the third day of incubation tin; series of nhvimis mynlnim's may often 9611 tO !'" prolnn-e.l forward (866 Fig, 236) I'.v faintly visible agreeing in size and exactly in series \\ith ihe m\ot<>mes. iv MYOTOMES OF HEAD 211 These blocks m.iv be indistinguishable in ordinary thin sections but quite distinct in stained preparations of the whole embryo. It will require strong evidence to justify the refusal to ^ivo them the inter- pretation that at once suggests itself — that these slight condensations of tin; mesenchynie are as it were the ghostly remnants of once existing myotomes which in Birds have ceased to become functional. An important side issue of their presence to be borne in mind is that the slightly greater resistance of the more condensed portions of mesenchynie must necessarily exercise pressure upon the soft surface of the rapidjy growing brain and produce a modelling of its surface which may be adequate to explain at least some of these appearances of segmentation of the brain-region which are included under the term neuromery. The blocks in question extend well forwards — in the specimen figured (Fig. 236) there are four distinguishable anterior to the middle of the otocyst and they may be taken as additional evidence in favour of there being not one but a number of myotomes repre- sented in the region of van Wijhe's " fourth " myotome. It is of interest to note that in the Lampreys the blurring of the segments immediately posterior to the third of van Wijhe's series seems not yet to have come about and there is an undoubted simple " fourth " myotome (Koltzoff, 1901). We may justifiably associate this with the low degree of cephalization in these creatures which has involved a persistence of, or more probably a reversion to, an apparently archaic condition of this myotome and its immediate successors in the series. The relations of segments I, II and III to the eye-muscles have been worked out in a number of Elasmobranchs and similar conditions have been described in Eep tiles and Birds. Our knowledge of the holoblastic Vertebrates in this respect is still fragmentary. In the case of Lepidosiren and Protopterus the eye-muscles develop out of compact masses of mesenchynie in which it is impossible to recognize definite segments (Agar, 1907) while on the other hand in Ceratodus (Gregory, 1905) these segments make their appearance much as in Elasmobranchs. Before leaving this part of the subject it should be pointed out that not all morphologists are convinced that segments I, II and III are actually serially homologous with the undoubted mesoderm segments or myotomes of the trunk-region : the blurring of the mesoderm arrangements between them and the admitted myotomes, and more especially their late appearance in ontogeny, at a time when the anterior members of the occipital series have already degenerated, are. brought as evidence against the more generally accepted view. The present writer does not feel inclined to attach great weight to these objections. (1) The break or blurring of the series immediately behind III seems adequately explained by the disappearance of functional muscles in this region and (2) the relatively late appear- ance of myotomes I to III is explicable by the fact that the 21S EMBRYOLOGY OF THE LOWER VERTEBRATES - 11. N functional muscles derived from them are connected with the eye- ball an organ which becomes complete and functional only at a relatively late period of development. HYPOBRANCHIAL OR HYPOGLOSSAL MUSCULATURE. — In addition to the musculature already indicated the Vertebrate head possesses on its ventral side a system of hypobranchial muscles which have the appearance of a prolongation forwards of the longitudinal muscles of the ventral body-wall. This hypobranchial musculature as a matter of fact does arise in ontogeny as a prolongation forwards of the anterior trunk and occipital myotomes, as is well shown by Lcpidosiren or Protopterus (Agar, 1907). About stage 29 the ventral ends of myotomes y, z and 1 are seen to be growing out at their ventral ends into a long slender prolonga- tion (Fig. 116). These processes grow outwards in front of the pronephros and undergo complete fusion at their tips. The fused apical portion c.h soon separates from the parent myotomes and grows forwards, on each side of the pericardiac cavity, until it reaches the hyoid arch. It now spreads ventrally until it meets its fellow below the pericardiac cavity. The common mass so formed be- comes converted into a sheet of longitudinal muscle-fibres, attached posteriorly to the shoulder girdle and anteriorly for the most part to the hyoid arch (coracohyoid muscle, Fig. 117, cor. liy\ the branchial arches being reduced in the fishes in question. As the muscle on with its development the an- terior boundary of the portion belonging to myotorue 1 becomes marked by a connective -tissue intersection, while in some specimens a similar intersection appears to demarcate y from z. I n oi her Vertebrates the hypobranchial or hypoglossal musculature appears to originate in the same way — difference occurring <>nl\ in the number of niyotomes which take, part. Five appears to be UK- most usual number (,SV //////////, doming: Teleosts, Harrison). KI.K- TKICAI. ORGANS. — The conspicuous sign of a muscle becom- ing active is that it- Changes its shape: an inconspicuous accomi>ani- ment of this change of shape is tin- production of a slight electrical disturbance In tin- case of most electrical organs we ha\e t<> do with portion^ of the HIM dm in which the function of contraction ha> hem reduced to a subsidiary role «>r abolished KK;. 11G. Dorsal view of anterior myo- tomes of a Protopterus of stage 29. (After Agar, 1907.) •-.It. coracohyoid muscl<- ; .V, notochord inuscl.'-bud to pectoral limb; ?/n, p.f, KLKOTKICAL OKGANS 213 entirely, while UK; production of electrical disturbance has become predominant, We have here an excellent example of the principle of "substitution of functions," which is constantly at work during evolution, the previously predominant functions of organs becoming subsidiary <>i% tailing in ID abeyance and being replaced by functions which were previously subsidiary. The development of the electrical organ can be conveniently studied in the case of the Skate (Raid) of which the most complete description has been given by Ewart (1888, 1889, 1892). In this animal the electric organ forms an irregularly spindle-shaped body which lies embedded in the lateral muscles on each side of the tail region. It varies in size in different species and is distinguishable to the naked eye from the muscle by its more gelatinous appearance. FIG. 117. — Side view of skull and niyotonu-s of Lepiifoxire/t, stage 38. (From Agar, 1907.) Cartilage dotted, myotomes indicated by outlines, nerves black, a.o.p, antorbital process ; "'/,/. caps, auditory capsule ; br.n, brachial nerve ; c.ph.n, nerve to dorsal portion of constrictor of pharynx ; car. hi/, coracoliyoid muscles; hy, hyoid arch; hi/po\YKR VERTEBRATES CH. el ,/ being now still thicker and the whole fibre having assumed the shape of a inace. In the expanded head portion of the mace the cross striation is becoming closer, while in the slender handle the striation is becoming blurred and in the portion next the head is almost disappearing. The end -plate forms a very definite layer of uniform thick- ness covering the truncated an- terior end of the mace. It is crowded with large nuclei and to it pass nerve- fibres whic.h show a regular dichotoinous branching. In the fibre shown in Fig. 118, D, taken from the same 10 cm. embryo, the head «»r the mace is still more ex- panded as compared with the strm. The main portion of the head, in \\liidi the muscle striation had become closer, now forms ii thick plate bent into a 1 18.— Develi.jiim-iit "1 the rli-.-tri.- ui-.ui .if ll"« '"'//.». (Alter K\\ar1. 1888.) A an r an. I D IViini an embryo about in cm. in i • ; cm in I.-,, -tii. 0.1, . . . : ..'. ekctric layer; • :. ttnuUni iv ELECTRICAL ORGANS 215 sain *T shape with its concavity posterior and composed of numerous closely packed lamellae. It forms what is termed in the fully de- veloped organ the striated layer. On its anterior face the striated layer is covered by the end-plate, now known as the electric layer, while its posterior face is covered by a thick layer of richly nucleated protoplasm which, from the deeply pitted character of its posterior surface, is known as the alveolar layer. From this passes back- wards the main part of the muscle-fibre which shows symptoms of degeneration especially in the portion next the alveolar layer where it becomes vacuolated. Whether the alveolar layer represents, as seems probable, a localized thickening of the sarcolemma is not clear from the descriptions. In the fully developed condition (Fig. 118, E) the muscle-fibre has become converted into the functional electroplax (Dahlgren, 1908) or electrical unit. What was the head of the mace in earlier stages is now expanded to form a broad thin circular disc, lying perpendicular to the long axis of the body — the stem of the mace having degenerated into an apparently insignificant and functionless vestige (Fig. 118, E) or having disappeared entirely. The electro- plax is formed of the striated layer which is almost flat except round its edge where it is bent in a tailward direction. It is com- pletely ensheathed in syncytial protoplasm, that on its posterior face forming the alveolar layer, probably nutritive in function, that on its anterior face forming the electric layer. Into the latter there pass the numerous end -twigs of the nerve-fibres, the superficial (i.e. headward) layer showing a characteristic fibrillation of the protoplasm in a direction perpendicular to the surface (nervous layer — Ewart) in contrast to the deeper portion in which the proto- plasm is granular and nucleated (nuclear layer — Ewart). The tail- like vestige of the posterior portion of the muscle-fibre is directly continuous with the striated layer. With the latter it represents the contractile portion of the original muscle-fibre, while the ensheathing protoplasm whether electric layer, or alveolar layer, or sheath of the tail-like vestige, is probably to be regarded as repre- senting the superficial portion of the sarcoplasm. As the muscle-fibres pass through the above-described modifica- tions, the connective tissue between them increases in quantity and becomes condensed between the electroplaxes in such a way that each electroplax becomes enclosed in a disc-shaped compartment. The walls of this fit close to the electroplax round its edge while, on the other hand, the anterior and posterior walls are separated, especially the latter, by a wide space from the face of the electro- plax. This space is occupied by connective tissue with sparsely scattered cells and a jelly-like appearance. That on the anterior side is traversed by the very numerous nerve-fibres which branching dichotomously pass towards the electric layer, while that on the posterior side is traversed by blood-vessels. During the earlier stages of development the electric organ 216 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. increases in size, partly by the adding on to it of new electroplaxes formed at its periphery, hut the marked growth which takes }>la< -e in the organ later on is due to actual growth of the individual units which form it. Thus comparing a skate of 180 cm. length with one of 45 cm. the individual electroplaxes are found to have increased in size practically in the same proportion as has the body as a whole. The above description deals with the development of the electric organ as it takes place in Raia batis. In other species of skate the process appears to be similar as regards its main features, but it is interesting to notice that the relative expansion of the front end of the muscle-fibre to form the electroplax is much less pronounced in cer- tain species than is the case in R. batis. Of the species so far investigated fi. radi- ata shows the least ad- vanced stage of evolu- tion. In this species (Fig. 119, A) the elec- troplax is, as in vari- ous other species (e.g. R. circularis and It. fullonica, Fig. 119), in the form of a cup rather than a disc. In R. radiata the wall of the cup is very thick and retains throughout life only slightly modi- fied muscle structure. The electric layer is relatively feebly devel- oped, the thick alveolar layer is represented by hardly modified IK ..leiiiina and the tail is only comparatively slightly degenerate. 'I 'he skate has been taken as the basis for the description of the development of the electric organ since the phenomena concerned have been particularly clearly worked out in this lish. In the Torpedoes tin; electric organ develops from muscles in the region of ihe viseeral arches l.y very similar stages. As regards the electric "i I :> : ..ur knowledge is still very insufficient. In .Mormyrids and in (,'yttun'fnx they are clearly modified portions of i IK- lateral muscles as in i In- skat <•; in ^«fro«0ptt0(Dahlgren)theyaie i"-lieve I to be derived from eye - muscles ; while in Mn/o/iffruriis though ;_'<• ne rally believed to be modified skin glands they are • -I b\ I'alil-ivii ;md Kej.ner (I'.H'S to In- more probably of muscular origin. Fi<;. 119.— Illustrating the adult condition of the electro- plax in (A) Ruin ,;i,li,it(i.t (B) R. circularis, (< //. ;md (D) R. batis. (After Ewart, 1892.) iv LATERAL MESODERM 217 LATERAL MESODERM. — The lateral mesoderm forms th<- lining of the splanchnocoele. Its superficial layer persists throughout life as the coelomic ur peritoneal epithelium, while its deep surface produces by proliferation abundant inesenchy me cells which forma con- nective-tissue backing to the epithelium. The development of muscle- fibres, which is so characteristic a feature of the coelomic lining in the dorsal or myotomic region, is here to a great extent suppressed, this portion of the mesoderm no longer playing any part in the muscularization of the body-wall. It still however takes place in restricted areas, smooth or striped muscle-fibres being developed in those portions of the mesoderm which invest particular organs such as heart and blood-vessels, alimentary canal with its appendages, oviduct. The development of the musculature of the heart will more suitably be treated in the chapter dealing with the vascular system. As regards the muscles of the gut-wall we have little detailed knowledge, what there is being related mainly to the musculature of the skeletal elements contained in the visceral arches. In the larva of Lepidosiren the important point has been estab- lished by Agar (1907) that the sheath of muscle which forms the con- strictor of the pharynx is of double origin, its ventral and larger portion being a development of the splanchnic mesoderm covering the pharynx, while its dorsal portion arises as an outgrowth from one (y) or more of the occipital niyotomes. The fact that muscular tissue derived from myotomes may join the splanchnic muscle to form part of the muscular sheath of the alimentary canal is of importance (1) by impressing upon us that an apparently homo- geneous muscular apparatus may really be heterogeneous — muscular- ized from two quite distinct sources, and (2) by indicating the possibility of splanchnic musculature being replaced by myotomic or conversely. Obviously the muscular sheath just mentioned might, by reduction of one or other of its component parts, become purely myotomic or purely splanchnic. As will be gathered later (Chap. VII.) the point is an important one from its bearing upon the discussion of certain problems of morphology. RENAL ORGANS. — In triploblastic Metazoa the function of excreting nitrogenous waste products is commonly carried out by tubular organs to which Lankester (1877) gave the name nephridia. Under this term were included the excretory tubes of Chaetopods, Molluscs, Rotifers, Trematodes, Turbellarians and Vertebrates. Subsequent research soon brought to light an important structural difference as regards the inner ends of these nephridial tubes in different groups of animals. In certain groups the tube possesses at its inner end an open funnel or nephrostome l which leads from the coelome into the cavity of the tubule, while in other groups the inner end of the tubule is without any coelomic funnel but is on the other hand provided with an arrangement of flame- 1 Gtoodrioh terms such runnels " coelomostomes " and uses the word nephrostome in a special restricted SCUM'. 218 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. cells. The appreciation of this difference gave rise, not un- naturally, to a suspicion — which would now appear to be unfounded — that under Lankester's name nephridium were included excretory tubes of two morphologically distinct types and the use of the word nephridium was often restricted to the one of these types in which the coelomic funnel was present. Later researches brought out the fact that in some cases — certain Polychaete worms — the excretory tube may possess both flame-cells and coelomic funnel. And finally the hypothesis was developed — by Meyer and especially Goodrich (1895) — that the nephridial tube and the coelomic funnel were originally quite distinct organs with separate openings to the exterior. On this view the primitive excretory tube or protonephridium (Goodrich) was provided with flame-cells at its inner end, while apart altogether from it and opening independently to the exterior was the coelomic funnel which formed the primitive exit for the reproductive cells. In the course of evolution there came about a fusion of the two structures, the coelomic funnel becoming as it were grafted on to the nephridium and in many cases shifted up the wall of the tubule right to its inner end. Such a compound organ (Nephromixium — Goodrich) might retain for a time both flame-cells and coelomic funnel — as in the Polychaetes alluded to above — or the flame - cells might, as is more usual, disappear leaving an excretory tube possessing at its inner end a coelomic funnel which shows no trace of its morpho- logically independent origin. To support the hypothesis which has just been outlined there is brought in the evidence of embryology which testifies (see Vol. I. p. 158) that the main part of the excretory tube is developed as an ingrowth of the ectoderm, while the coelomic funnel arises as an outgrowth of the mesoderm. This hypothesis has met with very general acceptance not merely with regard to the excretory organs of Annelids alone but also as a theory of the morphology of excretory tubes in general. As, however, the writer of this volume takes up a somewhat different standpoint it will now be necessary to state shortly what that standpoint is. The word nephridium will be used in the original sense as meaning an excretory tube whether possessing flame -cells or a coelomic funnel at its inner end. Physiologically the open funnel and the flame-cell appear to be associated primarily with two different sets of spaces. The funnel is associated with coelomic spaces and it serves to transmit to the exterior the products of the liniug of such spaces — fluid, excretory, or reproductive. Th<- flame-cell is associated rather with the meshes of the mesenchymatous spongework: it serves to filter oil' from these spaces watery fluid containing excretory salts in solution. The activity of the "flame" is in direct relation to the pressure of fluid within .ices: if the pressure is lowered by making a minute |.im«-i im- in th«- hody wall tin- mov-nu'iit at once ceases — to jv KENAL ORGANS 219 commence again when pressure is restored. This association of flame-cells with the spaces of the mesenchyme is seen in the more lowly forms in which they occur and it is therefore justifiable to regard it as primitive in spite of the exceptional cases in which flame-cells occur in coelomic cavities. The question of the relative antiquity in evolution of coelomic funnel and flame-cell is one which cannot be decided with certainty, depending as it does in turn on the unsolved question as to whether coeloine or mesenchyme was evolved first. Looking to the occurrence of mesenchyme in Coelenterates (e.g. the Alcyonarians), in animals in which there is not as yet any closed-off coelome, the balance of probability seems to be on the whole in favour of the flame-cell having originated first, in other words in favour of the original nephridium being of the type called by Goodrich protonephridium. The fact that existing excretory tubes of this type arise from the ectoderm is also an argument for its antiquity, as it seems natural to suppose that primitively excretory products were got rid of at the outer surface of the body. In the primitive ancestral form the genital cells, formed by the lining of the enterocoelic pouches, would reach the exterior through the protostoma or primitive mouth but as evolution proceeded and the coelenteric pouches became separated from the enteron to form a closed coelome1 another mode of exit would have to be evolved. The natural mode of such exit would be by rupture of the coelomic wall at its weakest spot. Such weak spots would be provided at points where the cavity of the nephridial tube came into proximity with that of the coelome. At such points rupture would take place and the tendency would be for such a temporary rupture, at the time of maturity of the genital cells, to be replaced by a permanent 2 opening from coelome into nephridium. This permanent opening would be the coelomic funnel (coelomostome, nephrostome in the original sense). The coelomic funnel, though originally developed to transmit the genital cells, would necessarily also serve as an exit for super- fluous coelomic fluid, and the fluid so transmitted would necessarily serve incidentally to flush out excretory matters passed into the lumen of the tube by the activity of its walls and would thus fulfil the function originally fulfilled by the fluid drawn in by the flame- cell. The function of the flame-cells being in this way otherwise provided for they would tend to disappear. The nephridial tube thus came to transmit (1) the reproductive cells and (2) the highly poisonous excretory products. There is, as it appears to the writer, ample evidence that under such circuni- 1 The argument involves as will be seen the assumption that the coelome was in its evolutionary origin enterocoelic. This assumption appears to be justified by the numerous cases in which the coelome so arises in ontogeny. - An actual case where such a temporary rupture, brought about at parturition, llM come to l»e a fixed character of the species and develops independently of mn-luinical rupture, is seen in the "median vagina" of en-tain Marsupials. 220 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. stances the tendency of subsequent evolution would be to separate from one another the paths to the exterior of the genital cells and of the poisonous excretory products respectively. It might fairly be anticipated for physiological reasons that there would be such a tendency but that it actually exists is demonstrated by the facts of comparative anatomy and embryology. Over and over again we find cases where such separation has undoubtedly come about. For example in the evolution of the Gasteropoda the right nephridium has lost its excretory function and come to be merely a genital duct. In Vertebrates there are several familiar examples of parts of the renal system which have to do with transmitting genital cells becoming separated off from those which retain a renal function.1 The present writer then believes the balance of probability to be in favour of the evolutionary origin of the type of nephridial tube commonly met with in coelomate animals, possessing a coelomic funnel or nephrostome at its inner end, having come about in the manner outlined above. The essential difference between the view here outlined and that developed by Goodrich is that it rejects the idea that evolution has brought about a more and more intimate connexion between originally independent genital funnel and nephridial tube as opposed to physiological probability. On the contrary it regards the funnel as having opened into the tube at the time of its first appearance, the progress of subsequent evolution having been in the direction of separating genital funnel and nephridial tube and not of uniting them. Even in the case of Polychaete worms the arguments against interpreting the anatomical arrangements in different genera as illustrating evolutionary sequence in the reverse order to that believed in by Goodrich seem unconvincing and insufficient to counterbalance the weight of physiological probability. In the case of a tube leading from the coelome to the exterior the two ends are almost of necessity inesodermal and ectodermal in their nature respectively. Consequently , the fact that the " nephromixium " has such a twofold origin in ontogeny does not appear to the present writer to constitute evidence of any particular 'it that it actually arose in phylogeny by the fusion of two pre- existing independent Drains. As regards the proportion derived from the two layers the probability would be that the specially excretory portion was mininally ectodermal — excretory products being naturally got rid of by the outer surface — and that the portion specially concerned with the M-.-tiiu^ rid of coelomic products \\mild be mesodermal — arising as a hul^in^ of the coelomic lining. Accepting as a working hypothesis that the ncphridial system of tuhi-s with their ii'-phroxioiiies arose in the manner outlined above, it is important to U-ar in mind how greatly the system would he influeneed in its suhsequent evolution by the establishment of Ion "I ili'- Mullrri.-oi ilin-t II.HII UK- kiiliiry lyitaiO "r the Hcparation of tin- im,,l ••nlli-Hiux tiil.i-s I'mm tin- Wollliaii dud. iv NEPHK I J > I A L OKGANS 221 circulating mesenchyme or blood. This would render possible the shortening up of tin- ni-pliridiiil tubes and the more definite localiza- tion of the excretory tissue. Whereas the original flame-cell type of excretory apparatus was diffuse — the flame-cells being scattered throughout the mesenchyme sponge-work — as it is still to be seen in the more lowly organized forms— it would now become compact, the waste-products being brought to it by the movements of the circulating blood. NEPHRIDIAL ORGANS OF VERTEBRATES.-^-Before passing on to the details of development of the renal organs in Vertebrates it is necessary to notice one or two points of general importance regard- ing the morphology of these organs within this particular phylum, and also to define precisely the sense in which certain technical terms will be used. In the first place the kidneys or renal organs of Vertebrates are built of tubules each of which is a nephridium according to the original definition of the term. The conclusion, already arrived at, that the ancestral Vertebrate possessed a completely segmented coelome, carries with it the further conclusion that in all probability a pair of nephridial tubes originally opened to the exterior from each segment. A characteristic feature however of the Vertebrates (with the exception of Amphioxus) is that the nephridia open not directly to the exterior on the surface of each segment as in a typical Annelid but into a longitudinal duct which passes back along each side of the body and communicates at its hind end with the cloaca. The whole series of nephridial tubes on each side of the body is known as the archinephros l and the duct as the archinephric duct. In the embryos of Vertebrates development takes place from the head end backwards. We should therefore expect the nephridial tubules to appear in regular sequence from before backwards. It is however highly characteristic of the Vertebrate that the tubules, instead of developing in this regular sequence, develop in three batches one behind the other — an anterior, a middle, and a posterior. These constitute respectively the pronephros, mesonephros, and metanephros (Lankester, 1877). In many of the lower Vertebrates there is no separation between mesonephros and metanephros, the two forming a continuous structure which acts as the functional kidney. Such a type of renal organ consisting of the series of tubules corresponding to mesonephros together with metanephros may conveniently be termed the opisthonephros.2 Of the four types of kidney just mentioned the first — the pro- nephros— forms the functional kidney in larval Vertebrates. It is well seen in the larvae of Crossopterygians, Actinopterygians, Lung- fishes, and Amphibians, while, as Sedgwick first pointed out, it is 1 Archinephron, Lankesiter (1877). Price's term Holonephros is also frequently iii the same sense. 2 In analogy with the use of the word opisthosoma in the group Araclinida. 222 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. reduced in forms with richly yolked eggs where the development is not larval. Its reduction or disappearance in the last-mentioned forms may probably be due to the facilities afforded for getting rid of excretory products by simple diffusion from the blood circulating on the surface of the yolk-sac into the surrounding medium. The opisthonephros forms the functional kidney in the adults of most if not all anamniotic Vertebrates. Distinct mesonephros and metanephros are found in the Arnniota — the mesonephros being functional during the later embryonic period and in the Reptiles during the first few months after hatching, B FIG. 120. — Renal organs of the Frog (Rana temporaria) as seen from the ventral side after the ventral wall of the splanchnocoele and the portion of the alimentary canal contained within it have been removed. (After Marshall, 1893.) A, 12 rnm. tadpole ; JJ, 40 nun. tadpole ; C, froj? at time of metamorphosis. A, dorsal aorta ; A.r, aortic root : /•', fatty body ; sr.mnrnts ; ('. li> segments ; 1). •_'? segments. .!<-hin«'plirir duel ; /m, pronrpl.rir tubule. Tin- Human figures are placed The first signs of the pronephros make their appearance — in embryos with about 9 or 10 mesoderm segments — in the form of biil^iiiLrs (»ui wards of the outer or somatic wall of the nepliroiimie of segments IV and V. These outward 1 minings are the rudiments »f th'- proiK'plmc tubules. A third soon appears in segment VI Fi'_r. 1'JL'. A., of, also Fig. 123, A, pn). The three rudiments urn\\ actively in length pushing their way tailwards along the body just external t" the n&phrotomee, They emne 1<> he in close contact and presently fuse to l«»rm a rod-liko structure (Fi-\ I--. 1'^ which continues in extend backwards tu\\ards the tail and becomes tubular Umni i, cavity secondarily in its interior. This, at first Holid, rod-like structure (Ki.LT. 1-U, i\ u.n.nt of /V/////v/-//.s stairs 20, 23, ami 'JH j»as>iiiK Iliron^'li tin- nnllim-nl »( n<-pln osluinr B, which ift seen )irnj«-i-tiii;r out wants from tin- wall of 111.- iicpin-' ,il aorta; '''«•/, co«lmii<- MVlfe) : M6, uiyoooela ; ; .HirliMal \.-il, ; split, re].' In other Verte- 1 »rates possessing a functional pronephros the appearances seen in early stages are readily reconcilable with those described above for Hypogeo- phis, and we may take it that, apart from variations in detail, this represents the normal mode of development of the organ. CROSS OPT ERYGII. - In% Crossoptery- gians, so far at least as Polypterus — the only member of the group investigated — is concerned, the first rudiments of the pro- nephric tubules are in the form of pro- jections which pass outwards and back- wards from the exter- nal side of each of the anterior nephrotomes (Fig. 125, A). The number of these tubule rudiments pre- simiiilily varies, seven being seen in one specimen and nine in another. Apparently the tubule rudiments become fused at their outer ends to form a solid mass — the rudi- ment of the aivhi- ncj.liric duct. In the stages shown in V\g. L26, A and B, five lulmlo rudiments are si-en passing ;it their outer ends into the IV PKONEPHROS duct rudiment. As development goes on however the tubules belonging to ne])hrol.oiues I, III, and IV, those labelled A, C and I) in the figure, become reduced in size and finally FIG. 126. — Dorsal view of pronephros of Polypterus at stages 20, 23, 24 + , 25 and about 28. a.n.d, archinephric iluct. Tin' tubule rudiments arc indicated by letters, the nephrotomes by l!nni;iii numerals. disappear, while B and E on the other hand increase in length and become the functional tubules. The anterior end of the archinephric duct becomes gradually modelled out of the solid rudiment already referred to in the way indicated in the figure. ect. ~an.d. ncB end ncY. Fie. 1-J7. — Part of ;i longitudinal vertical section through the series of nephrocoeles in the pronephros of Polypterus — stage 24+. a.n.d, archinephric duct; ect, ectoderm : rml, endnderm : /(-•./.'. nephrocoele •' IT ; nephrocoele "F." After it has assumed its definitive tubular form this front part of the archinephric duct commences to grow actively in length, it becomes thrown into complicated coils and forms a large fraction of the entire bulk of the pronephros in its early functional stages. 230 EMBKYOLOGY OF THE LOWER VERTEBRATES en. •: and 10 15 In its later functional stages the pro- nephros reaches relatively enormous bulk, occupying the whole thickness of the body- wall, but in these later stages the two tubules become much elongated and coiled as well as the duct itself. The nephrocoeles belonging to the various nephrotomes which develop tubules form a series of closed cavities lying in a row one behind the other (Fig. 127, nc.B, nc.F}. They are for a long time, in Polypterus, the only coelomic spaces which are widely open (Fig. 125, B). As development goes on the nephrocoeles connected with the func- tional tubules (B and E) become more and more dilated, their' wall becoming thinner as they do so, and the floor bulging into the cavity to form the glomerulus. Eventually the cavity of the nephrocoele becomes continued ven- trally, a split spreading downwards to form the splanchnocoele, into which the nephrocoele opens freely. The portions of splanchnic mesoderm to which the glomeruli are attached, i.e. the floors of the original nephrocoeles, become folded in towards one another, as the splanchnocoelic cavity dilates, to form the dorsal mesentery so that the glo- meruli are eventually borne by the mesentery one on each side. Meanwhile the nephrocoeles belong- ing to the tubules which atrophy gradually shrink up and disappear, ami as they do so the two large functional nephrocoeles increasing still more in si/e meet and their cavities as well as their glomeruli become continuous. No definite constrictions (peritoneal canals) are formed between nephrocoeles and splanchnocoele, unless possibly during late stages, but the dorsal portion of. the more posterior nephrocoele becomes rut oif from the spl.inelinocoele by another method — the free ed^e of the ^lonierulus (••lining t«i fuse \\itli the soinatoplemv " Metotic ".posterior to the otocyst. so as to form a floor to the nephrurorle. 20 25 30 39 I i... 128.— Renal organs of th -i-li- ul a l'p>t'i|)ti-iii> larva of stage in ;i i.-. on-tiiirtioii by M. 'luH ; ..y,, '/, arcnuii'pniic auci , --y, opumo- IM-JIMI «• lubuli^ : y«/i. pi"m-pli! (uipit-il !••! I.-IM iii'liriil'- iii-plirii>tuim-* ami tin- li-iiH-.i in.-1 .;im-iits. iv PRONEPHROS 231 This posterior nephrocoele is still in wide communication with tin* splanchnocoele indirectly by way of the anterior nephrocoele. DIPNOI. — In Lepidosiren and Protopterus1 the fully functional pronephros of the larva possesses usually two tubules (Fig. 128, B and D). These are the surviving members of a series of tubule rudiments extending through at least the anterior 4-7 segments but probably extending much further back. The tubules which In-come fully developed are normally "B" and "D" i.e. those corresponding to the second and fourth mesoderm segments. Thus the second tubule does not correspond with the second tubule in the fully developed pronephros of Polypterus. The tubules appear to originate (cf. Fig. 129, A, B) as in Hypogeophis except that the outgrowths from the nephrotomes are solid as in Polypterus and such is the case also with the archinephric duct rudiment. The nephrocoeles of the two main pronephric tubules undergo fusion as in Polypterus so as to form a large pronephric chamber on each side. This is continuous with the pericardiac portion of the splanchnocoele and the two glomeruli as usual become fused together to form a compound glomerulus.2 In Lepidosiren the fusion of the pro- nephric chambers takes place before the appearance of the glomerular rudiments. These appear first on the floor of the continuous cavity (Fig. 129, C, gl) and very soon undergo fusion themselves. By differential growth the root of attachment of the glomerulus becomes gradually shifted towards the mesial plane and dorsally (Fig. 129, D and E) so that it comes to hang down into the pronephric chamber or nephrocoele from a point in close proximity to the dorsal aorta. The pronephric chambers are at first perfectly continuous with the splanchnocoele which spreads outwards from them. Later on the pronephros becomes greatly enlarged and bulges across the splanchno- coele until it comes in contact with the mesodermal sheath of the oesophagus. Fusion then takes place (at the point marked with * * in Fig. 129, E) between the surfaces in contact so that the glomerulus conies to be enclosed in a secondary pronephric chamber, which however remains freely open to the splanchnocoele at its hinder end. The glomerulus becomes firmly slung diagonally across this chamber by its tip undergoing fusion on the ventrolateral side of the chamber with the mesoderm investing the pronephros. In Ceratodus (Semou, 1901) the pronephros probably develops in a manner similar to that described in the case of the other two Lung- fishes. The organ in its first stage is a solid projection of the meso- derm the appearance in section being similar to that figured for Lepidosiren. The portions of the rudiment corresponding to the individual tubules are in such close apposition as to be at first indistinguishable (as is often the case in the other two Lung-fishes) : 1 A large part of the investigations upon which this account is based were carried out by Miss Muriel Robertson in the University of Glasgow during 1904. 'J The word glomus is often used for such a compound glomerulus. 232 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. it is only when nephrocoeles begin to appear (in the regions of the dte FI l.m. N: >.nd nc. enl. .MJ. nit-lit ol' tin- |iroiu-i>lii-,.> in btgridosiren as -lm\\n in transverse si'rlit.n>. A, it I -'-''X1' '21 : (', HtaKe '.'44-. ".«.'/, an-liiin'idiric >hid ; •/(•/, .•ntltnli-nii ; - nt, i-iit.Ti.- cavity ; yl, ^lonn-riilus ;/.//-, lat' : iii\nt»iiii' : A, nutui'l I; m: iirphnii -.x-lf : /•, hj.-/, Mi-lerotome. lift}) ;uid sixlli thai. I In- -«•;_• incnl.Ml nature of t lit- nulniii'iil IV PRONEPHROS 233 becomes apparent. The fully functional pronephros has two tubules on each side, corresponding to the segments above mentioned : it may be presumed that these are the survivors of a once greater number, though there is no record of other rudiments having been actually observed. puc. FIG. 129A. — Development of the pronephros in Lepidosiren as shown in transverse sections. D, stage 30 ; E, stage 31+. A, dorsal aorta ; a.n.d, archinephric duct ; end, endoderm ; gl, glome- rulus ; M, liver ; my, inyotome ; X, notochord ; nc, nephrocoele ; oes, oesophagus ; pn, pronephric tubule; p.v.c, posterior vena cava ; >•/>/<•. sjilanchnocoele. ACTINOPTERYGII. — The acquirement of a thorough knowledge of the development in the more primitive members of the group — the ganoids — is an essential preliminary to the proper comprehension of the development of the more highly evolved Teleosts but un- fortunately our knowledge of renal development in the ganoids is still far from complete. 234 EMBRYOLOGY OF THE LOADER VERTEBRATES CH. The tubule rudiments appear to arise in normal fashion, as out- growths of the lateral wall of the nephrotome. These outgrowths show the familiar variation of being sometimes hollow sometimes solid. Thus in Amia according to Felix (1904) the anterior three rudiments are hollow pockets while those farther back are at first solid. Tubule rudiments make their appearance from segment III to segment XIII but here as elsewhere only relatively few of these complete their development and are to be found in the pronephros at the height of its functional activity. Thus in a six-day Acipenser larva Jungersen found six functional tubules while in Amia Felix finds only a single tubule functional. In the latter case the tubule opens from a large pronephric chamber apparently formed by the fusion of at least three nephrocoeles. The tubule belongs originally to the most anterior of these and corresponding to it there is present a single open peritoneal canal. Later on this becomes replaced functionally by another peritoneal canal situated farther back. In Lepidosteus the functional pronephros has at least three tubules each with its nephrocoele (Felix, 1904). As in the case of the Lung-fishes the dorsal part of the splanchno- coele in the pronephric region becomes floored in by the approxi- mation of the mesial surface of the pronephros to the lateral surface of the oesophagus (cf. Fig. 129 A, E) so as to form a secondary pro- nephric chamber. In Lepidosteus this forms a widely patent cavity with which the first nephrocoele becomes completely merged and which remains ventrally in continuity with the main splanchno- coele by a narrow richly ciliated tubular channel. In Acipenser the first nephrocoele undergoes a similar modification while the remain- ing five are fused with one another but isolated from the splanchno- coele. TELEOSTEI. — The development of the renal organs has been worked out in detail in the case of the genus Salmo by Felix (1897). In this case the myotojnes are already separate from the more ventrally situated portions of the mesoderm at a very early stage. The first rudiments of the pronephros are in the form of a series of somewhat conical, segmentally arranged, solid projections from the median edge of the lateral mesoderm towards the mesial plane. These projections — five in number (segments 3-7) in a 26-day Trout — are probably to be regarded as nephrotomes which have been precociously separated from the myotomes, if indeed they ever were continuous. These five nephrotomes soon come into intimate contact so as to be no longer distinguishable. They now together form a continuous mass of mesoderm the so-called pm iirpliiir fold. The dorsal and outer portion of this mass Invmiit's nipped oil' to form the anterior portion of the an-hiin-phric duct except at one point uln-m a connecting isthmus remains to form a in I •ill.-. Tin- nicHial portion of the mass becomes the wall of the single pronephric chamber. iv PRONEPHROS 235 The whole mass is at lirst solid, the cavity of duct, tubule, and proDephric chamber, developing secondarily. The cavity of the pronephric chamber is for a time continuous with the split-like splanchnocoele, but it soon becomes constricted off from it and forms a completely closed cavity. Bearing in mind the segmented condition of the pronephric rudiment in its first stage of development and the process of fusion of successive nephro- coeles which takes place in Ganoids, we may conclude that the pronephric chamber of the Teleost probably represents a number of nephrocoeles fused together. The single pronephric tubule is very possibly the same member of the series as that which occurs in Amia although this has not yet been actually determined. A remarkable peculiarity found within the group Teleostei is that in a few genera (e.g. Fierasfer, Zoarces, Lepadogaster) the pronephros retains its renal function throughout life (of. Guitel, 1901, 1902). 1 AMPHIBIA. — In Amphibians other than Gymnophiona the pro- nephric rudiment first becomes visible as a solid swelling of the somatic niesoderm at the level of the anterior mesoderm segments (Rana segments 2-9, Triton alpestris 1-6, Mollier). Though at first no segmentation is to be detected -by the ordinary methods of observation in this swelling it is reasonable to interpret it as representing morphologically a series of closely apposed or fused nephrotomal projections like those of Hypogeophis. This pronephric rudiment gradually becomes demarcated off from the rest of the mesoderm by a split which becomes apparent on its ventral side at first laterally and then spreads inwards. The rudiment now forms a thick flap (cf. Lung-fishes, Fig. 129, A and B) hanging down on the outer side of the mesoderm, and continuous with the somatic mesoderm along its dorsal and median edge. Segmentally arranged coelomic splits make their appearance along the line of attachment of the pronephric flap and these we may interpret as incipient nephrocoeles. The split already mentioned as demarcating the pronephric rudiment ventrally spreads round its median edge, so as to detach it completely from the (nephrotomic) niesoderm except at segmentally arranged points where a connect- ing isthmus remains as the nephrostomal end of a tubule. The pronephric rudiment now undergoes a kind of modelling process similar to that occurring in Crossopterygians and Lung-fishes, its outer portion being gradually cut off from behind forwards so as to form the archinephric duct, while the part nearer the mesial plane forms the recurrent portion of the duct with the tubules connected with it. The whole rudiment is at first solid. The earliest coelomic spaces to appear are the nephrocoeles and from these split -like extensions spread outwards in each tubule rudiment, while towards the outer margin of the rudiment the continuous longitudinal cavity of the archinephric duct develops. Of the tubule rudiments, as usual, only a few become functional— 236 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. in Anura commonly 3, in Urodeles commonly 2 (in Amphiuma 3 according to Field). Probably here as elsewhere the number is really a variable one. As the tubules develop they show active increase in length so that they become much coiled and the same applies to the part of the archinephric duct lying in the pronephric region. It is only when they first appear that the nephrocoeles show a segmental arrangement : later on they become merged in the general splanchnocoele. Along the inner wall of the dorsal portion of this cavity, i.e. the portion which represents the fused nephrocoeles, the glomerulus develops as a continuous laterally projecting fold of splanchnic mesoderm. Usually the portion of the body cavity con- taining the glomerulus becomes for a time incompletely shut off from the rest to form a secondary pronephric chamber as in Lung- fishes, the mesoderm covering the lungs undergoing fusion with that covering the bulging surface of the pronephros. The secondary pronephric chamber may in turn be subdivided by the* edge of the glomerulus fusing with the mesoderm covering the pronephros. MEROBLASTIC VERTEBRATES. — As a rule, in the Meroblastic Vertebrates the pronephros never becomes a functional organ, and correlated with this it shows a reduction in its structure. Possibly, as already indicated, this may be due to the presence of the large yolk-sac with highly vascular surface in contact with the external medium, which will facilitate the getting rid of excretory material by diffusion outwards. ELASMOBRANCHII. — In Elasmobranchs the ventral ends of certain of the anterior mesoderm segments, usually commencing with segment VII, become dilated to form vesicular cavities (van Wijhe, 1889) which are probably to be interpreted as nephrocoeles. The tubule rudiments appear as thickenings of the somatic wall of these nephrocoeles which grow outwards and being in close apposition form at their outer ends, apparently by fusion, a solid continuous pronephric swelling. The tubule rudiments make their appearance in sequence from before backwards. Different workers vary in their statements as to the number of rudiments in different forms [Scyllium, 5 — Riickert, 3 — van Wijhe; Pristiurus, 5 — Riickert, 4 — Rabl, 3 — van Wijhe; Eaia clavata, 5 — van Wijhe; It. alba, 8— Rabl ; Torpedo, 7— Riickert (Fig. 130)] from which we may conclude safely that the number of tubule rudiments is very liable to variation both as between different species and different individuals. This variability may be taken in correlation \\illi the fact, observed 1 iv van Wijhe, that in /Vv'.s/ /i/en/i/iix presents to us a mode of development IV AECHINEPHRIC DUCT 239 of the archiiicpliric duct which is relatively primitive. In a number of Vertebrates there appear to be distinct traces of the formation of the front end of the archinephric duct by fusion of the outer ends of tubuh; rudiments in a manner essentially the same as that which holds for Hypogeophis. As will have been gathered from the pre- ceding pages t his is the case with such different groups of Vertebrates as Elasmobranchs, Crossopterygians, Lung-fishes, Reptiles and Birds. end. coel. Qc '/C. -eel. P FIG. 131. — Diagram illustrating a possible mode of evolution of the archinephric duct. A, the coelomic compartments are bulging towards the nephridial tubes; B, the compartments have come to open into the nephridial tubes and the tlaine-eells have disappeared ; C, D, the external openings of t he ne]>ln idia are becoming shifted backwards so as to give rise to the archinephric duct ; E, the archinephrie duct is completely formed and communicates with the enteron through one of the segments retaining, or reverting to, its primitive enterocoelic connexion. a.n.d, archinephric duct; c.f, coelomic funnel; coel, coelomic cavity; d.o, cloacal opening of archinephric duct; ect, ectoderm; end, endcMlenn; f.<-, flame-cell ; ?i, nephridial tube. If we are justified in looking upon this mode of formation of the duct in ontogeny as relatively primitive, it obviously affords strong support to the second of the two above-mentioned hypotheses as to the evolutionary origin of the archinephric duct : the bending back of the tubule rudiments would then be interpretable as a develop- mental reminiscence of the backward shifting of their external open- ings which took place during phylogeny. 240 EMBEYOLOGY OF THE LOWEK VEETEBEATES CH. The independent backgrowth of the remainder of the duct in Hypogeophis is probably to be regarded as a case of accelerated or precocious development to allow the anterior tubules to become functional at an early stage of development before those farther back have developed. As regards the ontogenetic development of the main part of the duct in other Vertebrates we find the most divergent statements and it seems clear that this divergence can only be explained by the actual facts not being always the same. In the Sauropsida it is admitted that the main part of the duct is formed as in Hypogeophis by independent backgrowth. Amongst the Anamnia the same is said to be the case in Elasmobranchs by Balfour and by Eabl, and in Alytes according to Gasser, but other authors describe two other methods of formation as occurring. The first of these is found in Elasmobranchs according to van Wijhe, Beard, Elickert and others. According to these investigators, the archinephric duct makes its first appearance as a longitudinal ridge - like thickening projecting inwards from * the ectoderm. This becomes split off as a solid ectod.ermal rod which develops a cavity secondarily and forms the archinephric duct. Such a mode of development would be of great morphological interest as it would lend decided support to the view that the archinephric duct originated in evolution as an ectodermal groove — it being a common ontogenetic modification that what is morphologically a groove develops ontogenetically in the form of a solid ridge-like ingrowth. It has however to be borne in mind that there exists a serious source of possible error in making observations upon the archi- nephric duct in early stages. The duct lies between ectoderm and somatic mesoderm — the two cell-layers mentioned fitting close round it. During the various processes to which the embryo is subjected preparatory to being cut into sections the ectoderm usually separates slightly from the mesoderm, and the archinephric duct tends to adhere firmly to one or other of these layers. This is the case more particularly at its tip, where it is pushing the ectoderm and meso- derm apart as it grows back and is therefore in particularly intimate contact with them. It is exceedingly difficult in studying sections to distinguish with certainty between such intimate contact and actual organic continuity. In cases where the hinder part of the duct is adherent to the ectoderm an appearance is produced which simulates closely a development by splitting off from the ectoderm. ;i matter of fact C. Eabl's very careful investigations (1896) fail to confirm the ectodermal origin of the duct in Elaamobfanoha and upon the whole in the writer's opinion there does not appear to v longer justification for accept in- it us ,-ictimlly occurring. The other mode, by which the extension of the archinephric duct backwards has been described as taking place in tin- Anamnia. is that the nd. above but, making lull allowance for this it seems impossible to escape the admission that in many forms (Petromyzon, Lung-fishes, most Amphibians, Teleosts and probably actinopterygian Ganoids) the duct is prolonged bar.U wards by a process of this kind. It being accepted that in a number of Ananmia a large part of the archinephric duct arises in development by being split off from the mesoderm, we are faced by the problem how this mode of develop- ment is to be correlated with the mode of development by fusion of the outer ends of tubule rudi- ments. It may be suggested that what has happened is that the development has been accelerated — as often happens —by skipping over the early stages. The mode of develop- ment-in question may have been derived from the more primitive mode by the omission of the separate tubule stage and the passage at once to the stage in which the tubule ends are fused into a continu- ous structure. In some cases however the primitive mode of development has undergone a further modi- fication. This is exemplified by Polypterus (Graham Kerr, 1907) where the hinder portion of the duct appears to be formed by bodily conversion of the series of nephrotomes. These are not segmented but form a continuous structure which becomes converted directly into the archinephric duct. In whichever way the archinephric duct completes its extension backwards, it eventually comes to open into the cloaca. This is, in the great majority of Vertebrates, described as coming about by fusion of the previously freely-ending tip of the duct with the cloacal wall. It is obvious that such a process cannot correspond with what happened during evolution as the duct must have had its posterior aperture throughout in order to perform its function. It is possible that a clue to the evolutionary origin of the com- munication between archinephric duct and alimentary canal is given by Polypterus. It has already been mentioned that in this animal the hinder part of the archinephric duct arises by bodily conversion of the row of fused nephrotomes. Fig. 132 shows that the opening of archinephric duct into the alimentary canal presents a striking VOL. II E •net. FIG. 132. — Transverse section through Polypterus of stage 23 at level of cloacal opening. a.n.il, opening of archinephric duct into cloaca; c/, opening of cloaca to exterior; end, alimentary canal rudiment; »».*/, myotome ; N, notochord ; s.c, spinal cord. -2±*2 EMBBYOLOG1 OF THE LOWEK VERTEBRATES CH. resemblance to the primitive communication of mesoderui segment with enteron, and it is suggested that it actually is this primitive communication which has remained patent while in all the other segments it has disappeared. The two archinephric ducts open at first separately into the cloaca, one on each side. In some groups of Vertebrates however their terminal portions become gradually approximated and eventu- ally fused together into an unpaired dorsal vesicle which may undergo various modifications. In Elasmobranchs it forms the urinogenital sinus which bulges forwards and on each side becomes prolonged into the sperm-sac. In Lung-fishes it forms the cloaca! caecum : in Teleostei the urinary bladder. It is noteworthy that in the adult Lung-fish the communication of the kidney ducts with the caecum is close to the posterior opening of the latter, so that a small amount of shifting would cause these ducts to open into the cloaca independently of the caecum. This suggests a possible evolutionary origin of the allantois. It is conceivable that a caecum similar to that of Lung-fishes arose by a fusion of the terminal portions of the kidney ducts ventral, instead of dorsal, to the alimentary canal and that the ducts then came to be emancipated from the caecum which remained as a ventral diverticulum of the cloaca to form the allantois. We have no definite evidence as to the evolutionary origin of the allantois and it is well to bear in mind the possibility here indicated in addition to the simpler and perhaps more probable hypothesis that the allantois was from the beginning simply a bulging outwards of the ventral cloacal wall as it is actually in ontogenetic development. DEGENERATION OF THE PRONEPHROS. — The role of the pronephros as the functional renal organ is usually confined to comparatively early stages in development and at the end of this period, when its function is being taken over by the opisthonephros, the pronephros commences to undergo characteristic degenerative processes which normally culminate in its almost complete disappearance. In the frog (Marshall and Bles, 1890) these processes become apparent in the tadpole of about 20 mm. in length. The archi- nephric duct becomes more or less obstructed behind the pronephros and as fluid continues for a time to pass into the tubules the latter me greatly distended in places, their lining cells assuming a cloudy appearance, the cell boundaries becoming indistinct and their inner surfaces losing their smooth outline and becoming ragged. Tlx- whole orjan shrinks in size, becomes invaded by leucocytes, the nephrostomes close, one after the other, and by the end of the first year tin; whole nr#in with the adjacent portion of iln- aivhinrphrir duct has practically disappeared. Mri.ui;i\> DUOT. Throughout the series of gnathostomatoua Vertebrates, with the exception of the teleoetomatoufl iishrs, tin- oviducts are admittedly homologous. They — the Miillrrian ducts — are above ail rhara»-i»M-i/»-«l by UK- lad I hat they open freely iiit«> the iv MITLLEKIAN DUCT 243 splanchnocoele ;it their anterior eud by an open funnel (ostium t'ubae). There exists in some of the more archaic fishes what appears to be distinct evidence that the Miillerian duct has been evolved out of the tubulus juul duct of the pronephros and it will therefore be convenient to consider this evidence now. The Elasmobranchs are the fishes in question. In Torpedo (Riickert, 1888) as the pronephros degenerates its tubules become reduced to the three hindermost. Of these* three the two posterior degenerate while the other — tubule E — persists and its enlarged nephrostonie becomes the coelomic funnel of the Miillerian duct. Other workers (e.g. van Wijhe and Eabl), working on other Elasmo- branchs (Pristiurus), trace back the coelomic funnel of the Miillerian duct also to an opening derived from the pronephros and nephro- stomal in its nature, but they believe the opening to be formed not by the persistence of a single enlarged nephrostome but rather by the fusion of three or four nephrostomes together. That it is morpho- logically a single nephrostome is however rendered more probable by what we now know regarding the development of the pronephros in those of the more archaic fishes in which it develops as a functional organ. It will be recalled, for example, how in Polypterus tubule E (like B) becomes enlarged as compared with A, C, and D. A pro- nephric tubule enlarged in this manner in correlation with purely excretory needs would provide an obviously adequate beginning for the evolution of a funnel for the transmission of the eggs like that at the front end of the Miillerian duct. While the funnel of the Miillerian duct is nephrostomal in origin the main part of the duct is developed in the Elasmobranchs (Semper, 1875 ; Balfour, 1878) from the archinephric duct. The latter undergoes a process of splitting from before backwards into a dorsal and a ventral tube, the latter being at first a solid thickening of the ventral wall of the archinephric duct. Of the two tubes so formed the ventral is continuous with the pronephric funnel, while the dorsal carries the openings of the kidney tubules farther back in the series : the former becomes the Miillerian duct, the latter persists as the functional duct of the opisthonephros (Fig. 133, C, W.d). This mode of development is satisfactorily explained by the assumption that the relatively archaic fishes in which it occurs are repeating the process by which the Miillerian duct arose in evolu- tion. Such a splitting of an originally common duct into two, so as to separate the routes by which two different products reach the exterior, is probably of frequent occurrence in evolution. Good examples are seen in the splitting of the common genital duct of hermaphrodite gasteropods (e.g. the ordinary snails) to form a separate oviduct and vas deferens. It appears then justifiable to accept as a working hypothesis that the Miillerian duct arose in evolution by being split oft' from the archinephric duct and that its coelomic funnel is a persistent pronephric funnel. Turning to Vertebrates other than Elasmobranchs, well-marked 244 EMBKYOLOGY OF THE LOWER VEETEBIiATES en. pn. a.n.d. H B Wet. differences are found to exist between the phenomena as described for different groups and even for members of the same group by different observers. While some of -os. these may be due to observations being pushed to within the limits of prob- able error it is impossible to avoid the conclusion that great differences do actually exist in the details of develop- ment of the Miillerian duct. It is possible on general embryo- logical principles to arrive at an idea of the kind of variations which might be expected to show themselves from the supposedly primitive mode of de- velopment. I. The Miillerian duct might con- tinue to arise in an unmodified manner by splitting from the arctiinephric duct, its funnel being a persisting nephro- stome. II. In correlation with the fact that the one derivative of the archi- nephric duct (duct of the opistho- nephros) is required to be functional at a very early period, while the other (Miillerian duct) does not function until adult life, there would be a tendency for the two ducts no longer to keep exactly abreast in their development but to become separated, the Wolffian duct developing relatively earlier, the Miillerian relatively later. To enable this to take place, the primitive stage in which the two ducts were still one would tend to be more and more curtailed until it was eventually eliinin- 133. -Arrangement of archi- ated and the t wo ducts were independent from the beginning. III. The independently arising Miillerian duct might retain the mode Aln:i1"17' •:"-''-.naiei»mn1.;c) of exten8ioil backwards by intrinsic growth, eventually reaching and fusing \s it.h the wall of the cloaca. IV. Its separation from t lie somatic mesodt-rm mi-lit take place relatively lat.-r than its extension backwards so that it would arise in develop- ment comparat i\m I ••-!'« m- m-phric duct, etc., in embryos of F Pristiwus. (Based on Rabl's figui. l--ru;ilt- 27 mm. '•/, cloaca ; .M.d, Mulli-rnin ilnct ; lomjc ojx-iiiux of .Mnlkrian ment of pronrphros and opislhonephros, e.g. the tubule rudiment makes its appearance relatively later in the case of the latter — at a period after the nephrotoim- has become constricted off from the splaiirlmoeorlie id-Hi. A furthrr diffrrenr.- lirs in the fart llial there takes in ihr OpiflthonephrOB a -real inereasr in the number of its tubules — secondary, tertiary, eta tubules brill-- added to thosr of the ari e in characteristic fashion, An out-To\\ih hum t.li.- :urdiaii portion of the nrphrotome and iv OPISTHONEPHEOS 247 becomes constricted off as a small round vesicle with thick wall, (•(imposed of tall epithelial cells, and a small lumen. This is a secondary nephrotome. It remains for a time without change but eventually behaves very much as the original (primary) nephrotome, one wall becoming invaginated to form a glomerulus, and pocket-like outgrowths giving rise, one to a tubule rudiment, the other to a peritoneal canal. An important difference in detail is seen in the behaviour of the duct, which sends out a tubular projection of con- siderable length to meet the secondary tubule. This outgrowth arises from the duct some distance behind the point where the primary tubule opens into it. The secondary nephrotome in turn buds off a tertiary nephrotome which again behaves as before and its tubule is met by a projection from near the tip of the outgrowth of the duct which has already developed in relation to the secondary nephrotome. Consequently secondary and tertiary tubules open into the archinephric duct by a common collecting-tube formed of this outgrowth. Apparently new generations of subsequent nephrotomes may go on being formed in a similar fashion each from the preceding one until there may be as many as eight in a single segment, all of them, except the primary, opening into a common collecting-tube. The degree of development reached by the opisthonephric units is different in different parts of its length. They attain full develop- ment in the manner above described in the region of segments 50- 100. In the region in front of this (segts. 30-50) the secondary nephrotomes and their derivatives never become functional and their rudiments degenerate. Still further forward (segts. 24-29) even the primary units as a rule degenerate without completing their develop- ment. Apart from differences in detail it is clear that the primary units of the opisthonephros present the most striking resemblance to those of the pronephros and the evidence that they are serially homologous seems convincing. Normally there is a gap of a few segments between the hind end of the pronephros (segt. 15) and the front end of the opisthonephros (segt. 24) but Brauer found that even in these segments there makes its appearance a distinct nephrotome, with the vestige of a glomerulus, although it does not proceed with its development. Consequently the units of pronephros and opisthonephros (primary) are to be regarded as members of a once continuous series. That this series once extended back beyond the present limits of the opisthonephros is indicated by the fact that distinct nephrotomes are present in segments 101-104, but as was the case in the intermediate zone between pronephros and opisthonephros these do not proceed to develop tubules. ELASMOBRANCHTI. — It will be convenient now to consider shortly the development of the opisthonephros in the Elasmobranch fishes as they have provided the material for a large proportion of the most 248 EMBKYOLOGY OF THE LOWER VEKTEBKATES CH. important work dealing with the morphology of the Vertebrate kidney. It was in the opisthonephros of Elasmobranchs that \vick (1880) made his classical discovery — which forms the foundation on which our present-day knowledge rests — that the nephridial tube of the Vertebrate is a development of the coelomic wall, of that part of it which we now call nephrotome or proto- vertebral stalk. Since the date of Sedgwick's work the opistho- nephros of Elasmobranchs has formed the subject of detailed studies by Riickert, Eabl, van Wijhe, and other well-known investigators. Owing to the lower end of the myotome in these fishes becoming displaced in a lateral direction, through the accumulation of ruesen- chyme between it and the mesial plane, the protovertebral stalk becomes rotated outwards so as to assume a nearly horizontal position (Fig. 134, A, nt), the originally dorsal end of the stalk becoming now ect nt my. FIG. 134. — Origin of opisthonephric tubule in Elasmobranchs. A, Pristiurus (after C. Rabl, 1896) ; B, variation observed in Torpedo (after Riickert, 1888). A, dorsal aorta ; a.n.d, archinephi ic duct ; ect, ectoderm ; my, myotome ; nt, nephrotome ; sjHc, splanchnocoele ; t, tubule rudiment. external, and the originally external side coming to be ventral. The duct (a.n.d) thus comes to lie ventral to the nephrotomes instead of being on their outer side as was the case originally. The nephro- tomes become isolated from the myotomes by their ends next the myotomes breaking up into mesenchynie. The result is that the nephrotomes now form a series of blindly ending pocket-like pro- jections of the coelomic epithelium which curve outwards dorsal to the duct. Each pocket has an epithelial wall and it is noticeable that tin- somatic portion of the wall is markedly thicker than the splanchnic, the cells of tin- former being taller and more columnar in shape. As development goei on it is found that the thicker more columnar e.-lled portion of the wall of the pocket extends for some distance on to its dorsal wall, ami this is interpreted by Kiiekert and L'aM as meaning that tin- somatic epil helium is spreading inwards towards tin; mesial phm-. n-plan irj splanchnic, epithelium as it does so. In view of what we know regarding the development «>l other groups it more le.isonahle to <-\phin thr appearance as being expn iv OI'JSTHONEPHROS 249 of outward growth on the part of the somatic wall of the nephro- toine — the terminal portion becoming the tubule rudiment. Riickert (1888) figures a remarkably interesting variation which he came across in Torpedo in which the separation of nephrotome from myo- tomc had been delayed. In this (Fig. 134, B) the nephrotome still forms a distinct stalk continuous with the myotome and the tubule rudiment is visible as a pocket-like projection of its somatic wall, agreeing exactly \\itli tin- assumedly primitive type of tubule rudi- ment as it occurs in the pronephros of one of the lower holoblastic Vertebrates. To correlate this specimen with the normal condition all that is necessary is to imagine the portion of the stalk next the myotome to have disappeared by becoming resolved into mesenchyme. The, rest of the stalk together with the tubule rudiment would then remain as a curved blindly ending pocket l the tip of which would represent the tip of the tubule rudiment. This curved pocket-like structure increases in length, its tip conies into contact with, and later fuses with, the dorsal wall of the duct and it is in this way converted into a short tube opening at its inner end into the splanchnocoele and at its outer into the duct. The tubular structure so arising does not retain its simple tubular shape but undergoes the series of changes shown in Fig. 135. Its cavity dilates in the middle to form the definitive nephrocoele, the cavity of the Mal- pighian body (m.b) ; its splanchnocoelic end becomes relatively narrow to form the peritoneal canal (p.c) 2 : its outer end becomes also relatively narrow and it is this outer portion which undergoes an immense increase in length and becomes the functional tubule. Opisthonephric rudiments appear in the fashion above indicated throughout the greater part of the length of the body where the splanchnocoele is present. They commence behind the pronephros (about .the 8th or 9th segment) and extend back to the cloaca or a few segments posterior to it. In the latter case the postcloacal rudiments do not come to anything. Their occurrence is to be looked on as a reminiscence of a period when the alimentary canal and splanchnocoele extended farther tailwards. It is to be noted also that the group of tubules at the front end which subserve a genital function in the male similarly appear only as transient rudiments in the female. The portions of the opisthonephros which perform an active renal function increase much in bulk and this, as elsewhere, is brought about not merely by the great increase in length of the individual tubules, but also by the addition of numerous new tubules, each with its Malpighian body etc., of the second, third and so on, order. Probably (Balfour, 1878) these arise by a process of budding of the nephrotomes of a similar type to that which occurs in fft/po- 1 Care should be taken to avoid the not uncommon error of referring to the whole of this structure as the " tubule-rudiment." 2 Attention has already been drawn (p. 227) to the undesirability of applying the misleading adjective " nephrostomal " to this canal. 250 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. S^\ geopliis though in the case of the Elasmobranchs the details seem to be more obscure and the descriptions are conflicting. The male Elasmobranch is an excellent example of a Vertebrate in which the nephridial system is responsible for the function of conveying to the exterior both the renal excretory materials and the reproductive cells and we find a well-marked tendency to separate the routes of those two products to- wards the exterior. This separation is brought about by the shifting back- wards of the openings of the collecting- tubes of the posterior, purely renal, part of the opisthonephros, so that instead of being spaced out along the course of the duct they come to be coincident with its opening into the urinogenital sinus. This backward shifting is most pronounced, and it also makes its appearance earliest in ontogeny, in the most anterior of the tubules in question. It is accompanied by a fusion together of the terminal parts of the collecting -tubes into a continuous ridge-like projection of the dorsal wall of the duct, in which the individual lumina are for a time greatly reduced or even completely obliterated. Eventually, as a rule, the ridge splits up and the terminal parts of the collecting - tubes regain their indi- viduality— forming a group of -distinct tubes, varying in number in different forms from about 4 (Spinax) to about 15 (Acanthias), and converging so as to open close together into the urino- genital sinus. In some cases the split- ting apart is not complete and more or fewer of the tubes may be united to- gether to form a longitudinal "ureter. ' The mode of origin of the Mal- piirhian body — the definitive condi- tion of the nephrotome — from which each opisthonephric tubule leads has already been indicated. It is for a time rounded in form (Fig. 135) but eventually one portion of its wall — varying greatly in position — comes to bulge inwards to form tin L!lonierulu8 containing a loop of blood-vessel. Tin- peritoneal canal during development lengthens ()llt consider- ably Mij L36 I'; and becomes narrower. This nairo\vin- is most marked in the posterior third of the <.pisthonephros and in this we FIG. 135. — Illustrating the later development of a segmental unit of the opisthonephros in male J'iMiuru.s. The figure is in each case a view from the mesial side. (After C. Rabl, 1896.) A, 1'jtli unit ol 17 iiiin. i-mln-yn ; 15, Ifith tt'6 Him. ; r, l.Mli unit <.t L'.V:I nun. ; I), '-'Mil unit from saim- I'nibiyo as C. ni.li, Malpi-liian luxly ; /*.<-, peritoneal canal ; //./, pi-ritonral funnH ; /. luliiilr. iv OPISTHONEPHROS 251 see what is probably the expression of a general tendency for the portion of coelome containing the glomerulus to become more and more completely isolated from the main splanchnocoele as the renal unit becomes more and more highly evolved. Eventually, in the adult of the majority of Elasmobranchs, the peritoneal canal becomes completely obliterated, but in a considerable number of others l this happens, if at all, only towards the anterior and posterior ends of the opisthonephros so that the greater part of the organ retains open peritoneal funnels throughout life. Bles (1897) has made the interesting suggestion that there is a physiological correlation between the persistence of open peritoneal funnels and the absence of abdominal pores — secondary perforations of the wall of the splanchnocoele in the neighbourhood of the anus which make their appearance, at a late period of development, in various Elasmobranchs and other Vertebrates. URODELA. — The third type of development of the opisthonephros amongst the more primitive Vertebrates is found in the Amphibians, especially in the Urodeles. The excellent account given by Fiir- bringer (1877) still forms a thoroughly adequate basis for the description. The Amphibians possess, as has already been shown, a large and highly developed pronephros amply sufficient for their excretory needs during early periods of development. In correlation with this there is marked delay in the development of the opisthonephros, the myotomes having already become separated and their stalks or nephrotomes breaking up into mesenchyme before the opisthonephric units make their appearance. The rudiments of these units — the nephrotomes — become reconstituted in the midst of the mesenchyme as solid cellular strands which may retain their rnetameric arrange- ment (Amphiuma — Field, 1891; anterior segments in Triton, AmUystoma, etc.) but usually have completely lost it. Each of these nephrotome rudiments is a solid strand of cells which curves out- wards dorsal to the duct. In the anterior region where, as is specially clear in Triton, the inner end of the strand is for a time continuous with the lining of the splanchnocoele, the general arrangement is clearly the same as that of the Elasmobranch (cf. Fig. 136, A, with Fig. 134). The splanchnocoelic end of the nephrotome disappears for a time while the main portion develops a cavity in its interior and becomes converted into a vesicle with epithelial wall lying immediately dorsal to the duct (Fig. 136, B). This vesicle becomes elongated in a mediolateral direction (? by active growth of its outer wall) and then assumes a characteristic curvature first ^- and then v/vlike in shape (Fig. 136, C). The mesial end of the gives rise to the Malpighian body, the remainder to the actual tubule, its outer end undergoing fusion with the wall of the duct (Fig. 136, D). The tubule grows rapidly in length and is forced into complicated 1 E.g. Cestracion philippi, Rhino, squatina, Scyllium canicula,S. slellarc, Pristiurus Spinax niyer, Acanthias vulgar is, Scymnus lichia — (Bles, 1897). 252 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. n and *plc. }'.ji'>. Transverse sections showing various staffs in the development of the OpictbooephroC. (After FurLiin^-i, 1877.) • 14 MIDI 'lulu. 17 MUM. ; I 'inn ml ni Mrrlun.-j.lnic d n:i«l ; •/,//-, sjilaiiHn.'.rc .<-|i- ; /. tMl.nl.-; n, I'-i, /:(, pin,,., lllljlll.- Illiir coils and windings as it does so (Fig. 136, E) while the Malpighian body dilates and its dorsal wall becomes invaginated to form the glomerulus. As a rule the primitive con- tinuity of nephrotome with the splanchnocoelic lining disappears in the Amphibian as already indi- cated, but it becomes re-established by a peritoneal canal developing secondarily (Fig. 136, D) as an out- growth, arising in Urodeles usually from the neck of the Malpighian body and in Anura from a point farther down the apparent tubule, which grows towards and fuses with a thickening of the coelomic epithelium. Such displacements of the communication between Mal- pighian coelome and splanchnocoele are probably of a similar nature to those mentioned in the case of the pronephros of Hypogeophis (see p. 226). In those parts of the opistho- nephros which are actively renal in function, i.e. the hinder portion in Urodeles and the greater part of the whole length in Anura, there takes place great increase in bulk, associated with the development of generations of subsequent tubules. Such secondary, tertiary, etc. tubules make their appearance amongst the mesenchyme in the form of cellular strands which resemble closely — both in their original appearance and in the series of changes which they j»a.ss through —those from which the primary dm, cuts arise (Fig. 136, F, /;). Kyentiially the secondary tubule comes to open into the primary luluilc. I In- terminal section of which MHIS I'nnns a collecting- 1 ul>f com- mon t<> both, while Mie tertiary tubule .similarly comes (<> oj.eii into iv OPISTHONEPHEOS 253 the secondary. As tin- various generations of tubules go on with their development, undergoing the same histological differentiation ju id increasing enormously in length, they become inextricably mixed up together to form the compact fully developed opisthonephros of the adult. Kventually, in the Urodele, the duct is slightly displaced out- wards so as to leave a distinct gap between it and the opisthonephros across which pass the terminal parts of the collecting-tubes. In the male Urodele the openings of these become, as a rule, shifted back- wards to the hind end of the duct as in Elasmobranchs. The Amphibia alone among tetrapod Vertebrates retain the relatively primitive feature of possessing open peritoneal funnels in the adult, and they can be excellently demonstrated with their actively moving flagella by examining the slender anterior portion of the excised and still living kidney of a female Urodele in normal salt solution under the microscope. In the anterior genital portion of the opisthonephros of the male they as a general rule (not in Spelerpes, Spengel) remain however obliterated. In the Anura (Nussbaum, 1886) the peritoneal canals at an early stage of larval life lose their connexion with the Malpighian body or tubule and establish a secondary connexion with the blood spaces between the tubules, thus affording a route by which the fluid in the splanchnocoele is returned to the blood, analogous to that provided by the lymphatic system in higher Vertebrates. AMNIOTA. — In the Amniota the opisthonephros of the Fishes and Amphibians is represented by the mesonephros and metanephros — and it will be convenient to consider the mesonephros first. MESONEPHROS OF BIRDS. — As has already been pointed out one of the marked differences between Amphioxus and the Craniata is that in the latter segmentation is no longer apparent at any stage of development in the ventral or splanchnocoelic region of the meso- derm. The Amniota show a further accentuation of this difference inasmuch as the loss of mesodermal segmentation has extended so far towards the dorsal side as to involve the region of the nephro- tomes. In the early ^embryo of the bird the nephrotomic part of the mesoderm has the form of an unsegmented mass — the intermediate cell-mass — showing more or less distinct traces of being composed of a somatic and a splanchnic layer continuous with the correspond- ing layers of the splanchnocoelic mesoderm and of the myotome. Although the intermediate cell-mass no longer consists of discrete nephrotomes, traces of its primitive segmental nature persist in its connexions with the segmentally arranged inyotomes and in the fact that its connexion with the lateral mesoderm is not continuous in a longitudinal direction. As regards the mode of origin of the actual mesonephric units differences exist, as was shown long ago by Sedgwick (1880), which are of much interest owing to the fact that the less modified mode of development found at the front end of the series is readily correlated 254 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. with that which is found in the Anamnia, while the more highly modified mode of development occurring posteriorly is equally readily correlated with what happens in the metanephros of the Amniota. In the anterior region (approximately segments 12-15) the inter- mediate cell-mass is compact, recognizably two layered, and the split which separates the two layers may be obviously continuous with the splanchnocoele (Fig. 137, A). It separates at an early stage from the myotome, but it remains continuous at intervals with the lateral nt. nc. glom. p.c.v. -p.c.v. splc. P.C.U glom. Ki<;. 137. — Sections illustrating the development of the mesonephros in Birds. (A and B, after Sedgwick, 1881 ; C, D, E, after Schreiner, 1902). A, 22-segment chick at level of the 15th segment; B, 34-sr^niml chick at level ,,r \:\\\\ or itth segment (combined from' two sections) ; C, 45-.se^nient duck at level ,,t -juti, segment : l>. •ir>-s.--nu-nt duck at level of 25tli segment ; K, 46-Mgmenl 'luck at level (>f -Jlth segment. .1, dorsal aorta; n.n.il, UfehinaphriC duet ; glom, fjlomerulus; nc, nephrocbele ; «./, nrplirotonie ; j>.<; peritoneal canal ; /i.e. r, I>u>ti-iioi cardinal vein; sjilt; IplMlchnoCOele J t. (abide rudiment. mesoderm. The intermediate cell-mass becomes closely Jipjmsed to and very soon directly continuous with the duct by a narrow isthmus in each segment — the tubule rudiment (Fig. 137, A). Ventrally, i.e. near its jmn-tion with the splanchnocoele, the split between thr tun layers of the nephrotome dilates and I'm-ius a definite m^jdinx «•,•!,• ujiicli nj.cii- iiiln the splanchnocoele by a wide peritoneal canal (Fig. 137, B,2?.c). The tubule rudiment develops a lumrn leading tY«mi inln duct.1 and the dorsal wall of the nej»lirnr tin- iin-soiii-pliric or Wolffian duct as its t'u ii'-t i<»n i- ic -l i ic i, .| 1 1, drain in- the metOQephlOl OF " \V«'l!li;iu IKH|V." iv MESONEPHROS 255 into the cavity to form a, glornerulus ((/lorn) which may become much enlarged so as to extend right out into the splanch- nocoele. As the process of development is traced back into the second region of the mesonephros (stretching approximately from segment 16 to 19 or 20) a distinct modification becomes .apparent. The inter- mediate cell-mass in this region becomes loosened out into mesen- chyme, and amongst this loose tissue what may be termed the definitive nephrotomes make their appearance secondarily in the form of roundish condensations of cell elements. Each of these becomes more and more sharply marked off from the surrounding mesenchyme, its cells assume a radial arrangement, and presently a small rounded cavity appears in the centre. This cavity dilates and the result is a hollow vesicle with a wall composed of a single layer of cells — the definitive nephrotome. In the third, hinder, region of the mesonephros, extending from about segment 20 or 21 backwards, the process in the Fowl, though not in the Duck, has undergone the further modification that the intermediate cell-mass is from an early period completely isolated from the peritoneal epithelium. The peritoneal canals have here completely disappeared, except for faint vestiges, the cells of the peritoneal epithelium still showing here and there traces of the same arrangement as they have further forwards where they are passing into a peritoneal canal. Apart from this separation from the peritoneal lining, the process is similar to that already described. Here also the intermediate cell-mass becomes separated out into loose mesenchyme in which the definitive nephrotomes make their appearance secondarily. An important feature in the above described processes of develop- ment is the obliteration of the primitive segmentation of the nephro- tome region. When the definitive nephrotomes become visible, and so bring into view the metameric segmentation of the mesonephros, a further modification becomes apparent in that the mesonephric segments, except towards the front end of the series, are more crowded together than are the primitive mesoderm segments as represented by the myotomes (Sedgwick, 1880). Thus in the Duck Schreiner (1902) found in the region of myotome XX, 4 or 5 meso- nephric rudiments, in that of myotome XXV — 7, in that of XXVI —9, in that of XXVII as many as 13. As development proceeds, the mesonephric elements become still more crowded together inasmuch as from segment 21 or 22 back- wards "subsequent" nephrotomes make their appearance in the mesenchyme. These closely resemble in appearance the primary nephrotomes, with which they are at first in close proximity if not in actual continuity, and they develop in succession one over the other, each series forming a vertical row over its primary nephrotome. The number of subsequent tubules is greatest posteriorly where there are commonly four to a segment. 256 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. The later development of the individual nephrotome of the meso- nephros takes place in the Birds along lines exactly similar to what takes place in lower forms such as the Amphibia. The tubule rudiment originates as an, at first solid but later pocket-like, outgrowth of the lateral wall of the nephrotome (Fig. 137, C, f). The tip of this presses against the mesial wall of the duct and, as the tubule grows in length, fusion takes place and the lumina of duct, tubule rudiment and nephrotome — which together form a characteristic ru-shaped structure as seen in a transverse section — become continuous (Fig. 137, D and E). The portion of the r\j nearest the mesial plane represents the nephrotome in the strict sense, i.e. the forerunner of the Malpighian body, and has assumed a watch-glass shape, its dorsal wall being involuted into the cavity as the rudiment of the glomerulus (gloin). The further development of the mesonephric unit, which need not be followed out in detail in this book, consists in (1) the immense growth in length of the tubule, which leads to its becoming in- extricably intertwined with its neighbours, (2) the histological differentiation of its wall, and (3) the differentiation of the Malpighian body. It should be mentioned that where the tubules are much crowded together they do not all establish a communication with the duct in the typical manner above described. Some, even of the primary tubules, come to open into neighbouring tubules. In the case of the subsequent tubules, some open into the duct in the typical fashion, others open into neighbouring tubules, while the majority become con- necteol with pocket-like outgrowths from the duct. These outgrowths are greatly developed in some birds (cf. Duck, Fig. 138, B), becoming much elongated and taking the form of branched collecting-tubes into each of which open a series of subsequent tubules (cf. Hypogeophis), the whole condition distinctly foreshadowing arrangements presently to be mentioned in the metanephros. The mesonephros acts as the renal organ only for a short period during the early stages of development. In the Fowl it begins to develop about the end of the second day of incubation, it reaches its maximum about the 7th or 8th day, and almost immediately thereafter begins to show signs of degeneration as the renal I'mu - tion becomes concentrated in the metanephros. The mesonephros never completely disappears though it ceases to be of any importance as a renal organ: its persistence is correlated with the fact that this portion of the opisthonephros has already in the forerunners of the Amniota important functions connected with reproduction. Its modification in relation to these functions will be gone into later. METANEPHHOS OF BIRDS. — The continuous mass of mesen- chymatouw tissue representing the nephrotomes or proto vertebral stalks does not cease at the hinder limit of the im'si.ncphros at segment XXX: itlH continued on through sr-mrnts XXXI, XXXI I, IV METANEPHROS 257 ur ur XXXIII and XXXIV to the level of the opening of the duct into the cloaca. The nephmtnmal • tissue in the segments mentioned remains for a time passive but eventually it gives rise to the definitive nephrotomes of the nietanephros. The metanephros is thereinn; ontogenetically as was indicated long ago by Sedgwick (1880) in its origin simply a taihvard continuation of the 'mesonephros. In the terminology used in this book it consists of the greatly enlarged posterior segment or segments of the opisthonephros. The develop- ment of the metanephros is inaugurated by the appearance of the rudiment of the ureter or meta- nephric duct. This arises as an out- growth (Fig. 138, B, ur) from the dorsal wall of the meso- nephric duct near its posterior end. The outgrowth ex- tends in a dorsal direction and then spreads out at its tip, projecting very slightly tailwards but growing much more actively in a headward direction along the outer side of the hinder or metanephriC por- FlG 133. —Reconstructed outlines of hind end of mesonephric tion 01 the nephro- duct and ureter in Bird embryos as seen from the left side, tomal mesenchyme. (After Schreiner, 1902.) This latter becomes A, duck embryo with 48 segments ; B, duck embryo with 50 segments ; •aonnnrlarilv fnVinnf C> ,-|r hy a nearly strai'jli! peri iv OPISTHONEPHKOS 259 toneal canal. The earlier developmental material does not suffice to show definitely whether or not, as is probable, this peritoneal canal is a secondary connexion with the peritoneal epithelium. The peritoneal funnels exist only for a time during larval life : in specimens 90 mm. in length they had disappeared. In Calamichthys (Lebedin- sky, 1895) the peritoneal canals have been found still persisting in a larva of 15 cm. ACTINOPTERYGIAN GANOIDS. — Here again the definitive nephro- tomes appear as solid masses of cells arranged segnien tally. The gap separating them from the pronephros is in the more primitive Sturgeons al>out 3 or 4 segments, in the more highly evolved Amia 16 or 17 (Jungersen, 1893-4). The rudiment grows in length, develops a lumen secondarily, joins on to the duct, by its lateral end while its mesial end dilates to form the Malpighian body — all in the usual fashion. At a late period — after the Malpighian bodies have already assumed their characteristic features — they develop peritoneal canals as outgrowths from their walls which meet and fuse with the peritoneal epithelium secondarily. Later on the peritoneal canals become again obliterated and appear to be absent in the adult except in the case of Amia. TELEOSTEI. — In the Teleostean fishes, as is indeed the case to a certain extent in all the members of the Teleostomi, the opistho- nephros is delayed in development in correlation with the prolonged functioning of the pronephros. According to Felix (1897) in the Trout the first opisthonephric units or definitive nephrotomes begin to make their appearance about 70 days after fertilization as rounded clumps of cells, in the centre of which a small lumen appears. These lie immediately dorsal to the duct, in the connective tissue trabeculae which at this stage of development traverse the cavity of the interrenal vein. These rudiments appear first about the middle third of the duct and gradually spread backwards, those in front being segmental in position while those farther back are no longer segmental and fuse together into irregular masses. Each rudiment grows actively in length and goes through the usual series of changes before joining up to the duct. To the primary units just described are added secondary and tertiary units. These develop exactly as do the primary except that in the case of the tertiary set the tubule may fuse either with the duct directly or with an already developed tubule. As the tubules increase enormously in length they become inex- tricably entangled together extending even across the median plane so that the substance of the two kidneys becomes continuous through the interrenal trabeculae. It is further characteristic of the tele- ostean kidney that there takes place in it a great development of round-celled pseudolymphoid (Felix) tissue. This forms a packing tissue between the tubules and appears to be formed by proliferation from the walls of the interrenal venous spaces. The opisthonephros extends back for a short distance behind the 260 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. cloaca. This postcloacal portion drains into a pocket-like prolonga- tion which grows back from the duct, usually on the right side only. A remarkable peculiarity has been observed in certain Teleosts (Lepadogaster, Guitel, 1901) in which, correlated with the persist- ence of the enlarged glomerulus of the pronephros, the Malpighian bodies of the opisthonephros have, at least in the adult, completely disappeared. l)tpNOL — In the Lung-fishes the development of the opistho- nephros closely resembles that in Amphibia. In Lepidosiren and Protoptcrus the units appear as rounded, at first solid, masses inde- pendent alike of myotome and of peritoneal epithelium. In Protop- terus they commence about segment 14-18 but in some specimens they appear to be represented by slight condensations of mesenchyme right forwards as far as the hind limit of the pronephros. The rudi- ments extend back to about segment 36 i.e. to about one segment in front of the cloaca. They are roughly segmental in position and remain so during the greater part of larval life. 'Each rudiment gives rise to a typical Malpighian body and a tubule which joins on to the duct secondarily. There does not appear to be any trace of peritoneal canals developed although they are for a time present in Ceratodus. The development of the primary units is followed by the develop- ment of subsequent ones but the origin of these has not so far been worked out. In Protopterus, though not in Lepidosiren, the hinder ends of the kidneys become continuous across the mesial plane and this fused portion becomes gradually marked off conspicuously by its pale colour the cortical region of the paired kidneys becoming crowded with amoebocytes laden with melanin which settle down there and give it a coal-black appearance. REPTILIA. — In the Reptiles we find, as we should expect, that the process of development follows upon the whole the same lines as in Birds but at the same time shows various features in which the con- dition remains more primitive. Thus in Lacerta Schreiner (1902) finds that, except in the hinder portion of the opisthonephros, the units arise directly from typical nephrotomes or protovertebral stalks. These become isolated from the peritoneal mesoderm and then from the myotome. Each develops a lumen and becomes vesicular and its lateral wall gives rise to an outgrowth which becomes the tubule rudiment and fuses with the duct. No peri- toneal canal- are developed, though vestiges of these may appear — a vestigial peritoneal funnel appearing as a. slight- projection from the splanchnocoelc into the ventral end of the nephrotome (Lacerta), or the latter remaining fora time con neeted with the peritoneal lining Ik representing the peritoneal canal (J ////// v',s). In the posterior segments the nephmtoines are no longer distinct: they form a continuous mass of inesenehy me .-I retching uninterruptedly from se-nimt to segment. In this, cellular conden- iv L'KXAL ORGANS L'«;I sations occur which ui\<- rise to definitive nephrotomes and these also are no longer strictly segmental, there being about 2 to each segment from segment 25 to 30. The definitive nephrotomes pursue the normal course of develop- ment. The first t»> appear are towards the ventral edge of the nephrotomal tissue but later other subsequent units appear in succession more dorsally. GENERAL Moui'iioi.ocY OF THE RENAL ORGANS OF VERTE- i SKATES. — The main problem connected with the morphology of the renal organs is that which deals with the serial homology of its con- stituent elements. Lankester (1877) clearly implied this homology when defining his terms archinephros etc. while, looking at the matter from a more strictly ernbryological standpoint, Sedgwick (1881) formulated the view that pronephros, mesonephros and meta- nephros are simply successive portions of a single elongated ancestral excretory organ possessing a duct and segmentally arranged, serially homologous, tubules. In discussing this archinephros theory it is necessary to bear in mind the following points: — (1) The names pronephros, mesonephros and metanephros accord- ing to their original definitions signify three sets of renal structures i'( inning a succession along the length of the body in a tail ward direction : — (a) an anterior or headward set, (&) a middle set and (c) a posterior or tailward set respectively. It is inadmissible by the terms of the original definition to use them in any other sense and to do so is bound to lead to confusion. (2) In addition to the anteroposterior series of renal units there may develop a sequence of elements within the same body segment— i.e. the development of the primary unit may be followed by the production of a series of subsequent units, secondary, tertiary, quaternary and so on, probably derived originally from the primary nephrotome by a process of budding. The extent to which such subsequent units may develop differs greatly in different animals and in different segments. In the pronephric region there are commonly none, in the opisthonephros of Hypogeophis there may be as many as twenty in a segment, while it is possible that the metanephros of the Bird is to be looked on as a gigantic mass of subsequent tubules belonging to a single segment. It is obvious that in comparing renal elements of different parts of the series care must be taken that the comparisons are made between elements of the same order, and it is further obvious that a danger to be guarded against is involved in the theoretical possibility of the suppression of the tubules of one order — say the primary tubules — in some particular region. The comparison of mesonephros with pronephros involves then these two fundamental questions : (1) Does the mesonephros contain a set of units of the same order as those of the pronephros ? — i.e. in this case primary elements and 262 EMBEYOLOGY OF THE LOWER VERTEBRATES CH. (2) Are these elements serially homologous throughout the length of pronephros and mesonephros ? From the facts of development as stated earlier in this chapter it is clear what the answer to these two questions must be. It has been shown that in Hypogeopliis and other forms the first tubule to appear in each segment of the opisthouephros arises as a direct out- growth from the nephrotorne exactly in the same way as the pro- nephric tubule : it is clearly then a primary tubule, and its Mal- pighian body, arising directly from the main part of the stalk, is also primary. The evidence then seems conclusive that in Hypo- geopliis the pronephric and opisthonephric tubules form a homologous series, and naturally if this is true of Hypogeopliis it is, in all proba- bility, true of other Vertebrates. Yet the view has been strongly advocated and is still held 1 »y many rnorphologists that there is no precise homology between the units which build up pronephros and opisthonephros. Riickert, van Wijhe, Field, Semon, Boveri, Felix, have been among the more important protagonists of this view. They have brought forward such arguments as the following : — (1) While the pronephric tubule arises as an outgrowth of somatic mesoderm, the mesonephric is derived partly from somatic and partly from splanchnic. (2) The pronephric tubules arise relatively early and in continuity with the archinephric duct, the mesonephric tubules arise much later and in discontinuity with the duct. (3) The glomerulus of the pronephros is unsegmented and lies in the general splanchnocoele : that of the mesonephros is segmental and lies in a special chamber the cavity of the Malpighian body. These arguments however do not appear any longer to have the weight which formerly attached to them. (1) The evidence of Hypogeopliis that opisthonephric tubules arise as outgrowths of the somatic wall of the nephrotouie just as do the pronephric tubules seems quite convincing. (2) In Hyrpogeopliis all the pronephric tubules except the first three join up to the duct secondarily precisely as do the opistho- nephric tubules. Further the precocious completion of the archi- nephric duct is a physiological necessity, in view of the early fund inn- ing of the pronephric tubules, and this in turn involves as a necessary consequence that the tubules belli in 1 those \\hich first function become joined to it secondarily. (3) The glomerulus of the pronephros /.s se-inental and the pronephric chain UTS are also segmental at first in some of the more archaic forms and the unsegmented condition is purely secondary. Another line of argument is directed not against the view that prone] ihi-os and mesonephros are built, up of serially homologous units DUt rather againfll tin- strict homology of the functional part- of these units. Thus it is stated that in the region of the pr<>nepliros in addition to tin; mam tubules there occur rudiments of other iv KENAL ORGANS 263 tubules which rescinl.lc more closely those of the mesonephros and similarly that in tin- region of the mesonephros, in addition to the ordinary tubules, there occur vestiges of another set of tubules resembling more closely those of the pronephros. Consequently, of the set of potential tubules (primary, secondary etc.) which is repeated in each segment, it is not the corresponding jnember which becomes the functional or main tubule in the pronephric and opisthonephric regions respectively. To the present writer the various observations which have been brought to support this argument do not appear to be anything like so convincing as the very clear evidence afforded by Hypogeophis and he consequently holds that in the present state of our knowledge there is no adequate reason to refuse to accept the precise homology of the first-appearing (" primary ") tubules of the opisthonephros with those of the pronephros. The idea of the primitive continuity between mesonephros and metanephros is less open to attack than that between the pronephros and the anterior (mesonephric) portion of the opisthonephros. Apart from the evidence of embryology we find in various of the lower vertebrates (Elasmobranchs, Urodeles) an elongated opisthonephros in the adult which shows in the clearest possible manner an incipient stage in the differentiation of the organ, into an anterior genital region and a posterior renal region, of precisely the same kind as we believe to have taken place in the Amniota. Further we have seen that in actual ontogeny the tubules of mesonephros and metanephros arise from an at first perfectly con- tinuous mass of nephrotomal mesenchyme. As regards the minor problem whether one or more primary tubules still persist in the metanephros among its immense mass of subsequent tubules there is, as yet, no adequate evidence. Accepting then the idea of the archinephros as a sound theory of the primitive condition of the renal system of Vertebrates we may sketch out the probable course of the modifications which have come about in its development somewhat as follows. Primitively its tubules developed — in accordance with the develop- ment of the body-segments generally — in regular sequence from before backwards. The disappearance of segmentation in the ventral portion of the coelorne enabled the early-formed tubules — those towards the head end — to drain the whole length of the splanchnocoele. Correlated with this these tubules became greatly enlarged and their efficiency greatly increased. This high development of the anterior tubules to drain the whole splanchnocoele enabled them to cope with the entire excretory needs of the developing animal for a prolonged period and the tubules behind them in the series being unnecessary were either delayed in their appearance or ceased entirely to develop. Thus a gap arose separating off the precociously developed tubules as the pronephros. Within the pronephros itself there was 264 EMBKYOLOGY OF THE LOWKi; VERTEBRATES < 11. a tendency for functional activity to become specially marked in certain tubules these becoming enlarged in comparison with the others. The increase in size of pronephric tubules was accompanied by increase in the size of their glomeruli, which con- sequently came into contact and fused together. As the pronephric tubules drained the whole splanchnocoele the peritoneal canal leading to their nephrocoeles became wider and wider until at last they ceased to be marked off from the rest of the splanchnocoele. The opisthonephric tubules — the renal functions being still for a time undertaken by the pronephros — developed in regular sequence from before backwards. With the acquisition of new outlets for fluid in the splanchnocoele, such as abdominal pores, or connexions with lymphatic or blood vessels, the peritoneal canals leading from it into the nephrocoeles (Malpighian bodies), in which the secretion of coelomic fluid was specially concentrated, became gradually reduced and finally disappeared, there being no longer any physio- logical need for them. Within the series of opisthonephric tubules, the excretory function became more and more concentrated in the segments nearest the cloacal opening. In these segments the opisthonephros increased in bulk owing to the specially active budding processes which gave rise to successive generations of subsequent (secondary, tertiary, quaternary and so on) tubules. The final stage in this process was reached in the Birds, where renal activity became concentrated in a single segment close to the cloacal opening. In this segment an immense hypertrophy of the opisthonephric elements took place, successive generations of tubules being added on in front. Thus the opisthonephric mass belonging to this segment came to extend headwards dorsal to the anterior portion of the opisthonephros (mesonephros) and became the definitive kidney or metanephros. ORIGIN OF THE NEPHRIDIAL DUCTS. — As already pointed out the nephridial tubes in craniate Vertebrates open primitively into a longitudinal archinephric duct — the presence of this duct being the most conspicuous feature which differentiates the renal system in Vertebrates from the presumably ancestral condition as exemplified l.y Annelids, where the tubules open separately upon the external surface. Two possible ways in which this duct may have originated in evolution have already been indicated and it has also been indicated that on the whole, the balance of probability seems to be in favour of the view that it came into being through the ha<-k\\anl shifting of the external openm- of each tubule till it became coincident \\iih the ii'-\i I chind it. Those who take this view usually assume that, tin- archinephric. duct originally opened posteriorly upon the outer surface of the hody ami thai iN opnir -ndarily shifted into the NEl'HIMDIAL DUCTS 265 cloaca, l.ut as ahva support for this view. Everywhere the archinqihric opening is at first within the endodermal part of the alimentary canal and this siiuucsts thaj the communication of duct with cloaca has come about in some other way. The evidence of Polypterus suggests as already indicated that the opening into the cloaca represents the persistent primitive communication of a mesoderm segment with the enteron. It is quite conceivable {/hat & secondary communication between archiiiephric duct and gut may have come about in this way, in correlation with the pronephric part of the archinephros reaching the actively functional condition at a period when the mesoderm segment at the level of the anus had not yet been completely separated from the endoderm. Once this secondary opening was estab- lished it would be a natural consequence for the post-anal portion of the nephridial system to atrophy and disappear. The hypothesis indicated in this descrip- tion derives the nephridial apparatus of the Vertebrata from an ancestral condition resembling that characteristic of Annelids — the main difference being that in the Vertebrates the nephridial tubes open into a longitudinal duct which at its hinder end communicates with the alimentary canal. It is of great interest then to find even within the group of the Annelida clear expressions of the tendency for the nephri- 'posterioVTmiTthe body of dial tubes to open into such a duct. The best marked case of this known up to the present appears to be that of the Earthworm Alloloboj)hora antipae described by Kosa (1906).- Here (Fig. 139) in the posterior portion of the body the nephridial tubes lead into a longitudinal duct which fusing posteriorly with its fellow opens into, the alimentary canal on its dorsal side and near its posterior end. In other words in this particular case an arrangement precisely like that of the vertebrate has been evolved out of an ancestral condition in which segmontally placed nephridial tubes opened independently upon the outer surface. In regard to the origin of the typical metanephric duct or ureter as seen for example in a Bird there are two obvious possibilities. If the metanephros represents a number of nephridial segments its special duct may have originated by such steps as are represented by the adult condition in male Urodeles and male Elasmobranchs i.e. by the openings of the collecting-tubes into the original duct becoming displaced hackwards. Or on the other hand if the metanephros . I llolubophora antipae as seen from the dorsal side to show the relations of the renal organs (shown in black) according to Rosa (1906). 266 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. represents the greatly enlarged tubule system of a single segment the ureter would probably have arisen simply by the enlargement of the collecting-tube of that segment. When one studies the facts of development as now known (see p. 258 and especially Fig. 138) the balance of probability appears to be decidedly in favour of the second of these hypotheses representing the method by which the ureter has actually arisen in phytogeny. THE GONAD. — The great mass of the cells which constitute the body of a Vertebrate or any other of the higher Metazoa are specialized for the performance of particular functions in the ordinary life of the individual, and, correlated with this specialization, such cells have lost the power of giving rise to reproductive cells or gametes. The main mass of the body constituted of such specialized cells is known technically as the soma. At one or more points in the body there remain however patches of cells which have not undergone this specialization for ordinary vital functions and which retain the power of giving rise under favourable circumstances to gametes. The sum total of such cells con- stitute the gonad. The word gonad is commonly used in a loose sense as an equivalent of ovary or testis but it should be borne in min'd that each of these organs contains a large proportion of immigrant tissues — connective tissue, blood, nerves and so on — which are strictly speaking part of the sorna. The problem of greatest general importance attaching to the development of the gonad of Vertebrates is that which concerns the origin of the cells (gonocytes) which constitute it. And the interest of this question rests especially on the fact that in certain invertebrates the germ-cells have been traced back to blastomeres specially se.t apart at early stages of segmentation. All the probabilities seem to indicate that such a process if it occurs in the animal kingdom at all, is of a fundamental character and that indications of the same process may be confidently looked for in other groups. The most however that we seem to be justified in asserting to be definitely established for Vertebrates is that genital cells are derived from the mesoderm of the coelomic wall. Apart from the actual facts of observation such development of gonocytes from coelomie lining fits in well with Lieneral morphological ideas. It is clear that ire must believe that in the simplest diploblastic ancestor of the Vertebrates the gonocytes were derived from epithelial cells. It is also clear that, on the view that the Coelomata passed through an Actino- e during their evolution, we must, rega.nl it as probable that during that Stage tin- 'jmiocytes were sit uated, as in existing AHiiiM/oa, in the endndernnl epithelium lining the packets between the mesenteries an epit helium \\hieh.nn that view, is represented l»\ the rndodri m nf the enterocoelie pmieh of an AmpUoxus embryo and i rivative tin- eodnmie mesoderm of an adult Amphioxus or oth<-r Vertebn In Amjilti- i <>l the adult shows special peculiarities IV GONAD 267 which mark it off from all other Vertebrates. Bearing in inind howuver that tin- general arrangement of tlie mesoderm of the adult Amphioxus, \\hicli also shows striking peculiarities, is preceded by a condition in ontogeny which there is reason to regard as more nearly primitive than OOCUT8 in any other Vertebrate — the possibility at once suggests itself that this may also be the case with the gonad. Consequently it becomes important to enquire what are the early conditions of the gonad in Amphioxus and whether it is reasonable to interpret the conditions in the more typical vertebrates as being modifications of those illustrated by Amphioxus. The earliest so far recognized stage of the gonad (Boveri, 1892 ; /arnik, 1904) consists of a thickened portion of coelomic epithelium at the ventral end of the mesoderm segment i.e. in the region where at an earlier stage the segmented part of the mesoderm was continuous with the portion which loses its segmentation. The thickening lies close to the headward boundary of the segment and within its ventral angle. As the segment has already become nipped off from the lateral mesoderm it is not possible to say from actual observation that the thickening belongs to the splanchnic rather than the somatic wall though this is probable from the condition in the more typical vertebrates. The genital thickening is repeated over a number of segments (from about the 9th or 10th to about the 34th or 35th— Zarnik). There are then three important points to be gathered from the study of the origin of the gonad in Amphioxus : — (1) It arises as a thickening of coelomic epithelium i.e. it shows the mode of origin characteristic of coelomate animals in general, (2) It arises close to the boundary of segmented and unsegmented mesoderm, and (3) It arises on the dorsal side of that boundary. In the more typical Vertebrates the ovary or testis first becomes clearly recognizable as a rule in the form of a longitudinal ridge — the genital ridge — which runs along the dorsal wall of the splanchno- coele on each side, at a varying distance from the line of attachment of the dorsal mesentery, and projects into the splanchnocoelic cavity. The genital ridge commonly extends over a greater antero-posterior extent than does the functional gonad later on — e.g. in the Salmon of the 185th day it extends from about the level of the fourth trunk myotome back to behind the anus (Felix). The restricted portion of the ridge which is destined to develop into functional ovary or testis is termed by Felix the gonal portion to distinguish it from the portions in front (progonal) and behind (epigonal) which remain sterile. The relatively great anteroposterior extent of the gonad during early stages in its development is probably to be regarded, along with the greatly elongated condition in the adult of some of the more archaic Vertebrates, as evidence that at one period of evolution the gonad extended throughout the whole length of the splanchnocoele. 268 EM 1*. 1 ! Y o L< M ; Y o \- T 1 1 K L< ) \\ Kit VERTEBRATES CH. As development goes on the genital ridge increases in depth and is now termed the genital fold. This is composed of peritoneal epithelium covering a supporting and, later on, vascular core of mesen- chyrnatous connective tissue. The rudiment of the actual gonad in the strict sense consists of a thickening of the peritoneal epithelium covering the genital fold — the germinal epithelium. This thickened germinal epithelium may extend over both mesial and lateral surfaces of the genital fold as in most Amphibians, Reptiles and Birds or it may be restricted to its lateral (Ichthyophis 9 , most Teleosts) or median (Elasmobranchs except in very early stages, Ichthyopliis $ ) surface. Of the more primitive holoblastic Vertebrates the Amphibia are the only group on which detailed observations on the origin of the gonad have been recorded. We shall accordingly summarize the early stages in the development of ovary and testis in this group and where possible in its more primitive subdivision the Urodela. Fig. 140 illustrates the earliest stages of the gonad so far identified in Urodeles, as described by Schapitz (1912) for the Axolotl. Fig. A is taken from an embryo in which the protovertebral stalk or nephro- tome is not yet completely restricted off from the myotome. On its outer side is seen the rudiment of the archinephric duct. The stalk is continuous ventrally with the lateral or splanchnocoelic mesoderm. In its inner portion certain of its cells (e.g. the two adjoining cells in the figure in which the nucleus is shown in a darker tone) are begin- ning to show recognizable indications of nuclear and cytoplasmic features which are characteristic of the gonad later on. It will be borne in mind that the wall of the protovertebral stalk is morpho- logically part of the coelomic wall to which therefore these gonad cells also belong. In the sections shown in B and 0 the mass of cells showing these peculiarities has become more and more distinctly marked off from the lateral mesoderm (mes) and may now be spoken of definitely as the gonad. In the stage illustrated by D the lateral mesoderin is seen to be spreading inwards towards the mesial plane ventral to the gonad and it is beginning to show here and there a di-tinct split separating its somatic and splanchnic layers. In the later stages (E, F, G) this split becomes a patent cavity — the splanch- nocoele (splc) — and the gonad is seen to lie on the dorso-lateral side of this, separated from the actual cavity by the somatic layer of peritoneal epithelium. In the last stage figured (G) the gonad is causiiu a slight bulging of the peritoneal lining into the splanchno- coele : this hiil-ini; is the im-ipient genital ridge (g.r). During the earlier "f bl illustrated the gonocytes gradu- ally acquire i In- -u j.i-rlicial historical characters of ^erni-eells. The oeU-boay is l.ir^-r than that of the other cells, it remains full of yolk - les. ami in the spaces between the latter are to he seen line granules of dark pi-jment. The nucleus is elongated or lolu-tl iii shape, the chroma! in distributed in line particles BO that the iiueleus OS a who!- deeply than do the nuclei of other cells, ami GONAD 269 large round nucleoli aiv ] .resent, frequently corresponding in number with the lobes of the nucleus. The embryonic gonad during the stages which have been described FIG. 140. — Origin of the gona.l in Atnhljiatoma. (After Schapitz, 1912.) A. 7-8 days after fertilization ; B, 10 days ; C, 12 days ; D, 18 days ; E, 17-18 days ; F, 19 days ; (!, just after hatching. it.n.il, arrhinr|ilirir duct ; irti'.! by soim- as arrrssorv ^uiun-\ tea is not as a rule continuous from end to end. On the contrary it consists of isolated pieces and these in many cases show distinct traces of metameric arrangement, the pieces being directly opposite 270 EMBRYOLOGY OF THE LOWER VERTEBRATES « 11. the inesoderm segments. The discontinuity becomes less marked in the later stages but even in an 18-day embryo Schapitz found the gonad still consisting on one side of the body of metamerically arranged blocks while on the other it had become a continuous strand, except for a single small isolated piece posteriorly. From what has been said it seems clear that the gonad of the Urodela is a derivative of the coelomic wall lying close to the boundary between the segmented and the unsegmented (lateral) portions of the mesoderm. As in early stages it consists of blocks with a roughly seginental arrangement it would appear to lie on the dorsal or nephrotomal side of the boundary mentioned. There is no apparent reason for declining to interpret this early segmented stage of the gonad as a persistent trace of a primitive segmental arrange- ment like that of Amphioxus. The tendency for the segmented condition to disappear in the typical Vertebrates is adequately explained by the gradual dorsal- ward encroachment of the unsegmented splanchnocoele. The boundary between segmented and unsegmented (lateral) mesoderm has altered much in position during the course of evolution, and there is no adequate reason to suppose that this boundary is riot still a fluctuating one and if it is so we may expect varying traces of the original segmented condition to present themselves during develop- ment. The gonad has been described as being paired throughout but it may be mentioned that various observers have noticed an unpaired condition at one or other period during the early stages of develop- ment. This appears to be adequately interpreted as a secondary fusion similar to that occurring between the right and left opistho- nephros in a Teleost or in Protopterus rather than as a primary condition. We have traced back the gonad to its first recognized beginnings in one of the relatively primitive holoblastic Vertebrates. Before passing on to its farther development it has to be noticed that there exists a considerable volume of evidence pointing to the existence of additional germ - cells which arise independently of the coelomic lining and some of which migrate into the germinal epithelium and may give rise eventually to functional gametes. It is not proposed to describe this evidence as it has not as yet, in the present writer's opinion, reached tin; stage of liein^ convincing. It does not appear to have been imily demonstrated that the supposed extra gonocytes are really gonocytcs at all rather than somatie eells. What is needed to provide such a demonstration is a careful study by skilled eytolou'ists nf the nuclear features of these cells, to determine \\ln-lln-r there an- any delinite nuclear characters (such as Boveri discovered to he piv-.-ni in tin- x"ii'»cyh-s of Ascaris mcynlo- ,-,1,1,11111 ) showing 1 IM-III to IK- heyond d<»ulit gOnOOVtea and affording a means of tracking them d«.\\n in their supposed migration. Mere and stain in •_• capacity of i he nucleus as a wlmle. i.r presence of iv GONAD 271 nucleoli, do not seem sufficiently definite characters as these are probably directly related to volume and metabolic activity of the ri-ll. Cytoplasinic features — of which much use is made in this con- nexion— such as richness in yolk or roundness in shape are also unreliable. As regards the first of these, the study of the develop- ment of embryos rich in yolk brings out clearly the fact that the cells in particular tissues do not, by any means, all keep pace with one another in their developmental processes. Individual cells lag behind, and one of the commonest characteristics of such cells is that the yolk stored up in their cytoplasm remains unaffected for some time after that in the neighbouring cells has been completely used up. Obviously in such a case richness in yolk, even when occurring along with greater size due to less active division, does not constitute evidence of any weight as regards difference in morphological nature between the heavily yolked cell and those round about it. Again there is reason to believe that yolk may be stored up secondarily in particular cells or portions of tissue of a developing embryo as a preparation for future needs quite apart from the actual germ-cells. As regards approximation to a spherical shape, it should be remembered that there is a usual tendency for irregularly shaped or branching cells, such as those of ordinary mesenchyme, to assume temporarily a rounded form at the period during and about mitosis. Such cells are apt to assume an appearance misleadingly like that of young germ-cells. The various features above indicated occurring together are sufficient to give a characteristic appearance to the cells in the main gonad but they form hardly definite enough criteria to prove that cells elsewhere are germ-cells in face of the strong probability that the whole mass of germ-cells in the body are of a common origin. GENITAL KIDGE AND GENITAL FOLD. — The genital ridge was left as a slight bulging inwards of the peritoneal epithelium covering in the gonocytes. As development goes on the ridge becomes con- verted into a prominent fold — the genital fold. The peritoneal epithelium at first passes continuously over the surface of the strand of gonocytes but soon a change comes about in their relative positions the gonocytes coining to be incorporated in the thickness of the epithelium which may now therefore be spoken of as germinal epithelium. The gonocytes are to be seen first along the free edge of the fold (Fig. 141, A) and this during subsequent development swells out greatly and forms the functional ovary or testis, while the proximal portion acts merely for suspensory purposes. The gonocytes increase in number by mitotic division but are also reinforced from small apparently indifferent cells lying between them (Fig. 141, C, gc1}. We may take it that these small cells are in all probability to be interpreted as cells of the original gonad which have lagged behind in development, though it is naturally difficult from mere observation to make certain that they are not ordinary peritoneal cells. At a particular stage in development (between 26 and 33 mm. in 272 EMBRYOLOGY OF THE LOWER VERTEBRATES < n. Rana tewjKinii-ia — Bouin) the total number of gonocytes in the gonad undergoes a remarkable reduction, e.g. from between 200 and som. ffc. Fl«.. 111. I)«\i-l«ij.iiiriit of the -.in.'i.l in Ainpliiliia as srcn in transverse sections. (A, .-Ht.-r SrliHi.it/, I'.Mii; li K, after Bouin, 1901.) A, lai\a ot A\"l"ll (.!,„• ma) ; H. lM 0 nun. t;i.l|'..|,- ; C. -.'I nun. t;i.l|.ol»> ; in. l.-i'tpol.- ; K. .r, nun. tain|il<-( ,-l\ u >\\ of appai'-iUh inilillfifiil r.ll iiyinc; M-//I, nomalir nn'sn.lci in ; •-]'/, >|.l!iiii-linic IIJ.-MX|«-!-III ; .//, yolk. 250 in a frog tadpole nl L'<; nun. t«. II«-|\\(H;II 37 and I<1 in a tadpole nf ?>'.', nun. r.nuin . Curing this process indi\idual gonooTtee iv GENITAL OKGANS 273 • lr;_rl u/itirns ami ]>ml>.tl>ly (MI below), iv TELEOSTEAN OVARY 277 probability seems on the whole in favour of the first mentioned, for the formation of folds or grooves of the fertile surface of the genital fold, so as to give increased area, is a very usual phenomenon, and the formation of a single longitudinal groove would readily lead to the first-mentioned condition. On the other hand the replacement of this condition by the second is also readily* understandable. The ovary passes without a break into the oviduct which is simply the posterior sterile portion of the genital ridge in which a cavity develops secondarily — not always continuously — from before backwards. The oviduct differs greatly in length in different Teleosts : in some (Zoarces, Cyclopterus) the ovary itself may stretch right back to the genital pore. Although the above description h'ts in with the normal con- ditions, there are various Teleosts in which the processes of fusion connected with the ovary do not take place and in which the ovary remains as a genital fold hanging down into the splanchnocoele, B Fi<;. 14'J. — Diagram illustrating the conversion of the genital fold into a closed ovary in the Teleostean fishes. e.g. in the case of the Salmon fusion of the ovarian edge with the body wall takes place anteriorly for a short distance and again in the posterior sterile region, but the greater part of the, fertile region of the ovary hangs free. In such cases the eggs are shed into the splanchnocoele and pass to the exterior by genital pores (compare Cyclostomata, p. 246). Unfortunately we are still in almost complete ignorance regard- ing the development of ovary and oviducts in the Ganoids. From the little we do know it would appear that in Lepidosteus (Balfour & Parker, 1882) the ovary becomes enclosed in the same manner as in Cyprinoids (Fig. 142, B). Posteriorly it is continuous with the oviduct as in Teleosts generally. In the other Ganoids the ovary retains the form of a genital fold hanging down into the splanchno- coele while the oviduct is provided anteriorly with a coelomic funnel. The position of this funnel, far removed from the front end of the splanchnocoele, is sometimes used as an argument against the homology of this opening with the ostium of a true Miillerian duct, but such an argument carries little weight as we know from 278 EMBRYOLOGY OF THE LOWER VERTEBRATES « n. the higher vertebrates that the ostium of an undoubted Miillerian duct is liable to undergo secondary shifting into such a position. Again the fact that the opening lies on the mesial side of the ovary is adduced as an argument in the same sense but in this case we have definite embryological evidence from Polypterus (Budgett, 1902) that this position is secondary, the early rudiment of the duct lying external to the ovary and immediately ventral to the Wolffian duct as is the case with a typical Miillerian duct. Consequently there is no sufficient reason to doubt that these oviducts with open ostia in ganoids are really Miillerian ducts. PHYLOGEXY OF TELEOSTEAN OVIDUCT. — The facts of development show clearly that the main part of the Teleostean oviduct is of the same morphological nature as the ovary with which it is continuous. It arises from the hinder part of the primitive ovary which has become sterile and assumed a merely conducting function. The main difficulty connected with the morphology of the organ is that of accounting for the joining up of the part of the oviduct of ovarian origin with the cloaca or exterior. Balfour suggested that this had come about by the oviduct becoming fused with the lips of the " abdominal pores." As an objection to this was adduced the observation by Hyrtl that in Mormyrus abdominal pores exist along with oviducts. This objection disappears, however, if we remember that in Balfour's time there were confused together under the same name two different types of .aperture — true abdominal pores and genital pores. Substituting genital pores for abdominal Balfour's view seems still the most feasible. The probability seems to be that the main steps in the evolution of the Teleostean oviduct were as follows : — (1) The primitive oviduct or Miillerian duct underwent gradual atrophy becoming gradually shorter l until eventually nothing was left but its external opening — the genital pore. This process would doubtless be correlated with the loss of its glandular function and this in* turn may have been connected either with the adoption of pelagic spawning, in which special tertiary investments for the e-^s were no longer required, or with a special development of primary envelopes within the 141-0 up. A stage was thus reached which is ivpiv.-enied by the condition in fialmo. Of course we do not know whether Salmo has retained this condition or has reverted to it; the latter is more probable. (2) Tin- portion of splanchnocoele along the n viperous surface 06 ••in -Insed so as to form a cavity which served to conduct- tin* :-h''d ova back into the neighbourhood of the genital pore. Anteriorly the ovarian surface ahuttm^ on this cavity remained fertile, \\ hile ri.irlv ii became Sterile, so that the posterior portion of the • nied merely a conducting function (oviduct (3) The lip-- bounding the posterior end of the oviduct front 1 \Vi- may see early stages in thin JIMM-I-SS illustrated l.y the (Jium ids .//,//t her \Yrlehnites has a ver\ eharactenslie iv THE SUPRARENAL ORGANS 283 chemical or physical reaction, in that it takes on a deep yellow or brown rolour when treated with salts of chromic acid. Hence it is eon vniirnt, and usual, to apply to it a name expressive of this reaction— such as Chromophile (Stilling), ChromaHinu (Kohn) or Thaeochrome (Poll). The cortical tissue has also characteristic features — in particular the fact that its cytoplasm contains numerous granules of lipoid or fat-like substance, soluble in Ether, Xylol, etc., staining deeply with various Aniline stains, and giving the characteristic black with Osmic Acid. For masses of this tissue the name Interrenal organ may be used (Balfour) which although a topographical term like cortical substance has the advantage of being correct for vertebrates in general during at least the early stages of their development. Of the more primitive groups of gnathostomatous Vertebrates only the Elasmobranchs and the Amphibians have been studied carefully in regard to the development of these organs and we shall consequently use them as illustrating the general mode of develop- ment which, with variations in detail, holds throughout the groups dealt with in this volume. ELASMOBRANCHii.1 — The Interrenal organs are here interrenal in position through life, forming either one (Sharks) or a pair (Skates and Rays) of elongated bodies lying in the region of the mesial plane and extending for some distance opposite the hinder part of the opisthonephros. In Scyllium (Poll) the interrenal makes its first appearance (7 mm. embryo) in the form of a number of irregularly distributed thickenings of the splanchnic mesoderni in the region of the root of the mesentery, just ventral to the dorsal aorta. The possibility of metanieric arrangement in the very earliest stages does not seem to be absolutely excluded but there is no evidence of this so far. The rudiments are most numerous in the genital region but they occur as far forwards as the hind end of the pronephros and back as far as the cloaca. The rudiments of the two sides, projecting towards the median plane, meet and become continuous, and as antero- posterior fusion also comes about, the rudiment takes the form (10 mm. embryo) of a cellular rod lying beneath the dorsal aorta and above the mesenteric root, and for a time still continuous with the splanchnic mesoderm which gave it origin. For a time there is close apposition, amounting to apparent continuity of tissue, between this rod and the opisthonephric nephrotomes lying on either side of it, but it is doubtful whether any special morphological significance is to be attached to this. In embryos of 16-28 mm. in length the interrenal organ gradually becomes separated in a tailward direction both from the coelomic epithelium and the nephrotomes, and assumes its definitive form. Only the tailward part of the series of original rudiments 1 The best general account of the development of the Suprarenal organs is that by Poll (1905). 284 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. completes its development in the way described. The whole series extends through about 25 segments but of these only about the posterior half take part in the formation of the interrenal rod : the anterior ones either atrophy completely or develop into small accessory interrenals. The chromophile organs of the Elasmobranch (Swale Vincent, 1897) are small, rounded, segmentally arranged bodies lying ventral to the intercostal arteries — the anterior few on either side forming a continuous structure which was regarded by the earlier workers as an accessory heart (Duvernoy). These bodies are, as Balfour showed (1878), derivatives of the sympathetic ganglia. In a Scy Ilium of about 53 mm. the lateral part of the ganglion rudiment begins to show differentiation from the rest, its cells being relatively smaller than those which are destined to become ganglion-cells, and their protoplasm not only staining more deeply with ordinary stains but also developing the characteristic chromic acid reaction. In the Scy Ilium of 90 mm. the chromophile organ has assumed its definitive rounded form. Intrusive connective tissue forms a sparse stroma and capsule and in the former a capillary network is present. The series of segmentally arranged chromophile masses undergoes much modification in subsequent development — some, particularly at the ends of the series, aborting, others undergoing fusion. The details vary in different genera, the result being a striking variety in the adult arrangements in the various members of the group. AMPHIBIA. — Brauer (1902) in his work on the renal organs of Hypogeophis gives a clear account of the development of the supra- renals. The interrenals appear as in Elasmobranchs in the form of cellular proliferations of the coelornic epithelium, in this case a little external to the root of the mesentery. These proliferations are paired and segmental in their arrangement, and extend from the region of the pronephros - to that of the cloaca. The cellular buds become constricted off from the coelomic epithelium and lie above it as rounded masses embedded in the mesenchyme. As the two j posterior cardinal veins approach and fuse the interrenal buds become displaced upwards so as to lie between the cardinal vein and the dorsal aorta. As development goes on processes of fusion lake pla<-<- I.etween the rudiments more especially anteriorly where they come to form an unpaired elongated mass lying below the dorsal aorta and for tin- most part dorsal to the posterior vena cava (i.e. tin- 1'ii-ed po-ii-rior cardinals) but here and there passing laterally round bne vin to its ventral surface or even piercing it the fusion between the two cardinals having been obstructed at sin-li points. In th«' posterior half of the organ the several rudiments retain their distinctness and lie on the ventral face ol the oj.istlm- nephroi. Tho\\er. IMlhel' 1 ha II as heillg the BOf some primitive pocket like di\ ertieulum of unknown funei ion. iv THE COELOMIC ORGANS 287 LITBBATUBE Abramowicz. Morph. J.ilirh., xlvii, 1913. Agar. 'I1! ;u is. Roy. Soc. Ediu., xlv, 1907. Balfour. Monograph on the development of Elasmobranch Fishes. London, 1878. Balfour and Parker. Phil. Trans. Roy. Soc., clxxiii, 1882. Balfour and Sedgwick. Proc. Roy. Soc., xxvii, 1878. ' Bles. IW. Roy. Sue., Ixii, 1897. Bouin. Arch, do Biol., xvii, 1901. Boveri. Anat. Anz., vii, 1892. Brauer. Zool. Jaltrb. (Anat.), xvi, 1902. Braus. Morph. .lalirb., xxvii, 1899. Budgett. Trans. Zool. Soc. Loud., xvi, 1902. Dahlgren and Kepiier. Animal Histology. New York, 1908. Ewart. Phil. Trans. Roy. Soc., 179 B, 1888. Ewart. Phil. Trans. Roy. Soc., 179 B, 1889. Ewart. Phil. Trans. Roy. Soc., 183 B, 1892. Felix. Anat. Hefte (Arb.), viii, 1897. Felix. Hertwigs Handbuch der Entwicklungslehre, iii, 1904. Field. Bull. Mus. Comp. Zool. Harvard, xxi, 1891. Fiirbringer. Entwick. der Amphibienniere. Heidelberg, 1877. (See also Morph. Jahrb., iv, 1878.) Goodrich. Quart. Journ. Micr. Sci., xxxvii, 1895. Guitel. Bull. Soc. Sci. et Med. de I'Ouest, x, 1901, and xi, 1902. Gregory. Sumons Forschungsreisen. i, 1905. Hochstetter. Morph. Jahrb., xxix, 1900. Jungersen. Arb. zool.-zoot. Inst. Wiirzburg, ix, 1889. Jungersen. Zool. Anzeiger, xvi and xvii, 1893, 1894. Kerr, Graham. Proc. Zool. Soc. Lond., 1901. Kerr, Graham. The Work of John Samuel Budgett. Cambridge, 1907. Kerr, Graham. Quart. Journ. Micr. Sci., liv, 1910. Koltzoff. Bull. Soc. Imp. Nat. Moscow, xv, 1901. Lankester. Quart. Journ. Micr. Sci., xvii, 1877. Lebedinsky. Arch. mikr. Anat., xliv, 1895. Marshall. Vertebrate Embryology. London, 1893. Marshall and Bles. Studies Biol. Lab. Owens College, ii, 1890. Mollier. Anat. Hefte (Arb.), iii, 1893. Muller, E. Anat. Hefte (Arb.), xliii, 1911. Nussbaum. Arch. mikr. Anat., xxvii, 1886. Poll. Hertwigs Handbuch der Entwicklungslehre, iii, 1905. Poole. Proc. Zool. Soc. Lond., 1909. Rabl, C. Morph. Jahrb., xxiv, 1896. Rabl, H. Arch. mikr. Anat., Ixiv, 1904. Rosa. Archivio zoologico, iii, 1906. Riickert. Arch. f. Anat. u. Entwick., 1888. Schapitz. Arch. mikr. Anat., Ixxix, 1912. Schmalhausen. Zeitschr. wiss. Zool., C, 1912. Schreiner. Zeitschr. wiss. Zool., Ixxi, 1902. Sedgwick. Quart. Journ. Micr. Sci., xx, 1880. Sedgwick. Quart. Journ. Micr. Sci., xxi, 1881. Semon. Keibels Normentafeln. iii, 1901. (Also in Zool. Anzeiger, xxiv, 1901.) Semper. Arb. zool.-zoot. Inst. Wiir/burg, ii, 1875. Vincent, Swale. Trans. Zool. Soc. Lond., xiv, 1897. Wijhe, van. rix-r die Mesodermsegmente und die Entwicklung der Nerven des St'larhirrkojit'es. Vi-rhaml. Akad. Wet. Amsterdam, xxii, 1883. (Reprinted, Groii- ingen, 1915). Wijhe, van. Arch. mikr. Anat., xxxiii, 1889. Zarnik. Zool. Jahrb. (Anat), xxi, 1904. CHAPTER V THE SKELETON THE skeletal tissues of the animal body show a variety which is at first sight quite bewildering. Closer scrutiny however reveals certain general principles which are at work. In a very restricted set of cases we see that the supporting structure consists of a row or rod of cells which is rendered stiff through the individual cells being blown out or distended with fluid. Such turgor of cells is a far less con- spicuous feature in the animal kingdom than it is in the vegetable. It is well seen in the axial row of endoderm cells which supports the tentacles of the Hydrozoa. In the Vertebrate it is seen in the notochord. Far more usually the support is given by a definite supporting substance with such physical qualities as rigidity, tensile strength, elasticity, as may be required in the particular case. These supporting substances of the animal body again show the greatest variety in their morphological nature but they may all be classed between two extremes — in one of which the supporting substance consists clearly of modified cells or portions of cells and in the other of dead intercellular substance. Examples of the former are seen in the remarkable phagocytic organs of nematode worms where an emu -mous cell becomes developed into an immensely Com- plicated branched structure of stiff horny consistency upon the terminal twigs of which are perched innumerable minute blobs of ]»li;iMocytic protoplasm. A good example of the second type is seen in the skeleton ol' ordinary coral — a mass of hard calcareous material lying clearly outside the limits of the living cells. It is necessary to emphasize the fact, which is frequently lost siirht of. that the differences between these two types are superficial ralh'T than !'iinl the more v THE SKELETON 291 primitive Wrtcbrates (Lampreys, Elasmobranchs, Teleostomes, Dipnoi, Amphibia) and the early stage of development at which it appears, that tin1 hvpoeliord is an organ of great antiquity in the Vertebrate stem, but we have no definite knowledge of its ancestral significance. The fact that it does not occur in Amphioxus has rendered possible the suggestion that it represents the longi- tudinal groove which in this animal runs along the mid-dorsal line of the pharyngeal wall. But this idea is negatived by the fact that tin1 hypochord extends right back to the tail and is not merely a pharyngeal organ, and the probability seems to be that it has come down from a period in evolution long before the appearance of Amphioxus. It is perhaps simplest to regard it merely as an accessory notochord. Whereas the true notochord plays an important physiological role — as the main part of the axial skeleton during early stages, and as the foundation for the vertebral column of later stages — the hypochord has no such justification for its persistence. It lasts only for a short time and eventually breaks up and completely disappears. In the Amniota there is no typical hypochord developed but it is possible that a thickening of the mid-dorsal endoderm which is frequently found in the pharyngeal region (e.g. in the Second day Fowl embryo) may represent a last vestige of it. SKELETAL DEVELOPMENTS OF THE CONNECTIVE TISSUE Whereas the notochord is derived directly from the endoderm, the cartilaginous and bony components of the skeleton on the other hand are modifications of the mesenchyme or connective tissue, which forms a considerable proportion of the entire bulk of a typical vertebrate. Connective tissue in its least specialized form may be seen in practically any late vertebrate embryo as a reticulum or sponge work— a syncytial framework — of much-branched cells, the processes of which are continuous from one cell to another, while the meshes are occupied by a clear fluid or jelly-like matrix. Masses of this tissue form a kind of packing between and around the various epithelia of the body, while it also, in the form of discrete wandering cells, actually invades the epithelial tissues and colonizes them. Such immigrant elements are found for example between the muscle-fibres, in the substance of the central nervous system, and even frequently between the epithelial cells of the epidermis. The primitive or embryonic connective tissue undergoes gradual differentiation in accordance with the physiological role which it has to play in different localities. This differentiation finds expression in .such superficial features as shape and arrangement of the individual cells and more fundamentally in the peculiarities in metabolism which lead to its storing up particular substances in its protoplasm 292 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. —pigment of chromatophores, fat of the cells of adipose tissue — or again in the influence exerted by the metabolic activity of the cell upon the character of the matrix. This matrix is commonly described as intercellular, which is «juite correct, but the important point is not the question whether it is inter- or intra-cellular but the fact that it is in immediate relation to, and under the influence of, the living protoplasm of the cell. The portion of matrix in con- tiguity with one of the irregularly shaped connective-tissue cells [is comparable with an intracellular vacuole the outer wall of which has thinned out and disappeared. The matrix has been formed by the breaking down of living substance and it seems merely a matter of phraseology whether we speak of it as modified protoplasm or as dead " formed " material. The most familiar differentiation of the matrix of connective tissue consists in the development within it of thin tough fibres, characterized by the physical property that they soft'en and dissolve, yielding gelatin, under the action of boiling water, and that they become further toughened by the action of tanning agents. These fibres run indiscriminately in all directions or, in the more specialized conditions, are definitely orientated, as in the case of tendon where they are parallel and arranged in longitudinal strands, or of aponeuroses where they are arranged in thin layers, those of one layer perpendicular to those of the next. Other portions of the matrix take the form of elastic fibres — characterized by their elasticity, by their connexion together to form a network, by their being much less easily affected by boiling water, and by their not yielding gelatin. The amount of matrix present differs greatly in different localities. It may be reduced to a very small amount — to a mere demarcating line — between closely fitting plate-like cells, as in the case of the endothelium covering the surface of a tendon, or it may be large in amount and comparatively rigid as in the case of the two great skeletal tissues cartilage and bone. CAKTILAI.INOCS OR CHONDKAL SKELETON. — The cartilage is char- acterized by its cells taking on a rounded form and becoming separated hy an abundant semitransparent, elastic, chondrin-containin^ matrix. The process of chondrificatiOD homines apparent first of nil in the M.mcwhat dense parking tissue. f" skeletogenous layer") ] immediately surrounding the notochord. This connective tissue becomes locally modified to form little blocks of rartilajjv known as the arch-elements farcualia (Jadnw, 1895), lyin^ just outside the primary sheath ami arranur'*d in lour Longitudinal rows, two dorsal composed of the rudiments of the neural arches, l\\" ventral the rudiments of the haemal an-h'-s. Tin-.- arch elements are apparent ly in t lie primitive erudition duplie;ited in c inent. i.e. \\ilhin the limits of a myOtOme Or 'demlomcj then; are situated t\\o pairs of neural and 1 wo pairs ol' haemal ;iidi (dements. 1 The. ter^ ivoeboodnd ia applied to th< rtilagi • .11!. ii in.ii i ' ace. TIIK SKELETON 293 m.c. ce There n<»\v takes place iii two of the more primitive groupfl ••!' Vertebrates tin- Kl;i>iinihrain-hii (including tin- I Inlnr.-phali and the Dipnoi — a remarkable process \\ hereby the secondary sheat h <>!' the notoclionl In -comes converted into a sheath of cartilage. Certain of the cartilage cells in the arch rudiment take on an amoeboid character and burrowing their way through the primary sheath, apparently by the help of a digestive ferment, invade the secondary sheath (tig. 145, m.c). Continuing their migration they become distributed equally throughout the whole substance of the secondary sheath, including those portions, in the head re- gion which will later on form part of the cranium. The immigrant cells fin- ally settle down in the substance of the second- ary sheath and the latter becomes a cylinder of cartilage. It is important, with an eye to the evolution of the vertebral column in Vertebrates higher in the scale, to bear in mind that this invasion of the Secondary sheath by im- Fic. 145< _Part of a transverse section through a migrant Cartilage Cells Lepidosireu of stage 38, traversing one of the neural takes place at four points arch rudiments. in the transverse plane, corresponding to the bases of the four arch rudi- ments, and that this arrangement is repeated twice within the limits of one segment owing to the arch rudiments being so repeated. Consequently if we suppose the colonization of the secondary sheath to be restricted to the neighbourhood of the trans- verse plane in which the arch rudiments are situated the result would be the formation of two rings of cartilage within the limits of a single segment. In the case of Lung-fishes and Holocephali the chondrified secondary sheath undergoes no further modification but in typical Elasmobranchs it becomes divided up into segments, which form the centra or bodies of the vertebrae, in the manner to be described later on. In this process the originally uniformly flexible notochord with its sheaths becomes replaced physiologically by a series of rigid masses, flexibility being given to the whole by the presence of the intervening joints. As this jointed condition of the vertebral column originated in evolution at a time when the longitudinal muscles of the body were already divided into myotonies, we may ••.••. n-tndmr.Ial .-pinn-lium ;•//(..-, migrating cart ilau.' cell ; * 294 EMBRYOLOGY OF THE LOWER VERTEBRATES « n. take it as probable, for obvious mechanical reasons, that the rigid skeletal masses arose in a position alternating with the muscle segments. The individual vertebral centra were in other words from the beginning intersegniental in position in relation to the general body metamerism. In sketching out in somewhat greater detail the further develop- ment of the vertebral column the assumption will be again made use of, as it was in dealing with the mesoderm segments, that the trunk region has in all probability departed least from the primitive con- dition, and the facts quoted will in the main be taken from this region of the body. The student who goes on to peruse original memoirs will notice that this rule is by no A. B. A B A. B. means always accepted. Some if fi tt . . «ii i_ " n T writers will, be found to assume that the caudal re- gion is more nearly primitive, and, in accordance with this assumption, to interpret the phenomena observed in the trunk vertebrae by those observed in the caudal, in- stead of vice versa. In this connexion it must be borne in mind that the Vertebrate is above all ,o,t or spinal nerve ; K, surface of notochord ; *, inter**- ^ essentially a COelomate iin-iital l>lo<*l-v«'ssel ; v.r, ventral root of spinal nerve. . o J 1 animal. JSo one doubts that •whatever the common ancestor of the Vertebrates was like it was at least coelomate. And most morphologists would admit further that the weight of evidence indicates that in this ancestor the splanchnocoele extended throughout the greater part of its length and that the existence of a considerable stretch of body towards the hind end devoid of splanclmocoele (i.e. a tail region) is secondary. P.m. if the caudal region has in this way undergone profound secondary modification of its structure it is clear that it is not in this re-inn of the body that we should expect to iind persisting primitive modes of development of the axial skeleton. It is now necessary to follow out the fate of the arch-elements. i\ mentioned the primitive arrangement of these appears to have heei i i w<» pairs to each segment, above and hel..\\, so that eon-e *pondin;j with each mvntnme there were, on each side, t\\<> neural elements aO anterior A _, and a posterior p, ,-md two liaemal elements an anterior // .,ml a pOftteripT (I). NI-.I I;AI. Ai;< BBS.1 Apparent Iv t he most nearly pri mil ive arrange- l'i<;. 146. — Arrangement of dorsal arch -elements in hinder trunk region of a Petromyzon larva 95 linn, in length. (After Schauinsland, 1906.) A, anterior, B, ]>ostf'riorin>unilarch-flcine7its ; d.?-, dorsal 1 In writing tli»-si: »'<-li<.iih «.n tlir '. < oliiinli nilicli i. Bfl ni:i«l<- «•!' S.'liail: ulnrh (In- -tinlriil i- Idrllvd lor ;i limlr drt Jllli-il ible. NEURAL ARCHES 295 ment of the an-hrs is that \\hich occurs in tin- liiinli-r trunk re of the Lamprey (Fig. 146). In this animal, as is well known, the dorsal (sensory) and ventral (motor) nerve-roots are still separate and are spaced out alternating with one another at approximately equal distances along the sides of the spinal cord. The dorsal arch elements alternate, in their turn, with the nerve-roots, so that there are, on each side, an anterior (A) and a posterior (B) neural arch- element within the limits of a single myotome.1 It should be noticed particularly that of these the anterior is situated between the sensory and the motor nerve-root belonging to the segment. This suggests a possible explanation of the later evolutionary history of these cartilages (A) which in the typical Fishes tend very usually B FIG. 147. — A, arrangement of arch-elements in mid-trunk region of a Carcharias embryo 85 mm. in length ; B, do. in anterior region of a Sturgeon (Acipenser huso) 36 mm. in length. (After Schauinsland, 1906.) A, anti'i ioi neural arch-element ; JB, posterior do. ; a, anterior haemal arch-element ; ft, posterior do. ; d.r, sensory nerve-root ; v, blood-vessel ; v.r, motor nerve-root. to become reduced in size, even to the point of disappearance. It may be that this reduction in size is connected with the fact that in the Fishes, as indeed in all gnathostomatous vertebrates, the two nerve-roots have become approximated together to form a common sensori-motor spinal nerve. On the other hand this explanation would leave untouched the fact that a similar reduction in size may occur in the corresponding ventral or haemal arches. The reduction in size of the "A" elements, which is of so frequent occurrence amongst the typical fishes, is well shown in Figs. 147, A and B, which are based upon Schauinsland's recon- structions. This marked reduction is by no means of universal occurrence. The two common Dog-fish — Sci/llium and Acanthias— are familiar examples of fishes in which the "A" elements 1 In the anterior trunk region the arrangement is apt to be modified— the inter- segmental vessel, which forms the anterior limit of the segment, coming to lie on the taihvard side of the A cartilage of that segment (Schauinsland). 296 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. (" intercalary pieces," " interdorsals ") remain nearly as well developed in the adult as the " B " elements.1 In Lung-fishes (here and there) and in Urodele amphibians the "A" pieces can still be recognized (cf. Fig. 148); they have also been observed in the embryos of various Reptiles. In this case they usually lose their individuality at an early period, becoming completely merged in the definitive neural arch formed by the " B " elements lying next to them on their headward side, but in some cases, e.g. in the tail region of Lacerta, they have been found to persist as discrete structures even in the adult, forming a vestigial second neural arch behind the main arch. The neural elements become prolonged dorsally and meet so as to form a complete neural arch and the apex of this becomes pro- longed as an unpaired piece in the mesial plane, to form the neural spine. The complete neural arch formed in this way frequently becomes segmented up into separ- ate pieces of cartilage. The arcualia in such cases become each divided into a larger basal (basi- dorsal — B, interdorsal — A,Gadow) and a smaller apical (supradorsal) portion. The spine may segment into three superimposed rod -like portions. HAEMAL ARCHES. — In the Cyclostomes typical haemal arches are absent, although possibly vestiges of them are represented by a continuous ridge of cartilage occurring in the tail region of Petromyzon where the neural arches have also been reduced to a similar continuous ridge (Schneider). Of haemal arch elements there were apparently primitively two pairs to a segment just an in the case of the neural arches. This seems to be clearly indicated by Callorhynclius (Fig. 149). It is ;ils<> well shown in the young Sturgeon (Fig. 147, B) where the antcri..r • •It-UK Hi " in each segment has undergone reduction in size exactly rafl tin- case \\iih the corresponding neural element (A). A similar condition is found in many Elasmobranohe, though not in all. the "" " elements lu-in-j in some cases apparently completely nhsnit, 1 The e.NHinination of one of these Dog-fishes brings out another point of gfiin.il importance namely th;it tin .uvh-eloinent ;is it increases in lisa is apt to spread round a nerve mot in it> neighbourhood. The result is that in tin- :u lull t In- n< rve roots may pass out, nut l.i-i\\n-n tin- aivh .-lenient.-, Imt through them. The leMOB to be leai lit from this is that the topo^r.iphieal relation of skeletal elements to m-r\e trunks in not to Lit Jal.. n a ml.tlliMr .-\ id. -HIT ;is ID the primitive situation of siieh « ,« in> M . FIG. 148. — Arrangement of arch-elements in anterior caudal region of a Siredou 50 nun. in length. (After Schauinsland, 1906.) in Fiji- 147. HAEMAL ARCHES 297 In various Fishes, for example Laemargus and Amia (also in some Amphibia, see Fig. 153, B), the haemal arch-element in the trunk region segments into two pieces — one of which carries the rib and may become shifted dorsally, while the other becomes displaced in a ventral direction. The ventral pieces come to form projections downwards from the centrum of the vertebra on each side of the aorta and have been termed "aortic supports." They may be termed haemal processes as they appear to be homologous with the knob-like structures bearing this name which are to be seen in the caudal region of e.g. Laemargus, projecting inwards from the haemal arch into the tendinous septum which underlies the caudal aorta. In the caudal region the haemal arch-elements are commonly much longer than in the trunk. They bend round to meet one another and are prolonged into a haemal spine. These features are associated with the extension of the body in a dorsal and ventral direction correlated with the use of this region of the body for the purposes of movement. Towards the head end of the series it not uncommonly happens in Cartilaginous fishes that the haemal arch - element becomes broadened out over the surface of the notochord indicating the beginnings Of the evolution Of FIG 149. -Portion of vertebral column of a °. , P , 10 cm. embryo of CaRorhynchus a few seg- perichordal centra (see below). ments posterior to the hind end of the This is Well shown by Callorhyn- skull. (After Schauinsland, 1906.) chus (Fig. 149) where incipient centra are distinctly seen, formed by the much -enlarged and fused haemal arch-elements (a and &). In the air-breathing vertebrates there are no longer double sets of complete haemal arch - elements but a distinct trace of this condition is seen in such a Urodele Amphibian as Siredon (see Fig. 148) where a large perforation through the haemal arch element, traversed by the intersegmental blood-vessel, betokens its double origin. A characteristic feature of the Amniota is that the haemal arch (the cartilaginous forerunner of the " chevron - bone ") tends to become displaced forwards so as to assume an intervertebral position or even to become fused with the vertebra lying in front (Angins — Goette). VERTEBRAL CENTRA. — Except in the case of Cyclostomes, Holo- cephali and Lung-fishes, the elastic notochord becomes replaced physiologically during development by the series of vertebral centra. In the various subdivisions of the Vertebrata we find two distinct Reference letters as in Fig. 147. 298 EMBRYOLOGY OF THE LOWER VERTEBRATES < 11. methods by which vertebral centra are produced (1) by the segmentation of the cartilaginous secondary sheath (sheath centra ; chorda centra — Gadow) and (2) by the enlargement of the bases of the arch-elements which grow round the notochord and give rise to centra outside the primary sheath — (perichordal centra ; arch centra — Gadow). SHEATH CENTRA are seen in Elasmobranchs. In the region which will develop into a centrum the chondrified secondary sheath becomes thickened so as to bulge inwards and constrict the noto- chord (Fig. 150). A more deeply staining " middle zone " soon becomes distinguishable in this thickened part of the secondary sheath (Fig. 150, m.z) having a shape something like that of a dice-box, its central part lying much nearer to the axis of the FIG. 150. — Part of sagittal section through the secondary sheath of a Scyllium of 61 mm. total length showing an early stage in the development of a centrum. (After C. Rabl, 1893.) i.z, inner zone ; 7/1.2, middle zone ; 0.2, outer zone ; N, notochord ; si, primary sheath. notochord than do its two extremities. This middle zone becomes the main part of the wall of the amphicoelous centrum, its substance becoming usually strengthened by the calcification of its intercellular matrix. The inner zone (Fig. 150, i.z) may grow in thickness so as to cause greater and greater constriction of the notochord. This process attains to its maximum in the Skates where it extends inwards to the axis and causes the formation of thick septa \\hirh divide the notochord into isolated intervertebral fragments. More usually however the inner zone does not UH,ii:it. alopment of the vertebral centre ia utoanin lniri/..nt:i! • ' . • '. i' •-> l»y Srliiiuiiislan.l, 1HOG.) ' / rilOII, H'« nm cailila-f ; -V, c,.iincrlivc UHHU«- ; /-.. noUx hordal • ••!. . v VERTEBRAL CENTRA 301 Tin; notochord becomes more or less constricted by the ingrowth of the intervertebral or joint cartilage which pushes the sheath in front of it. Besides this constriction of the whole notochord with its sheath the substance of the notochord becomes eventually, sometimes at a relatively late stage of development, interrupted by the development of 'mtfravertebral cartilage which may form a com- plete cartilaginous partition across the notochord at about the middle of each vertebra (Fig. 151, B, nc). The origin of this cartilage is disputed. Some (Lwoff, Zykoff, Gadow) derive it from immigrant cartilage cells which have penetrated through the notochordal sheath from outside, while others (Gegenbaur, Field, Ebner, Klaatsch, Schauinsland) believe it to originate by the metamorphosis of actual notochordal cells, probably cells of the notochordal epithelium. In spite of a possibly greater volume of evidence supporting the latter view it is difficult to avoid the impression that the former has in its favour the balance of a priori probability. The Reptiles are commonly regarded as the least specialized of the three subdivisions of the Amniota and it may therefore be con- venient to let them form the basis of our description. Schauinsland's work may be referred to for more minute detail. The sclerotome tissue grows actively and comes to be specially concentrated immediately round the notochord to form the peri- chordal layer. This layer is at first — in accordance with its origin from the sclerotomes — segmented (Fig. 152, A, scl) but the original segmentation soon disappears so that it forms a perfectly continuous investment to the notochord. A secondary segmentation now becomes visible in as much as the perichqrdal layer is decidedly thicker in a position corresponding to the middle of each original segment. These thickenings mark it off into a series of reel-shaped pieces each of which is a primary vertebral body (Fig. 152, B and C, p.v.l)}. It will be understood that the hinder half of each primary vertebral body is derived from the front half of a sclerotome while the front half of the same primary vertebral body is derived from the posterior half of the next sclerotome in a headward direction. In other words each primary vertebral body is formed from the adjoining halves of two original segments, and as a result of this the primary vertebral bodies necessarily alternate in position with the myotomes, each myotome running from about the level of the middle of one primary vertebral body to a level about the middle of the next in the series (Fig. 152, B). The portions of the sclerotomes lying outside the perichordal layer undergo fusion also. This outer part of the sclerotome bulges out between the myotomes while it extends dorsalwards so as to arch over the spinal cord. It is in the wall of the tunnel so formed that the neural arch-elements make their appearance while the sclerotome tissue ventral to them takes part in the formation of the body of the definitive vertebra. The superficial part of the vertebral body arising in this way from sclerotome tissue outside the perichordal layer 302 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. 152, C, S) is best developed laterally (Sphenodon) though it extends as a thinner layer over both the dorsal and ventral sides of the perichordal layer. Eventually chondrification takes place and the vertebral body, derived partly from perichordal and partly from sclerotouie tissue lying outside and continuous with the neural arch portion, becomes converted into a mass of cartilage in which the only clue to its compound origin is the somewhat flattened shape of the cartilage cells in the inner part derived from the perichordal layer (Fig. 152, C, p.v.b). During the development of the vertebral centra the notochord becomes constricted across much as in Urodeles. A complete septum of notochordal cartilage is formed across the middle of each I .v. •'•••"& Nf p.v.h &££. FlG. 152. — Diagram illustrating the mode of development of the vertebral centra in a Reptile as seen in horizontal sections. (Based mainly on Schauinsland's figures of Sphenodon, 1906.) my, rnyotomi- ; A, notochord ; j'.r./i, primary vi-rd-hral body ; N, superficial portion of centrum .'it«id«- prrirhurdal luyi-r ; f.'.i, spinal ^an-lion ; *<•/, sclcrotonn- ; -•, blond-xrss.-I. hi (•.•m|«rinK the Hegmentui relations of A and H tin- int.-rs.-ni'-ntal blood-vessels (<') form usHul landmarks. vertebra in Sphenodon and in the Lacertilia. In the ordinary Li/.ards this appears to arise as a ring-shaped ingrowth of cartilage \\hieh (-'districts the uotochord, pushing the primary sheath in front of it (fiadow, 1897) while in Sphenodon and also in the (Jeekos the cartilage makes its appearance internal to the notochordal sheath (Howes and Swinnerton, I'.iOl;. It may l»e suspected tliat in the latler case iliinii.Lji.ml cartilage cells ha\c made their \\ay through the notOfilmnl.-Ll sheath though this ha- not so far lu-eii demons! rated. 1,'r Tlicj ribs are long cartila-mous projections from the vertebrae \\-hieh run outwardi and \etiiiall\ m t IMJ substance ol the myoKcpta and serve t<> support and strengthen the wall of the IpWcnnOOOela As Groette •. ISV.S, 1879) first showed, there are included under I he name "ribs" I \\ •• morpholo-ieally ililfereiil v RIBS 303 structures, which may he distinguished by the names dorsal or upper ribs and ventral or lower ribs. In Polypterus both sets of ribs are well developed — the dorsal ones larger towards the head, the ventral larger towards the tail. In other vertebrates the rule is that only one set is developed, though the otln-r may be represented by more or less distinct rudi- ments or vestiges. Thus in Actinopterygian ganoids, Teleosts and Dipuoans the ribs are ventral ribs while in Elasmobranchs, Amphibians and Amniotes they are dorsal ribs.1 Both types are associated with the myosepta but whereas the dorsal ribs lie at the level of the horizontal septum which divides the lateral musculature into a dorsal and a ventral half, the ventral ribs, on the other hand, lie along the peritoneal edge of the rnyo- septum where it abuts on the lining of the splanchnocoele. Probably both sets of ribs are to be interpreted morphologically as outgrowths from the vertebrae and the balance of evidence appears to favour the view that both are fundamentally outgrowths from the series of haemal arch-elements. Ventral Ribs. — This is clearly the case with the ventral ribs which are simply the ventral prolongations of the haemal arch- elements, frequently jointed off from the basal stump of the arch (transverse process) by the conversion of a thin layer of the cartilage into fibrillar material. In the skeleton of a Lung -fish, a Crosso- ptvrygian, or an Actinopterygian the ribs are seen to form a perfectly continuous series with the haemal arches of the tail region. Dorsal Ribs. — The nature of the dorsal ribs tends to be obscured by the fact that their point of attachment to the vertebra shows much variation e.g. they may appear to arise not from the haemal but from the neural arch. That we have to do here with a secondary shifting in a dorsal direction is indicated by various considerations. Amongst the Rays it can sometimes be seen that the ribs towards the head end of the series become more and more displaced dorsally, so that they come to project from the neural arch. Then it will be remembered that in various fishes the haemal arch -element becomes divided into a ventral part (haemal process) and a dorsal part which latter carries the rib and may undergo a considerable displacement in a dorsal direction. In Urodele Amphibians Goeppert has shown that the apparent attachment of the rib to the neural arch has come about in a some- what complicated fashion as illustrated by Fig. 153. The most nearly primitive condition is that shown in the larva of such a perennibranchiate form as Necturus (Fig. 153, A). Here the haemal arch -element (li.a) sends off a strong outgrowth (r.b), the "rib- bearer," which passes in a dorsal direction closely applied to the 1 Distinct traces of dorsal ribs occur in various Teleost-s, e.g. Salmonids and Clupeids. The numerous little bones found in the myosepta of various Teleosts in addition to the true ribs are probably to be looked on as independent and secondarily developed "tendon bones." 304 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. b. surface of the neural arch, from which however it is marked off by a thin fenestrated layer of bone (6). It will be seen from the diagram that the bases of the neural and haemal arches and the base of the rib-bearer enclose a space through which runs the vertebral artery (r.tt). The haemal arch-element passes horizontally outwards beyond the base of the rib-bearer and the rib itself forms merely a prolongation of the haemal arch-element, becoming segmented off from its proximal portion (" transverse process ") by the de - velopment of an intercalary zone of fibrillar joint tissue. A little way out from its base the rib grows out into a projection which is directed dorsally and ^towards the median plane. This dorsal process is prolonged into a liga- ment which is attached at its end to a mass of bony tissue developed on the outer side of the rib-bearer and indicated in the diagram by the dia- gonal shading. In the larva of Salamandra macu- losa the condition is found which is illustrated by Fig. 153, B. The most important difference from the condition seen in Necturus is that the basal part of the haemal arch-element has become greatly reduced, and is now attached to the notochordal sheath merely by a thin thread of bone. FIG. 153.— Illustrating the attachment rrn ^u jLarpr arnwq nilf frnm thp of rib to vertebra in the Uro.l.-lu , ' grows according to Goeppert (1896). haemal arch -element as before but it A, trunk v-rt-bn, of AW*,,,,,, larva; B, ^.shorter and is more completely fused trunk vifi-tebra of macuiosa with the neural arch. The dorsal larva; C, t rank vert ,-\a-A nt Triton alputris prQCCSS of the rib has increased in larva, fc.ibone ; It. a, haemal Arch-element ; , i i i • i , i A.a'.haemal ,„„— ; V. noto.-hord ; n.a, Strength aild nOW extends tight to the neural ai.-h: r, rib; . s-//-/'.s- the condition is found which ^illustrated h\ 53 (' The original ha sal part of t he ha'-inal arch element \\hich lay ventral to the vertebral artery has disappeared, so far as earl ihiu'i1 is coneerned, its jdace j»ein^ taken by a thin thread of hone. The rib hearer i- shorter and stouter than in tfulu nm mini and its fusion with the neural arch still nmiv , [Tli« diagonally shade-l ,.} v RIBS AND STERNUM 305 complete. The double-headed rib has all the appearance now of simply articulating with a massive projection of the outer side of the neural arch : its original connexion with* the haemal arch would never be suspected. The question naturally arises whether in other Amphibians in which the transverse process and rib projects from the neural arch, the dorsalward shifting has come about in the same manner as is apparently the case in Urodeles. The probabilities appear to be against this. In the remaining two groups of Amphibja — the Anura and the Gymnophiona — the transverse process, though it springs from the neural arch, lies still ventral to the vertebral artery, which suggests that there has taken place here a simple shifting dorsalward of the whole of the haemal arch carrying the rib, including its basal portion. In the case of the Amniota, Schone (1902) has carefully investi- gated the development of Reptiles and has failed to find anything corresponding to the rib-bearer of Urodeles. In all probability here as in Anura there has taken place a simple dorsal movement of the rib and transverse process. The Amniote rib appears to arise generally in continuity with the anterior half vertebra (a) i.e. from material derived from the posterior half of the sclerotome. In the case of Sphenodon (Schauinsland) the sacral and usually the caudal ribs, on the other hand, appear to contain material derived from both halves of the vertebra, the ribs being in these regions much broader than they are elsewhere and marked by a longitudinal groove indicating their double origin. In the last vertebrae of the tail these may give rise to two separate transverse processes attached to each side of the vertebra tipped each one by a small rib-rudiment. The uncinate processes on the ribs of certain Reptiles and Birds arise as independent centres of chondrification. They may later on ossify and fuse completely with the rib (most Birds) or they may never show complete fusion (Apteryx, Sphenodon). In Sphenodon they become simply calcified without undergoing true ossification (Schauinsland). STERNUM. — The sternum of the Amniota arises typically by the fusion together of the ventral ends of a number of the anterior rib- rudiments into a continuous plate on each side. The two lateral plates so formed undergo fusion across the mesial plane to form the definitive unpaired sternum, a plate of cartilage still continuous with the ribs. Eventually the sternum becomes segmented off from the ribs and may become calcified by the deposition of limy particles in the intercellular matrix (Reptiles) or replaced by bone (Birds). In Amphibia also the sternum arises by the fusion together of two longitudinal band& of cartilage but no connexion can be traced between these and the ribs. This peculiarity, as compared with the Amniota, is apparently to be correlated with the comparatively short extension of the ribs in a ventral direction which is characteristic VOL. II X 306 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. of this group of Vertebrates. lu the Fishes the sternum has not yet made its appearance. SKULL. — The skull is a mass of condensed and strengthened mesenchyrue serving essentially to support and protect the organs of the head. It protects the brain and sense organs : and it forms a support and framework for the masticatory and other apparatus connected with the mouth and pharynx. In correlation with this its characteristics in detail are secondary to characters of the brain and other organs. The skeletonization of the mesenchyme does not take place continuously but commences in irregular patches which gradually spread and eventually join together. Though there is frequently considerable agreement between different Vertebrates in the position of the centres of skeleton formation in the head there are in other cases equally well-marked variations between forms known to be phylogenetically closely related. It is as a rule impossible to say definitely whether or not the first appearance of skeleton at particular points is of phylogenetic significance or is on the other hand related merely to existing arrangements of the adult. Under the circumstances all that will be attempted here is a short sketch of the general features of cranial development without entering at all into minute detail. For a full and detailed description reference should be made to the admirable work of Gaupp (1906). As has already been indicated there is a marked tendency for the arch - elements to undergo fusion towards the head end, the axial skeleton being necessarily rigid instead of flexible in the brain region. Eventually towards the front end of the series both neural arches and vertebral centra become completely fused together to form part of the skull. The skull consists in its simplest form primarily of a chondro- cranium — a trough of cartilage, the cavity of which is occupied by the brain and more or less open on its dorsal side. Somewhere about the middle of the floor of the chondrocraniuin there exists a recess in which rests the infundibulum of the brain, and the portion of cranial floor lying behind this is distinguished by having the notochord embedded in it — this organ having its anterior limit just behind the tip of the infundibulum. We are thus brought into touch with a deep-seated distinction between the posterior or epichordal (cliordal — Kolliker) region of the cranium a ml the anterior <>r prechordal (Kolliker). We are probably justified in iv^anlinu the epichordal region of the minium .is hrin^ morphologically a metamorphosed | .mi ion of vertebral column in which the processes of fusion, already indicated as frequently occurring in the anterior n-.'imi. have ;itt;iiued to their maximum. As will be explained later the process of incorporation of a few vertebrae (the number varying in dillrivni ^roujw into tin- hinder i \- raj.snlt's. of interest \\IHMI oorrelated \\ith the persistem f this portion of the chondrocranial roof in /'rotopterus — suggest in- that v CHONDBOCKANIUM 315 this is probably the most archaic portion of the cranial roof of the Vertebrate. At the same time the possibility must not be lost sight of that instead of being of ancestral significance this feature may be associated merely with particular activity of cartilage- formation in the region of the otocyst, connected with the need of protecting that superficially placed organ of sense. DEVELOPMENT OF THE CHONDROCRANIUM IN BIRDS. — According to Sonies (1907) the first cartilage to make its appearance is an unpaired plate arranged in a frontal plane and surrounding the notochord at its highest point in the cerebral or mesencephalic flexure. Sonies terms this (Fig. 157, acr) the acrochordal cartilage and states that it makes its appearance in the 5-day embryo of the chick. What appears to correspond to it in Apteryx is described by T. J. Parker as the prochordal cartilage, though in this case it lies quite anterior to the notochord. Very soon after the acrochordal cartilage, the parachordal makes its appearance — ensheathing the notochord. As this is thickest laterally and very thin ventrally and especially dorsally (where indeed it may be absent) it presents when viewed as a transparency from the dorsal or ventral side a misleading paired appearance. In Apteryx however the parachordals have apparently retained the actual paired condition. For a time the parachordal and acrochordal cartilages are separated by a wide gap but later (11-12 mm.) this becomes filled in by the development of the paired elongated mesotic (basiotic) cartilages (Fig. 157, B, mo). In the Duck these are at first independent, but in the Chick they appear to be, even at the time of their first appearance as cartilage, continuous with the parachordals. Extending forwards they become continuous with the acrochordal, bounding upon their mesial side a space in which no cartilage is present — the posterior basicranial fontanelle (Fig. 157, E, ./?.&/). Postero-externally the mesotic cartilage fits round the lagena, form- ing the rudiment of the cochlear part of the auditory capsule. The parachordal cartilage spreads out on each side forming the basilar plate of cartilage and in embryos of about 7 days (13-14 mm.) two pairs of neural arch-elements make their appearance as lateral projections near its posterior end (Fig. 157, E, n.d)— the posterior, situated between the Hypoglossal and the First Cervical nerve, developing first. In the Kestrel (Tinnunculus alaudarius) Suschkin (1899) found three such occipital arches (Fig. 158, n.d} and Gaupp looks upon this as probably the typical number for Birds. The acrochordal spreads out and forms a transversely situated plate of cartilage. The trabeculae appear in the chick embryo of about 11 mm. as paired parallel rods of cartilage underlying the fore-brain. Posteriorly each passes into a swelling lying lateral to the pituitary body and as in the Duck and Starling (Sturnus) this forms at first an inde- pendent piece Sonies terms it the polar cartilage. Even in the Duck embryo this polar cartilage (Fig. 157, C, p) becomes very soon 316 EMBKYOLOGY OF THE LOWER.VERTEBRATES CH. continuous with the trabecula in front and with the acrochordal cartilage behind. The connective tissue between the anterior ends of the trabeculae gradually chondrifies in continuity with them in both Chick and Duck (Sonies). In the Kestrel Suschkin found a, for a time independent, intertrabecular plate of cartilage in this position (Fig. 158, itr). This intertrabecular tract of cartilage serves to bound anteriorly the fontanelle (Fig. 157, D, p.f) in which the pituitary body lies and through which pass the two internal carotid arteries. Posteriorly this fontanelle is demarcated from the posterior basicranial fontanelle by the acrochordal cartilage later the posterior boundary of the sella turcica. It appears to be character- tr a.c n.a. a.c. FIG. 158. — Early stage in the development of the chondrocranium of the Kestrel (Tinnuncid'u^ alaudarius). A, side view; B, dorsal view. (After Suschkin, 1899.) o.c, auditory capsule ; itr, intertrabecular cartilage ; n.a, neural arches ; tr, trabecula ; - III, foramen for oculomotor nerve. istic of Birds that this dorsum sellae undergoes a considerable amount of reduction during later development. In Chick embryos of about 12 mm. a patch of cartilage has made its appearance external to the otocyst between the lateral and the superior (anterior) semicircular canals which gradually spreads forming the external wall of the auditory capsule and closely moulded to the surface of the canals. This periotic carlil.im- ir»7, K, <'.c) is for a time separated l»y a wide g.-ip from the basilar plate but this gap gradually becomes UK >re and more encroached upon until reduced to a narrow fissure through wliie.h cranial nerves IX, X, ami XI fiml lln-ir exit. Apart from this lissmv the basal and periotic cartilages become continuous. As the wall of the auditory capsule extends dorsally it remains incomplete at two points \\hriv p.-rfonil.-d |,y i|,,. |-'ari;il and Auditory mrvrs. The roof of tin- ( ImndiMi r.niium is represented by a quitr v CHONDROCEANIUM 317 inconsiderable tectum synoticum, which originates as a pair of at first separate cartilaginous rods (Chick -21 mm.). These very soon become continuous with one another and with the auditory capsule. DEVELOPMENT OF CHONDROCRANIUM IN GENERAL. — The three examples of chondrocranial development which have been dealt with will suffice to give a general idea of the process with its variations. A survey of the known facts in Vertebrates generally shows that the first rudiments of the chondrocranium consist of paired elongated pieces of cartilage (preceded by prochondral tissue) lying on each side of the mesial plane and on the morphologically ventral side of the brain. These rudiments are divisible into a (para)chordal portion lying at the side of the notochord, and a prechordal portion lying anterior to this. A break in the continuity of the cartilage frequently occurs somewhere about the limit between these regions and this had led to the regarding of the portions so separated — trabecula in front and parachordal behind — as being fundamentally distinct morphological elements. As a matter of fact the break, when it does occur, appears to vary in position : thus in Petromyzon the "trabeculae" extend back for some distance beyond the tip of the notochord, so that their hinder parts are parachordal in position. In many cases the break is visible only for a very short period, while in others (Lepidosireri) there is complete continuity between trabecula and parachordal. On the whole it appears justifiable in the present state of our knowledge to regard the break in continuity between trabecula and parachordal not as marking a demarcation between two originally distinct morphological elements but rather as a secondary solution of continuity correlated with exaggerated cerebral (mesencephalic) flexure. The parachordal cartilage in the case of the Elasmobranchs passed backwards by perfectly insensible gradations into the cartilage of the vertebral column. In that portion (occipital region) which lies between the hinder limit of the definitive cranium and the vagus nerve there appear for a time evidences of segmentation, corresponding with that of the vertebral column, and it is therefore justifiable to regard this portion of the parachprdal cartilage as representing a region of fused vertebrae. In the anterior or mesotic portion there are no visible metameric swellings but, as the relations to the notochord are otherwise identical, it is difficult to refuse a homology in this case which is granted in the case of the hinder portion. Here again, then, we should be inclined to regard the distinction between the mesotic and the occipital portions of the parachordal as merely a secondary differentiation in what was once a continuous structure or series, of structures : in other words we should regard the whole of the parachordal region of the cranium as representing a modified portion of vertebral column which has been absorbed into the cranium. The foundations then of the vertebrate chondrocranium are laid 318 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. in the form of paired basal cartilages which are eventually continuous throughout parachordal and trabecular regions but which may for a time consist of separate portions lying one in front of the other. As chondrification spreads from each of these primary elements, they become united together in a continuous plate of cartilage, forming the floor of the chondrocraniuni. From this in turn chondrification spreads upwards to form the side walls and roof, and forwards into the ethmoid and nasal regions. To the brain-case so formed there become added the protective capsules of the olfactory organ and otocyst. As each of these organs is a development of the external skin, we may assume with a con- siderable degree of probability that their cartilaginous capsules were originally independent of the cranium. Any repetition however of this completely independent stage of the sense-capsules in question has apparently become obliterated from ontogenetic development. Portions of the sense capsule may arise from separate centres of chondrification e.g. in the case of the auditory capsule the first rudiment may be in the form of an independent patch of cartilage in the region of the lateral semicircular canal. Even in such cases however the inner portion of the capsule develops in continuity with the chondrocraniuni. Again the Dipnoan arrange- ment, where the otic capsule is without any wall upon its mesial side so that it takes the form merely of a bulging of the lateral cranial wall, is to be looked upon as secondary. SKELETON OF THE VISCERAL ARCHES. — The anterior portion of the alimentary canal forms a tube leading from the mouth back underneath the cranium, its lateral walls perforated, and therefore weakened, by the visceral clefts. The coelomic space being no longer present in this region somatopleure and splanchnopleure are in continuity, a continuous mass of rnesenchyine extending from ectoderm to endoderm. This mass of tissue is divided by the clefts into the series of visceral arches and each of these is characteristically strengthened by a tract of tissue in its interior undergoing con- densation and chondrification to form half-hoop shaped cartilaginous arches. These arches are named according to the mesenchymatous arch in which they lie— Mandibular (I), Hyoid (II) and First branchial (III), Second branchial (IV) and so on. The skeletal branchial arches differ in number in different verte- 1 miles, just as do the corresponding meseuchymatous arches (see p. 153). In the Lamprey,1 which probably in this respect shows 1 he most nearly primitive arraiiL'ement, tin- two half-hoops of a pair lurnmo con- tinuous with one another ventrally. In tin- gill-hjva.t hinu fishes ihe hoop typically l>ecomes divided by joints into fn each side, with a median ventral copula — nodouht an adapt i\v arrange- ment t" facilitate the movements of respiration. Where branchial respir reduced the arch lias reverted to its primitive un- 1 Then: is in the writ' i "pinnm n<> sullieient evidence to doubt that the visceral skeleton of HOB ia homologous with that of Unathostomes. v VISCERAL ARCHES 319 segmented condition (Lepidosiren, Arnniota) and no trace of segmenta- tion appears during ontogeny. In Elasmobranchs (JJohrn, 1884) chondrification begins on each side and then spreads dorsally and ventrally. Segmentation takes place first into a dorsal and ventral half and later each of these segments again. The gill rays develop independently of the hoop and only come into contact with it later. The hyoid arch corresponds closely with the branchial arches in its mode of development. The arches so far dealt with — branchial and hyoid — having to do primarily with the function of branchial respiration show their typical development in Fishes. With the disappearance of this func- tion they become degenerate. This degeneration makes itself manifest in (1) reduction of segmentation, (2) tendency to fusion between successive arches and (3) reduction in number from behind forwards. Thus in a Newt four cartilaginous branchial arches make their appearance but they are for a considerable period continuous dorsally and ventrally with their neighbours in the series, and they develop only one joint upon each side i.e. the half-hoop consists of two segments instead of four. In a Lizard only two cartilaginous branchial arches make their appearance, and in a Bird only one. The hyoid and the anterior branchial arches have probably been saved from complete disappearance in the higher Vertebrate by the fact that they have taken on important functions in connexion with the tongue and have become specialized in accordance therewith. Thus in the case of the frog tadpole there is found, when the branchial apparatus is at the height of its development, a continuous cartilaginous hyobranchial skeleton, in which may be recognized parts corresponding to hyoid arches, copula between these, and 4 pairs of branchial arches continuous ventrally. At the time of metamorphosis this becomes greatly modified to give the adult condition (Gaupp, 1894): the mid-ventral portions become greatly expanded to form a flattened plate — the so-called " body of the hyoid " : the hyoid arch becomes an elongated slender rod which serves to suspend the apparatus from the skull : the branchial arches disappear except the ventral end of the second which persists as a stump (" Postero-median process "). MANDIBULAR ARCH. — The usually accepted idea of the mandibular arch is to regard it as a half-hoop shaped cartilage resembling the other arches, to which is added a forwardly projecting outgrowth — the palato-pterygoid bar — which forms the primitive upper jaw skeleton. In actual ontogeny there is always a less or greater amount of departure from this general scheme. In the Amphibians and Lung-fishes the hoop-like character of the main portion of the arch has been most completely retained. Here (Fig. 155, A) the arch develops on each side as a curved bar of cartilage — a mid-ventral copula having been detected in certain cases. The cartilage soon becomes completely continuous at its 320 EMBKYOLOGY OF THE LOWER VEETEBRATES CH. upper end with the chondrocranium and its dorsal end becomes segmented off, as the palato-quadrate cartilage, from the larger ventral portion — Meckel's cartilage — which forms the primitive skeleton of the lower jaw. In the animals mentioned the lower jaw remains throughout life connected with the cranium through the dorsal portion of the original arch. This must be looked on as in all probability the primitive mode of attachment of lower jaw to skull and such skulls may there- fore be termed protostylic.1 Both in Lung-fishes and Urodele amphibians the palato-pterygoid process is much reduced. In Urodeles it makes its appearance only at a late stage of development and is of comparatively small size. In Lepidosiren and Protopterus it has become eliminated almost entirely from development, being represented for a short time by a slight condensation of tissue which never becomes chondrified. This is probably to be interpreted as a modification of development induced by the precocious development of the bony skeleton of the upper jaw which in the forms mentioned replaces functionally the originally cartilaginous skeleton. The Elasmobranch fishes do not exhibit this reduction of the palato-pterygoid bar for this becomes the functional upper jaw. On the other hand an important modification of development has taken place in correlation with the fact that in these fishes the original dorsal end of the mandibular arch has lost its primitive function of suspending the jaw, this function having been taken over by the enlarged dorsal end of the hyoid arch (Hyostylic type of skull). In correlation with this the portion of the mandibular arch lying above the pterygoid outgrowth is, all through development, greatly reduced. It is apparently represented by the prespiracular cartilage, which develops comparatively late. The mandibular arch makes its appearance in Acanthias (Sewertzoff, 1899) as a C-shaped rod of cartilage lying in the rim of the buccal opening on each side (Figs. 156 and 159). The lower half of this segments off as Meekel's cartilage, while the upper half, which develops from behind forwards, clearly represents the pterv^o- Ojiiadrate bar. This becomes continuous with and later articulated towards its anterior end with the trabecula — a doubtless secondary connexion with the cranium seeing (1) that it arises from the an i erior and later developed portion of the palato-pterygoid outgrowth and (2) that in primitive sharks such as Notidanus, in Luii^-lisli.-s, and in l:rodclc amphibians, the attachment of mundibular arch to skull is further hack in tin- auditory region — in fact in the region of tin- original dorsal end of tin: mandihular arch. In the lower Vertebrates apart from those mentioned the <>t the caiiila^inous mandibular arch takes place on thiiiii K. -rr, 1908. Attrntiuii is draw n in I hi |.;i]>n tu tin- nn-d of an additional .•• more primitive tyj>< "I ..... ll.-d uut<>siyli<- skull. A similar suggestion had, lr " mad»- 1>\ <;p -ui-y [II TIIK SKKLKTON 321 similar lines. In the Reptiles and also in Birds the palato-ptery- goid outgrowth is again reduced in size — in correlation with the tact that in the Tetrapod Vertebrates the tooth-hearing function of the original upper jaw <>r palato-pterygoid bar has been taken over by the secondary upper jaw composed of bones such as the maxilla and premaxilla. BONY OR OSSEOUS SKELETON Bone, like cartilage, is a modified connective tissue. In its typical form it differs from cartilage in the facts, that its matrix yields on being boiled a larger proportion of gelatine, that the matrix is rendered rigid by being strongly calcified, and that the cytoplasm projects as slender branching processes which ramify FIG. 159. —Skeleton of visceral arches and pectoral girdle of 20 '5 mm. embryo of Spinax. (After Braus, 1906.) /,i. /;•"'. branchial arcln-s ; lln, liyoid ; /, labial cartilage; M, mandibular arch; p, palato-pterygoid I»ar ; /'./, rudiment, of jicctm-al tin ; p.g, pectoral girdle ; Q, knob for attachment to trabecular region of skull. through the matrix and are commonly continued into those of other cells. Many different varieties of bony tissue exist. In ordinary bone the cell elements are completely surrounded by the calcified matrix. On the other hand some of the cells may have the main part of their cell- body outside the calcified mass, only a slender prolongation being surrounded by it (Bones of Lepidostens and Amia'). Or this peculiarity may apply to all the cells (Dentine of higher Vertebrates) or finally no cells or parts of cells are enclosed within the hard matrix — as is often the case in early stages of development and as occurs in the adult condition in many Teleostean fishes. Probably the most archaic type of bony skeleton in existing Vertebrates is seen in the Placoid scales of the Elasmobranchs and consequently the mode of development of these will logically fall to be considered first. The appearance of the scale is foreshadowed by a localized condensation of the dermal connective tissue immediately beneath VOL. n Y 322 EMBRYOLOGY OF THE LOWEE VERTEBRATES CH. the epidermis. Presently this begins to bulge upwards like a dome into the epidermis. The epidermal cells immediately bounding this little dermal elevation take on a columnar shape : they constitute the enamel epithelium (Fig. 160, e). The (dermal) cells on the surface of the dome or papilla, immediately underlying the enamel epithelium, also become distinct and form a definite layer of odonto- blasts. The hard substance of the scale makes its appearance as a cone of dentine fitting over the surface of the dermal papilla and in turn ensheathed by the enamel epithelium. The dentine cone, which usually becomes directed tailwards, gradually thickens, encroaching upon the dermal papilla or pulp which it surrounds. It lies FIG. 160. — Longitudinal vertical section through the skin of an embryonic Shark to show a developing placoid scale. (From Balfour's Emlniology \ figure by Gegeubaur a tin Hertwig, 1874.) / . .-]•!. I.TIIIIS : • . <-namel epithelium; o, enamel; /<. di-imal papilla. immediately outside the odontoblasts and as it increases in thickness the outer portion of some of the odontoblasts persists as a fine thread of cytoplasm extending out through the substance of the dentine, so that when dried the dentine is seen to be traversed by innumerable fine slightly diverging canals each of which contained a protoplasmic thread. Tin: hard nuiterial of the dentine is commonly regarded as calcifi<-• except the v PLACOID SKELETON 323 enlarged spine-like scales in front of the dorsal fins, on the anterior face of which it is well developed. The enamel is in turn covered on its surface by an extremely thin membrane-like layer — the tMiiimel cuticle — and Huxley (1859) made out the important point that this is continuous with the basement membrane of the epidermis outside the limit of the scale rudiment. In the teeth of the higher animals, which as will be- seen later are simply modified placoid scales, the enamel is sharply marked off from the dentine and it is usual to regard it as of totally different origin namely as a kind of cuticular formation by the inner ends of the enamel epithelial cells. The chief reasons for this view are the sharp differences in appearance and composition from the dentine in these higher Vertebrates, and the fact that the cells of the enamel epithelium undergo a shortening as the enamel layer thickens — as if the inner ends of the epithelial cells were undergoing conversion into enamel from within outwards. It is however curiously difficult to find evidence sufficiently con- vincing to justify the almost universal acceptance of this idea even as regards the higher Vertebrates. And in the case of the Fishes the evidence — such as the location below the basement membrane and the frequently quite gradual transition between the so-called enamel and the dentine — strongly supports the idea that the former is simply a modification of the outer layer of the dentine. The basal edge of the cone of dentine comes to spread outwards all round parallel to the surface of the skin as irregular trabeculae forming a strong basal plate by which the scale is firmly fixed in the derails. This basal plate is usually of homogeneous appear- ance but its substance shows a gradual transition to the typical dentine of the spine, and in the case of Callorhynchus (Schauinsland, 1903) the basal plate as a whole shows, just as it does in the ancient fossil Coelolepids, dentinal structure. There seems then no reason to doubt that the basal plate is in its nature closely allied to dentine or in other words that it is bone in the broad sense of the term. TEETH. — A section across the jaw of an ordinary Dog-fish is sufficient to demonstrate the important morphological fact of the homology of the teeth and the placoid elements of the skin. Teeth are simply placoid elements belonging to that portion of the outer skin which is carried inwards to form the stomodaeurn. Or con- versely the spines of the placoid scales are simply teeth which have not been carried inwards into the stomodaeurn. In accordance with this the placoid scales were long ago (1849) named, by Williamson, dermal teeth. The demonstration of the homology in detail will be found in a classical paper by 0. Hertwig (1874). The lining of the buccal cavity being morphologically part of the outer skin the. probability is that originally teeth or placoid elements were distributed equally all over it. But in the evolution of the Vertebrata there has clearly taken place a restriction of the teeth to particular parts of the lining where they can be most 3'24 EMBKYOLOGY OF THE LOWER VERTEBRATES < 11. effective. In some of the lower fishes (many Elasmobranchs, e.g. Acanthias) teeth of a simple character, practically unmodified placoid elements, are still to be found scattered over the roof of the buccal cavity and even extending back into the pharynx. Un- fortunately the development of these has not been worked out in detail. In Teleostean fishes however a very simple type of tooth development has been described e.g. in the Pike (Esox lucius). The teeth are here no longer scattered equally over the buccal lining ; they are restricted to the dentary, maxilla, vomer, palatine, and the inner surface of the visceral arches. The teeth on the roof of the mouth arise as simple conical dermal papillae which project into the epidermis and develop enamel, dentine, and an irregular trabe- A. FK;. 161. — Early stage in the development of the tooth in (A) Ceratodn* and (B) Lepidosiren. (A after Semon, 1899.) '/, dentine; u<1, oclontoblasts. cular bony base on the same general lines as described above for the typical placoid element. Relatively primitive conditions are, found again in the Dipnoi and Amphibia (Urodela and Gymnophiona) in the latter of which the teeth may be very widely distributed e.g. 011 premaxilla, maxilla, vomer, palatine, pterygoid, parasphenoid (N/Woywj), as well as on the deutary and occasionally on the splenial. In the simplest cases the tooth originates as a simple conical or rounded dermal papilla which projects upwards into the ectoderm (of. Ceratodus, y ;i narrow stalk or isthmus. The tooth is built up of precisely the same elements as the placoid scale — dentine, enamel and basal plate. Its modifications are such as to make it more efficient for its special purpose. The projecting spine becomes exaggerated to form the functional part of the tooth : it remains conical, or it becomes a flattened blade with plain or serrated edge, or it becomes a low flattened crushing plate. To secure greater strength the pulp may become traversed by hard trabeculae (vaso-dentine). Kegarding each of the three elements mentioned above there is a certain amount of controversy. As regards the dentine there is the question of its origin — whether it is to be regarded as calcified matrix or as modified cytoplasm. The evidence of Lepidosiren — which on account of the size of its cell elements is always of weight in such questions — seems very clearly on the side of the latter view. As shown in Fig. 161, B, the cytoplasm of the odontoblast passes uninterruptedly into the calcified dentine, the spaces between the odontoblasts on the other hand dying away as the dentine is approached. But if in a relatively archaic creature like Lepidosiren the main part of the dentine is undoubtedly modified cytoplasm this at once raises a strong presumption in favour of the same being the case in the higher Vertebrates even if it be not actually obvious. Again as regards the enamel it is taught practically universally that it is formed after the manner of an internal cuticle by the cells of the enamel epithelium. This idea has come down to us from the days of the early investigators who devoted themselves especially to the investigation of Man and those Vertebrates most closely allied to him. In those days the structure of the lower animals was interpreted according to the data obtained from Man and his allies. The whole outlook was the opposite of that which holds in these evolutionary days when the accepted principle of all morphological work is to interpret the higher and more complex animals by data obtained from those lower in the evolutionary scale. Applying this principle to the case of the teeth of the most archaic Vertebrates we see in the Elasmobranch fishes that the outermost layers of the dentine develop the special modifications already alluded to — extreme denseness and hardness, transparency and high refraction, reduction of the proportion of organic material, reduction of the tubular cavities. Here the enamel is undoubtedly modified dentine. But if this be so there are only two alternatives open to us in interpreting the enamel of the higher Vertebrates. It is either to be regarded as a further stage in the differentiation of the outer layer of dentine or it is to be regarded as something quite new, a new substance formed by the enamel epithelium. This latter is the generally accepted view and in accordance with it the hard layer on the teeth of fishes was given by Williamson the name 326 EMBKYOLOGY OF THE LOWEK VERTEBRATES CH. Ganoine to distinguish it from the true enamel of the higher Verte- brates. As regards the basal plate the main question at issue is the evolutionary one whether or not the view of Gegenbaur should be accepted that these basal plates constitute the h'rst phase in the evolution of the bony skeleton. This question will more suitably be discussed in connexion with the bony skeleton in general. Lastly questions of general interest are raised by these excep- tional cases where the developing tooth cannot be. traced into immediate relationship with the ectoderm. In Lepidosiren and Protopterus as well as in the Urodele Amphibians portions of the lining of the buccal cavity which give rise to the teeth have the appearance of being derived in the embryo from endoderm. Again in Teleostean fishes teeth are developed far back in the pharyngeal region, in other words in a portion of the alimentary canal which is lined with endoderm. Such cases obviously cause serious trouble to those who apply the germ layer theory rigidly. They explain them by supposing that there takes place in development an actual spreading inwards of ectoderm over the surfaces on which teeth will develop. As indicated in Chapter III. in dealing with the buccal lining of Urodeles and Lung-fishes the writer of this volume believes that the evidence adduced so far of the ingrowth required by this explanation is not to be relied upon. He would rather explain such cases as due to the more or less broad debatable zone between the ectoderm and endoderm, the influence of one layer being liable to spread into the other and there being no sharp line the position in regard to which decides definitely to which layer a particular organ belongs. EGG-TOOTH OF REPTILIA. — In the embryos of Reptiles there appears a precociously developed "egg -tooth" at the tip of the upper jaw which has for its function the rupture of the egg-shell. In Geckos there are a pair of these present, attached to the pre- maxilla close to the mesial plane. In other Reptiles the left egg- tooth appears only as a transient rudiment and the functional (right) tooth takes up a practically median position so that it appears to be unpaired. It is of interest that this holds also for snakes in \\hieh there are no definitive teeth in the premaxillary region (Rose, 1894). To i SON FANG OF VIPERIDAE. — The poison fang of the Viperidae is highly specialized for the injection of poison, its pulp hein^ traversed by ;i ImiLMtudinal tube, composed of dentine and attaehed t<> tin- outer wall of the tooth along its anterior face. The inner tube — or poison canal — passes at each end into an open groove the openings so formed s> rvin.ur l'»»r entrance and exit of the poison respectively. The main features oi'the development are illustrated hv the transverse sections sho\\n in Kig. 163 (p. 329). The poison ! makes its ;i].|M-aranee as a lon^it ndiniil infolding of the dent ine. the eetoderm <>l the tooth-germ seeming to push tin- dentine in U;tore jl to form a groove (Fij_: L63 V , The grOOVe deepens and ils THE TEETH 327 lips meet (6) so as to convert it into a tube. From its mode of formation this tube is at first tilled, with ectoderm of the tooth- germ. Eventually however this ectoderm disintegrates and leaves an open tubular cavity (2). Up till this stage the tooth is still enclosed in the ectodermal germ which has increased much in size (2) but eventually this ectodermal mass also disintegrates with the exception of its outer- most layer, so as to give rise to the cavity of the sheath in which the functional tooth is contained. As the tooth becomes functional this cavity comes to communicate with the duct of the poison-gland so that it receives the poisonous secretion, and owing to the poison- caiml retaining the form of an open groove towards the basal end of the tooth it in turn receives the poison from the cavity of the sheath. FIG. 162. — Diagram illustrating tooth succession in an Elasmobranch (A, primitive, B, existing condition) ; an Amphibian (C) ; and a Reptile (Lacerta) (D). <>th---erms in TIIK TKKTII 329 r;ij)id succession, ;is many as tm being visible at one time in tin- ordinary Viper. As the tooth-germs develop and approach the surface they take up a position in two rows (3, 5, and 2, 4, in Fig. 163). The maxilla, which carries the functional faiijj, has two bases of attachment for teeth, an inner and an outer, and lhcs<- arc made use of alternately — a functional 9 f 7- dl p.c 6, ir tooth with external attach- ment being succeeded by one with internal and con- versely. The replacement takes place approximately syn- chronously in the two maxillae a pair of teeth attached to the right-hand bases of attachment of the two maxillae (in other words attached to the outer base of attachment on the right maxilla and to the inner base of attachment on the left maxilla) being replaced by a pair attached to the left-hand bases of attachment (inner on right maxilla, outer on left max- illa). In consequence of this arrangement the individual teeth of a functional pair are the same distance from One another as their prede- which is present as a thin sheath over the apical end of the cessors and their successors. tooth is not shmvn\ In. 7,the 1>oison ;,^'!al is.seen ar\ an open In this modification of the primitive P dl FIG. 163. — Part of transverse section through upper jaw of a young Viper. (After Rose, 1894.) The developing teeth are numbered in order of sequence. No. 8 is not shown. Dark tone = ectoderm ; pale tone = iiiesem-hvme. Dmtine is shown in black. The enamel arrangement of the replace- groove along the side of the tooth, filled with ectoderm ; in C this has become converted into a tube (p.c) still tilled with linear ectoderm cells; in 2 the cells have degenerated, leaving a clear lumen ; in 1 all of the ectoderm external to the tooth except its outermost layer lias broken down to form the teeth We have dOUbt- cavity of the tooth-sheath (S). The functional tooth (1) in a mechanism tO SeCUre thesection here liguml belongs to the outer series t., which more rapid succession, and in this connexion it is ol interest to notice that the replacement of the functional tooth is not dependent upon its having already suffered injury or become worn out but takes place at regular intervals (about six weeks in the case of the European Viper, Kathariner) while the snake is leading an active life. In the Crocodiles the dental lamina becomes broken up into a network, and finally reduced to a strand of tissue running longitudin- ally along the jaw, slightly to the inner side of the tooth-bases. A 330 EMBEYOLOGY 01 II IK LOWER VERTEBRATES CH. succession of tooth-germs are produced from this strand, each one lying to the inner side of a functional tooth. As the success! mi a 1 tooth develops it causes absorption of the inner wall of the functional tooth, and gradually comes to lie within the base of the latter. Finally the old tooth is shed and its successor remains in its place. In toothed Birds, so far as is known (ffesperornis, Marsh, 1880), the tooth replacement seems to have taken place in the same way as in Crocodiles. In modern birds a slight transitory ectodermal thickening has been interpreted as the vestige of a dental lamina (e.g. Terns — Rose, 1892 ; Carlsson, 1896) but the evidence is not convincing. Careful researches in this direction in the less highly specialized birds are highly desirable. TOOTH-PLATES. — In many of the lower Vertebrates instead of, or in addition to, teeth of the ordinary conical shape adapted for piercing, there are present massive plate-like structures adapted for crushing. Large tooth-plates of this kind were conspicuous structures in many of the extinct fishes and lower Tetrapoda. Amongst living Vertebrates they are exemplified by many of the Skates and Rays, by the Holocephali and by the Dipnoi. Embryological study has shown that these plate-like teeth may arise in either of two possible ways. In the Skates and in the Holocephali the tooth-plate is a single much enlarged and flattened tooth. In Callorhynclius Schauinsland (1903) describes how the tooth-plate originates in a widespreading dental papilla of a depressed dome shape. The outer layer of this develops a cap of dentine in the ordinary way. Below and con- tinuous with this there develops a trabecular spongework of calcified tissue which shows a transition from ordinary dentine in its super- ficial parts to a tissue closely resembling normal bone except that the cell-bodies remain superficial, only their branching processes becoming embedded in the calcified material. As development goes on the individual trabeculae become thicker and more numerous and the intervening meshes filled with ordinary vascular iiiesenchyme become less and less conspicuous until the tooth as a whole assumes its definitive strong and massive character. No typical enamel seems to be formed but Schauinsland points out that the enamel epitheliiM: - to exercise a distinct modifying influence over the superficial layer of dentine which becomes hard and glassy (Vitro- dentine wherever it is in contact with the enamel epithelium. In the case of Lun^-lishrs Semon (1899) has given a beautiful demonstration, which I can fully confirm, that the tooth-plates date not by th«- enlargement ;md modification «>!' single teeth but by the fusion of a number of originally separate denticles. Tin1 evidence <(f Talaeontolnj y it, may he mentioned is in complete a ment with the <;ml»ryMl.i;jie;i| evidence furnished hy Cri-iffm/n* on point and we may lake iht- latter at a particularly uood example Of the recapitulation of phylogenetic c\<»lution during the develop- ment of ih,. individu il TOOTH-PLATES 331 The originally separate denticles develop as already explained (p. 324) in typical placoid fashion, giving rise to little hollow cones • if dentine. Tr.iU-rulae of Imny 1 issue (" trabecular dentine/' or ^ olf FIG. 164. — Illustrating the dental arrangements in young Lung-tislics. (A audB after Semon, 1899.) A, roof of mouth of a GVratodns of stage 48, showing the separate conical teeth ; B, teeth of roof of mouth from a slightly younger specimen (stage 46) after the soft tissues have been cleared away by the action of dilute alkali ; ('. /.f /'/.•-•)/> //. macerated upper .jaw of voting specimen, showing the i>oint«d cusps still piesent on (Jie tooth-plates ; o//l, anterior nai-is ; "//-. jiosterior naris. " pulp dentine ") spread inwards from the bases of these cones through the underlying mesenchynie, so as to join up the various denticles by a loose calcified spongework. As development goes on the trabeculae of this thicken, the pulp-filled meshes become pro- 332 EMBKYOLOGY OF TIIK LoXVKR VERTEBKATES CH. portionally reduced, and the trabecular mass becomes the compact substance of the adult tooth. In the functional tooth the tips of the original denticles have completely disappeared. In Lepidosiren and Protopterus the separate denticle phase of development is not so distinct as in Ceratodus but a reminiscence of it is seen in the pointed cusps which are present on the teeth in early stages (Fig. 164, C). THE BONES IN GENERAL. — The view is now accepted by many morphologists, following Her twig and Gegenbaur, that the true bony skeleton has come about in evolution by the spreading inwards of bone-forming activity from the skin, where it arose in association with the coating of placoid scales which occurs in the lowest Giiatho- stomata. The probability of this view being correct is rendered apparent by a survey of the phenomena of development of some of the bones in the lower Vertebrates. Both in Lung-fishes and in Amphibians the bones of the skull which carry teeth are found to arise in development in the form of more or less trabecular bony tissue which spreads outwards from the tooth-bases in the same way as has already been described as occur- ring in the development of the com- pound tooth in Lung- fishes (Fig. 164). 0. Hertwig (1874*) found for example that the vomer, palatine andopercular of Urodele Amphibians are developed in this way, forming perforated bony plates studded with 1874*.) conical teeth (Fig. 165). In the case of dentary, maxilla, and premaxilla, part of the bone arises in exactly the same way, while part on the other hand spreads through the mesenchyme without having teeth on its surface. It is to be noted that these bones at first, as frequently happens in the development of bony tissue, have no cells actually enclosed in the calcified substance. Later on the teeth in some cases disappear, leaving behind merely the basal plate of bone which gradually increases in thickness. On turning to the Anura it is found that the bony trabeculae develop precociously ;tiid form the basil plate of bone while the teeth belonging to it are d'-layed in their appearance and may even be omitted. The embryology of tin- Amphibia then teaches us (1) that typical may l>« developed from the basal trabeculae connected with placoid elements and (2) that a secondary modification may ariati in which th<- tooth formation is delayed or suppressed, tin1 trabecular basal plate simply developing by ii^-ir ami becoming converted into the definitive hone. The f.i> i hy Hertwig for Amphibia do nut stand alone. < )n the contrary an exactly similar mode of dr\clopment is seen in v OKIGIN OF THE BONY SKKLKTON the "membrane" bones of the roof of the mouth in Teleosts, and in i In- tooth- hearing hours of Lun^-iisln's. Again there are present minute enamel-tipped teeth scattered over the surface of the dermal bony plates of (Jrossopterygians and various Siluroid Teleosts such as Loricaria, Hypostoma, Callichthys. Such facts as those just enumerated seem to justify the acceptance, as a working hypothesis, of the view that at least the more superficially placed dermal bones of the Vertebrata have actually arisen in the course of evolution from the basal trabeculae or plates connected with placoid scales. Admitting this a further question presents itself. What was the evolutionary origin of the more deeply situated masses of bony tissue, for example those which replace cartilage ? Has the tissue which gives rise to these gradually been infected with bone-forming activity which has spread inwards from the skin ? Or has this bone-forming power in the deeper tissues arisen independently ? It is in this connexion extremely instructive to study the gradual spreading of the irregular shreds of bony material from the tooth-base of a Lepidosiren. They gradually spread onwards through the connective- tissue matrix like crystals forming in a fluid, and there is no apparent reason why such spreading should not continue for relatively great distances, provided the necessary pathway of connective tissue is present. It appears in fact thoroughly reasonable to regard the deeper portions of the bony skeleton, like the more superficial, as having arisen in evolution by the spreading inwards of bone-forming activity from the skin. In considering this important morphological problem, the origin of the bony skeleton, it must be borne in mind that the all-important fact, which far outweighs all other evidence available, is that in the Elasmobranchii, the group of gnathostomatous Vertebrates which is admittedly the most archaic, the placoid scales are the only elements of the osseous skeleton which have as yet made their appearance. There is no suggestion that the ancestors of existing Elasmobranchs ever possessed a bony skeleton apart from the placoid scales. Con- sequently in the Vertebrate groups which have been evolved sub- sequently to the Elasmobranchs the bony tissue must either be a further development of the bony basal plates of the placoid scales, or else a new independent development. If the former view is shown to have in its favour a reasonable degree of probability we are bound to accept it as our working hypothesis until a better is suggested, for it alone of the two views mentioned is really constructive, the other offers no explanation but merely the negation of an explana- tion. In the opinion of the present writer the reasonable degree of probability has been amply demonstrated by the facts which have been quoted. It is also necessary to avoid attaching too great importance to the differences in detail which have arisen in the evolutionary history of bony tissue under different circumstances. Such differ- 334 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. ences may become conspicuous and highly characteristic — for example the difference in relation to the calcified material — whether the cell elements are completely surrounded by it as in the ordinary bone of the higher Vertebrates, or have merely a prolongation of the cell-body embedded in it as in ordinary dentine. Such differences in detail may be of great interest in themselves. For example the bony tissue forming the scales of Lepidosteus is characterized by the fact that some of the bone - cells show the dentinal characteristic that the main part of the cell-body lies on the surface of the calcified material and only a prolongation of it is enclosed within the hard substance. Now Goodrich (1913) has made out the important fact that this peculiarity is not confined to the scales but extends to the whole of the bony skeleton. Such a fact is obviously a strong additional evidence of intimate evolutionary relationship between the scales and the rest of the bony skeleton. Again such detailed differences may raise interesting problems, for example whether the " ordinary bone " type or the dentinal type (as is perhaps probable) is the more primitive type of bony tissue. Interest in such details must not be allowed to obscure the main conception of bony tissue as contrasted with cartilaginous, or the problem of its evolutionary origin. As regards that origin we seem justified in believing that bone formation has during the evolution of the Vertebrata spread from the dermis — from the neighbourhood of the placoid scale bases — into the deeper tissues and so given rise to the deeper portions of the bony skeleton. On the other hand we do not appear to be justified in regarding the evolution of the deeper parts of the skeleton as being due to a sinking downwards of actual individual placoid elements. Nor, in the author's opinion, is there reliable evidence, so far, bearing on the further problem whether or not the first scleroblasts or bone-forming cells of the Vertebrata were i in migrants from the ectoderm. This view, which was supported by Gegenbaur,- has a considerable amount of a priori probability in its favour in view of the facts of skeleton formation in the lower invertebrates. It is no longer possible in the present state of knowledge to classify bones, as did the older workers, simply into two sharply defined sets — membrane hones and cartilage bones. The most iliat we can do is to recognize various stages in the process of shifting inwards from tin; skin, from which as already indicated they probably arose in the early their evolution. Firstly we have, the most primitive type which may be termed dental bones, which an- superficial iii position and which still are cnnner,t,c(l at one j icrioi I or nt I ier with teeth. Typical examples a iv the bones alivadv referred tit in the roof of the imnit h in A mphihians. A second Category COnflistfl of bony plates which have lost their tooth struct HITS and have sunk do\\n to a deeper level. These frc'jiient ly hecuine applied to t he surface of l he car! Harmons skeleton, iMi-aled however from the cartilage by a layer of im- BONES AND SCALES 335 tissue. Such may be termed investment bones (Allostoses, (Jaiipp . Finally a third category of bones are the substitution bones (corresponding roughly to the old group of cartilage bones; Auto- stoses, Gaupp). In these the formation of bone has spread into the connective tissue in immediate contact with the cartilage, and as the tissue is formed, room for it is made by the destruction of the pre- viously existing cartilage, which it therefore comes to replace. While it is convenient to recognize these three types of bone development, and probably justifiable to interpret them as represent- ing successive steps in the evolution of bone, it must not be supposed that they are absolutely distinct : intermediate forms occur frequently and a single bone of the adult may arise during ontogeny in part according to one type and in part according to another. Bony tissue being rigid and inextensible, it is essential to the functions of movement and growth, that it should not be continuous throughout the body. It consequently takes the form of separate bones, the junctions between which are specialized either for move- ment, or for addition of new bony tissue at their margins. Each bone arises by the spreading outwards of bony tissue from one or more centres of ossification. The study of the arrangement and honiology of the various bones constitutes an important part of the science of Comparative Anatomy — particularly important for the reason that it is the bony skeleton alone which is as a rule preserved in the fossil remains of Vertebrates belonging to past phases of Evolution. It should be borne in mind that a single bony plate in such a part of the skeleton as the skull may represent ossification which has spread out irregularly from the bases of a large number of the original placoid elements. In view of this it will be realized that great caution must be exercised in homologizing apparently similar bones in different groups of the lower Vertebrates. Thus the same name — implying homology — is commonly given to similar bones in the skull of a Crossopterygian, an Actinopterygian, and a Lung-fish or Amphibian. There is no guarantee of any precise homology in such cases and the student should be on his guard against taking very seriously the nomenclature of such bones as expressing exact and well-determined homology. FISH SCALES. — In the Fishes, that is in those Gnathostomata in which the skin has not yet become specialized for Respiration (Amphibians), or for protection against desiccation (Reptiles), or for diminishing loss of heat (Birds and Mammals), there is commonly present a coating of dermal bones which most usually take the form of scales. Such scales are in the most general terms simply plates of bone in one or other of its varieties. The development of what is probably the most primitive type — the placoid scale — has already been dealt with. It need only be added that individual scales, inter- spersed regularly amongst the others, pause in their development, and 336 EMBRYOLOGY OF THE LOWER VERTEBRATES en. only proceed with the process when room is provided by the already developed scales becoming spaced out during the growth of the body. While the placoid scale is simply an individual dermal tooth the ganoid scales as seen in the surviving Poly pier us or Lepidosteus are on the other hand tooth-plates, numerous minute denticles being associated with each scale. In these fishes also there is a certain amount of independence between the dermal plate of bone and the actual denticles which are at first quite separate from it (Nickerson, 1893 ; Goodrich, 1908). The reduction in size of the dental cones and the loss of their attachment to the bony plate are steps towards their complete disappearance which has been reached in the scales of ordinary Teleostean fishes. In the ganoid scale of Polypterus or Lepidosteus the protective power of the bony plate has been greatly increased by its superficial layers undergoing modification of an analogous kind to that of the superficial layers of the dentine cone in the teeth of fishes. This portion of the scale is extremely dense, hard and enamel-like and is without cells embedded in it. Like the corresponding layer in the tooth of a fish it is commonly known by Williamson's name Ganoine. The advisability of using this name, rather than enamel, rests mainly upon the assumption that enamel is a substance fundamentally different, derived from a different cell-layer, from bone or dentine. If it be the case however that enamel is merely the superficial layer of dentine which has undergone secondary modification then there seems no particular harm in adhering to the custom — until recent years quite general — of using the word enamel for the superficial layer of the ganoid scale. Ganoid scales are still comparatively thick and bulky struc- tures but in the typical Teleosts the scales have become very thin plates of bone so modified as to be very tough and flexible, and overlapping like slates on a roof so as to be able to slide over one another during flexure of the body. This overlapping has been rendered possible owing to the surface of the scale being no longer inseparably linked to the ectoderm by the development of teeth. The scale is developed in the thickness of the dermis and it is only at its posterior edge, if anywhere, that it is even in early si connected with the epidermis. Vestiges of dermal denticles have been described in Teleostean fishes and these deserve fuller investigation. Marett Tims (L906) describes a sta^e in f,W//.s in \\hieh tin- sc.;i.le consists"!' separate platele! each \\iili a tooth-like spine projecting from it, while in such South American Siluroids as CW//V///7///N ami L»ric!' the l.one diller- on t he v THE SKELETON '337 inner and outer surfaces, that formed on the outer surface being the enamel or gunoine already referred to. In the highly evolved Teleost, where the scale has increased in area at the expense of thickness, the addition of new bone on the^tat inner and outer surfaces of the scale is relatively small in amount as compared with that round the edges. In accordance with variations from time to time in the metabolic activity concerned in the production of the new bone the latter tends to show variations in rapidity of growth, density and other characters, and consequently to show a more or less distinct layered arrangement. Where there are periodic variations in the metabolism of the fish — associated it may be with sexual activity or with food supply or with changes in the physical environment (e.g. seasonal changes of temperature) — these variations may be duly chronicled in the contemporary layers of the scales. Such scale records are often particularly distinct and easily observed in the scales of the Teleostei owing to their thin flat character and the predominance of growth at their edges. The development of the Cycloid scales, of Dipnoi has not been investigated in detail. So far as the main features of their develop- ment are concerned they apparently resemble the scales of Teleosts. Like them they are, except at their posterior edge, deeply embedded in the dermis. On their outer surface they are prolonged into numerous, often recurved, spines which in all probability represent true denticles although they have lost their primitive relation to the epidermis. BONY VERTEBRAL COLUMN. — In all gnathostomatous Vertebrates, except the Elasmobranchs (including Holocephali) and Sturgeons, the vertebral column becomes in great part bony. The process of ossification is found in its first beginnings in the Lung-fishes, where the arches become ensheathed in bone. In the bony Ganoids and Teleosts ossification usually commences in the connective tissue bounding the surface of the arcualia, the first shreds of bone being completely cell-less. From the arches the bone spreads over the surface of the chordal sheath (in Amia it develops here first — Schauinsland) to form the rudiment of the bony centrum. In Coregonus it is stated (Albrecht, 1902) that for a time two bony rings can be distinguished round each centrum (cf. variation in Amia mentioned below on p. 339). From the thin superficial sheath of bone an irregular spongework of bony trabeculae spreads outwards and forms the bulky centrum of the definitive vertebra. As this process goes on the basal portions of the cartilaginous arches become surrounded by bone and may persist as four tracts of cartilage running outwards through the bony centrum (e.g. Esox — Pike). Most usually the arches become completely bony : the original bony sheath covering their surface becomes perforated on its median side by invading vascular connective tissue which destroys the cartilage and deposits bone in its place. VOL. II Z 338 EMBKYOLOGY OF THE LOWER VERTEBRATES ni. The neural spine even when segmented in the cartilaginous condition becomes ensheathed in a continuous layer of bone. In the case of Teleosts the cartilaginous stage of the haemal arches is frequently completely eliminated from development, the arches being laid down as bone in the connective tissue. In the Urodele Amphibia bone makes its appearance as a cell- less sheath round the surface of the centrum, which gradually increases in thickness and becomes cellular, enclosing connective- tissue cells, and also spreads over the surface of the arcualia. The cartilage becomes gradually absorbed and replaced by the bone. In Sphenodon, which may be taken as an example of the more primitive Reptiles, a bony sheath similarly develops round the centrum, but according to Schauinsland it consists at first of a distinct dorsal and ventral half. The bony tissue of the dorsal portion spreads upwards so as to enclose the bases of the neural arch- elements but the main portion of the arch-element on each side becomes enclosed in an independent bony sheath of its own. This latter appears first on the outer side of the cartilaginous arch and may persist as a separate bony element for a long period, even throughout life in the Crocodiles and various other Reptiles. Bone formation also spreads inwards into the substance of the cartilaginous centrum along what possibly corresponds to the boundary between the primary centrum and the chondrified tissue external to it (Fig. 152, p. 302). Thus arises a deep-seated centre of active bone formation. From these various centres ossification spreads, the cartilage being gradually supplanted by bone. Not the whole of the bone so deposited is permanent : a great part of that lying outside the primary centrum becomes again absorbed, leaving a superficial tract connected with the more central portion only by sparse bony trabe- culae, the meshes being occupied by intrusive connective tissue. An interesting adaptive feature is found in the tail region of certain Reptiles (Lizards, Sphenodon) which enables the possessor to break off its tail suddenly when seized by an enemy. In these animals the halves of the centrum derived from successive sclero- tomes have reverted to a condition of incomplete fusion — the ossifica- tion IK-MILT more or less interrupted in the plane of contact of the two successive sclerotornes by a transverse septum of cartilage. As at the same time the corresponding connective-tissue septum between consecutive myotomes remains weaker than usual a violent contrac- tion of the caudal muscles is able to tear across both cartilaginous and connective-tissue septum and break off the distal portion of the tail. In the case of the Birds it would appear that, the main centre of ••atioii of the centrum corresponds to the deep-seated one in Sphenodon, the superficial hone-forming activity bein^ much reduced. A '•h.iracterislic feature of t he I'.irds, associated primarily with their peculiar respiratory movement*, is the extensive fusion \\hich takes i Mreen ih(- vertebrae of the trunk region. THE DEFINITIVE VERTEBRA 339 Tin-: COMPOSITION OK TIIK DKHNITIVE VKRTEBRA. — A fascinating but difficult chapter in Vertebrate morphology is that which deals with tli<- composition of the definitive vertebra. We have already, in describing the development of the cartilaginous vertebral column, mentioned the elements which go to build it up — neural, haemal, and central. The difficulties of interpretation arise from the fact that great variety shows itself in the ultimate fate of these elements and in the manner in which they undergo fusion with their neighbours. This can perhaps best be illustrated by the case of Amia as described ABA A. a. b. FIG. 166. — Variation in vertebral column of .-!////, neural arch-elements ; «, /*, haemal arch do. ; a, ft, central do. by Schauinsland. Here in some cases two amphicoelous centra (a and fl) are developed corresponding to a single segment, each one carrying its pair of neural and pair of haemal elements, those attached to the anterior centrum (A and a) being relatively small, those on the posterior centrum (B and &) on the other hand well developed. Variations from this diagrammatic arrangement are found in different parts of the body. In the tail region (Fig. 166, A) the original condition frequently persists although in aged individuals the arch-elements (A, a) of the anterior vertebra of the segment are liable to become completely over- grown and hidden by bone. On the other hand there frequently 340 EMBEYOLOGY OF THE LOWEK VEETEBEATES CH. takes place fusion between adjacent centra so that compound centra are produced. Most usually in this case it is the two centra (a, /?) of one segment which undergo fusion but in some cases the posterior centrum of one segment fuses with the anterior centrum of the next so that the resulting compound vertebral centrum (/? a) belongs to two successive segments. Again in some cases as exemplified by the specimen figured (Fig. 166, A) three successive centra may undergo fusion. Towards the front end of the tail and throughout the trunk region the two centra of one segment undergo fusion but apparently the hinder centrum has undergone reduction in size with the result that its neural arch-element (2?) becomes displaced on to the top of the smaller anterior element (A) (Fig. 166, B, C). Towards the extreme front end of the trunk the neural element B becomes practically intervertebral in position (Fig. 166, D). It is to be noticed that in these cases, where the element B has been dis- placed, the bony splints which develop on its surface never spread downwards from it, so that it remains throughout life without any continuity of structure with the rest of the vertebra. Towards the tip of the tail there is apparently no regularity, all kinds of fusions and modifications of the various elements taking place. To sum up we see in Amia two complete potential vertebrae corresponding to each segment,1 each with its central, its neural and its haemal elements. The two vertebrae of a segment may be represented by the A B formulae a and /3 where a and /? are the centra, A and; 5-the neural ~a b arch-elements and a and I the haemal arch-elements. In the trunk region the ordinary compound vertebra may be represented thus AB afl but in occasional cases fusions take place so as to produce vertel'rae db BA ABA ARAB of the type /3a or of the type afia or a/Zap . Such a fusion as ba aba abab that shown in the last formula produces a \ertehra with a very lon^ body upon which may persist four sets of neural and four sets of haemal element^. The great range 'of variation seen in Amia from the presumedly original condition, even in different regions of the vertebral column of one individual, emphasizes the need of much caution in the laying down of ^<-in-r;tl principles n-^; inline; the composition of the delinitive vcrtel.ra. It seems just iliahle to admit the two pairs of neiir.il and two pairs of haemal arch elements into the general 1 Diplonpondyloni <-<>n I M-CI .i 1 1 i 1 1 - cnshcatlicd by splenial and dentary (the latter taking on the tooth -bearing function) \\iih such other Unes as angular, supra - angular, and coronoid. In 1 Tin- pleuroccipital li'im of (he I)ijnmi :n [o t In76t1 nt of the occipital arch (Agar, 1000). v BONES OF SKULL 343 the region of the Hyoid arch the Teleostwnatous fishes with their greatly developed opm-nlum develop a series of opercular bones. Of these various bones mentioned in relation with the buccal cavity and pharynx the majority show more or less distinct evidence of their dental origin [ Premaxilla, Maxilla, Dentary, Palatine, Pterygoid, Vnmer, Parasphenoid, Opercular — of. Hertwig, 1874*]. In some cases this may be apparent only in part of the bone, the rest developing as an ordinary investment bone, while in a few cases bone which is in one part of the investing type may in another part present all the features of a replacement bone. The student should recognize that the ossification of the skull, though differing greatly in degree in different Vertebrates, is never complete. In the adult Elasmobranch the cranium is entirely cartilaginous, bone being confined to the placoid scales : in a Sturgeon there is still a well-developed chondrocranium but the surface of the head is covered with large bony plates : in such a Teleost as the Salmon the chondrocranium also persists to a great extent but extensive tracts of the cartilage are replaced by bone while the superficial plates of bone are now in much more intimate relations to the surface of the cartilage : in such a Teleost as a Cod again the cartilage is reduced in the adult to such an extent as to be quite inconspicuous. It never however completely disappears and the macerated skull of a Vertebrate as seen in an osteological or palaeontological collection is imperfect, being without parts which may be of great morphological significance. AUDITORY SKELETON. — In those Tetrapoda in which a tympanic membrane is present the vibrations of this membrane are trans- mitted through the tympanic cavity to a movable portion of the wall of the auditory capsule by a special arrangement of skeletal structures. These reach their highest development in the auditory ossicles of the Mammalia which have attracted much attention from students of mammalian anatomy and have been the centre of much controversy as to their phylogenetic origin. In the non-mammalian Vertebrates the two outer members of the chain of ossicles — the malleus and incus — have not yet made their appearance so that we are only concerned in this volume with the inner or stapedial portion which is represented in the Sauropsida and most of the Anura by the columella auris. It will be convenient to study the develop- ment of this in the case of the Lacertilia in which it has been recently investigated by Versluys (1903), Cords (1909) and Good- rich (1915). It will be recalled that the tympanic cavity is the dilated outer end of the spiracular or hyomandibular gill pouch, and the Eustachian tube is the inner or pharyngeal portion of this pouch. The pouch is for a time open to the exterior, forming an ordinary spiracular cleft, bounded in front by the mandibular and behind by the hyoid arch. In the hyoid arch is situated the main branch (hyomandibular) of the Facial nerve and from this, near its dorsal end, there cornea off a branch — the chorda tympani — which runs in a ventral direction 344 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. behind the cleft to its lower limit and then curves forwards beneath the cleft towards the region of the lower jaw and floor of the buccal cavity. The external opening of the spiracular cleft gradually closes, from below upwards, as is usual with this cleft, a stage being passed through in which only the dorsal end of the cleft is open — precisely as in the adult of an ordinary Elasmobranch. As the lower limit of the opening gradually shifts dorsalwards the chorda tympani remains in close relation with it so that the portion of the nerve on the ventral side of the opening assumes a more and more dorsal position. Eventually even the dorsal vestige of the cleft closes so that the spiracle has now reverted to the condition of a pouch. Owing to the shifting in position of the chorda tympani as it followed the retreating lower edge of the spiracular opening this nerve now passes forwards dorsal to the main portion of the pouch, instead of entirely ventral to it as it did originally. The dilatation of the outer end of the pouch to form the tympanic cavity is brought about mainly by active growth of the lower portion of its posterior wall. This bulges outwards and spreads forwards and dorsalwards beneath the epidermis, from which how- ever it remains for a time separated by a considerable thickness of mesenchyme. Later on this thins out relatively so that the three layers bounding the tympanic cavity on its outer side — endoderm, mesenchyme, ectoderm — form a thin membrane — the tympanic membrane. As the tympanic dilatation goes on expanding in a dorsal and anterior direction the chorda tympani becomes displaced in front of it still further from its original position. In the mesenchyme of the hyoid arch there takes place a gradual condensation to form the rudiment of the skeletal arch. The lower and main portion of this condensation becomes the cartilage of the definitive main cornu of the hyoid. Its dorsal portion also becomes converted into cartilage, taking the form of a stout rod the inner (" stapedial ") end of which fits into the fenestra ovalis — a vacuity in the wall of the auditory capsule — while its outer portion (" extra-columella," Gadow) extends outwards towards the skin, embedded in the mesenchyme of the posterior wall of the*piracnlar pouch or tympanic cavity. Finally the lining of this cavity pn\\s actively dorsally and \entrally to the columella so that it bulk's backwards both above and below the columella. The pockets of tympanic lining so formed meet round the columella and fuse together so that the cnlumella, instead of bein^ embedded in the hind wall of the cavity, n«»\v passes ri^lit through it, enclosed in ;i delicate slii-al.il ol' mescnchyme covered with endoderm. Tin- exten- sion of the tympanic cavity backwards past the cnlmnella causes an extension <>f the tympanic, membrane in I! n- direction so that. the point at which the lip <.)' the extra culumella reaches the skin, instead of being singled behind the tympanic membrane as it \\as to be a bunt the centre of that membrane. v AUDITORY SKELETON 345 The general mode of development of the colnim-lk and the cavities associated with it as seen in Lacerta Appears to be typical <>!' the Saurnpsida in general. It is now necessary to refer to a i'rw additional details. The inner end of the coluinella (stapes; iits into the fencstra ovalis. It is for a time, during prochondral or cartilaginous stages or both, continuous with the wall of the auditory capsule and is probably to be interpreted as a portion of this wall which has become separate and movable. Chondrification of the columella commences in the Lacertilia from three centres according to Versluys and it is to be noted that the separation between extra-columella and stapes arises secondarily within the region of cartilage which develops from the innermost centre. In Birds an interesting variation has been discovered (Goodrich, 1915) in the relations of the chorda tympani. In the Duck these are normal, agreeing with what has been described for Lcwerta. In the ordinary Fowl and the Turkey however the stage in which the chorda tympani is posterior to the hyomandibular cleft is omitted from development. Even in early stages it is found to pass in front of the pouch or cleft. This is one of those cases which emphasizes the need of caution in regarding the course of a nerve as a necessarily deciding factor in discussions as to the morphological nature of particular organs. Position in regard to a particular nerve-trunk often affords us most valuable evidence regarding the primitive position of an organ. Here, however, we have it impressed upon us that we must never rely absolutely upon such a piece of evidence taken by itself. Were we to do so in this case we should be led into the absurdity of con- cluding that the tympanic cavity of the Turkey is not homologous with that of the Duck. As a matter of fact nerve-trunks do not always form impassable barriers to the evolutionary change in position of organs. A skeletal structure may spread round a nerve-trunk (e.g. neural arches of Dog-fish) and becoming absorbed behind it may come to be transposed entirely pa^t the nerve. In the case of the chorda tympani and the tympanic cavity it is clear that the nerve lay primitively behind and below the cavity and we may probably take it that, in accordance with the general principle that nerve-trunks tend to shorten and so economize material, in the course of evolution it became shifted dorsal wards through the mesenchymatous middle layer of the outer wall of the tympanic cavity before it became thin and membranous, so as eventually to lie completely dorsal and anterior to the tympanic membrane. Incidentally the variation from normal development occurring in the Turkey and Fowl is one of those cases apparently impossible to explain on the outgrowth theory of nerve-development, but readily understandable on the view of nerve - development supported in 346 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. Chapter II., according to which new nerve -paths may arise in response to the short circuiting of nerve-impulses. A tympanic cavity with membrane and columella occurs in many of the Anura — as for example the ordinary Frogs and Toads (Rana, Bufo) — while in others such as the genera Boinbinatbr and Atelopus (" Phryniscus ") it is absent. In the Urodele Amphibians the tympanic cavity and membrane have not yet made their appearance. The inner end of the columella is however represented by a movable plate of cartilage fitted into the fenestra and in various genera the extra-columellar portion is represented by a rod-like outgrowth from this. The former apparently develops from the auditory capsule while regarding the latter there is much difference of opinion as to the extent of its relation to the cartilage of the hyoid arch. The disagreement between different observers probably means that there are actual differences between different genera of Amphibia. This is quite what is to be expected, for whenever we find a single organ of which part is derived from one embryonic source and part from another the proportion contributed by the two sources is liable to vary, so that in one case it may be the portion derived from the one source which is conspicuous and in another case that derived from the other. The tympanic ring within which the tympanic membrane is stretched arises in the form of an outgrowth from the rudiment of the quadrate, i.e. from the upper portion of the skeleton of the mandibular arch. This outgrowth separates off and grows round the outer end of the hyomandibular pouch in the form of a crescent the two horns of which eventually meet to form a complete ring. SKELETON OF THE MEDIAN OR UNPAIRED FINS. — The median fin, thin and membranous as it is in its most highly evolved condi- tion, is supported by characteristic skeletal arrangements. Into these two distinct elements enter, one mesial represented by rays of cartilage or bone (radials), the other superficial and of dermal origin. The mesial fin -rays are frequently in close relation to the neural and haemal arches and it is reasonable to suppose that in the process of evolution, as the hind end of the body became extended in a dorsal and ventral direction, so as to attain to the flattened form conducive to efficiency in propelling the body, the neural and haemal spines underwent a corresponding extension for the purpnsi s of support. This view is corroborated by the existing Dipnoi in which the mesial fin remains a comparatively slightly differentiated extension of the body dorsally and ventrally and in which the mesial supporting elements are simply the prolonged neural and : lines, each secondarily subdivided into three segments. The same is the case in Fishes generally so far as the ventral port nm of the caudal fin IS concerned in whieli the mesial supports develop also for the most part as typieal haemal spines. The mesial support^ of the dorsal portion of the median tin «>n I!M contrary do not ii, I nerally nlmw this relation to the MKDIAN FIN SKELETON 347 vertebral arches. They arise, e.g. in Elasmobranchs, in ontogeny as independemt rods of cartilage without .definite relation to the metamerism of the body and later on become segmented into three pieces. In those cases, so far as they have been investigated, in which the radial elements are connected witli a continuous basal plate of cartilage, this latter appears to arise in ontogeny as a continuous plate, though there is no reason to doubt that it arose in phylogeny by the fusion together of the basal portions of originally separate rays. This want of correspondence of the mesial elements of the dorsal fin skeleton with the vertebrae is probably sufficiently explained as a secondary result of the prolonged working of the general principles which have governed the evolu- tion of the median fin and which find their ex- pression in the tendencies (1) of the continuous fin to become specially devel- oped at particular points and to die away in the intervening spaces, (2) of the resulting separate fins to have their base of attachment to the body shortened and (3) of these fins to be situated on the body at the points where they are mechanic- ally most effective. DERMAL SUPPORTS OF MEDIAN FINS. — The median fins being pri- marily mere extensions of the body in the vertical plane it would only be reasonable to expect that they would show traces of skeletal elements comparable with the placoid elements or their derivatives characteristic of the rest of the surface. And in fact the dermal skeletal supports of the median fins can, some of them, be clearly recognized as homologous with scales, while in others although this may no longer be recog- nizable their origin is found to be closely associated with the basement membrane as was the case with the dermal teeth. It will be convenient to^ consider first of all the dermal skeletal elements in which the direct relation to scales is most clear. Such are the bony fin-rays of Crossdpterygian and Actinopterygian fishes. In an ordinary Teleost (Fig. 167) the fin-rays of this type (lepido- trichia, Goodrich) appear in their earliest stage, as shown by FIG. 167. — Two successive stages in the development of the lepidotrichia of Salmo. \. Salmon (after Harrison, 1893); B, Trout (after Goodrich, 1004). li./n, basement membrane; ect, ectoderm ; I, lepido- trk-hial rudiment ; w»x, infsrnchynn- of dcrinis. 348 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. Harrison (1893), in the form of a localized thickening of the basement membrane underlying the ectoderm. This thickening becomes more and more marked and eventually separates off round its edges in the manner shown in Fig. 16*7, B, mesenchyme insinuating itself all round between the ray and the basement membrane, so that the former eventually lies free from the basement membrane (or in some cases still connected with it by narrow bridges) deep down in the mesenchyme. The ray soon becomes calcified. New layers are deposited on its inner and outer surfaces, mesenchyme cells become included within its substance and it becomes a plate of ordinary bone. The rays are elongated structures which develop from the fin base towards the tip. They often become jointed, either by calcification being interrupted at intervals (Goodrich) or by a secondary solution of continuity (Harrison). Rays are formed in the manner described on each surface of the thin membranous fin. The rays of opposite sides correspond exactly in position and become later on fused across the mesial plane so as to form a single unpaired ray whose paired origin is indicated only by its forking at its inner end to embrace the tip of the median radial, the process of fusion between the two elements not taking place at this proximal end. In the dorsal and anal fins and in the ventral part of the caudal fin the lepidotrichial fin-rays correspond segmentally with the true median skeletal elements the tips of which they embrace as indicated above. In the more primitive Teleostomes the identity in nature of these fin-rays with the scales which cover the rest of the body is still more obvious. In Polypterus and Lepidosteus they develop a coating of ganoine and even bear distinct small denticles on their surface. It is also interesting to notice that in the anal fin and ventral part of the caudal fin of Polypterus the fin-rays at their proximal ends merely pass in beneath the edges of the body scales and do not take on any relation to the true median skeletal elements. The palaeontological fact may be recalled in passing that in some of the extinct fishes a perfect gradation can be traced between the fin-rays and typical body scales (see Goodrich, 1904). In addition to the fin-rays just described, the homology of which with scales may be taken as well established, it is very usual to find another type of fin-ray in which this homo- logy is not so obvious. This is exemplified by the horny fin-rays (see Goodrich, 1904) which occur in the fins of Elasmobranchs (including Hnlnciipliali), in the "adipose" fin of Salmonids ;mal side pn -ceding those on the pnstaxial or definit i\ely ventral. Each ray Kpreads out from the prochondral tissue between two segments of the axie and it is nntr\\<.rihv that rays develop IVom the. first (proximaL of these interseginental joints although in v I'WTOIIAL LIMB SKELETON 351 the course of further development these, doubtless to give greater mobility to the tin, disappear. In the details of its development each ray repeats that of the main axis. In addition to the normal rays, which are attached to the axis at the level of the intersegmental spaces, occasional rays make their appearance opposite the segments themselves. According to Semon these sprout out from the thin superficial layer of the axial tissue which like that between the segments persists for long in an unchondrih'ed condition. These extra rays are most frequent on the postaxial side of the limb which in the pectoral limb becomes ventral, in the case of the pelvic limb dorsal (see Chap. VII.). Growth of the fin and of its enclosed skeleton continues for a long period — even after the adult condition is attained. As regards the skeleton this continued process of growth takes place by two methods (1) by a simple continuation of the extension at the apex, and (2) by the already formed elements of the cartilaginous skeleton, axial or radial, continuing their individual growth in size. As the definitive condition of the skeleton is reached, the inter- segmental tissue chondrifies, towards the apex forming soft hyaline cartilage with a sparse matrix and towards the. body taking the form of fibro-cartilage. Towards the base of the limb this fibro-cartilage develops many fluid-filled cavities so as to assume an almost spongy character and in this way give greater mobility. This is specially marked at the junction with the shoulder girdle and between the first and second segments of the limb-axis and in these two cases the apposed surfaces of hyaline cartilage are curved and concentric so as to afford a distinct development in the direction of a true ball and socket joint. The details of development of the pelvic limb skeleton apparently agree with those of the pectoral. In this case also the girdle arises in the form of two originally separate halves. ELASMOBRANCHII. — The earliest stage in the development of the pectoral limb and its girdle in Elasmobranchs is that described by Huge and by Braus (1904) in Spinax where there exists a condensa- tion of connective tissue in the form of a curved rod on each side of the body close under the skin, in the position shown in Fig. 159, p.g (p. 321). This forms the rudiment of the pectoral girdle. From it there grows outwards a short projection into the limb-rudi- ment which as it is clearly homologous with the axial cartilage of Ceratodus we may call by the same name. The girdle rudiment increases in length both dorsally and ventrally, and ventrally the two rudiments come to be in apposition. On each side a tract of cartilage* now develops in the prochondral rudiment : the t wo cartilaginous rudiments show a similar dorsal and ventral extension and presently they also come into apposition ventrally and form a continuous structure across the mid-ventral line. In other Elasmobranchs (see Mollier, 1894) the conditions appear to be similar on the whole to those described in Spinax. 352 EMBRYOLOGY OF THE LOWER VERTEBRATES « n. The prochondral rudiment of the axial cartilage extends out into the liinb-rudiment, forming a broad plate which tapers oft' posteriorly (Fig. 168). The condition is in its essentials the same as that in Ceratodus except that here the axial rudiment is laid back along the side of the body. The prochondral fin-rays arise, as in Ceratodus, in the form of outgrowths from the axial element. These are restricted to the outer (preaxial) side of the limb. They develop in Spinax (Braus) in series from before backwards except that anteriorly, in the region which will give rise to mesopterygium and propterygium, a few rays develop in the opposite sequence from behind forwards. The chondrification of the limb skeleton appears to take place in Mustelus and Torpedo continuously but in Spinax it sets in first in the axial portion and then in the rays in the same succession as they first appear. The separate segments of the rays in Spinax also FIG. 168.— Section through develop in succession as separate centres of pectoral fin of Torpedo chondrification. embryo, parallel to sur- ^0 understand the morphological relations Momer! 1894.) of these earl7 stages ifc is advisable to refer back to the paddle type of limb as it exists The prochondral rudiment . ,, . -.•• c >-\ -rn of the skeleton is shaded. m the ancient sharks of the genus Pleura- canthus. Here (Fig. 169, B) we find a limb resembling generally that of Ceratodus but differing from it in two conspicuous details. (1) The skeletal axis has become relatively larger and clumsier, its original elements having probably under- gone extensive processes of fusion both with one another, as shown by the fact that the cartilages of the axis are in places less numerous than the lateral rays, and also with the basal portions of these lateral rays. (2) The rays on the postaxial side of the limb are much reduced in number, only a few persisting towards the limb apex. The tendency of the postaxial rays to disappear in these archaic sharks (and the same tendency is seen in Lung-fishes) justifies us in believing that the external side of the pectoral limb-axis in the young Elasmobranch is morphologically preaxial. This conclusion the interesting question — Are there any vestiges of postaxial rays to be found in existing Elasinobranchs ? This .[iiestion lias i«> be answered in the affirmative. In Centrophorus (Fig. 169, C) Braus finds a number of postaxial rays near [he tip «>!' tin- tin in a elopment; in Spinax at least one similar piece of • •art ila'je urcurs; and even in UK- adults of various Sharks Gegenbaur and Bunge found similar vestiges. A& vestigial m^ans are notori- ously variable more extended invest i^-at ions into the occurrence of BUCh ve^li-ial ]»«,>!, ixial rays are very desirable. They should he carried out on as many different species of Shark as possible and SKELETON OF THE rAlllKI) FINS 353 on as 1 .1 r-v ;i.s possible a number of individual specimens of each species. The skeleton of the pelvic girdle arises in a manner similar in its main features to that of the pectoral girdle. It is however character- istic of Elasmobranchs (except Holocephali) that the portion of the girdle dorsal to the attachment of the limb undergoes atrophy in later stages of development. As in the pectoral fin an axial cartilage appears with fin rays sprouting from its external side. Here also a separate cartilage develops anteriorly with a few rays attached to it but it is doubtful whether it is justifiable to homo- logi/e this in detail either with the propterygium or the meso- pterygiuni of the pectoral fin. The cartilaginous skeleton of the clasper arises in continuity Fio. 169.— Pectoral fin skeletons of: A, Ceratodus (Semon) ; B, Plcuracanthus (Fritsch) ; C, Centrophorus embryo (Brans); D, Acanthias (Gegenbanr) ; E, Cladosdache (Bash- ford Dean) ; F, Polypterus larva (Budgett) ; G, Polypterus (Wiedersheim). The outer or prt-axial side of the limb is to the left, except in A. with the rest of the fin skeleton and appears to consist of the tip of the limb axis with possibly a few modified rays. The claw-like structures are simply modified placoid scales. TELEOSTOMI. — As regards Polypterus, commonly regarded as the most archaic of existing Teleostomes, our knowledge of the develop- ment of the limb skeleton is fragmentary. In the larva of stage 36 (Fig. 197) the skeleton of the pectoral limb is in the form of a thin lamina of cartilage with small irregularly scattered perforations. This is connected with a shoulder girdle rudiment consisting of a simple curved rod of cartilage. In the 30 mm. larva described by Budgett (1902) the girdle has become shortened into a compact block of cartilage and the cartilaginous plate lying within the limb itself has become thickened along its anterior and mesial edges. These thickened portions are separating off to form the rod-like "proptery- VOL. II 2 A 351 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. giuiii " and " metapterygium " (Fig. 169, F). The main portion of the plate is becoming split up towards its margin by a series of slits into a number of radiating pieces which represent the separate radii of the fully developed fin (Fig. 169, G). The close correspondence between the fin skeleton at this stage of its development and the fin skeleton of a shark is obvious from Fig. 169. In Actinopterygians the pectoral limb skeleton is in the pro- chondral stage a continuous mass, of which the anterior and mesial part separates off to form the pectoral girdle, while the distal portion spreads outwards to form the rays. The pelvic limb skeleton shows a fundamentally similar origin from a continuous prochondral rudiment, but here the skeletal rudi- ment appears first in the projecting limb and only secondarily spreads within the body wall into the region of the pelvic girdle. It appears to the present writer that no special weight need be attached to such cases where the skeleton develops earlier in ,the limb than in the body wall : they are probably to be regarded simply as special cases of the frequent tendency for highly specialized organs to be laid down precociously in development. II. LIMBS OF THE TETRAPODA. — In the Amphibia also the pectoral girdle and the skeleton of the limb itself are foreshadowed by a single condensation of rnesenchyine which extends in a dorsal and ventral direction to form the girdle and out into the limb to form its skeleton. No general rule can be given as to the relative time of development of the various parts. In Bombinator according to Goette the girdle rudiment appears first and the limb skeleton sprouts from it : in Proteus according to Wiedersheim the limb skeleton appears first and the girdle later. In the girdle rudiment the dorsal or scapular portion becomes apparent first. Chondrification takes place separately in the girdle and the limb, the joint remaining unchondrified. The cartilaginous pectoral girdle of the Amphibian, as of other quadrupeds, takes on the form of a A upon each side of the body— the three branches of the A being known as scapular, coracoid and precoracoid portions respectively and the glenoid articulation for the limb being situated at the meeting point of the three portions. As the two ventral 1 tranches of , the A are in some cases continuous with one another at their tips through a strong membrane, il sinus not improbable that they had originally the form of a continuous flattened plate of cartilage, of which the central portion has now di>,ippeared, IcaMii'j the thickened marginal parts as precoracoid (anterior) and coracoid (posterior) respectively. On this view the epicoracoid when present would represent tin- persisting thickened ventral margin of the primitive girdle. In actual ontogeny 1 lie 1 hrec I. ranches spread gradually out \\ards from I he original rudiment, while the epicoraeoid when present, is formed by the coracoid spreading forwards at itn \entral end and ;lh the end of I he ] H'GCOraCOld. The two lateral halves of v SKELETON OF THE LIMBS 355 t-lii- uinlle coine to overlap one another in the mid-ventral line and in t ho case of the higher Anura complete fusion takes place. Amongst the Eeptilia the first rudiment of the pectoral limh skeleton has IK -en investigated hy Mollier "(1895) and found to consist of a condensation of inesenchyme in the glenoid region corresponding p.nilv to the glenoid portion of the girdle and partly to the basal portion of the limb skeleton — the two being thus again continuous at first. The girdle portion of the rudiment spreads ventrally to form the coracoid region, then dorsally to form tl it- scapular. The chondrification of the various parts takes place in the order of their appearance. In Chelonians the girdle takes on the typical A-shaped form with a more or less pronounced projection from the lower end of the coracoid forwards towards the lower end of the precoracoid which apparently represents the epicoracoid of Amphibians. In Sphenodon and in Lizards on the other hand the ventral portion of the cartilaginous girdle consists of a flattened plate which may become perforated by several foramina. Whether this flattened ventral portion corresponds to coracoid and precoracoid is doubtful. It seems on the whole more probable (Goette) that the precoracoid has disappeared in these forms owing to its functional replacement by the clavicle, a process seen in its incipient form in Anura. This view is supported by the occurrence of a distinct strand of condensed connective tissue in the position where the precoracoid should be though in this case it does not become chondrified but becomes replaced by bone (clavicle) at a later stage. In Birds the girdle forms a simple curved rod without any bifurcation into coracoid and precoracoid portions ventrally. Each lateral half of the pelvic girdle of quadrupeds is, like the pectoral girdle, typically of a A-shape, the three limbs being known here as ilium (dorsal, more correctly iliac bone or iliac cartilage), pubis (anterior) and ischium (posterior). The frequency with which the pubis and ischium are continuous at their ventral ends suggests that here also they represent the persisting thickened marginal parts of a once flattened plate-like ventral portion of the girdle. As in the case of the pectoral girdle the three processes are formed by simple spreading outwards from the original rudiment. In Amphibia chondrification takes place apparently from a single centre on each side (Triton, Bunge, 1880) giving rise to a pair of longitudinal plates of cartilage which meet ventrally. In Reptiles each half of the pelvic girdle passes through the typical A-shape. The ventral end of the pubis, like that of the ischium, meets its fellow across the mid-ventral plane forming a symphysis. In some cases, e.g. Sphenodon and certain Chelonia, the pubic symphysis becomes connected up with that of the ischia by a longitudinal bar of cartilage. In the Crocodiles the pubic portion of the girdle becomes eventually segmented off at its dorsal end from the rest of the girdle. 356 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. 9 In Birds the pelvic girdle during the prochondral stage passes through the A-shape, the right and left halves distinct from one another and each at first continuous with the skeleton of the limb. Pubis, ilium and ischiuin usually chondrify separately but in many cases (e.g. in the Common Fowl usually) ilium and ischiuin may Income chondrified in continuity, and less frequently all three elements chondrify in continuity. A highly characteristic feature of the avian pelvis is that the pubis swings in a tailward direction about its attached dorsal end until it assumes a position parallel with that of the pubis. In the pelvis shown in Fig. 170, B, this rotation is just commencing. The Cheiropterygium (Huxley), or skeleton of the limb in Amphibia and Amniota, consists of three distinct portions corre- sponding respectively to the Upper lr il. Arm or Thigh, the Forearm or Leg, ££v ^-^ and the Hand or Foot. As these por- tions are looked upon as homologous in the fore and hind limbs it is con- venient to have a morphological name xx;^ for the corresponding parts of the two P B is. se^s °f limbs, and such names have been proposed by Emery and Haeckel FIG. 170. -Side view of pelvis of Bird _ Stylopodium, Zeugopodium or Zygopodium, and AutSpodium. In V^i^^XCMglen; the autopodium there may further be recognized Basipodium (carpus or tarsus), Metapodium (metacarpus or metatarsus) and Acropodium (phalanges). The limb skeleton is typically at first quite continuous. A rod- shaped condensation of mesenchyme appears first in the limb stump — the rudiment of the stylopodium (femur or humerus) — and as the limb grows this spreads outwards, bifurcating as it does so to form the rudiments of the zygopodial skeleton : with further growth the two limbs of this unite distally to form the rudiment of the auto- podial skeleton. Chondrification takes place from the base of the limb outwards, each separate element of the adult making its appear- ance as a separate chondrification centre. The skeleton of the autopodium originates in a flattened plate- like extension of the prochondral zygopodial skeleton. In tin's the various carpal or tarsal elements make their appearance as s" pa rate centres of chondrification. It seems unnecessary in a general text-book like tbe present to go into the great variations in detail which are found amongst the various tetrapods in regard to the skeleton of carpus and tarsus. It, need only be said that the striking •ions found in different groups from the schematic arrangement, such as is illustrated by r-'ig. 171, seem lo have been brought about bv eidai'j'Miienl 01- reduction of individual elements, or ihe fusion of originally separate elements. v SKELETON OF THE LIMBS 357 From the plate-like rudiment of carpus or tarsus there spread out r;uli;itiii- extensions normally five in number to form the skeleton of the digits. In the Amniota these appear practically synchronously although in Amphibians there is a tendency for them to develop in n 'IT i ilar sequence according to the number of the digit (Rabl, 1901). In the substance of these the phalanges make their appearance as discrete chondrifications. In the Birds the loss of individuality of the digits involved in the conversion of the tip of the pectoral limb into a rigid support for the flight feathers has been accompanied by processes of reduction and fusion of the orginal elements. In the prochondral stage five digits are laid down but only II, III and IV proceed with their development. Of these metacarpal II becomes reduced to a small stump project- j[ IH. ]y ing from III : metacarpals III and IV , o o o y become fused with one another at both Q o" 0 o 0 ends : and the three distal carpals become 0 0 0 0 Q fused with the metacarpals to form the o 0 0 o ^ carpo-metacarpus characteristic of the Bird. O Oo®£...Q:lce In the hind limb of Birds there are c&-£35 Q , ..<>•• . ^ also laid down prochondral rudiments of ,--0 0 •-.... the five digits and again I and V become Q ^ reduced although not so completely as in the fore limb. V reaches the stage of a small metacarpal nodule of cartilage which FIG. 171. — Cartilaginous eie- however soon disappears. Metacarpals II, lneuts which develop in the III and IV fuse with one another and J^I^ESftSR with a cartilage which represents the distal (After Mehnert, 1897.) row of tarsals to form the characteristic W| centraie; ci-5, distal can«is; tarso-metatarsus. Metatarsal I disappears », intermedium; //, hum.-] us: //. except in its distal portion. And finally ;j;^; ' n the two proximal carpals which are visible for a time fuse with the end of the tibia to form the tibio-tarsus. BONY SKELETON OF THE LIMBS. PECTOEAL GIRDLE. — In the Sturgeons the original cartilaginous pectoral girdle persists, lying close under the skin of the posterior branchial region. Plates of bone corresponding exactly with those on the rest of the skin develop superficial to the girdle and serve to reinforce it. Of these bony plates there are two principal ones on each side, one in the region of the glenoid surface the cleithrum (Gegenbaur) and one extending ventrally to meet its fellow — the clavicle. In existing Crosso- pterygians where the evolution of the bony skeleton has reached a higher level than in the Sturgeons the same two bony elements develop but here the original shoulder girdle — its function being to a great extent taken over by the cleithrum — becomes relatively reduced in size. It lies on the inner surface of the cleithrum and its cartilage gives place in part to two replacement bones — the scapula dorsal, and the coracoid ventral. It is to be noted also 358 EMBEYOLOGY OF THE LOWEK VERTEBRATES CH. that the cleithrum sinks more deeply into the tissues while the clavicle remains superficial. In the higher bony fishes — Ganoids and Teleosts — the conditions are very similar to those of Poly pier us — the primitive shoulder girdle being small and usually becoming replaced in great part by bone (scapula and coracoid) and the main supporting function being exercised by the independently developed cleithrum. In the Dipnoi more nearly primitive conditions are retained as the original cartilaginous girdle remains well developed throughout life and retains its continuity with its fellow ventrally. Cleithrum and clavicle however are also developed and they show a higher condition in that they are developed in intimate contact witli the surface of the cartilaginous girdle, the clavicle ensheathing the anterior face of the coracoid portion. In Amphibians the scapula becomes replaced' incompletely or completely by bone which spreads dorsalwards from the region of the glenoid articulation. The coracoid may remain cartilaginous (most Urodeles) or become replaced by bone. The precoracoid also tends to be strengthened by the formation of bone. In the common frog (Rana) and Toad (Biifo} the bone (" clavicle ") is in the form of a splint lying along the anterior side of the precoracoid and originat- ing in the connective tissue some little distance from the cartilage. In other cases the bony tissue completely surrounds and to a great extent invades and replaces the cartilage. We may infer with con- siderable probability that the bone in question was originally in phylogeny a " membrane " bone and that becoming more and more intimately related to the precoracoid cartilage it has in the latter form become more or less completely a " cartilage " bone — a good example of the type of evidence which has led morphologists to minim i/.e the importance of the distinction between these two types of bone. In the Amniota scapula and coracoid are replaced nearly or quite completely by bone. A clavicle like that of Amphibians develops in relation to the precoracoid in Reptiles except Crocodiles. In Birds what appears to be the same element (furcula) is widely separated from the coracoid, probably for mechanical reasons con- nected with flight, while a separate centre of ossification appears at the apposed ventral ends of the two bones. In Reptiles a somewhat similar element — the episternum — makes its appearance and is con- tinued tailwards along the mid- ventral surface of the sternum ;mrli< >n oi' this and th.- keel of the sternum the rest. PKI.VM- ( IIIMM.K. —The cart ila-nmus pelvie girdle becomes replaced by bone less or more completely without !vt-ci\in- any reinforcement IVoin investing bones. The iliac, puhic and isehinl portions ossify each from its own centre except in Amphihia \\huv the pubic region remains c;irt il;c_!in.> In bony Teleoslomaloiis fishes eaeli hall' of the pelViC 'jinlle v THE SKELETON 359 becomes replaced by a plate of bone the morphological nature of which has been much discussed. Detailed studies of its development in a variety of different teleosts and in the more primitive ganoids .•in1 much needed. LITERATURE Agar. Trans. Roy. Soc. Edi.n., xlv, 1906. Albrecht, A. Ent \\irkrlung des Achsenskelettes der Teleostier. (Digs.) Strussburg, 1902. Braus. Morph. Jahrb., xxvii, 1899. Brans. Haeckrl festschrift (Jenaer Denkschriften, xi), 1904. Braus. Hi-rtwigs Handbuch der Entwicklungslehre, iii, 1906. Budgett. Trans. Zool. Soc. Lond., xvi, 1902. Bunge. Entwickelungsgeschichte des Beckengiirtels der Amphibien, Reptilicn und Vogel. (Diss. ) Dorpat, 1880. Carlsson. Anat. Anzeiger, xi, 1896. Carlsson. . Anat. Anzeiger, xii, 1896*. Cords. Anat. Hefte (Arb.), xxxviii, 1909. Dohrn. Mitt. Zool. Stat. Neapel, v, 1884. Gadow. Phil. Trans. Roy. Soc., B, clxxxvi, 1895. Gadow. Phil. Trans. Roy. Soc., B, clxxxvii, 1897. Gaupp. Schwalbes Morph. Arbeiten, iii, 1894. Gaupp. Hertwigs Handbuch der Entwicklungslehre, iii, 1906. Gibson. Anat. Anzeiger, xxxv, 1910. Goeppert. Gegenbaurs Festschrift. Leipzig, 1896. Goette. Arch. mikr. Anat., xv, 1878. Goette. Arch. mikr. Anat, xvi, 1879. Goodrich. Quart. Journ. Micr. Sci., xlvii, 1904. Goodrich. Proc. Zool. Soc. Lond., 1908. Goodrich. Proc. Zool. Soc. Lond., 1913. Goodrich. Quart. Journ. Micr. Sci., Ixi, 1915. Gregory. Biol. Bull., vii, 1904. Harrison. Arch. mikr. Anat., xlii, 1893. Hertwig, 0. Jenaische Zeitschr., viii, 1874. Hertwig, 0. Arch. mikr. Anat., xi, Suppl., 1874*. Howes and Swinnerton. Trans. Zool. Soc. Lond., xvi, 1901. Huxley. Todd's Cyclopaedia of Anatomy and Physiology, v, 1859. Kerr, Graham. Proc. Roy. Phys. Soc. Edin., xvii, 1908. Leche. Anat. Anzeiger, viii, 1893. Marsh. Odontornithes. Washington, 1880. Mehnert. Morph. Jahrb., xiii, 1888. Mehnert. Schwalbes Morph. Arbeiten, vii, 1897. Mollier. Anat. Hefte (Arb.), iii, 1894. Mollier. Anat. Hefte (Arb.), v, 1895. Nickerson. Bull. Mus. Comp. Zool. Harvard, xxiv, 1893. Rabl, C. Morph. Jahrb., xix, 1893. Rabl, C. Zeitschr. wiss. Zool., Ixx, 1901. Rose. Anat. Anzeiger, vii, 1892. Rose. Anat. Anzeiger, viii, 1893. Rose. Anat. Auzeiger, ix, 1894. Schauinsland. Zoologica, xxxix, 1903. Schauinsland. Hertwigs Handbuch der Entwickelungslehre, iii, 1906. Scheme. Morph. Jahrb., xxxi, 1902. Semon. Forschungsreisen in Australien, i. Jena, 1893-1913. * Sewertzoff. Kupll'ers Festschrift. Jena, 1899. Sollas, W. and I. Phil. Trans. Roy. Soc., B, cxcvi, 1903. Sonies. Petrus Camper, iv, 4, 1907. Suschkin. Nouv. Mem. Soc. des Naturalistes de Moscou, xvi, 1899. Tims. Marett. Quart. Journ. Micr. Sci., xlix, 1906. Traquair. Proc. Roy. Phys. Soc. Edin., xii, 1893. Versluys. Zool. Jahrb. (Anat.), xix, 1903. Wijhe, van. Comptes rendus 6me Congres internat. Zool. 1904, Berne, 1905. Williamson. Phil Trans. Roy. Soc., cxl, 1849. CHAPTER VI VASCULAR SYSTEM As has already been indicated the vascular system of the animal body consists of strands of highly specialized mesenchyme — the cells (corpuscles) along the axes of the strands being detached from one another and floating freely in a fluid intercellular substance (plasma), while the superficial cells are united together to form the walls of tubular channels — the vessels. The vessel walls are provided with a coating of muscle-fibres and this muscular coat becomes greatly thickened and specialized at one or more points to form hearts which serve as pumps to force the blood through the system of vessels. The fundamental plan of the Vertebrate vascular system appears to have been like that of an Annelid worm, with two main longi- tudinal blood-vessels, situated respectively one on the neural side of the alimentary canal and one on the side opposite to this, connected together by a series of half-hoop shaped vessels encircling the alimentary canal laterally. In the Vertebrate the longitudinal vessel on the neural side of the alimentary canal is the dorsal aorta and in it the blood runs in a tailward direction. The longi- tudinal vessel on the other (ventral) side of the alimentary canal develops the heart on its course: its precardiac portion is the ventral aorta, its postcardiac the subintestinal vein. In this ventral vessel the blood passes in a headward direction. Halt-In M.p shaped vessels lying in front of the heart and connecting ventral aorta and dorsal aorta are the aortic arches. ORIGIN OF TIM: HKAKT ANI> VKSSELS IN TIIK HOLOIM.ASTH VBBTEBBATE&— -Amongst holoblastic Vertebrates the first steps in the development, nf the vessels have been investigated in the Newt Triton] hy Mollier 'I'.'iii; , and his account will here he followed. In an embryo with six inesodcnn segments 1 lie lateral sheets of mesodcrni have m.-l v.-nt rally except in the region nf the liver where ih'-v terminate in a free edge. This free ed-jr is thickened and the thickening exl'-nd hack he mid \entral line t«. \\ards the cloaca as the rudiment of the snhintest inal vein the entire thickening hmn- l Ims a Y-sli.ipe (Fig, 173, A). CH.VI ORIGIN OF THE HEART AND VESSELS 361 At a stage with twelve segments this Y -shaped vasotdai rudi- ment is continued forwards as a couple of strands of cells, Ivii; each side on the inner surface of the splanchnic meBodenn ami apparently derived from it. These are destined to givr rifle in th»-ir hinder portions to the two vitelliue veins and in their anterior region to the first rudiments of the heart (Fig. 172, A, enc). At a stage with fifteen segments the paired strands of cells have assumed a disposition like that shown in Fig. 173, B. They approach one another as the mesoderni extends downwards and presently fuse across the mesial plane (Fig. 172, B and C), the fused portion being the rudiment of the heart while the two anterior limbs represent the first (mandibular) pair of aortic arches and the two posterior enc. FIG. 172. — Ventral portions of transverse sections of young Amphibians to illustrate the development of the heart. (Based on figures by Mollier, 1906.) A, B, D, E, Triton (A twelve segments, B sixteen do., D twenty do., B twenty-six do.); C, Sana. d.mc, dorsal rnt'soeardinin ; <•<•/, ectoderm ; enc, endocardium ; end, endoder'm ; me, myocardium : tnes, mesoderm ; pc, i>«>ricurdiuc cavity ; c.inr, ventral mesocardium. limbs the vitelline veins. The heart rudiment is at first extremely short in an antero-posterior direction being much broader than it is long. This is correlated with the shortness of the foregut. As the latter lengthens the heart rudiment keeps pace with it, and becomes elongated (Fig. 173, D). As it does so the tissue within the rudi- ment becomes loosened and takes the form of a syncytial network with wide meshes. In the meantime the mesoderm on each side, now containing a wide coelomic (pericardiac) space, has grown down to the mesial plane ventral to the heart, so as to give rise to a ventral meso- cardium which however only persists for a short time (Fig. 172, C and D, v.mc). About this same period fluid begins to collect in the interstices between the cells of the subintestinal strand, with the result that some of the cells in its interior assume a spherical 362 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. form and are recognizable as embryonic blood corpuscles. Mollier notes that about this stage the subintestinal strand comes into extremely close relation to the yolk-cells, there being in places apparently complete continuity between the two — hence the con- clusion on the part of observers who did not study earlier stages that the vascular strand was actually derived directly from the endoderm. About the stage with sixteen to eighteen segments the rudiments of the Duct of Cuvier and dorsal aorta become apparent, in the form of cells at first scattered and later joined into strands. The aorta cells anteriorly often show connexions with the sclerotomes and u.u a.al s.i.v. B. C D. Fi<;. 173. — Rough diagram to illustrate the form of the early rudiments of heart, vitrlline veins, and subintestinal vein in Triton as seen in plan. A, six mesoderm segment B, fifteen segments ; C, eighteen segments ; D, twenty segments. ".".I, MiHii-litiulai aortic arch ; cl, position of cloaca ; H, heart ; L, position of liver ; Mibintestinal vein ; v.vt vitclliin- vein. Mollier admits that some of them may actually be derived from the sclerotomes (see p. 364) though he considers that the main source of origin is the upper angle of the lateral mesoderm. At the stage with twenty segments a network of fine channels has appeared over the surface of the yolk, between it and the mesoderm, foreshadowing the vitelline network of blood-vessels. The subintestinal strand has become still looser in texture and prolongations may be found passing I'mm it in \\anls amongst the yolk-eells. The, heart has now attained the i'<>rm of a straight tul.c the protoplasmic Strands in its interior disappearing while its super fi'-ial cells t;ike on an endothelial character, and are recognizable as the, endocardium. The splanchnic, niesodenn has bcenme closely moulded round it venlrally and later;ill\ ( Kijj. 17'-', I > ) loriiiin- t he rudiment ni' the myocardium and the Utter l.e^ins to slm\\ con- tractions causing si i-j hi m<. \enient of the fluid contents of the vi ORIGIN OF THE HEART AND VESSELS 363 liy the t \vciit y-sc\c!i segment stage the anterior limbs of tin- suhintestinal strand have iM-mme definite ' (vitelline; veins \\ilh well-defined lumen tilled with fluid in which spherical ynun^ corpuscles float freely. The large flat culls forming the wall are prohably simply the modified superficial cells of the strand thmi^h Mollier thinks these may be reinforced by additional mesoderni cells from without. The vitelline veins are continued in front into the posterior venous limbs of the heart and the heart itself is seen in transverse sections (Fig. 172, E) to be now completely enclosed in myocardium, the inner wall ol% the pericardiac space having become moulded right over its dorsal side. Where the two sheets of mesoderm, one from each side, have met dorsal to the heart there still persists a septum — the dorsal mesocardium (Fig. 172, E, d.mc) which serves to sling up the heart to the ventral side of the foregut. The dorsal aorta is at this stage particularly instructive. Posteriorly it is represented by scattered cells, lateral in position, thus betraying their lateral origin. Further forwards these have approached the mesial plane and form a pair of cellular strands. Further forwards still — in the region of the first eight segments — these have become still more nearly mesial in position and over part of their extent have undergone actual fusion to form the unpaired aortic rudiment. About this stage the dorsal aortic rudiment is connected up to the vitelline network by a series of segmen tally arranged vessels (segments 5-17) which had made their appearance about the twenty- segment stage as segmentally arranged strands of cells. The rudiments of the ducts of Cuvier make their appearance even earlier than that of the dorsal aorta, in the form of cells derived according to Mollier from the somatic mesoderni at the cranial side of the pronephros. These rudiments develop extensions in a headward and in a tailward direction to form the cardinal veins. The vessels of the head region develop in situ from the rnesenchyme and the same may probably be said of the smaller vessels generally. The Crossopterygian fish Polyptei°us (Graham Kerr, 1907) is, apart from its generally archaic character, particularly suitable for the study of the first beginnings of the vascular system owing to the fact that the long axis of the embryonic body is straight, so that horizontal as well as sagittal sections may be made passing through practically the whole length of the dorsal aorta during its early stages, when in its hinder portion it has not yet taken definite form. The first conspicuous stage in the development of the dorsal aorta consists in the collecting together of irregular rnultinucleate masses of yolky protoplasm in a row beneath the hypochord (Fig. 174, A). Vacuolar spaces develop in these masses and fore- shadow the aortic cavity. The masses of protoplasm become more closely aggregated into a cylindrical shape while the vacuolar spaces increase enormously in size and eventually flow together to form the continuous aortic cavity. In the specimen figured in 364 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. Fig. 174, B, the cavity was perfectly continuous towards the head end, while posteriorly it was still in the form of isolated vacuoles. The cells which form the rudiment of the dorsal aorta are from their coarsely-yolked character clearly derived ultimately from the primitive endoderm, but the question remains whether they are derived from the definitive endoderm directly or through the iuter- vac. Fl<;. 17-1. -Portions of liori/ont;tl .sections through /'»(i//>f, ,'its larvar of ittgBI '_' 1 i A ) and '!',> (H) showing the rudiment of tin- aorta in loiuitinlinal M-rtion. A, ami ic nidiim-nt ; <••<.•. xaciioli-s. mediary of the mesoderm. Such a section ae tluit. slio\\n in Fig. 175 indicates thai, tin- latl'-r is the case. Tin; definitive endoderm shows a perfectly sharply denned surface, clearly market 1 off from the aortic, rudiment, \\hili- the mesodenn <•!' (he srlrn»ii»mr «>n the nt-ln-r hand is C(HitiiiUMiis \vi! h the aortic rudiinciit. We may sav t licivtm-c \\ilh hi^h j.rohahilit y th.-il 1 he aort !«• cells are derived IVoin tlie sclerotom- . A ivinarkahlr feature h,i heen noticed in I ho development of the vi ORIGIN OF DORSAL AORTA IN PtfLYPTERUS 365 j dorsal aorta of Polypterus which requires further investigation hoth in that genus and in any other Vertebrates in which it may be found to occur. In Polypterus in 'the -Stage immediately pre- ceding that in which the aortic cells collect together the position of the future aorta is distinctly marked out by the arrangement of the delicate reticulum that is visible connecting up the various organ-rudiments of the larva. This reticulum is usually regarded as an artifact caused by the action of the fixing and preserving solutions upon the albuminous substances contained in the fluids of the embryonic body but the fact that it becomes arranged in this peculiar fashion to foreshadow the future aorta at once raises the question whether it is not really a reticulum of living substance. The aortic cavity in Polypterus has been seen to originate by the fusion of intracellular vacuoles. The cavity is filled with clear fluid and this condition persists even after the main channels of the vascular system are laid down. The blood is at first simply fluid or plasma with- out corpuscles. This plasmatic con- dition may persist even after circula- tion has commenced and the heart propels through the vessels simply the clear cell-less fluid. Here we find repeated in ontogeny an extremely K ' archaic condition of the circulation. ' The plasma becomes peopled with FK;. 175. -Portion of transverse section corpuscles comparatively suddenly. The portions of vessel wall lying external to the lining endothelium appear to arise from mesenchyme cells. SOURCE OF THE CORPUSCLES. — The blood corpuscles are to he looked on, broadly speaking, as mesenchyme cells which have lost their connexion with their neighbours and float free in the plasma. Their precise sources in ontogeny appear to be various : — (1) They can frequently be seen in process of being budded off by the wall of the embryonic blood-vessel into its- cavity. (2) In other cases the vessel with its contents is seen to arise as a solid mass of cells, those at the periphery becoming the wall (endothelium) of the vessel rudiment while those more deeply placed round themselves off, becoming separated by chinks containing fluid, and develop into corpuscles. This may be regarded as a through Polypterus of stage 25 showing the relations of the aortic rudiment (A) to the sclerotome (scl). ent, enteric cavity ; my, niyotoim- ; N, notochord. 366 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. modification of (1) brought about by a hurrying on of the develop- ment of the corpuscles. (3) In still other cases the cells of the mesenchyme reticulurn in certain localities e.g. in the spaces between the tubules of the pronephros draw in their processes, round themselves off and are carried away in the blood stream as primitive corpuscles. This last mode of origin may account for the fact that the enc. enc rnc. D. Fi'.. 176. — Portions of transverse sections through Khisinobram-.h rmliryos illustrating the origin of the heart. A, Tor)w.lu, stage with one #ill cleft; B, 7'<>;/'-'/u. sia^'1 \\ith two xill clefts; e, 1'riatini-i^ with twenty-fhi: s'-iin-nts : l>. Pristiurvi, sl:i-«- with forty .sr.-nn-nts. (After li-mcs l.y Kuck.-it (1S88) and Moliier (I'.'OO).) »/.//(••. dorsal iiu-socardiiini ; enc, finlDcardium : '//(/, ciKlodt-i m of loi.-nt : : HI ; N, notochnrd ; /«•, pci icai iliac cavil \ ; 8.0, spinal cord ; >-;>/f, corpuscles have been observed to make their appearance suddenly in numbers in the circulating blood. The sudden setting Inv of large numbers of corpuscles is -possibly due to an epidemic d •i ihr nicH'iirhvnie cells, as it is well kno\vn thai the onset of tin; inilotir process I'l v. pimtl y iinlucus a retraction of tin- processes of the cell hody and its a^iimpt i«»u of an approximately spherical sha| 1 1 i- not propo.-ed i«. i race out in this \ohiine 1 he further develop- ment of the hlood c,,i-puseles ho\\ I he originally similar indilfereiil vi OEIGIN OF THE HEAKT AND VESSELS 367 corpuscles become differentiated with further development into speciali/ed strains — Krythrocytes or Ked corpuscles and the vari«» us types of Leucocytes ; for an account of this iu Lepidosiren the student may be referred to the beautiful memoir by Bryce (1905). The mode of origin of the vascular system in the holobla.\ .1 simple process of splitting-oil' of th«- superficial layer of cells. As development goefl on, the area of vascular rudiments spreads mvv.irds into regions ol the niesodenn \\hieh ha\e been for some time completely separated from the endoderm bya \\eii marked split. vi OKIGIN OF THE VESSEL- 369 (lc\( Inj.iuM in such regions are therefore clearly deriva- tives of the mesoderm and there is no possibility of the endoderm playing a direct part in their formation as might be the case peri- pherally in the region of the germ-wall. The vascular rudiments become joined up by strands of cells to ton 11 a network and this network gradually spreads inwards, its extension being brought about by a progressive differentiation in situ lit »m the mesoderm : there is no actual sprouting inwards of the already formed strands of the network as is suggested sometimes by the study of whole blastoderms and as was once supposed to take place. Of the network of cell strands which traverses the rudiment of the vascular area the bulkier portions give rise to masses of blood corpuscles surrounded by an endothelial wall, the more attenuated portions to endothelial tubes without any corpuscles in their interior. In the former case the superficial layer of the cells forming the blood- island becomes raised up from the main mass of cells, fluid accumu- lating beneath it in spaces which are at first isolated but later become continuous. The flattened cells which are raised up repre- sent the endothelial wall while the main mass of cells left behind represent developing corpuscles. It is to be noted that the endothelial wall separates from the mass of corpuscles first below (i.e. on the side towards the yolk) and laterally, so that after fluid has accumu- lated in the rudimentary vessel the mass of corpuscles still remains attached to its, as yet undifferentiated, roof. The narrower strands between the main blood-islands and also all those in the pellucid area, except sometimes a few near its posterior end, give rise simply to endothelial tubes containing fluid plasma. As the circulation begins the masses of embryonic corpuscles gradually break up, first in the region of the sinus terminalis,1 the individual corpuscles being whirled away by the current and carried to the heart and thence through the circulation. The origin of the vitelline network has also been investigated in Elasmobranchs (especially Torpedo) by numerous workers. It agrees in its main features with what occurs in the Fowl. As regards the peripheral vessels in general, of the Vertebrata, we may say that they take their origin as chinks within the niesen- chyme filled with a clear fluid secretion (plasma). These chinks are at their first appearance in some cases clearly intercellular while in others they at first have the appearance of intracellular vacuoles. As has already been pointed out in dealing with connective tissue (p. 292) this difference though at first sight impressive loses most of its apparent importance when regarded critically. In this particular case the protoplasmic masses in which the vacuoles appear are as a rule inultinucleate and it is clearly impossible to draw a sharp line of morphological distinction between spaces in such masses with 1 The topography of the vascular area will be found illustrated later in the special chapter ou the development of the Fowl. VOL. It 2 B 370 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. partially broken down walls, and the ordinary intercellular spaces of syncytial embryonic connective tissue. GENERAL CONSIDERATIONS REGARDING THE MORPHOLOGY OF THE VERTEBRATE HEART. — It may be regarded as a primitive characteristic of blood-vessels that their walls are contractile, peristaltic waves of contraction serving to propel the blood in their cavities. It is usually the case however in the more complex animals that this contractility becomes concentrated in one or more localized portions of the vessels, known as hearts, in which the vessel becomes much enlarged and its muscular coating thickened and rhythmically con- tractile. In the craniate Vertebrates there is one heart present and it represents an enlarged portion of the ventral vessel in the region immediately behind the gills. During ontogeny the heart still repeats the archaic evolutionary phase in which it was tubular in character. As development goes on the primitive heart, or cardiac tube, shows rapid increase in size within the perieardiac chamber of the coelome in which it lies. This chamber is relatively small in size and in the lower, fish-like, Vertebrates is bounded by rigid unyielding walls. The confined nature of this space in which the heart has been evolved has, by imposing restrictions upon it during its increase in size, exercised a profound influence upon the modelling of the vertebrate heart. It is therefore desirable to have a clear idea of the general relations of the heart to the perieardiac cavity, during its increase in size, before attempting to study its development in detail in the various groups of Vertebrates. The portion of vessel originally included between the anterior and posterior limits of the perieardiac cavity will be referred to here as the primitive heart or cardiac tube. As development proceeds the increase in size of the primitive heart reveals itself in (1) increase in length and (2) increase in diameter. (1) As regards the former, the cardiac tube is at its posterior and anterior ends — where it enters and leaves the perieardiac cavity respectively — firmly embedded in the tissues of the perieardiac wall. These ends being consequently in the lower, fish-like, vertebrates rigidly fixed in position, it has of necessity come about that the cardiac tube, while in the course of evolution it has increased in length, has lost its original straight form and has been thrown into a system of bends or kinks which have had an important influence upon the structure of the fully evolved heart. This hendinu process is repeated, though with obscuring of some of its detail, during onto- geny and it is an interesting morphological problem to endeavour to unravel the details of the process from the data of comparat ive anatomy and emhrvl' App.in-nlly the primary llexure of tin- cardiac tuhe is represented by a simple loop or huliiin^ toward- iln- ri-lit side of the liody. which il.le in the eiidu Lebratc during early stages of heart development. VI MOKPHOLOGY OF THE HEART 371 R B With increasing growth in length of the cardiac tu!>r this curvature becomes converted into a double tloxure the heart taking on a S-shape. Of the two curves which make up the 5 one which has its concave side towards the head represents the orig- inal loop, while the other which is convex towards the head has developed in the portion of cardiac tube lying posterior to the primary loop. Of these two curves the one last mentioned, that which is morphologically posterior, is in an approximately verti- cal plane. The anterior or primary curve on the other hand shows much variation in position in different Vertebrates. While on the whole it still bulges towards the right side, as did the primary loop, the portion of it formed by the originally headward section of the tube comes in many cases to lie ventral to the other limb of the curve. In other cases this, originally anterior, por- tion of the tube lies for a time dorsal to the other, as is the case in Salamandra. The difference will be appre- ciated by comparing the relative positions of c and V in Figs. 184, A, and 178. Of all the lower verte- brates in which the peri- cardiac space is still bounded by rigid inextensible walls it is the group of Lung- fishes that shows the heart at the highest level of evolu- tion. And in correlation with this fact we find that in these fishes the kinking of the cardiac tube attains Lepidosiren (see below, pp. cardiac tube (the "conns Fi«. 177. — A, diagram to illustrate the flexure of the cardiac tube in the adult Lepidosiren, as seen from tlie ventral side. The portion of the diagram above the horizontal line represents the conus : the portion below the horizontal line would ivpivsent the rest of the heart on the assumption that this portion of the cardiac tube ; similar curvature to that of the conus. Longi- tudinal lines drawn along the tube mark the originally dorsal (D), ventral (I"), right (R), and left (L). H shows the spiral twisting produced by straightening out a tube possessing the sam« flexure as the couus portion in diagram A. its maximum. In a fully developed 376-378) the anterior portion of the arteriosus ") has developed a further 372 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. flexure in addition to those already described. The nature of this flexure is shown in the upper portion of Fig. 177, A, which repre- sents diagrammatically the conus of Lepidosiren as seen from the ventral side. The extreme anterior end of the tube, being fixed firmly at its exit from the pericardiac cavity, retains its primitive morphological position : its originally dorsal side is actually dorsal. Traced backwards the tube is seen to become sharply bent upon itself in a headward direction, in such a way that the side of the tube which was originally on the left side conies to be ventral, as is indicated by the finely dotted line L in the figure. Tracing the tube onwards a second sharp flexure is found and the tube resumes its antero - posterior direction. This second flexure involves a complete reversal of the tube. Its originally right-hand edge (indicated by the coarsely dotted line K), which -had come to be dorsal as a result of the first flexure, is now ventral. The changes in the position of the tube caused by the two flexures may be summed up by saying that the half of the tube which was originally dorsal, and which remains dorsal at its anterior or headward end, has come to be situated on the right side at the posterior or ventricular end of the portion of the tube now under consideration (conus). Similarly the half of the tube which at the headward end is ventral, has come to be at the ventricular end on the left side. The lower half of the diagram represents the portion of cardiac tube which gives rise to the main part of the heart and it is to a certain extent hypothetical, inasmuch as it does not rest on a complete series of observations, but it is clear that the morpho- logically right side of the cardiac tube, which is topographically ventral in the middle part of the figure, has to get back to its original right-hand position at the hinder end of the cardiac tube (which like the front end is firmly fixed in position), and it is reasonable to infer that the flexure of this portion, which gives rise to the atrium and ventricle, would be found, were its unravelling possible, to be symmetrical with the anterior flexure already dealt with. It is interesting to take such a model as that represented in Fig. 177 and subject the conus portion to a process of straightening out — such as would happen in nature if the conus were to shrink in length, its anterior and posterior ends remaining fixed. The result is shown in \:\£. 177, B. The conus assumes a twisting in a right-handed spiral through three ri^ht. angles. In the Anmiola it will be found that the representatives of the conus <»f the Lun^-tish the roots of the great arteries, pulmonary and systemic —as they bead Wardfl from the ventricular part d' the heart, twist round one anol her in ju>l such a spiral. (2) As regards the increase in diameter of the cardiac tube, it is characteri-t n thai this does not take place e.pially t hrmi-hoiit,. At vi MOEPHOLOGY OF THE HEART 373 certain levels the increase in diameter is much less pronounced than it is elsewhere, with the result that the' tube appears to be constricted at these points while it bulges out between them. This development of a series of dilated portions of the heart-tube is the first step in its segmentation into a series of chambers. Of these cliamlu-rs there are typically in the lower vertebrates four — sinus venosus, atrium, ventricle and conus arteriosus. Allusion must be made in passing to an unfortunate confusion of nomenclature which is apt to prove a stumbling-block in the way of the student who is trying to get his ideas clear regarding the morphology of the heart. The name conus arteriosus was first used, so far as the comparative anatomy of the lower Vertebrates is concerned, by Gegenbaur (1866) who used it to designate the structure lying between ventricle and ventral aorta in Elasmobranchs and Ganoids, and characterized by its possessing a muscular, rhythmically contractile wall and by its containing longitudinal rows of pocket valves. The name was introduced in order to accentuate the supposedly fundamental difference, already suggested by Johannes Mliller (1845), between the structure in question and the bulbus arteriosus of Teleostean fishes. This latter is not provided with striped muscle in its wall, it is not rhythmically contractile : in other words it does not form physiologically a part of the heart. Objections, and quite valid ones, have been raised against the use of Gegenbaur's name from the side of Human anatomy, it being pointed out that the " conus " of the lower fishes corresponds rather with the "bulbus" of the human heart. Hum;m anatomists working at the embryology of the vertebrate heart in consequence commonly use the name bulbus cordis for the part of the heart under discussion. Gegenbaur's name however has come to be so universally used by comparative anatomists in reference to the heart of the lower vertebrates as to indicate the desirability of using it in a work on comparative morphology such as this. It will be understood then that the name conus arteriosus is used in this volume as equivalent to what is by many writers termed bulbus cordis,1 without prejudice however to the question whether or not Gegenbaur was justified in his belief that conus arteriosus and bulbus arteriosus are fundamentally distinct structures. ELASMOBRANCHII. — The Elasmobranch heart passes through the typical early stages, first as a straight tube (see Chap. XI.), then as a tube which bulges towards the right side, and then as a tube with the characteristic 5-shaped double flexure already alluded to (Fig. 178). The three limbs of the S during further development become converted into (1) atrium with sinus venosus, (2) ventricle, and (3) conus arteriosus. The well-marked constriction which demarcates atrium from ventricle forms the auricular canal. The progress 1 I avoid in this book using the term truncus arteriosus as it is unnecessary and is liable to cause confusion owing to the want of precision with which it is commonly used. 374 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. towards the condition of the fully developed heart is marked mainly by the increase in relative size of the atrium and ventricle. Whereas however the increase in the size of the atrium takes the form mainly of a mere process of dilatation, that of the ventricle is accompanied by a much more marked thickening of its wall. This is brought about by the inner surface of the myocardium forming numerous projections into the lumen which, becoming more and more pro- nounced and interlacing and fusing with one another, form eventu- ally a spongework and encroach considerably on the ventricular cavity round its periphery. The endocardium fits closely over the surface of each of these myocardiac trabeculae. The physiological meaning of the formation of the trabeculae during the evolution of the ventricle probably lies in the fact that a bundle of muscle which has for its function the pulling together of the ventricular wall can carry out this function more efficiently if it runs straight between its two ends, in other words if it is in the position of a chord to the curve of the ventricular wall rather than simply a portion of that curve. Attention must now be directed to a very characteristic and important proliferation of ^ne endocardiac cells which makes itself apparent in particular regions. In the conus such proliferation takes place along the course of four longitudinal lines, giving rise to cells F,o.i78.-Two stages in the whjch Ue in,.the sPfe between endocardium development of th.- i..-:.rt and myocardium. As this proliferation goes of Acantkias seeu from the on the endocardium is eventually made to vr!ttralsiiQn«lAfterHoch" bulge- int0 the lumen as four prominent stetter, 1906.) ° ,. . _ T . .*•**** endocardiac ridges. In Acanthms (Gegen- .» ; T,im"inuH •*££!"£ baur> 1894)> one of. the four ridges-that which de.' is ventral in position — is reduced in size. In the auricular canal similar endocardiac proliferations take place, one upon the headward and one upon the tailward wall respectively of the canal. Here also each causes a prominent l'ul'_riri;_r nf the endocardium into the cavity — the atrioventricular cushion (anterior .UK I posterior). Both the ridges of the conus and the atrioventricular cushions constitute a valvular apparatus in that, by the contraction of the myocardium lying outside them, they can be jammed t<>-vth< T so as to occlude the lumen into which they project. In both cases, as development goes on, they underi-n ni»'i;iniori»liosis into a pmvlv mechanical and automatic valvular apparatus. In the conus each rid-."- hcci \cavated into a number of pnckct- valves (w semi- lunar" valves), the cavities of \\hich «.pm in a head\\ard direct ic n. (in-il and Ot ! l.iin these cavities as bein^ ]>roduccd simply hv the had re (»f t,he blood hut it Ifi M!\I ahlr to regard VI iii-Airr <>r ELASMOBRANCHS 375 such simple mechanical explanations of developmental phenomena with suspicion. Similarly the atrioventricular cushions become excavated <>n their ventricular side and form the two atrioventricular valves of the adult. A pair of laterally placed valves also develop r.a. ra i la V FIG. 179. — Views of the heart of />/ „ a- >ci/n from the morphologically ventral side. (B and C after J. Robertson, 1913.) A, sta^e :i-J ; H, st;ip- ::i ; ('. sta^r y:>. ranch — sinus venosus, atrium formed from the posterior limb of the cardiac loop, ventricle formed from the apical portion of the loop, and conus formed from the greater part of the anterior limb. The dilatation of the walls of the several chambers is not uniform. In the case of the atrial wall the dilata- tion is most marked dorsally and more especially laterally : the posterior wall on the other hand lags behind in its growth and the result is that the sinu-atrial and atrioventricular openings remain comparatively close to one another (compare adult condition as shown in Fig. 180). Similarly in the case of the ventricular portion of the heart the increase in size is mainly on the ventral and lateral sides, the dorsal wall lagging behind so that the communica- tion between atrium and ventricle and that between ventricle and conus also remain in close proximity (cf. again Fig. 180). After the demarcation of the chambers there come about t\\o important changes in the general form of the heart — the first is the assumption of bilateral symmetry on the part of the ventricle, correlated with a rotation of the ventral side of the ventricle towards the animal's left side. The other consists of a very marked increase in the length of the conus which, owing to the fixation of its two ends, is made to assume the characteristic double flexure already described and illustrated (see also Fig. 179, C). DEVELOPMENT OF SEPTA IN THE HEART. — By far tin- most important feature of the Dipnoan heart, as compared with that of the Elasmobranch, is that now, for the first time, there comes about that separation of the heart into an arterial and a venous half, which is so characteristic of the higher Vertebrates. In Lepidosiren this separation is inaugurated at a relatively early stage of develop- ment (stage 27)— ;tt ;i time \\ln-n the cardiac tube, as yet, shews no indication of a division into chambers — by prolif. -rat i«n <»l tlm cells of tin; endocardium on its outer surface. This takes place aloii-- a line which passes along the posterior wall of the U-shaped heart from the Icll Bide of tin- nuil-atrial opening, through the auricular canal down towards the ,ipe\ of the cardiac loop. As this prolifera- tion goes on i t oau '• 1 1 1< endocardium to bulge into the lumen so aa to form a prominent ridge traversing the hinder wall nl'airium and vi HEART OF LEPIDOSIREN 377 ventricle. It is of morphological interest to notice that this atrio- ventricular ridge extends IVmn its dorsal end not directly ventral- w.inls luii towards the ;iniin;irs right side, so that, if the ridge in question he takm as marking an originally longitudinal line along the \vall of the cardiac tuhe, it indicates that this part of the cardiac tube has undergone a process of twisting like that of a left-handed screw, in other words a twisting of the kind which might be expected on the hypothesis that the flexure of the atrioveutricular portion of the heart was originally that suggested in the discussion on p. 372. A second endothelial proliferation takes place along the atrial wall facing that on which the atrioventricular ridge has developed. The projection formed in this way grows towards and fuses with the atrioventricular ridge to form the atrial septum which divides the atrium into a larger right and a smaller left auricle. Owing to the left-handed position of the atrioventricular ridge at its dorsal end the sinus opens into the right auricle. The pulmonary vein as it develops comes to open on the left face of the septum, i.e. into the left auricle. The ventricle becomes similarly divided into a right and a left chamber1 the foundation of the septum consisting of the atrio- ventricular ridge already mentioned. In this case however Robertson does not describe any endocardiac proliferation vis-a-vis to the ridge but says the septum is completed by muscular trabeculae growing towards and eventually fusing with the edge of the ridge. The conus arteriosus is characterized by the development of a series of longitudinal endocardiac ridges similar in nature to those of Elasmobranchs. A conspicuous difference in detail is that each ridge is markedly discontinuous, the portions situated in the anterior and in the posterior section of the conus developing independently. We may take it that the ridges were primitively in the Vertebrata longitudinal and continuous and the secondary discontinuity visible in Lung-fishes and also in the higher Vertebrates may be associated with two probable causes : (1) interference with the development of the middle region of the conus by the flexure into which it is thrown, and (2) the tendency, as seen in Elasmobranchs and Ganoids, for the terminal members of the longitudinal rows of valves to become enlarged relatively to the rest. There can be no doubt that the longitudinal ridges as we see them in the Lung-fishes and the higher Vertebrates are revertive rather than persistent primitive features. In other words the ancestors of these Vertebrates passed through the phase of evolution in which each ridge had become converted into a row of pocket valves. This seems clearly indicated by the fact that the latter condition holds in modern Elasmobranchs and primitive Ganoids. But if so then the tendency for the terminal valves of the row to be specially developed in that earlier phase of evolution may show itself 1 The separation does not normally become quite complete in the adult either in the rase of atrium or ventricle. 378 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. on reversion to the ridge condition in more or less great suppression or diminution of the middle portion of the ridge. In the anterior section of the conus four longitudinal ridges develop, situated respectively on the right-hand side (1), dorsally (2), on the left-hand side (3), and ventrally (4). This anterior end of the conus retains as already explained its primitive position and we shall therefore always refer to the four ridges according to the position they have in this undisturbed portion of the conus as Right, Dorsal, Left and Ventral respectively, the adverb morphologically being understood before the adjective in each case. The right and left ridges make their appearance first and they alone become prominent, forming thin shelf-like structures which project right in to the centre of the cavity so that their edges overlap. For a short distance at the extreme anterior end they become fused together so as to form a continuous septum. The left ridge is comparatively short, tapering off posteriorly, but the right extends back through the anterior and middle section of the conus. At the point of flexure between middle and posterior sections there is a break during early stages but later on the ridge becomes continuous with a portion of ridge which projects from the ventral wall of the posterior section of the conus. There is no reason to doubt that this is really part of the same morphological structure as that with which it is in line in the anterior section of the conus and we shall therefore term it the posterior portion of the right ridge. The whole of this right ridge forms what is often called the spiral valve of the conus. The dorsal and ventral ridges of the anterior section of the conus are later than the lateral ridges in making their appearance and soon disappear again. The ventral ridge is especially feebly developed. In the posterior section of the conus the right ridge — now ventral in position — is alone well developed. The other three appear as rudiments, they are at no time prominent and they become resolved into vestigial pocket valves which may still be detected in the adults. This latter fact justifies the conclusion already readied that during the ancestral history of the Lung-fishes a stage was passed through during which the conus was provided with longitudinal rows of functional pocket valves, in other words that the primitive liil^cs seen in the conus of the modern Lung-fish are revertive iMt ho- than persistent. Y.M.VK.S OK TIIK HK.\I;T. The Binu-auricular opening is guarded on the right side by a \alve. This develops out of the inpushed fold of the oardiac \\all in the constriction between sinus and atrium. Tin- at ri"\r!i! ncular opening is guarded by a highly characteristic hcvcll,-.! plu^ |.'j;j. 180, AV./< which when the vi-ni rieli- contracts is pulled downwards so as completely to occlude the opening. I)evelo],up ntally this plug arises as a thickening <>r the atrioventricular ridge This ridge,. which, as already indicated, VI HEAET OF LEPIDOSIREN 379 forms the common foundation of auricular and ventricular septum, traverses the auricular canal, projecting into it from behind so as to give the atrioventricular opening a horse-shoe sliape. It is the part which lies above (dorsal to) the opening which becomes thickened and eventually assumes a cartilaginous character to form the plug. The plug is to be regarded as the homologue of the posterior atrioventricular cushion of Elasmobranchs but Robertson failed to find any trace of an anterior cushion. The conus in the completely developed state is characterized by the absence of the functional valvular apparatus found in its homologue in other Vertebrates. On the one hand the endocardiac ridges, functional in the young Elasmobranch or Ganoid, are no s.vr c.v Fi<;. 180. — Heart of an adult Lepidosiren with the right side removed. (After J. Robertson, 1913.) AV.p, atrioventricular plug; c.v, coronary vein (cut); d.O, ducts of Cuvier; j>.r, pulmonary vein ; p.V.c, posterior vena cava at its opening into the sinus vcnosus ; s.A, atrial septum ; s.r, ventrirular septum ; s.v, sinus venosus (its opening into the right auricle indicated by an arrow) ; sp, spiral valve ; III, VI, aortic arches cut near their ventral ends. longer in a condition to fulfil their original function and on the other the pocket valves are here vestigial. MUSCULARIZATION OF THE HEART. — Bud-like projections from the myocardium grow into the cavity of the heart, meet together and in the ventricle form a muscular sponge work of the same type as that seen in Elasmobranchs. In the case of the ventricle numerous trabeculae arising in this way converge upon the free edge of the atrioventricular ridge and become continuous with it. As develop- ment goes on this spongy mass of trabeculae undergoes condensation and acquires the solid character of the fully developed septum. The septum is continuous dorsally with the atrioventricular plug and it forms a muscular apparatus by which the plug is pulled down so as to fit into and close the opening. The myocardium of the auricular 380 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. canal and of the con us never forms a spongework but remains compact layer of muscle. In the case of the conus this muscular coat is very feebly developed in the middle and cephalic portions — in which fact we may probably recognize a degenerative feature see- ing that in the Elasmopianch the conns musculature is well developed right to its front end. The meshes of the ventricular spongework, as development goes on, come to spread somewhat round the auricular canal and round the ventricular end of the conus, so that each of these structures has the appearance of being drawn into the ventri- cular cavity. SAUROPSIDA. — The most exhaustive account of the development of a Sauropsidan heart is that dealing with Lacerta by Greil (1903) and upon it the following description is based. In its early stages the heart passes through the familiar tubular form and becomes bent, first bulging in a simple curve towards the right and then assuming a double S-like curvature just as in the Elasmobranch. About stage 17-18 the constriction of the heart into sinus venosus, atrium, ventricle and conus becomes apparent — the three last mentioned chambers bulging outwards between -the grooves which limit them. The atrial portion does not in these early stages take up the purely dorsal position seen in the Elasmobranch or Lung-fish but remains for a time well to the left. The conus, in its early stages, much reduced in relative size as compared with that of the Elasmobranch, undergoes a marked increase in length, which causes it to assume a bayonet-shaped curvature in which we may see a reminiscence of the sharp double flexure seen in the conus of the Lung-fish. In the Lizard however this curvature of the conus is merely temporary. As development goes on the increase in length of the conus instead of being more pronounced than that of the heart as a whole becomes less so with the ivsult that between stages 21 and 26 the anterior flexure of the conus becomes pulled out and replaced by a right-handed spiral twist. DEVELOPMENT OF SEPTA. — The septation of the heart is inaugurated by the appearance of localized proliferation of the eudocardiac lining. In the auricular canal, which runs in an antero-posterior direction rather than dorsi-ventrally, owing to the atrium lying anterior to the ventricle, instead oj1 dorsal to it as was the case in the Lung-fish. there develop two endocardiac cushions, one dorsal (posterior), the other ventral (anterior). Of these the ventral or anterior one which was not apparent in Lrpidosiren is well developed and is continued aa an endueanliar ridge round the anterior (headward) wall of the atrium on bo Itfl fOOl COmpaitt l''i.L,r. 1 83, C, at.s). As development, goes on t.his projects n,niv and limn- pr«»niinent ly into t lie cavity of the atrium and forma lln- main part of t he sept urn between thel\\o auricles. Hy about sta,^e L'li il has -Town hall \\av across the atrial Cavity, and by a I >< , ;he aiiric.ulareanal. While in c i he at urn is now compl, nol o physiologically vi HEAKT OF LACERTA 381 as secondary perforations have made their appearance in the septum so as to keep the two auricular (-avitii-s in free 'com niunication. The two endocardiac cushions of the auricular canal become joined together by a bridge of endocardiac tissue which forms the free edge of the auricular septum. This is followed by a complete fusion taking place between the middle parts of the two cushions, so that the atrioventricular opening becomes completely divided into a larger right and a smaller left portion. The, at first, thick mass of tissue which separates these two openings becomes gradually converted into a thin plate, situated in the plane separating atrium from ventricle, and therefore perpendicular to the plane of the atrial septum. This plate is divided sagittally into a right and a left half by its line of attachment to the septum. The free edge of each half is concave and projects freely into the corresponding auriculoventricular opening — forming the mesial or septal valve of that opening. In the meantime a new endocardiac cushion develops on what were the right and left sides of the auricular canal. These also become thin flaps and form the lateral auriculoventricular valves. It will be noticed that there have developed round the original atrioventricular opening four proliferations of endocardium — the same number as was found in the conus of the Lung - fish and as will be found in the conus of the Amniota, thus supporting the idea that there are four longitudinal endocardiac ridges potentially present throughout the cardiac tube of the higher Vertebrates though they may become actually apparent only in the conus region. In the Lizard the atrioventricular ridge, which was so conspicuous in the Lung- fish, has practically become reduced to the portion lying within the auricular canal — the dorsal (posterior) endocardiac cushion. The ventricle is undivided. The conus on the other hand undergoes a complete and somewhat complicated process of septation. This is inaugurated by localized proliferation of the endocardium to form longitudinal ridges. As in the Lung- fish these arise discontmuously there being distinct anterior (headward) and posterior rudiments. Anteriorly the normal^ four ridges develop, the dorsal and ventral appearing in this case at an earlier stage (1*7 or 18) than the lateral ones. Towards the ventricular end two ridges first make their appearance in a dorsal (ridge B, Greil) and ventral situation (A, Greil) respectively. Of these the ventral one becomes eventually continuous with the right-hand anterior rudiment. It clearly corresponds with the similarly situated ridge in the hinder portion of the conus of Lepidosiren and like it is to be interpreted as the hinder portion of the morphologically right- hand ridge. The ridge vis-a-vis to that just mentioned, here dorsal in position, would similarly represent the hinder portion of the morphologically left-hand ridge. Later on a small and transient ridge (0, Greil) makes its appearance on the right-hand wall of this hinder portion of the conus and this would represent the hinder portion of the morphologically dorsal ridge. 382 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. As development goes on the wall of the conus becomes changed in histological character, its striped muscles become replaced by smooth and in general it takes on the ordinary features of arterial wall so that it resembles a portion of the ventral aorta rather than of the heart. As may be seen in a living embryo this histological change is accompanied by a physiological one, for the rhythmic contractions of the heart are seen now to extend forwards as far as the anterior limit of the striped muscle but no farther. Altogether the superficial appearance is just as if the ventral aorta (" truncus arteriosus ") were extending backwards at the expense of the conus, and the word truncus is frequently used to include the whole as far back as the limit for the time being of the smooth non-striated muscular wall. It must however not be forgotten that in the strict morpho- logical sense all that part of the heart is conus which corresponds to the conus of Lepidosiren. The special criterion which identifies it is A. Ki-;. 181. — Diagrammatic transverse sections through conus of Lacerta (A) and (B) to show the endocardiac ridges and the pocket valves. 1, morphologically ri^ht ridge ; 2, dorsal ; 3, left ; and 4, ventral, a and b, problematical ridges discussed in text ; p, pulmonary cavity ; S, main systemic ; /.*, left systemic cavity. the appearance of double flexure or the resultant spiral coiling during its development. The muscular coating, so characteristic a feature in the vertebrates below the Amriiota, is associated with a definite type ©f functional activity : in the Amniota that type of functional activity has disappeared and with it the characteristic type of wall. Tin- ventral aorta is in its hinder portion, where it becomes continuous with the front end of the conus, divided1 into a dorsal (pulmonary) and a ventral (aortic) cavity by a horizontal septum and this is prolonged backwards aloiiu tin- \vall of tin- eonus by the right and left rid^-s. Of these the right is very lar^e, it, projects across iln- lumen and gradually fuses with tin- left rid^e i. Kig. 181, A). This ridj^e (:',) is low ami double and it is with its dorsal portion, i.e. the portion next tho dorsal rid-e. thai, the fusion takes place. By the spreading backwards of this process of fusion of the n-ht and Mi i to hori/ontal septum of the ventral aorta becomes prolonged bark as 'a septum in the conus — no longer 1 See below, p. 398. vi HEAET OF LACERTA 383 however but spirally twisted owing to the twisting of the conus already mentioned. Parallel with and preparatory to this process of fusion the distal ridge rudiments spread backwards, pursuing a spiral (•oiirsc and thus making evident the spiral twisting of the conus as a whole. It is interesting to notice that the line of insertion of the ridges, and therefore of the septum formed by their fusion, becomes marked on the outer surface of the conus by a distinct incision — a preliminary step towards the complete splitting of the conus in the plane of the septum which takes place in Birds and in Mammals. In addition to the dividing of the cavity of the conus into a pulmonary and an aortic portion in the manner just described there takes place also, in the Lizard, a splitting of the aortic portion into two parts, corresponding to the right and left halves into which the systemic portion of the ventral aorta is divided.1 The septum separating these becomes prolonged backwards at its hinder end, on the one hand, into the ventral ridge of the conus (Fig. 181, A, 4) and on the other into a quite similar ridge developed upon the surface of the septum which separates the aortic from the pulmonary (Fig. 181, A, 1). These two ridges facing one another across the aortic cavity gradually extend backwards and undergo fusion just as in the other case so as to form a complete septum dividing the aortic cavity into two (S and l.s). In this way then the original conus becomes replaced by a set of three tubes twisted spirally round one another, forming the roots of the two systemic aortae and of the pulmonary artery, still however enclosed in a common wall. VALVES OF THE HEART. — The right and left valves which guard the opening from sinus into atrium are formed simply by the exaggeration of the fold of the cardiac wall which delimits these two chambers from one another. The origin of the auriculoventricular valves has already been described. The pocket valves of the systemic aortae and pulmonary artery are derived from the endocardiac ridges of the conus as in the Elasmobranch. According to Langer (1894) the outer valve in each of the three vessels (pulmonary artery, left systemic aorta, right systemic aorta) are derived from the Dorsal, Left and Ventral endocardiac ridges (2, 3 and 4) respectively, while the inner valve in all three is derived from the hypertrophied Right ridge (1), which with its outgrowth takes part in the formation of all three vessels (Fig. 181, A). The question as to whether or not the pocket valve is formed from the extreme ventricular end of the conus ridge, or whether on the other hand a considerable portion of this end of the conus with its contained ridges becomes incorporated in the ventricle as maintained by Langer and Greil does not appear to the present writer to be satisfactorily settled. It is advisable that the point should be re-investigated upon abundant material. GALLUS. — (Figs. 182 and 183.) The most detailed investigations 1 See below, p. 393. 384 EMBRYOLOGY OF THE LOWER VERTEBRATES OH, of the development of the Bird's heart are those of Greil, of which unfortunately there is available so far only the abstract given by Hochstetter (1906). As we should expect from the close genetic relationship between Birds and Reptiles there is a close correspond- ence between the general features of the development of the heart in the two cases. It will suffice then to draw attention to the more important points in which the development of the Fowl's heart has Fia. 182.— Illustrating the development of the heart in the fowl. (Afti-r original ilra\\ in^s l>y (Iivil.) nt, atrium ; /'.'«./, biill>o-;iiiiicular tnld ; ., conns: /../. iHt am irl»- ; /./, left innominate artery; /./>, IHt pulmonar\ . LF, Ifl'l vi-nt i iHi- : /.". i i-lil auiii-1--: ./. li-ht iniiuiiiiiiati' aitn pulin. : ' mir aorta. been found to differ from that of Lacerta. S<> far as external form is concerned tin- mod Mrikin^ difference is that the sinus venosus loses its idi-iilit.y a- ;i iMi«-.| in ihc ri^lil nuriclc, all rxn-pt ils Iri'i portion which persists as the cardiac end of tin- iH'i dud nl (ji\ici- (.r iiit<;rior Vena CM An importanl Advance upulnmnar\ em-illation ihe period I.elnre the lungs are functional. It \\ill In- noticed in Fii^s. ISU. (' and I) lltal in the later stages vi HEART OF BIRDS 387 <>l its development the ventricular portion of the heart undergoes a certain amount of rotation, the right ventricle becoming displaced somewhat towards the left side, ventrally to the left ventricle. The result of this is to undo to a small extent the spiral twist of the conus. The conus of the Fowl develops typical endocardiac ridges, here however only three in number, and the individual ridges retain a more nearly primitive condition in that in early stages they are not so completely divided into two distinct rudiments, while in later stages two of the three are obviously continuous. One of these ridges is clearly the morphologically Right. Here again it is much • •nl.-irged (Fig. 181, B, 1) and grows right across the cavity to form a complete septum between the pulmonary (p) and systemic ($) portions of the cavity. Regarding the identity of the two other ridges there is some doubt. They are identified by Greil as the Dorsal (2) and Left (3) while the Ventral (4) is supposed to have disappeared. It appears to the present writer however that the possibility should be con- sidered whether they do not together represent the Left ridge, with which in Lacerta the free edge of the enlarged Right ridge comes in contact and which in the latter animal shows an incipient division into two parts by a longitudinal groove. The septum formed by the enlarged Right ridge follows a spiral course, its line of insertion being indicated by a spiral groove on the outer surface of the conus. In the Bird this groove gradually deepens into a slit which splits the septum into two halves and as a consequence divides the conus into two separate vessels which course spirally round one another — the roots of the pulmonary arteries and the systemic aorta respectively. No vestige of a septum subdividing the aortic cavity has so far been described. VALVES. — Pulmonary artery and systemic aorta are each provided with three pocket-valves at their ventricular end. These arise in the manner indicated in Fig. 181, B. Each vessel receives a valve split off from the enlarged Right ridge. The pulmonary and the systemic cavities receive further a valve split off from the endocardiac thickenings marked a and b respectively. Following Greil these would be attributed to the Dorsal and Left ridges, while accepting the alternative interpretation suggested above they would both be referred to the morphologically Left ridge. There remains a third pocket-valve in each cavity. That in the systemic cavity no doubt represents the otherwise missing Ventral ridge, while if it and 1) together represent the Left ridge then the third pocket- valve of the pulmonary cavity would represent the Dorsal ridge. As there is no reason to doubt the reliability of a pocket-valve as evidence of a once existing endocardiac ridge we should be driven — if we reject the explanation here suggested — to assume the former existence of an additional ridge between the Right and the Dorsal and there seems no justification otherwise for doing this. 388 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. The pocket -valves are stated not to develop at the extreme hinder limit of the ridges, the septum stretching back beyond them to become continuous with the interventricular septum. In the right auriculoventricular opening the inner or septal valve is not developed, the ventricular septum fusing with what in Lacerta becomes converted into the valve in question. The main features of heart development having been illustrated from these three different groups, Elasmobranchii, Dipnoi and Sauropsida, it will be convenient now to indicate the more important peculiarities which have been detected in other groups of the lower Vertebrates. It should be understood however that in the case of several of these, such as Cyclostomes, Qanoids and even Amphibians, apart from Urodeles, our knowledge is still fragmentary. In Polypterus (Graham Kerr, 1907) the cardiac tube when in the form of a loop shows a similar displacement to that which occurs in Lepidosiren — the lower end of the loop being pushed forwards in front of the yolk. In this case however the displacement has gone farther than in the Lung-fish so that the cardiac loop is completely inverted — its apex being directed forwards, while the ventral aorta passes off in a tailward direction. A similar displacement occurs in Teleosts. The conus of Ganoid fishes shows the usual endocardiac ridges which become converted into longitudinal rows of pocket-valves as in Elasmobranchs. In Polypterus these ridges are six in number, alternate ones being much reduced in size, with the result that in the adult three rows of large pocket-valves alternate with three rows of small ones. In all these fishes the endocardiac ridges and their resultant rows of pocket-valves run straight along the conus and there is no reason to doubt that this is the primitive condition. To determine the primitive number of the ridges in Fishes more research is needed although there is little doubt that four was the number present in the primitive Tetrapods. In the Tcleostran fishes we find in place of the conus arteriosus the structure known as the aortic bulk As already indicated (p. 373) this is distinguished from the typical conns by well-marked historical and physiological differences. And it is fre(|iient lv regarded as being morphologically a part not of the heart I ml of the irenl ml aorta. If however we take, as we arc probably justified in doin^, the point of exit from the pericardiac cavity as ln-in^ relatively lixrd and as marking tin- headward limit of tin- cardiac tube or primitive heart, then il becomes clcaf that, the aortic bulb, lyiii.n' as it dors within the p.-ricardiac ca\ily, is really a portion of the j.rimitivr cardiac tube and of that part «if it \\hich lay between the ventricle and the ventral aorta in ot her \\ords t he conus arteriosiis. What has happened in the cM.lnl.ioii of the Telcosiran heart is in all probability entirely analogous \\ith \\hat has taken place in the VI HEART 389 at Aiuniota, namely that the (•onus arteriosus has gradually lost its power of rhythmic contraction while pari passu its myocardiae, coating of striped uinsel.- has degenerated and its primitive histo- Logica] characteristics have been replaced by others resembling more closely those of the ventral aorta. During ontogeny it would appear from Hoyer's work on Salmo (1900) that the conns in the embryo possesses the characteristic features — a layer of striated muscle in its wall, and longitudinal ridges (two in number) projecting into its lumen — and differs from that of an Elasinobranch merely in the fact that these features do not extend throughout the whole of the distance between the ventricle and the anterior limit of the pericardiac space, but only through about the posterior half of that distance. In the adult the two ridges are repre- sented by the two pocket -valves. In Urodele amphibians (Sala- mandra — Hochstetter, 1906) the heart during the period when it is in the form of a tube with an S-Hke curvature is conspicuously different in appearance from that of the Vertebrates already described, owing to the fact that the two curves of the S lie in different planes from those which they occupy elsewhere. The morphologically posterior or tail- ward curve lies here nearly in the horizontal plane while the anterior FIG. 184.— Views of developing heart of Curve lies in a liearlv Vertical plane Salamandra as seen from the ventral -the limb of the curve which will side' atrium; c, conus arteriosus; d.C, duet ventricular portion, so that it is hidden in a view of the heart from the ventral side (Fig. 184, A). Special interest is lent to this curvature of the atrioventricular portion of the cardiac tube by the fact that it reproduces accurately the type of curvature which we inferred as being present in this portion of the vertebrate heart in our general discussion of its morphology (compare Fig. 184, A, with lower portion of Fig. 177). As development proceeds the left-hand end of the ventricular portion, i.e. its actually headward end. swings ventrally and tailwards so that the long axis of this portion of the heart conies to be per- pendicular to the sagittal plane of the body (Fig. 184, B). As the ventricular part of the heart shifts backwards the conns becomes visible in a view from the ventral side. The backward shifting of Cuvier ; p.v.c, posterior ven;i cava : sinus veuosus ; I*, ventricle. 390 EMBRYOLOGY OF THE LOWER VERTEBRATES Ctt continues until tin- ventricle mines t<> lie ventral to the sinus (Fig. 184, C) instead of well in front of it as it did originally. The ventricle is nn\\ F REPTILES 391 The hearts of Iieptiles in ^vm-ral a^ivc Hosi-ly in their develop- mental I'ra.tmvs with that of Lacerta. The most important varia- tions an- seen in the ( Yor.odiles (Hochstetter, 1906*). The conns hrrr is nf intrivst iii that it still repeats with particular clearness the sharp double llexmv seen in tin- Dipnoan (Fig. 185). The ventricular portion of the heart becomes completely divided into a ri^ht and left ventricle by a septum which is formed for the most part from trabecular projections of the myocardium but in part also from the endocardiac bridge which divides the atrio- \vntrirnlar opening as in the Bird. This septum becomes quite complete, the inter ventricular foramen which exists for a time dosing up and the interventricular septum "becoming continuous with the aortic septum of the conus. This is rendered a possible physiological arrangement in the crocodile by the fact that here the opening from the ventricle into that cavity of the conus which is continuous with the right systemic aorta, is farther to the left than in Lizards, while on the other hand the right atrioventricular opening is farther to the right. The result is that the opening of the right systemic aorta and the left auriculoventricular opening lie to the left of the septum, while the openings of the left aorta and pulmonary artery together with the right auriculoventricular opening lie to FlG- 185 —Heart of embryonic .e, , n i Tr-p Crocodile. (After Hocn- the right of the septum. Such differences stetter, 1906*.) in the position of the ventricular openings ,, conus ^teriosus . ^ venlride of the great arteries are regarded by Hoch- stetter as due to varying degrees of incorporation of the obliquely placed conus into the ventricle. The foramen of Panizza, that remarkable communication which exists in the crocodile between the two systemic aortae close to their point of exit from the ventricles, arises* as a secondary perforation of the aortic septum comparatively late in development just before the closing of the last remains of the interventricular foramen. The splitting up of the conus into independent vessels remains as a rule in the Eeptiles, as in Lacerta, in an incipient condition, indicated merely by a slight grooving of the surface. In Ophidia however the superficial groove becomes so deepened as to split the conus completely into an independent aortic and pulmonary root as in Birds. Passing in review the broad features of heart development in the lower Vertebrates the following general principles seem to emerge : — (1) The primitive heart or primitive cardiac tube is that portion of the ventral or subintestinal vessel included within the limits of the pericardiac coelome. (2) Annular segments in the wall of this tube lag behind in increase of diameter so that the tube becomes constricted into a 392 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. series of chambers sinus venosus, atrium, ventricle and conus arteriosus — the originally peristaltic waves of contraction tending to become reduced at each constriction so that the chambers come to contract in series. (3) The primitive valvular apparatus consists of a series <>!' longitudinal ridges. These are best marked in the conus where they were originally at least four in number. The presence of six in Polypterus and the occurrence of more than four rows of valves in various other relatively primitive Ganoids and Elasmohranchs suggests that the number may have been greater. Individual ridges may in various forms become reduced or disappear, while in other cases apparently an increase in number may take place. The ridges show very generally a tendency to develop from two distinct rudi- ments, an anterior one and a posterior one, but the continuity of the rows of valves in the lower fishes indicates the probability that this discHiitinuity is secondary. In the portions of the heart behind the conus there is no complete series of ridges like those in the couus though it is possible that vestiges of such ridges are represented by the endocardiac cushions and septal rudiments. (4) Using the ridges of the conus as marking morphologically longitudinal lines it is seen that the conus has in the Lung-fishes undergone a double flexure of such a kind as to produce when straightened out a right-handed twist of the conus through approxi- mately three right angles. (5) In tetrapodous Vertebrates the conus shows a similar right- handed twist and this is adequately explained by the relative reduc- tion in length which the conus has undergone if it be assumed that there was an ancestral condition resembling that of the existing Dipnoan (6) From the Lung-fishes upwards the originally right-hand rid-e ni1 the conus becomes hypertrophied and, either alone or by fusion witli its vis-a-vis, forms a longitudinal septum dividing the cavity of the conus into two parts — pulmonary and aortic — twisted spirally round one another. (7) Correlated with this process is a tendency for the wall of the conus to lose its striped muscle and its rhythmic contractility. Tins process takes place from before backwards and the portion which has suffered this change, and the wall of which has assumed the ordinary arterial character, is in common usage included under the n;uije truncus artcriosus. (8) The atrJo\rnirieul;ir part <,| the cardiac tube undergoes a doiihle I! 'ndar in nature to that seen in the conns of the Ijin^-li>li hut o| nidi a kind as would it' straightened out irive a Id! -handed I v, and septa of tin.- part of the heart originate in :idoeardiae OUShioHfl and in /,- / atrial septum and ventri- cular septum are at lirst continuous \\ith on.- another. (10) In the evolution o| 1 he valves <•!' tin- In-art, there ha- come vi HEART AND ARTERIES 393 about a substitution of automatically acting pocket- or flap-valves for the original endocardiac, ridge or cushions which for tln-ir functioning were dependent upon a complex and fallible neuro- muscular mechanism. ARTERIAL SYSTEM. — The conns artcriosns is primitively pi-ohm^-d forwards into the ventral aorta which gives off' on eacli side lin- geries of aortic arches. The most important evolutionary cl, which the originally simple tubular ventral aorta undergoes process of splitting whereby the stream of blood to the lungs is separated from that to the tissues generally. This splitting is seen for the first time in the Lung-fishes. Here a dorsal portion of the cavity is separated off, continuous behind with the pulmonary cavity of the conus and ending blindly in front by its floor meeting its roof. The anterior termination of this cavity is just in front of the point of origin of aortic arch V, and this arch along with arch VI, owing to the fact that they branch off from the ventral aorta somewhat dorsally, open from the cavity in question. The horizontal partition which forms the floor of this dorsal pulmonary cavity begins in ontogeny as a rudimentary ingrowth on each side between the origins of arches IV and V. The two rudiments grow back, fuse, and form a horizontal partition continuous at its hinder end with the Right and Left ridges of the conus which, as already explained, form the floor of its pulmonary cavity. A similar splitting off of a dorsal, pulmonary, part of the ventral aorta occurs in air-breathing Vertebrates in general. As regards the remaining, or systemic, portion of the ventral aorta, the main point to notice is its tendency to split into two separate halves in its anterior portion, a process correlated probably with economy of material, allowing as it does the origins of the aortic arches to be displaced outwards so as to shorten these arches. This splitting or bifurcation of the ventral aorta spreads backwards for a variable distance, commonly to about the level of aortic arch III or IV. In the Reptiles there comes about an independent splitting into two lateral halves of the aortic cavity posterior to the region of the bifurcation just alluded to. A vertical septum grows backwards from the anterior lip of the opening into aortic arch IV of the left side and becomes continuous with the septum separating the two systemic cavities of the conus. When this aortic septum is formed the ventral aorta in its hinder portion contains three cavities — a dorsal, pulmonary, leading to aortic arches V and VI of both sides, a left ventral leading to aortic arch IV of the left side, and a right ventral leading to aortic arch IV of the right side together with the paired anterior part of the ventral aorta and the aortic arches springing from them. Each of the three cavities is continuous behind with the corresponding cavity of the conus. In the majority of Reptiles (not in Lacerta) the separat i. m of these cavities is followed by splitting of the septa between them 394 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. so that tin- ventral aorta is resolved into three distinct vessels tunning portions of the common pulmonary artery and of the right and left systemic aortae respectively. THE AORTIC ARCHES AND THEIR DERIVATIVES. — From the ventral aorta there are given off on each side a series of half-he top-shaped aortic arches which pass in a dorsal direction, between successive visceral clefts, to open eventually into the dorsal aorta. In the region where it receives tin- aortic arches the dorsal aorta is i'requently paired (forming the aortic roots) either temporarily or throughout life. This assumption of the paired condition may not improbably be of similar significance to that of the ventral aorta i.e. have to do merely with the economizing of material. In any case the precise extent of the paired condition does not appear to be of any great morphological importance. As regards the aortic arches themselves the following general features are to be noted : (1) that they develop in order of position from before backwards in agreement with the general principle of development of the vertebrate body and (2) that individual arches tend to become reduced in size in correlation with diminution of functional activity. Thus the mandibular and hyoid arches having lost or at least greatly diminished their respiratory activity even in the lower fishes we find a corresponding disappearance or reduction of their aortic arches. An important point to notice is that where the particular visceral arch carries a true external gill (Crossopterygians, Lepido- siren and Protopterus, Urodele and some other Amphibians) the aortic arch passes out as a loop into the external gill. The aortic arch is in fact in these forms during early stages, before the gill- clefts are perforated, the vessel of the external gill. Such relations on the part of vessels of the fundamental morphological importance of the aortic arches are not to be dismissed lightly as modern adap- tive modifications. They appear to indicate that the archa it- function of the aortic arch was to supply the external gill with blood. As the external gill ceases to function a short circuit is formed at its base, through which the blood passes directly to the dorsal part of the arch without traversing the external gill. In Urodeles, according to Maurer, the short-circuiting vessel sprouts downwards from the dorsal iimb of the arch but in Lepidosiren Robertson finds it arising simply by the ciilar-emcui "!' pre-e\istin- chinks. Thus the definitive aortic, arch, in those cases in which an external -ill is fora time present, includes a portion secondarily intercalated in its course and derived from the short-circuit in- VOSSeL In the typical lishes. \\heiv respiration is carried on by the wall of t.he L'dl-cl.-fi. there (.ccomes j u terca l.i t ed in t he course of the aortic arch a respiratory net \\oi-k of capillaries, so that the arch is divided into a dislincl \.-nlial ''afferent ) and dnrsal (efferent) portion. In -uch a fish as /,, left pulmonary artery ; Z.s, left systemic; />, pulmonary; /•.;>, right pulmonary; .s', systemic aorta; >-, subclavian artery; v.'i, ventral aorta; <-..-. v.-ntral (external) carotid ; 1, II, etc., aortic arches. The longitudinal vessels with which the aortic arches are con- nected are prolonged forwards as the carotid arteries which supply i .In- head with blood. The ventral aorta is prolonged forwards as the ventral, or external, carotid while the prolongation forwards of the aortic root forms the dorsal, or internal, carotid. Of the alternative 396 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. names dorsal and ventral (Mackay, 1889) are to be preferred to internal and external, for the latter though in common use are less precise. During the development of the young individual there is laid down a general scheme of aortic arches and associated vessels agreeing with that just described, and the processes of modification whereby there becomes evolved out of this the complicated and very different arrangement of the great arteries of the adult a fiord material for one of the most fascinating chapters in vertebrate an. d.c. v.c. VI A. l-'i'.. ls7. — Illustrating modification of the carotid arteries, correlated of the neck region. nth elongation : i; (,1.,-- .-n:ii...- (/ ,,•;•;, inn,,tiix) ; p.c, primary carotid. <!..'_> \ . Th.- -j^ncral lines of these processes are best ill ust rated hy an outline of \\hat happens in the ^nmp Reptilia. Tin- iiiT.Mi'jviiinit \\hidi the main arteries assume in adult Reptiles shoWfl much variety. The relation which the ad nit ai-!-aii,Lreinls ami tui'iis • if tli< i. iin- straii'lileiiril out, !•'«• r example aivh 111 heconies a ioriion • lit Internal Miotid artery, In the dUgrami the 01 .tun- ..It lie an Ined l.-r t lie sake of clearnegs. ARTEKIES OF REPTILES 397 detail tlu-iv <-;m be '•••11 general agreement in the fate of various aortic ;irchcs .UK! of other parts of the primitive arterial scheme. Thus the ventral aorta is continued forwards to form the paired ventral ("external") carotid arteries (v.c) while the aortic roots similarly extend forwards as the dorsal ("internal") carotids (d.c). Aortic arches I and 1 1 »lis;ippear. Arch III persists as the root of the dorsal carotid while the portion of ventral aorta hehind it, when Kic. 187A. — Illustrating modification of the carotid arteries, correlated with elongation of the neck region. C, Crocodile; 1), Bird ; c, coeliac artery ; .s-, secondary suhclavian. (Other letters as in Fig. 186.) paired, is the common carotid (c.c). Arch IV is the Systemic arch which sends the blood to the hinder portions of the aortic roots (a.r) and thence to the dorsal aorta (A). Arch V is reduced, appearing only as inconspicuous and transient vestiges during development. Of arch VI the proximal portion becomes the root of the pulmonary artery ~(r.p and Lp) while its dorsal portion disappears. The chief differences in detail are as follows: those affecting the carotids will be given more fully later on. In Lizards and Chelonians the dorsal part of arch VI persists as 398 EMBRYOLOGY OF THK Ln\VKI! VEKTEBKATES MI. a duct of Botallus form ing a connexion between the pulmonary artery and the aortic root IV V VI A. just as is shown in l-'ig. A. L 186, B (d.B) for the Uro- dele amphibian. In other cases it may persist as a ligamentous vestige, as is the case on the left side in Tropidonotus. As a rule the portion of aortic root lying between arches III and IV disap- pears during development but in most Lizards (not in Chameleons and Moni- tors) it persists in the adult, so that in a dis- section arches III and IV appear to run into one another peripherally. In Monitors (Vara- nidae), in correlation with the elongation of the neck, arches III and IV become widely separated from one another and the interven- ing portion of ventral aorta shows a corresponding lengthening both in its paired (common carotid) and its unpaired (primary carotid) portions. I n Birds (Fig. 187 A, D) arch IV completely dis- appears on the left side and with it the portion <»f the left aortic root lyin^ posterior to it. Conse- <|iiently in the adult I'.ird there is only a sin-lr l.einir aortic ,-nvli and it passes dov.ii the ri-ht side of the hudy. Kl..\sM(H;i;.\Nci!ll. In the lchth\opsida. as we should expect, the tleparl ures I'nun the prinniive sehemeare le^sprmiouiiced: wc.ff. FKJ. 188.— Illustrating tli«- iii».liii."iii,,n..ra..rti. an hr- ill VI .hum- MiitoKi-n\ in M-i/Hiiim, an-.. i, Un- to Doll Ml. UK ln:il ; OH, ;in;istoni.,li.- vt'KKel ; e/, efferent i.ia elefta; III, IV, V, vi ARTERIAL SYSTEM 399 they iiiv mainly in (It-tails. In the Elasmobranohs perhaps the most conspicuous oi these is to he seen in the relations of the efferent •Is, each of which emerges, not from an ordinary aortic an-h traversing a gill septum, but from a vascular loop surrounding a gill-cleft, the general arrangement being that shown in Fig. 188, C. According to J)ohrn (1886) this arrangement comes, about in the following way. As the walls of the clefts form lamellae and develop respiratory activity, two branches grow downwards, one anterior and one posterior, from the dorsal end of each aortic arch (Fig. 188, A, VI). These branches become connected together by cross bridges as shown in the figure (an). The aortic arch now undergoes reduction and eventually becomes obliterated, just ventral to the point where the two branches are given off (Fig. 188, A, arch III), so that the arch is now divided into two distinct parts — a ventral afferent and a dorsal efferent — the latter prolonged ventralwards into the two branches. Of these the posterior branch of each pair becomes somewhat reduced in size, it develops a secondary connexion at its upper end with the efferent vessel next behind and loses its connexion with its original efferent vessel. The result is that, after this has happened, each efferent vessel again possesses two branches at its ventral end but these, instead of passing into the same gill septum, pass into two adjoining septa one in front of and one behind the intervening cleft (Fig. 188, B). Eventually the ventral ends of each pair of branches become joined so that the cleft is now surrounded by a complete efferent loop — the loops of successive clefts being connected together by a single persisting anastoniotic vessel (Fig. 188, C). In Chlamydoselachus the modification of the aortic arches just described does not take place. The number of aortic arches corresponds with that of the visceral arches and is normally six. In correlation with the presence of the pseudobranch on the posterior face of the mandibular arch in Elasmobranchs the first aortic arch in these fishes is well developed but its primitive relations with the arterial scheme become much obscured owing primarily to the development of large new afferent and efferent channels connected with the pseudobranch, which carries in its train the reduction of both the ventral and the dorsal portions of the original aortic arch. In the case of the second aortic arch, correlated with the fact that the anterior face of this visceral arch has lost its respiratory function, there is developed only a single, posterior, efferent downgrowth instead of two as is the case with the arches farther back. A wide anastomosis between this and the first aortic arch just below the spiracle provides the secondary afferent vessel to the pseudobranch which as already mentioned supplants the primitive afferent vessel formed by the ventral portion of the first arch. 400 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. CYCLOSTOMATA. — In the Lamprey it should be noted that according to Dohrn (1888) an aortic arch corresponding to aortic arch I of Gnathostomata makes its appearance and then disappears again. In Myxinoids the most important feature is that in them the number of aortic arches reaches its maximum for Craniata — up to 14 in Bdellostoma. CROSSOPTERYGII. — Our knowledge is in this case very incomplete. The chief peculiarity (Graham Kerr, 1907) is that, correlated with the large size of the external gill belonging to arch II, which forms the sole respiratory organ during early stages of larval life, aortic arch II makes its appearance relatively early and the development of the other aortic arches is postponed. Distinct vestiges of aortic arch I make their appearance. The succeeding aortic arches remain small for a prolonged period. Aortic arch VI becomes much enlarged in its ventral part in correlation with the fact that it supplies the pulmonary artery. ACTINOPTERYGII. — In the Teleostean fishes and- in the Ganoids that approach most nearly to them (Lepidosteus — F. W. Mliller, 1897 ; Amia — Allis, 1900) complicated changes, which need not be detailed, take place in arches I and II in relation with the blood supply of the pseudobrauch which in these fishes (p. 159) comes to lie on the inner surface of the operculum. DIPNOI. — In Lepidosiren (Robertson, 1913) aortic arches I and II never become complete well-developed vessels : they are vestigial and their ventral portions do not appear to develop at all. The remaining four aortic arches are well developed, each passes out into an external gill and in each an intercalary piece becomes developed to short-circuit the blood-stream at the time the external gills atrophy. In Ceratodus and Protopterus efferent downgrowths make their appearance as in Elasmobranchs but they remain connected with the dorsal portion of their own aortic arch and do not undergo fusion at their ventral ends so that the condition in the adult departs less from the primitive than it does in the Elasinobranch. AMPHIBIA. — In Urodela the arrangement of aortic arches (Fig. 186, B) closely resembles that in Lung-fishes. Arches I and II are reduced : the latter in fact is according to Maurer (1888) no longer to be detected at all in the case of Triton. Arches III, IV and V are prolonged outwards into external gills and in ea< -li case a short-cm-iiiiin^ piece becomes intercalated as the external nills lose thi-ir functional activity. According to Maun T the iuter- ralarv p<>rt urn makes its a ) i) » -a nt net • as a ilnwn^n'Wl li from the I «•!• efferent limb of the aortic arcli but this may perhaps be doubted in view of tin- lad that in Lung-fishes the corresponding piece of vessel develops b\ the widening out of pre-existing ohinka (Robertson, 19 13). iMIOTA. — Areh IV along with tin- aortic root, into \\hieh it forms tin- main systemic aorta <>n eaeh side. That of the ].-fl Bide i- Connected, in n.nvlalion ^ith the spiral twist of the VI AKTERIAL SYSTEM 401 conus and its derivatives, with tin- right side of the ventricular cavity in proximity to the opening of the pulmonary artery. With the separation of tin- two ventricles tin- arch in question remains connected with tin- riu'ht ventricle. With increasing dlicien< tlie pulmonary circulation the venous hlood of the right ventricle would he drawn oil' more and more to the pulmniiury artery and, correlated with this, \ve find in those Sanropsida in which metabolism is most active and respiration most efficient that this fourth arch on the left side with its aortic root disappears completely during development, leaving only the single right-hand arch and iv m acv ha. Fit;. 1 39. —Blood-vessels of Crocodile of stage 55-56. (After Hochstetter, 1906*.) Arteries are shown in outline, veins black. anterior cardinal vein; .!./•, aortic root; /<.", basilar artery; d.<\ a! carotid; at. otocyst ; I'.'-.i-, posterior cardinal vein ; /-./'. ri^ht, pulmonary artery ; r.c, ventral carotid ; lll-vi. aortic arches. root to form the proximal part of the systemic aorta (Birds, Fig. 187A,D). Arch V, in the Amniota, appears only transiently and so greatly reduced in size as to have completely escaped the notice of the earlier investigators. Hence in Eathke's classical scheme of the aortic arches which is given in the older text-books only five arches are shown, the posterior one being called the fifth. With our present-day knowledge of the homology of the lungs of Amniota with those of Crossopterygians and Lung-fishes, such a scheme is clearly erroneous, as it would involve the pulmonary artery, which is certainly the same vessel throughout, taking its origin in the Amniotes from the (ifth and in the Ichthyopsida from the sixth aortic arch. So without any special emhryological data we should VOL. II 2 D 402 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. be justified in believing that in the Ainniota an aortic arch has disappeared in front of the last one. The persisting vestiges of this fifth arch, which are now known to occur commonly in the embryos of the Amniota, were detected first by van Bemmelen (1886) in Reptiles and Birds. A good example of such a vestigial fifth arch is seen in the embryo of the Crocodile (Fig. 189). The cause of the reduction of this fifth aortic arch is probably t < • be recognized in the fact that it receives its blood from the pulmonary cavity of the conus and of the ventral aorta (Graham Kerr, 1907*). As a consequence of this, during the evolution of the lungs as the main organs of respiration a larger and larger proportion of the blood in the cavity mentioned has become drawn off to rthe lungs, leaving less and less for arch V, with the natural result that the latter has become reduced to the verge of disappearance. Before leaving the subject of the aortic arches it is necessary to . point out that their diagrammatic arrangement as shown in Figs. 186 and 187 is commonly much obscured in the adult. For during development there occur not merely the disappearance of large portions of the original scheme of arches and the straightening out of the unnecessary curves, but also other complications. The chief of these are due to the longitudinal vessels — ventral aorta or aortic roots — lagging behind in their growth in length. This leads, according to the position in which it takes place, to the crowding together of the ventral or the dorsal ends of consecutive aortic arches, and their mere approximation may be succeeded by actual fusion so that two or more arches may come to have a common root emerging from the ventral aorta, or a common terminal portion opening into the aortic root. PULMONARY ARTERY. — The pulmonary artery makes its first appearance in Crossopterygian fishes (Polypterus) as a branch from arch VI towards its ventral end which passes to the lung and adjoining parts of the pharyngeal wall. Throughout the series of lung-breathing Vertebrates it develops similarly as an outgrowth of the sixth aortic arch. A result of the main blood-stream of this arch passing oil' into t lie pulmonary branch is that the dorsal part of the arch lying beyond tin- point. nf origin of the pulmonary artery becomes as ;i rule reduced in size, forming the duct of Botallus. Except in the ease of certain Ileptiles j». 397) the duct of lintallus becomes in normal individuals of the Amniota completely obliterated soon after birth. In the Lun^-lislies the point ..I' origin of the pulmonary artery is displaced up to the dor>al end <>f arch V I \\ln-iv il is fused \\ith ai-c.h V. Thus arch V is able (<> carry blood direct Iv t.. the pulmonary artery and eorrela t . -d \\ith this it does not undergo the reduction in si/e \s liicli ha- taken plaee in the Amniola. In /.ij.ii/iuii'f./i and I'rotopterus a n import ant, development of the pulmonary Jtrt'-ry lakes place inasmuch as its area of distribution nds on to the luii'j belonging morphologically to the other side oi vi . AETEKIAL SYSTEM 403 the body. Tims Lin- ri^ht pulmonary artery conies to supply the dorsal side of both lungs, and the left artery the ventral side of both lungs. Each lung in other words receives a supply of blood from loth pulmonary arteries and this illustrates an initial step towards the condition in A m in \\hciv the right and left sides of the air-bladder — the honioloo-uc of the right lung — are supplied with blood directly by a typical ri^ht ami left pulmonary artery. In the Actinopterygian fishes apart from Amia the pulmonary artery has disappeared entirely from development and the air-bladder receives its blood-supply by secondary connexions with the dorsal aorta and its branches. CAROTID ARTERIES. — As has already been indicated the great longitudinal arteries — ventral and dorsal aortae — are prolonged for- wards into the region of the head as the carotid arteries — ventral and dorsal. Of these the latter, receiving as it does, in the case of the more primitive Vertebrates, blood which has been oxygenated by passing through the gills, becomes the more important and is responsible for supplying blood to the brain. The ventral carotids are found from the Lung-fishes onwards — perhaps in correlation with the reduction of the first two aortic arches. They are known commonly under the name external carotid ( = lingual artery of Amphibia) and supply blood to the ventral side of the head, though cases are known amongst animals no longer having func- tional gills (certain Mammals) in which they take over the blood- supply of the brain also. It will be convenient to consider first the carotids as they occur in the development of the Amniota. The simplest condition is found in an ordinary Lizard (Lacerta) where they are seen as apparent prolongations forwards of the aortic root and of the ventral aorta respectively. This simple arrangement becomes in other Amniota modified during the course of development in different ways, of which the following are the chief. In various Lizards e.g. Chameleons and Monitors (and the same holds for the great majority of Amniota —Fig. 187, A) the portion of aortic root between aortic arches III and IV disappears during development, the consequence being that arch III comes to form the posterior portion of the internal carotid artery, becoming drawn out in the process of growth so as to be in line with the front part of that artery derived from the aortic root. The paired part of the ventral aorta from which the third arch was given off now becomes the common carotid artery (c.c). The un- paired portion of the ventral aorta, from which the common carotids spring, is known as the primary carotid and in the long-necked monitors this becomes much elongated, the growth in length of the neck taking place in the region between aortic arches III and IV (Fig. 187, A). In the European Grass-snake (Tropidonotus, Fig. 187, B) an anastomosis is formed between the two internal carotids just behind the head (Fig. 187, B, an) and, correlated with this, the right common carotid as a rule disappears except for a small branch at its hinder 404 EMBRYOLOGY OF THE LOWEK VERTEBRATES « n. end which supplies the Thyroid gland (O'Donoghue, 1912). The blood-supply of the head- region tlierefore passes to it entirely by the persisting left common carotid (c.c). In Snakes other than Tropidonotus considerable variety exists in the condition of the common carotids. Thus amongst the Boidae the two arteries may remain of approximately equal size (Boa} or on the other hand the left may be reduced (Python). In Chelonians and Crocodiles (Fig. 187A, C) the growth in length of the neck takes place in the region in front of arch III so that here it is the portions of the carotid arteries in front of this level which undergo elongation. In both of these groups an anastomosis forms in the head-region between dorsal and ventral carotids and, correlated with this, the main blood-stream tends to pass to the head by the dorsal carotids, the ventral vessels becoming to a less (Croco- diles) or greater extent (Chelonians) reduced in size (van Bemmelen, 1887 ; Mackay, 1889). In the Crocodiles a still further modification takes place, inasmuch as the two dorsal carotids become for a considerable part of their length fused together into a single vessel, and following upon this aortic arch III of the right side atrophies, so that here as in Tropidonotus, though for a different reason, the main blood-supply of the head comes from the left side. In the Birds the condition closely resembles that of the Crocodile. Here also the ventral carotids become reduced — in this case to the point of complete disappearance — in the neck region as a consequence of an anastomosis with the dorsal carotids in the head. Here also the enlarged dorsal carotids approach one another on the ventral side of the vertebral column. In those birds which depart least from the primitive condition in this respect (Ostrich, Emu, Casuari, Tinamus, Penguins, Divers, Gulls, Plovers, Snipe, Rails and their allies, Fowls, Pigeons, Ducks, Ibises, Storks, Herons, Cormorants and some Gannets, Birds of Prey, Parrots, Hornbills, Motmots, Goat- suckers) the two definitive carotids merely lie in proximity to one another. In many birds however they become fused together into a single vessel over a great part of their length and in such a cast- there may be no further modification (certain Herons such as 1 In- common Bittern, some Cockatoos, some (Jannets), or, as is tin- uvneral rule, this fusion is followed by the disappearance of the third aortic arch of the right side just as was the case in the Crocodiles (Rliea, Apteryx, Gretas, Quails, sonic Cockatoos, Capitonidae, Toucans, Hoopoe. M<;ropidae, Tm^ons, Woodpeckers, most Swifts. Humming birds and 1'asH-rine birds). In a ic\v cases on the other hand it is the third aortic arch of the left side which becomes reduced to a small vestige Haminjn)) or disappears entirely (Eu/'<>f<* — the African Bustard). Amongst the ananniiotic Vertri > pical dorsal and vmlial carotids an- present m Amphibians and LUIILJ lishes. The arrange- ment in Urod.-le- i illustrated by I'V- ISO. I',; t he chief peculiarity . > that the po.Mrrior portinn ..I' the external carotid vi ARTERIAL SYSTEM 405 has disappeared in tin- ;i < I nil, tin • persisting anterior portion receiv- ing its lilninl tliruii^h a ne\v anastomofie channel (an) from the tliir.l aortic arch. The posterior portion of the "external carotid " or " lingual artery " of the adult Amphibian is really constituted hy this in -\v drvelopment. The connexion of this newly developed portion of vessel with arch HI is just at the point where the short circuit is formed between the afferent and efferent parts of the arch, and in Lung- fishes (Lepidosiren — Robertson, 1913) the blood -supply for the external carotid comes to it, for a time during early stages, from the do; sal cud of arch III or from the aortic root, through what seems to be a precocious development of this same short-circuiting channel. In this case the vessel in question is at first simply continued from its dorsal origin forwards into the external carotid : it is only later that it communicates with the ventral or afferent end of arch III so as on the one hand to form the short circuit, and on the other to permit the blood to pass to the external carotid from the ventral aorta. In the more typical fishes the ventral carotid is -not yet present. The dorsal carotids of the two sides develop an anastomotic con- nexion beneath the base of the skull so as to form with the aortic roots a complete " cephalic circle " which shows characteristic differ- ences in different Teleostean fishes (Ridewood, 1899). INTERSEGMENTAL ARTERIES OF THE BODY-WALL. — The dorsal aorta gives off on its dorsal side paired arteries which run out into the body wall between the myotomes. One of the most important features of this series of intersegniental vessels is that for a time during early stages of development they provide the blood-supply to the limb rudiments. The main artery of the fore-limb — the subclavian artery— a] >pears during the stages in question to be simply a prolongation into the limb rudiment from one of these inter segmental arteries — not necessarily the same artery of the series in different types of Vertebrate, or even in different developmental stages of the same Vertebrate. Thus in Lacerta it is said to be the seventh inter- segrnental artery (van Bennnelen, Hochs tetter, 1906) which becomes the subclavian artery and in the Fowl the fifteenth (eighteenth or ^nineteenth if the three or four intersegniental vessels in the head- region are included— Hochstetter, 1890). In the Duck, Rabl (1907) found that during the fifth day of incubation the subclavian varies from the eighteenth to the twenty-first intersegniental artery and that in some cases two or even three such vessels may pass out into the limb rudiment at one time. Probably we may take it that the general principle at work is t his —that the limb, as it became shifted along the side of the body in the course of evolution, received its blood-supply from successive inter- segmental arteries as it came to be opposite to them, and that during ontogeny there takes place an imperfect repetition of this process. The fact that the pectoral limb is supplied with blood by an 406 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. intersegmental artery raises the question whether or not this is to be regarded as the primitive mode of blood-supply. For if this question be answered in the affirmative we should be confronted with an important point which would have to be borne in mind in all specula- tions as to the evolutionary origin of the limbs of Vertebrates. As a matter of fact, however, recent investigations tend to answer this question in the negative. In the Chick (Evans, 1909) the limb rudiment in its earliest stages is traversed by an irregular network of blood spaces and this receives its blood-supply directly from the dorsal aorta by a number of slender channels — it may be as many as ten or eleven on the right side where they are commonly most numerous. These vessels are scattered irregularly over an antero-posterior extent of from three to five mesoderin segments and they take their origin from the dorsal aorta quite independently of and considerably ventral to the intersegmental arteries. As development proceeds a few of these supply channels — those which happen to be m6st nearly inter- segmental in position — become relatively larger and finally a single one, at about the level of the eighteenth intersegmental artery, becomes especially enlarged and carries the main stream of blood to the limb rudiment while the others gradually diminish in size and eventually disappear. The persisting enlarged vessel becomes the subclavian artery and secondarily its origin from the aorta becomes displaced in a dorsal direction until eventually it arises by a common root along with the intersegmental artery of which it now appears to form a branch. In the Duck similar observations have been made so we im- probably justified in stating that in the earliest stages of ontogenetic development the blood-supply of the pectoral limb is not metameric, and that the relation with the intersegmental artery observed in slightly later stages is a secondary acquirement. In most vertebrates the subclavian artery which arises in the manner above described (primary subclavian) persists throughout lifex. In Birds however a cross connexion develops between the primary subclavian just at the base of the limb, and the ventral end of the third aortic arch. This cross connexion gradually increases in size while the proximal part of the primary subclavian, arising from the aortic root, lire-nines correspondingly reduced and eventually disappears entirely. The result is that th<- permanent, artery of the foiv-liinh in the adult branches olf, imt from the dorsal aortic root hut from tli«- definitive (i.e. mo rplmlogically "internal") carotid, close in mder end. A similar substitution takes place in Chelonians and Crocodiles (Fig. 18?A, ( ') and this is tin- explanation. first -iv.-n hv Mackay (1889), of the otherwise pu//liiiL,r lad that in e.ertain Verte- brates the suhclavian artery passes mil. ventrally 1" tin- va-ns nerve ("Secondary suhelavian " instead of dnrsally as it. does normally. i iliac artery to tin- hind liml> has also heen braced hae.U iii at Li/ards and liirds I.. tin of mtei -M- mmtal arteries hut. vi ARTERIAL SYSTEM 407 In -iv again it would appear from later investigations that the dt'iinii i\<- artery is mm-ly a surviving and cnl-ir^ed representative of a number of original supply vessels (Evans, 1909). liVpivsniiaiivrs nf the series of intersegimmtal arteries are recogni/ahir in various Vertebrates in the hinder part of the head- region. Thus in Lacerta (van Bemmelen, Hochstetter, 1906) three have been detected in the head-region. Of these the first two dis- appear while the third becomes prolonged forwards immediately beneath the brain to become continuous with the internal carotid at the level of the mid-brain. These prolongations fuse together in the mill-line and form the basilar artery. A similar basilar artery continuous with the internal carotids is of common occurrence in Vertebrates though the relations of its paired fore-runner to the series of interseginental arteries differ in different forms. The vertebral artery of Sauropsida is in its origin intimately related to the intersegmental arteries. Thus in Lacerta the inter- seginental vessels posterior to the subclavian become connected together by a longitudinal anastomotic vessel, which persists and forms the cervical portion of the vertebral artery. In Snakes the two similarly -arising vessels apparently usually undergo fusion together in their anterior portion while farther back the unpaired condition is reached by the disappearance of the rudiment on the right side (Hochstetter, 1906). The arterial blood - supply of the urino - genital organs is also generally provided by branches from the intersegmental arteries of the embryo. MESENTERIC ARTERIES. — The digestive tract receives a varying number of branches from the dorsal aorta which may be at first paired and undergo fusion secondarily or may be unpaired from the beginning. In those vertebrates which have a bulky yolk-sac a pair of these are precociously developed as vitelline arteries. In some cases there is a remarkable relation between the chief mesenteric artery (coeliac) and the pronephros. Thus in Lepidosiren a connexion becomes established between the blood-spaces of the right pronephros and those of the gut wall, and the branch of the dorsal aorta which supplies the right pronephros persists as the root of the definitive coeliac artery. In such Teleostean fishes as the Trout the connexion with the pronephric arterial supply is only temporary, a new anastomotic channel arising farther back between dorsal aorta and meseuteric artery which remains as the definitive root of the latter vessel. VENOUS SYSTEM. — As an example of the development of the venous system in a holoblastic Vertebrate we will take that of Lepidosiren (Robertson, 1913).1 1 For comparison with /,.•/lk (stage 24). Anteriorly they become conjoined to form the hi -art while posteriorly they are continued into the rudiments of the vitelline network (stage 24-25). The appearance of the vitelline veins is followed almost immediately by the development of a longitudinal venous channel on each side anteriorly, superficial to the aortic arches —the anterior cardinal vein. At its hinder end the anterior cardinal is continued into a set of venous spaces in the region of the pronephros (pronephric sinus) and onwards behind this as the posterior cardinal vein. The pronephric sinus is continued along its outer edge, by a number of channels, into the vitelline network of venous spaces, lying in the splanchnic mesoderm over the surface of the yolk. In this network a conspicuous channel becomes apparent, leading from the anterior end of the pronephric sinus outwards' to the vitellmr vein, and so, by way of the anterior part of the vitelline vein, to the heart. This vessel so constituted, which at first makes a wide sweep over the lateral surface of the yolk, is the Duct of Cuvier (Fig. 190, b, d.C). As development goes on the Ducts of Cuvier become greatly shortened and at the same time widened until eventually they form in the adult very short wide channels for the conveyance of the blood from the cardinal veins into the sinus venosus (see Fig. 190, b and c and d). The posterior cardinal vein on each side appears about stage 24 in the form of spaces along the course of the archinephric duct which become joined up so as to form two longitudinal vessels running parallel to the duct, one on its mediodorsal the other on its ventrolateral side, the two vessels being joined round the duct by numerous anastomoses (Fig. 190, b and c, p.c.v). These posterior cardinal veins accompany the archinephric ducts throughout their length and just in front of the cloaca are joined by the hind ends of the bifurcated subintestinal vein (see below) and of the dorsal aorta. The vessel formed on each side by the union of these three elements is continued hack past the cloaca and unites with its fellow of the opposite side to form a vessel lying immediately beneath the post-anal gut. This vessel is to be interpreted morpho- logically as a post-aiiiil portion of the subintestinal vein and as \\ill be shown later it is destined to become the caudal vein of the adult. It will now be convenient to trace out the subsequent fate of what may be called tin- dorsal venous system, consisting primarily of tin- ;interior and posterior cardinal veins. POSTKKIOK CARDINAL VKINS. — The posterior cardinal vein was left in the forn; of a pair of vessels, an inner ami an outer, lying to the jirehinrphne duel and connected to-vtiier hy numerous ;Hi:i-1.om08es. As the opist I lom-phros develops between ilie.se channels i h«-y consequently come, ovei a considerable part of their length, to be CLjC.V v.V. pv.c. FIG. 190. — Development of venous system of L<-f><< /W/r//. (After Robertson, 1913.) A -!•'. vential jmrtion of venous system as viewrtl from tin* ventral side; b-e, dorsal ix)rtion as viewed from the dorsal side. A, staj:e -_M ; H, L'5 ; C, 30 ; I), :tt ; E, 85 ; F, young adult ; 1), 27-30 ; c, 30-31 ; d, 31+ ; e, youni,' adult. ".<•.,-. ant.-ridi- cardinal v»>in ; c« ' and ce", anterior and posterior cere- bial veins; osteri(ir vena cava ; ,-. rectal vein ; .-•, subclavian vein ; .«.i.r and .s-.i'.r", subintestinal vein; s. r. sinus veih'su> ; «p, sjilenic \ ein ; •. M'sti.uial front end of posterior cardinal ; r.r, vitelline veins ; * points at which vessels become discontinuous. 410 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. separated from one another by a considerable space in which the renal organ lies. As this happens the inner components of the two posterior cardinals become approximated and eventually undergo fusion with one another to form ,111 inter-renal vein (Fig. 190, d, ir.v). In Lepidosiren this fusion is only temporary and the two compon- ents again recede from one another — remaining, however, connected by a small number of anastomotic vessels (Fig. 190, e). There now takes place a severance of the continuity of the inner component at its hinder end (Fig. 190, e,*), and a little later a similar severance of the outer component at its front end. The physiological result of these interruptions of continuity is that the blood from the caudal region now reaches the opisthonephros entirely by way of the outer com- ponent, it then passes through the substance of the kidney and is drained away entirely by the inner component. In other words the outer component and its backward continuation has now become the renal portal vein. While these changes are going on in the opisthonephric region of the posterior cardinal the portion of that vein in front of the opistho- nephros takes on the form of a single channel, the original inner component becoming enlarged while the outer component becomes reduced and eventually disappears. The right and left posterior cardinal veins in this region in front of the opisthonephros become connected by numerous transverse vessels (Fig. 190, d). A short circuit now becomes established by which the blood from the right posterior cardinal can pass direct to the sinus venosus through the substance of the liver (Fig. 190, d, p.v.c\ This short-circuiting channel is of great morphological import- ance. It constitutes the intrahepatic or headward section of the posterior (" inferior ") vena cava, which in the higher vertebrates becomes the largest vein in the body. Its appearance here is followed by two important results. (1) The main blood-stream from the kidney region tends to pass to the heart, more and more by this direct channel, which in correlation with this becomes larger and larger. (2) The portion of right posterior cardinal vein lying behind its junction with the intrahepatic channel becomes correspondingly enlarged. These two components together constitute the definitive posterior vena cava, in which vessel therefore we recognize two fundamentally distinct pm-tinns, ;tn anterior or intrahepatic, and a posterior or cardinal. A secondary result of the diversion ol the Mood-stream from the right posterior cardinal vein through the hepatic com- ponent is that the anterior portion of the Cornier vein, Ivinu' in 1'ront of the junction of tin- t wo components, becomes relatively reduced in Size, It coniiniiii vvvvitli the ivst of the n-lit posterior cardinal ami pei the small vein shown at /• in Fi«j. 190,6. An important, due to tin- origin of the posterior vena eava in phyl< ii hy the eondition seen ill the adults of existing The opiMhoiH-phros in fchese vertebrates retains its vi VEINS OF LEPIDOSIREN 411 primitive elongated form, extending far forwards in the splanchno- coele, and tin- front end of the right opisthonephros is in immediate apposition to the tip of the liver which is also situated dorsally and on the right side. It is no doubt the approximation of the tips of these two organs, still persisting in the adult Dipnoan, which paved the way for the establishment of direct continuity betw«-«-n their vascular networks and the consequent short-circuiting of the renal blood through the hepatic vein into the heart (Graham Kerr, 1910). The ladder-like connexions between right and left posterior cardinals in the region anterior to the inter-renal vein gradually disappear in turn from before backwards (Fig. 190, d and e). ANTERIOR CARDINAL VEINS. — Apart from relatively less important details, the chief change which comes about in regard to the anterior cardinal is the diversion of the blood-stream about the level of the otocyst into a more laterally placed channel called the lateral cephalic vein, which eventually becomes intercalated in the course of the anterior cardinal, and to all appearance forms simply a portion of that vein (Fig. 190, d, l.c). The anterior cardinal vein at first passes back along the side of the head region (following the course shown by the dotted outline in Fig. 190, d), ventral to the otocyst and internal to the posterior cranial nerves, to join the front end of the pronephric sinus. Presently (stage 30) a branch of the anterior cardinal vein makes its appearance and extends backwards external to the ganglion of the seventh cranial nerve, bending inwards and rejoining the anterior cardinal between the eighth and ninth cranial nerves. A little later (stage 31) the vessel forming the outer side of this loop becomes prolonged back and forms a second loop external to the ninth cranial nerve and rejoining the main vessel between nerves IX and X. Finally (stage 31 + ) a similar extension backwards occurs external to the vagus, rejoining the anterior cardinal vein just behind it. The lateral cephalic vein develops from the outer portions of these three vascular loops, the development of each of the three segments being followed by the atrophy of the correspond- ing section of the original anterior cardinal, except in the case of the most posterior section which persists as a short wide vein opening along with the posterior cerebral vein (ce") into the definitive anterior cardinal. VENTRAL VENOUS SYSTEM. — In addition to the dorsally placed cardinal veins there exist certain important veins situated more ventrally and developed in relation with the vitelline veins. The vitelliiit' veins spread backwards on each side as a wide vessel con- sisting of an enlarged channel of the vitelline network which covers the whole surface of the yolk. Posteriorly they unite in the mid- ventral line to form a very short subintestinal vein in front of the anus (Fig. 190, C, s.i.v). Later on the space bounded by the two vitelline veins becomes bisected by a median ventral vein which looks like a prolongation forwards of the subintestinal vein and 412 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. is known by the same name (Fig. 190, D, s.i.v"). It will be realized that this anterior section of the subintestinal vein is developmeutally of a different nature from the posterior portion for it is formed l>y a short-circuiting of the blood-stream through the vitelline network, while the posterior portion represents rather the conjoined hinder ends of the paired vitelline veins (Fig. 190, C and D). This differ- ence in development is no doubt purely secondary and we may take it that the later condition, where the subintestinal vein is continuous right forwards to the heart, represents the really primitive condition of this vein in evolution. The continuity of the subintestinal vein at its front end with the heart is brought about in ontogeny through blood-sinuses which make their appearance in the liver (stage 31). The right vitelline vein and the anterior, secondarily formed, portion of the subintestinal vein now gradually disappear (about stages 32 - 35). The left vitelline vein ceases to form a continuous channel over the surface of the liver to the heart, so that the blood in it is forced to traverse the system of blood sinuses within the liver. In other words the whole of the blood which streams forwards in the subintestinal vein is diverted along the persisting left vitelline vein into the network of blood spaces in the liver. Subintestinal vein and left vitelline vein have thus come to constitute the hepatic portal vein. The latter becomes complicated by a branch sprouting out from the front end of its subintestinal portion. This branch s] .reads round the alimentary canal along the line of the spiral valve, fusing with the subintestinal vein at each point of intersection (Fig. 190, F). As the liver increases in length a special supply channel from the portal vein lengthens out along its left side, giving off numerous branches into the liver substance (Fig. 190, F). From this the blood drains by numerous efferent vessels into the intra- hepatic portion of the posterior vena cava. CAUDAL VEIN. — The post-anal portion of the subintestiual vein was let! (p. 408) at a stage when its anterior bifurcated portion w,-is continuous on each side not only with the pre-anal portion of the same vein hut also with the dorsal aorta and the posterior cardinal. As development goes on the first two of these connexions disappear so that the post-anal Bubintestinal vein is now continuous anteriorly only with the posterior cardinal. With the atrophy of the post -anal gut, it comes to lie immediately beneath the caudal portion of the dorsal aorta and is now known as the caudal vein, its anterior forked portion forming the hinder ends of the renal portals. The, veins which hav.- been described constitute the main trunks of the venous system : in the later stages of development a number of other import a n! vessels appear which are indicated in the diagrams. The subclavian vein (Fi^. 190, d, s) appears about, si age 31+ le;i(liu;j IVn)n the p.M-|o|,|| IJmh i II t o 1 lie proiiepliric SJllllS. As this sinus atrophies 1 lie point, of (.pen in- <.f t he si i be la via 1 1 \ein mines lated mi the anterior cardinal \<-in Fiu. MM), e, .s). From ii \viii .1 small lateral cutaneous vein (/./•) passes back VI VENOUS SYSTKM 413 in the body-wall. An inferior jugular vein (i.j) passes back from tin- head-region, lateral to the pericardiac cavity, and opens into the anterior cardinal close to its hind end and on its ventral side. In lain- stages the point <>f junction of the inferior jugular with the anterior cardinal comes to be shifted relatively forwards, as was the case with the subclavian. Farther forwards the anterior cardinal is joined j.y an anterior and a posterior cerebral vein (cef and ce") from a.c.u. uu y.s.u. p.c.v. an. v.v. y.s.u. B Fi<;. 191. — Diagrams illustrating early stages in the development of the venous system of Elasmobranchs according to Rabl (1892) and Hochstetter (1906). ii.f.r. anterior cardinal vein ; UH, fused portion of vitellinc veins behind liver ; cl, position of cloaca ; '/.'', duct of Cuvier; A.r, hepatic MMII; /<.<.,-. posterior cardinal vein; s.t.r, snbintcstinal vein; c.r. vitelline vein : t/.*.i\ main vein from yolk sac. the inside of the head. At the hind end of the system rectal (r) and pelvic (j)l) veins open into the renal portal. The former appear to be the persistent remains of the anastomotic brandies which in early stages connected the hind end of the subintestinal vein with the posterior cardinal. ELASMOBRANCHII. — It is instructive to compare with the develop- ment of the venous system in a holoblastic vertebrate the correspond- ing phenomena as they occur in the Elasmobranchs, the lowest of the meroblastic gnathostomes. An inspection of Fig. 191 brings out the most conspicuous difference, one that could be foretold a priori, 414 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. namely that, in agreement with the general principle that animals with a large supply of yolk tend to show a precocious development of the vessels on the surface of the yolk, the vitelline veins and their tin- liver entirely through I he ri^ht vitelline vein — in contrast with Lepidosiren where it did so by the left vitelline vein. Cardinal veins and duet <.i Cuvicr make their appearance, the chief dilT'-ivncr trom Lepidosiren being the comparative slmnm'SB of vi VKNOUS SYSTEM 415 the duct of Cuvier whic-h does not take the wide sweep over the surface of the yolk that it does in Lepidosiren during early stages. This dirieivnr.e is related to tin- fact that here the first rudiment of the duct of Cuvier opens into a portion of the vitelline vein which has fused with its fellow to form the hind end of the cardiac tube, while in Lepidosiren it opens much farther back, the result being that in Lepido- siren a considerable stretch of free vitelline vein becomes incorpora t<-d in the definitive duct of Cuvier (compare Figs. 191, C and 190, b). As in Lepidosiren the caudal vein becomes continuous with the posterior cardinals and loses its continuity with the pre-anal portion of the subintestinal vein. The inter-renal vein however develops In- iv simply as a forward extension of the caudal vein according to liabl. A number of anastomotic vessels connect up the inter-renal vein with the " posterior cardinal " — the equivalent of the external component of this vein in Lepidosiren. The posterior cardinal now becomes obliterated behind the anterior one of these anastomotic vessels while the inter-renal becomes separated off from the caudal vein so that the whole blood-stream from the latter has to pass through the kidneys to reach the inter-renal. The latter vein splits into a pair of vessels eventually, thereby revealing that the inter-renal vein here is homologous with that of Lepidosiren in spite of its different — no doubt secondarily modified — mode of development. The anterior cardinal vein here again becomes in part replaced by a lateral cephalic vein. In the lower vertebrates in general we may recognize the same main trunks as occur in Dipnoi and Elasmobranchs, with differences in detail. The following account gives an outline sketch of the development of the venous system in the various groups, the outline being filled in more fully in the case of Polypteriis on account of the very archaic character of this fish. CYCLOSTOMATA. — In the Lamprey, according to Goette (1890), the pair of vitelline veins appear first, spreading backwards on either side of the liver rudiment and meeting behind in the unpaired and much dilated subintestinal vein (Fig. 192, A). The vitelline veins break up into a network in the liver but on the left side the post- hepatic section of vitelline vein disappears, so that the hepatic portal vein is formed by the subintestinal and right vitelline vein (Fig. 192, B and C) — somewhat as in the Elasmobranch, and unlike Lepidosiren where it is the right vein which disappears. The vein of the " spiral valve " of the intestine arises comparatively late, at the time of metamorphosis according to Goette, and on the opposite side of the gut 1'rom that on which the subintestinal vein lies. This latter is no longer ventral but high up on the right side, owing to a rotation which the gut has undergone. The anterior and posterior cardinal veins present the peculiarity that they open at first separately into the vitelline veins. Later they become fused together to form the duct of Cuvier. Eventually the 416 EMBRYOLOGY OF THE LOWER VERTEBRATES < 11. left duct of Cnvier disappears completely (Fig. 192, C) the blood from the left cardinals passing to the right side by a new anastomotic vessel which develops ventral to the dorsal aorta (Fig. 192, C, an) — an arrangement presenting a remarkable analogy with what happens in certain Mammals. CROSSOPTERYGII. — In these archaic Teleostomes the main features of the development of the venous system have been investigated in Polypterus — the less specialized of the two surviving genera (Graham Kerr, 1907). In the earliest stage described there is a well-developed sub- intestinal vein which in front breaks up into a vitelline network. o.c.u. B. p.c.u r.v.u ....{ an a.c.v. S.I.U. s.i.is. FIG. 192. — Development of veins in Petromyzon as seen from the ventral side. ( After Goette, 1890.) anterior cardinal vein; (in, anastomotic vein ; osl -eloacal portion becoin*-- Converted into tin- caudal vein in the normal fashion as already dt-scril't-d I'nr /., /n'(/n the appearance of a direct forward prolongation of tin- caudal vein. Dorsally there develops on each side a cardinal trunk which swells out into a great irregular sinus (pri) in the region <»f the pi'oiirphros. On its outer side branches pass from the pronephric sinus into the in-neral vi tell i ne network. In this network a specially wide channel develops, starting from the pronephric sinus and sweeping outwards over the yolk to join the lateral vitelline vein and so reach the heart. This channel, which becomes gradually more and more sharply defined, is the duct of Ouvier (Fig. 193, A, d.C). The pronephric sinus is continued backwards into the posterior cardinal vein. This is at first distinct from its fellow but at an early stage fuses with it to form a median inter-renal vein (Fig. 193, B, ir) — the fusion being foreshadowed by the develop- ment of anastomotic connexions between the two veins while still separated from one another by a distinct space (Fig. 193, A, an). Toward^ the cloaca the posterior cardinals taper off and are con- nected by irregular anastomotic channels with the subintestiual vein, as already mentioned, and also with the dorsal aorta. Eventu- ally, as already indicated, the inter-renal vein and the caudal vein form a continuous vessel. During the later stages a striking asymmetry becomes apparent in the anterior, unfused, portions of the posterior cardinals — the left becoming greatly reduced as compared with the right (Fig. 193, D). The main blood-stream thus passes forwards on the right side, and upon this side a special direct channel 'develops on the ventral side of the pronephros, through which the blood-stream is able to reach the duct of Cuvier without passing through the tangle of pronephric tubules. The asymmetry affects also the ducts of Cuvier — showing itself first in that of the left side becoming relatively shorter than its fellow, which retains for a time its wide sweep over the surface of the yolk (Fig. 193, B and C, d.C). Eventually it too becomes shortened and its calibre becomes cousiderably greater than that of the left side (Fig. 193, D). From the pronephric sinus a branch (l.v) develops about stage 30 which passes dorsal wards and then backwards beneath the lateral line nerve — the lateral cutaneous vein. This, a large vessel about stage 33, becomes reduced to an insignificant vestige later on. The anterior cardinal vein runs along the side of the head region, passing through the angle on the ventromesial side of the otocyst, between the latter and the brain-wall. At its front end the vein dilates into a large sinus, which gives off irregular branches to the mesoderm of the head. At an early stage an anastomotic channel makes its appearance on the outer side of the otocyst continuous anteriorly and posteriorly with the anterior cardinal. When this channel lias been established (Fig. 193, A, l.c) the blood-stream from the head divides in front of the otocyst and passes backwards, part VOL. II 2 E a.c.tr a.c.u D . |»,-v.-lo|.iin-iit nf .Ini-iil vii. .us syst.-lil iif /', the i -;<: v a> -mi from (0); :!U linn, l.-t i \. -i '1 i . :ii.lin:il \rin ; - ; ; //.c. ln'jalir vrin ; /«.(/ •. .. mid \«r ju^ul;ir ir, Intwr-n-iial \i-in . I • ••plmlu- : /./•. l:iti-r;il i-iitain-im^ ; /-, ] •iilinnn:ii \ \ l ; pit, |.I.,M.-|^ I 11 i:m \.-in ; /A. th\n. -I. II J CH. vi VEINS OF POLYPTERUS 419 on its outer and part on its inner side. Eventually tin- inm-i channel becomes constricted across and finally completely severed at its liindiT end so that the whole blood-stream passes back by what was the outer channel. The inner channel persists as a small vein which opens at its front end into the definitive anterior cardinal. We see then that here, as in the Lung-fish or the Elasmobranch, the " anterior cardinal " vein of later stages has intercalated in its length a segment of lateral cephalic vein. An inferior jugular vein on each side (i.j) drains the blood from the ventral side of the head into the duct of Cuvier, and as develop- ment goes on these become asymmetrical, the left becoming greatly reduced while the right forms a large vessel, trifid at its front end where it receives the blood from the thyroid (Fig. 193, D, Th). The liver rudiment, at first a portion of the general yolk-mass, is at this stage supplied with blood by the portion of the general vitelline network which extends over it. The portal vein develops simply as an enlarged channel of the network on the left side of the hepatic rudiment. There is no information as to whether this is really the persisting left lateral vitelline vein as we might expect : nor is it known whether the hepatic vein is derived from the pre- hepatic portion of this same vein. An important feature is that there becomes established an anastomosis between the blood-vessels of the liver at the hinder end of that organ and the inter-renal vein, so that a portion of the inter- renal blood - stream becomes short - circuited direct to the heart through the hepatic vein, which in correlation with this forms a wide channel throughout the length of the liver (Fig. 193, E, h.ty.v.c')). This enlarged hepatic vein is of morphological importance as it is clearly the equivalent of a posterior (or " inferior ") vena cava. In the Crossopterygian this vessel takes a step in evolution beyond the condition in Lung-fishes, inasmuch as its posterior portion becomes denuded of liver substance, so that it runs for a considerable distance through the splanclmocoele as a naked vessel. Another important vein which makes its first appearance in the Crossopterygian is the pulmonary vein. The hepatic vein, which towards its headward end lies on the dorsal side of the liver, comes into close contact with the pharyngeal floor in the region of the glottis, and venous spaces developing in the mesoderm sheath of the pharynx come to open into it. These are apparently the forerunners of the pulmonary vein. In the 30-mm. larva, as in the adult, there is a main pulmonary vein which still opens into the hepatic vein on its dorsal side (Fig. 193, D, p). In addition to this main pulmonary .vein, formed by the fusion of two branches coming from the ventral side of the two lungs, there is a small accessory vein coming from the dorsal side of the root of each lung and from the adjoining parts of the pharyngeal wall. These also open into the hepatic vein just in front, and on each side, of the opening of the main vein. TELEOSTEI. — Amongst the Teleostean fishes we find the same 420 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. venous trunks laid down as in the Lung-fish or Elasmobranch. There is within tin- ^roup considerable variability but the variations are as a rule derivable from a primitive type like that of Lepidosiren. Thus in Salmo the subintestinal vein bifurcates in front into the two paired vitelline veins while in numerous other Teleosts (Esox, Belone, Syngnathus, Hippocampus, Gobius) it passes forwards into a median unpaired vitelline vein. Each of these conditions is obviously derivable from that illustrated by Lepidosiren, by the disappearance, on the one hand, of the median, and, on the other, of the paired vitelline veins. AMPHIBIA. — In Salamandra (Choronshitzky, 1900) two lateral vitelline veins are described, the right comparatively small in size. Behind the liver rudiment they lie close together near the mill- ventral line and passing forwards they diverge, passing one on each side of the liver rudiment to unite in front of it and form the hind end of the heart. The two veins undergo fusion behind the liver to form the subintestinal vein and in front of the point of fusion the right vein disappears so that, as in the Lung-fish, all the blood passes to the heart round the left side of the liver. The mesenteric vein develops as a branch of the right vitelline vein close to its front end and after the disappearance of the greater part of the right vitelline vein the mesenteric is seen replacing it as the right limb of a horseshoe-shaped arrangement of veins which embraces the tliver rudiment from in front. The portion of the vitelline veins in front of the mesenteric breaks up into the hepatic network. The vitelline vein shrinks to an inconspicuous vestige while the mesenteric becomes relatively large and forms the hepatic portal of the adult. The posterior cardinal veins (Hochstetter, 1888) run alongside the archinephric ducts, which they more or less surround, to the region of the pronephros where each dilates to form a large pronephric sinus. In the opisthoiiephric region the vein forms two main channels an inner and an outer (Fig. 194, B, op), the former eventually undergoing fusion with its fellow to form an inter-renal vein which becomes later the renal portion of the posterior vena cava. Tin- outer channel, as in the Lung-fish, becomes continuous with the caudal vein to form the renal portal, while at the front, end of the opisthoni'phrus it l'>ses its connexion witli the part of the vein lyinir t'art her forwards. A venous connexion is established between l'|,e t'roni ,.j,(l nt'thr inter-renal vein and the tip of the liver, and the chanin-1 which so arises, Commencing behind in the inter-renal \ein, Bring the substance ..f the li\vr and ending in the hepatic vein, fnrm< the pnslerior 76X18 Cava (Fig. 194, C, p.v.c). In later stages thi- liver n-iie di-appe.-ir- over the greater part «»r the posterior vena cava 80 that it 'ked thnui^li the spl;mehnoeoele. The anterior cardinal vein with its inten alated seetion of lateral ,•<•],!, the internal ju-jul.-ir vein of the adult. The iJurl ,,| ('uvi.-r IfltS in tlie adult and is m.\v termed the anterior vena cava. VI VENOUS SYSTEM 421 Tin- anterior abdominal vein arises as ;i pair of sin;ill veins in the ventral body-wall. These unite in trout in the region of the liver and open into the left duct of Cuvier according to Hochstetter. Later the two veins fuse into a single unpaired vessel except at their hinder ends where they become connected with the renal portal vein on each side. Anteriorly the opening into the duct *of Cuvier becomes replaced by an opening into the hepatic portal vein. sv. a c.i'. — P.IT.C cv. r.p. B Fia. 194.— Development of main venous trunks in &il, caudal vein ; l ('uvii'i : that i he wln»l«; 1 ilood -stream j. asses forwards tn tin- level of the dorsal anastoiiHJHM l.y 1li«- p.-i-sist-enl lel't, \.-in ( Ki.ir. l(.»r», I), l.r.v). lu the region anterior 1<> tin- d«»rs;il anastiuin»siK tin- 1«T|. \«-in now diniinislics in i/c and finally disappears, iirst in the region behind tin- hepatic vi VEINS OF LACERTA 423 network (Fig. 1.9;"), I)) and then in the region in !n»nt nf th,- n»-i \\nrk ( l-'i-. 195, E). Results of these changes are (1) that the hepatic network receives its blood supply by a single afferent vessel — the hepatic portal vein — which curves round the gut and is derived in great part from the left vitelliue vein — and (2) that its blood drains away to the heart by a single efferent vessel — the hepatic vein- derived from the front end of the original right vitelline vein. At a comparatively early stage in development a direct channel becomes established, by the widening out of the venous spaces almi.^ the middle of the hepatic network, so that a considerable proportion of the blood is able to pass forwards from the portal vein through the liver without actually traversing the network itself. This channel — the ductus venosus (Fig. 195, G, d.v) persists till nearly the period of hatching but then becomes obliterated so that all the portal blood has to traverse the hepatic network. The posterior vena cava makes its appearance as a gradually widening channel through the hepatic network towards its right side (Fig. 195, D, E, p.v.c). This portion of the liver becomes prolonged backwards as a slender lobe ensheathing a prolongation of the blood channel mentioned. This prolongation fuses at its tip with the tip of the right opisthonephros, continuity becomes established between the venous spaces of the two organs and finally, as in the Amphibian, the liver tissue disappears over a large stretch of the slender lobe already mentioned so that the vena cava is now for a considerable length free from either liver or kidney. At an early stage a branch of the vitelline vein develops close to its front end. This is the allantoic or umbilical vein (Fig. 195, r.all and l.all). These veins soon become asymmetrical, the left for a time being smaller than the right (Fig. 195, D, E). A little later however the left vein establishes a connexion with the hepatic network (Fig. 195, F); the portion of the vein posterior to this connexion becomes much widened, and the blood-stream from it courses by an enlarged direct channel of the network into the posterior vena cava (Fig. 195, F, G). The blood-stream being diverted through this channel, the portion of the left allantoic vein in front of it shrinks in size and disappears, as does the whole of the right allantoic vein (Fig. 195, F, G). The result is that there persists a single (left) allantoic vein which drains the blood from the allantois into the posterior vena cava near its front end. The allantoic vein - increases in size with the allantois but becomes obliterated at the time of hatching when the allantois is cast otT. The mesenteric vein develops as a branch of the portal vein (left vitelline vein) a short distance behind its entry into the liver: it increases in size as the vitelline diminishes with the consumption of the yolk and eventu- ally it alone persists as the peripheral portion of the definitive portal vein of the adult. The subintestinal vein is apparently present only in its post-anal portion which persists as the caudal vein of the adult. In front 424 EMBRYOLOGY OF THK LoWKII VERTEBRATES CR of the anus, where the ventral wall of the primitive alimentary canal has become intensely modified in connexion with the storage of yolk, the subintestinal vein has disappeared from the course of development. As regards the dorsal venous system (Fig. 196), the two posterior cardinal veins converge posteriorly and become continuous with the caudal vein. The portions in the region of the opisthonephros become resolved into their external and internal components connected by numerous sinus -like spaces and channels amongst the kidney tubules (Fig. 196, A). With the development of a capillary network a.c.u ac.v: C l'i'.. 196. — Diagram illustrating the development of the dorsal venous system in l.nc<'iin according to Hochstetter, as seen from the ventral M<1<-. a.c.v, anterior cardinal vein ; c, caudal vein; d.C, duct of Cuvier( = ant. vrna cava) : //, iliac vein ; p.c.v, posterior cardinal vein ; /,.<-.<•, posterior vena cava ; (, siibrlavian vein. in the substance of the opisthonephros the larger blood spares become divided into an afferent set connected with the external com- ponent and an efferent set connected with the internal one. The external channel remains continuous with the caudal vein and forms the renal portal vein. The two internal components fuse together in their {interior portion (Fig. 11)0, B) and become continuous \\ith t.he intrahepatie port KM, ol' tin- vena cava. 1'ostrriorly they remain separate and loae their continuity with ttfe eaudal vein (Fig. I!'*;. B,0) Tin- iilood j'rniu the kidneys bring now able t<» pass to the heart by the direct route through the posterior vena ea\a. the portions of posterior (ordinal lying in front of tin- kidneys are no longer n-«juire(l and soon disappear (Fig. 196, C). vi VENOUS SYSTEM 425 In the head legion the anterior cardinal becomes in great part by ji lateral cephalic vein in a ma nun similar to that already described lor Lepidosiren. I'.iUDS. — In Birds the development of the venous system pursues, as we should expect, a similar course to that already described for Reptiles. Amongst the differences in detail the most striking is that the two vitelline veins become completely fused into a single vessel, the ductus venosus, through the hepatic region, before there are any signs of a hepatic network. This may be regarded as a backward extension of the fusion of the two vitelline veins which gives rise to the heart. The ductus venosus secondarily becomes surrounded by the liver rudiment and a network of channels spreads out from it in the liver substance. The allantoic veins behave as in Lacerta except that a small vestige of the left is said to persist throughout life. The reduction of the tail region in modern birds has brought with it a modification of the caudal vein which is here paired, taking the form of a simple backward prolongation of the posterior cardinal. The main channel of the posterior cardinal runs along the outer edge of the opisthonephros but later on a slender channel appears along its inner edge — that on the right side being continuous with the posterior vena cava of which it forms simply a backward prolonga- tion. The two inner channels undergo fusion so that the blood from the kidneys can drain away entirely into the posterior vena cava and this is followed as in other cases by the atrophy of the portion of posterior cardinal lying in front of the opisthonephros. This atrophy extends as far forwards as the subclavian vein which in the Fowl opens into the posterior cardinal vein some distance from its front end. The portion of posterior cardinal lying in front of this point is consequently saved from disappearance and persists as a portion of the definitive subclavian vein of the adult. It will be understood that the blood of the outer channel of the posterior cardinal, which reaches it from the caudal vein, from the posterior limb, and from the body -wall, passes entirely through the opistho- nephric network towards the posterior vena cava, in other words that there is at this time a typical renal portal system. As the metanephros develops, its tubules are also mixed up with the sinuses connecting external and internal channels of the posterior cardinals, so that it too has for a time a functional renal portal system. Later on however one of the channels through the metanephros becomes enlarged and the blood-stream passes directly through it to the posterior vena cava without traversing the meshes of the network. A true renal portal system then no longer exists and the reason for its disappearance is no doubt to be found in the fact that the vascular network of the kidney has become connected with the arterial system. Obviously this will give a much more efficient circulation than tin- original one, owing to the higher blood pressure in the dorsal aorta and renal arteries than in the renal portal veins which have the 426 EMBKYOLOGY OF THE LOWEE YERTEBKATES en. systemic network of capillaries interposed between them and the heart. Further the quality of the blood supplied in this way to the kidney is better — being arterial instead of venous — and 'for both these reasons we can readily understand the tendency in the more highly developed vertebrates for the renal portal system to disappear. It has already been remarked that the posterior cardinals do not pass back into an unpaired caudal vein as in the Lizard. A vestige however of the unpaired condition may perhaps be recognized in the development of an anastomosis between the two vessels just behind the metanephros. From the transverse bridge so formed a connexion (coccygeo-mesenteric vein) is established with the portal vein in the mesentery. The anterior cardinal vein together with an intercalated portion « »f lateral cephalic persists as the jugular vein of the adult bird. The posterior cardinal vein undergoes in the Bird a curious change of position in relation to the root of the iliac artery which it crosses behind the uiesonephros. At first it lies on the ventral side of this artery : then it develops an accessory channel round the dorsal side of the vessel and finally the whole blood-stream passes by this dorsal channel while the ventral one disappears. This affords a good example of the way in which a vein may in the course of evolu- tion pass an apparent barrier formed by an artery, nerve or other organ. INTER - SEGMENTAL VEINS. — In the body wall there develops a series of veins corresponding with the intersegmental arteries and opening into the cardinal veins. YEINS OF LIMBS. — The vascular network of the limb-bud drains into the posterior cardinal vein. In the Bird (Evans, 1909) the drainage during its earliest stages is into the allantoic vein. Later numerous channels arise connecting the network with the posterior cardinal and presumably one or two of these become enlarged and persist as the definitive veins draining the hind limb. LYMPHATIC SYSTEM. — The venous system has its obvious roots peripherally in the capillary network of blood-vessels, but it is also provided with a much less conspicuous set of tributary channels which constitute the lymphatic system. This extension of the vascular system ivtainsa lower grade of evolution than the remainder, Its channels are less sharply defined, the lining endot helium over moil "I its • \tent having a much feebler development of the backing of connective tissue and muscle which forms the thick wall of the vein or artery, In its peripheral portions the lymphatic spares may have remained practically in the primitive condition of intercellular chinks of the nn-.-ncliyine, while in its central poll ions, as it approaches the points at which it opens into the oidmary \cin>. its \\.ill~ may !•«• well de\ eloped and muscular. The lymphatic system to drain oil' tin- plasma which has oo/ed nut IVoiu the capillary hlood-vessds and forms the internal medium bathing the surface of the living cell* «.f the l.ody, and to return il in the hlood-M ream. vi VASCULAE SYSTEM 427 The fluid is prop led by amoeboid corpuscles but is \\ithnut the red corpuscles which have no power to escape through tin- walls «it' tin- blood-vessels. Our present knowledge of the ontogeny of the lymphatic system is in trivat, part due to the labours of Huntington and McClure whose papers should be consulted as regards details. It seems clear that as a general rule lymphatic channels develop, later than the blood - vessels, as intercellular chinks in the niesenchyme which become continuous and form definite channels, the bounding cells becoming converted into thin endothelium. SPLEEN. — The spleen arises in Lepidosiren and Protopterus (Bryce, 1905 ; Purser, 1917), which may be taken as typical examples, in the form of a condensation of the mesenchyme of the gut- wall. Blood spaces soon make their appearance in the rudiment which later becomes intercalated in the course of the main venous channel leading from intestine to liver. Later on the main blood-stream passes to the liver by a direct channel, the spleen now lying on a lateral loop : later still the afferent part of this loop becomes replaced functionally by a new arterial connexion. The spleen rudiment frequently arises in close proximity to that of the pancreas and this has led to statements that the spleen is actually derived from the pancreas but the probability seems to be that such statements are based upon erroneous observation. LITERATURE Allis. Zool. Jalirb. (Anat.), xiv, 1900. Bemmelen, van. Zool. Anzeiger, ix, 1886. Bemmelen, van. Meded. tot de dierkuude. Amsterdam, 1887. Boas. Morph. Jahrb., vii, 1882. Bryce. Trans. Roy. Soc. Edin., xli, 1905. Choronshitzky. Anat. Hefte ( Arb. ), xiii, 1900. Dohrn. Mitt. Zool. Stat. Neapel, vii, 1886. Dohrn. Mitt. Zool. Stat. Neapel, viii, 1888. Evans. Aincr. Journ. Anat., ix, 1909. Gegenbaur. Jenaischer Zeitschrift, ii, 1866. Goette. Abhandlungen zur Entwickelungsgeschichte der Tiere, v. Hamburg and Leipzig, 1890. Greil. Morpli. Jahrb., xxxi, 1903. Hochstetter. Morph. Jahrb., xiii, 1888. Hochstetter. Morpli. Jahrb., xvi, 1890. Hochstetter. Morph. Jahrb., xix, 1892. Hochstetter. Hertwigs Handbuch der Entwicklungslehre, iii, 1906. Hochstetter. Voeltzkows Wissenschaftliche Ergebnisse, iv. Stuttgart, 1906*. Hoffmann. Morph. Jahrb., xx, 1893. Hoyer. Hull, intern. Acad. Sci. Cracovie. Comptrs ivndus lies Seances, 1900. Huntington and others. Anat. Record, ii, 1908. Huntington. Anat. Record, iv, 1910. Kellicott. Memoirs N.Y. Acad. Sci., ii, 1905. Kerr, Graham. The work of J. S. Budgett. Cambridge, 1907. Kerr, Graham. Proc. Roy. Phys. Soc. Edin., xvii, 1907*. Kerr, Graham. Proc. Roy. Phys. Soc. Edin., xviii. 1910. Langer. Morpli. Jahrb., xxi, 1894. Mackay. Phil. Trans. Roy. Soc., B, clxxix, 1889. KMP.lIYnl.ocy OF TIIK Lo\YKK VEKTEBRATES m. vi McClure. An.it. l{«v..nl. ix. 1915. Maurer. Morj.h. .lalnl>., xiv. 1888. Mollier. Hcrt\\ i^s Handlim-h d.-r Entwicklungslehre, i, 1906. Miiller. Abhand. Akad. Wi». li. rlin. 1845. Miiller, F. W. Arcli. niikr. Anat., xlix, 1897. O'Donoghue. Pnu . Zool. Soc. Loud., 1912. Purser, (^uart. .lourn. Micr. Sri.. Ixii. 1917. Rabl, C. Lt-iu-karts Frstsi-hrift. Lci).xi^. 1892. Rabl, H. Arch, niikr. Anat., Ixix, 1907. Ridewood. Pioo. /ool. Soc. Loud., 1899. Robertson. Ouart. .lourn. Micr. Sci.. lix. 1913. Rose. .Mtn-])h. .I.ilirl... xvi, 1890. Ruckert. Biol. Centralblatt, viii, 1888. Riickert. Hcrt \vi^s Handlmch dcr Kntwicklungslelire, i. 1906. CHAPTER VII THE EXTEKNAL FEATUEES OF THE BODY THE pRvrding chapters have dealt with the ontogenetic evolution of the various organ systems of the vertebrate body. The present chapter will sketch in outline the development of the external characteristics in so far as these have not already been referred to. (1) DEVELOPMENT OF GENERAL FORM.— The groups of vertebr in which the egg is typically holdblastic will be considered first. CROSSOPTERYGII. — Of the two surviving genera Polyptcrus alone has been studied (Graham Kerr, 1907) and the main features in the development of its body- form may be gathered from an inspection of Fig. 197. It will be seen that the head-end of the embryo is the first to project freely above the general surface of the body (Fig. 197, A). Tin1 tail projection soon however makes its appearance (Fig. 197, B) and during subsequent stages grows much more actively in length, the embryo soon assuming a somewhat tadpole-like shape, with a laterally compressed hinder region and a rounded swollen anterior region formed by the main part of the yolk-laden egg. Two organs, the cement-organ (c.o) and the external gill (e.g}, make their appear- ance as slight bulgings of the surface at a very early stage. During subsequent stages the hinder, laterally compressed region grows rapidly at the expense of the mass of yolk which becomes con- sequently mlueed in volume and at the same time loses its spherical shape s > that it projects less prominently. It will be noticed that during the later stages the post -anal region grows particularly actively, the anus thus coming to lie at a relatively greater and greater distance from the hinder end of the body and giving rise to a rapidly growing true "tail" region. During the later stages (Fig. 197, D, E, F) the head undergoes much increase in length, its a< forward growth beginning about stage 31. The mouth is at first widely gaping (Fig. 197, E) hut- by stage 34 the mouth-hinge becomes functional and it can i>e rinsed. By stage 36 the anterior swelling due to the yolk has ]>ractirally disappeared. ACTixoPTERVC.il. — Amongst the Actinopterygian fishes the Ganoids which still retain the holoblastic segmentation show very 429 430 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. T c.d B Kio. 19^. — Stages in tin- , operculuin ; p.f, pectoral tin ; /•/../; pelvic fin. the intestinal portion of the alimentary canal rudiment is relatively slender in form, arising by a process of actual backgrowth of the posterior trunk re-i<>n rather than by gradual modelling of the yolk as is the thick intestinal rudiment in the Lung-fish. Conspicuous characteristics of the actinopterygian Ganoid larvae are the presence of well-developed cement-organs and the absence of external gills. In the Teleostean fishes there has come about with the high development <>f teloleeithality a great reduction in the angular extent of the embryonic rudiment during its early stages. Consequently there is very slight ventral curvature round the yolk. In the 199, QywnarchM < A alt.-r Assla-l-.n.) i; li-nlli i|:i\ B iiiiKini\\ u ; I!. I'll t \ -I h CH.vu TIIK EXTERNAL FKA'ITIMvS OF THE BODY 433 (lanoids Amiii ;iii(l Li-/n'i/ux/<'tix the main mass of volk retain form for a considerable period, causing a gjeal liiil-in- of tin- \eiitral body-wall anteriorly. In the Teleost this is .still further accent- uated, the bulging forming (he yolk-sac which remains -prominent even in larvae sufficiently developed to he al>le to swim acti\el\ . An extreme case of the prominence of the yolk-sac is afforded by (,'i/ininii'i-Jmx (Fig. liMJ) where it shows a peculiarly elongate* 1 I'orm tor a certain j.eriod. Cement organs are, as a rule, absent in Teleostei : so also are external gills though in rare cases the latter have physio- logical representatives in filamentous prolongations of the -ill lamellae ( Fig. 1!)!). 1'. . Great \ariety of form exists amongst the larvae of Teleostean fishes, more especially amongst those of pelagic hahit. Familiar examples are seen in the pelagic larvae of. the lv-ls — much com- pressed from side to side, transparent and colourless — even the blood being tree from haemoglobin — and much greater in hulk than the immediately succeeding phase in the, life-history. The larvae of the Flat-fishes (Pleuronectidae — Flounder, Plaice, Sole, etc.) are again of special interest owing to the extraordinary asymmetry which they develop. They are at first quite symmetrical and in no way abnormal. The larva swims at this time with its laterally compressed body vertical alter the manner of a Bream but later develops the habit of swimming on its side. The side of the head-region which is below now grows more actively than the other so that the head becomes strongly asymmetrical and the eye of the lower side becomes gradu- ally transferred to the upper, the right and left eyes being now both on the same side of the head. Correlated with this asymmetry in form there comes about a corresponding asymmetry in colour, the chromatophores being collected together on the upper side and giving it its characteristic obliterative colouring. In some genera it is the right side of the body which is above, in others the left — while in a few species it appe irs to be indifferently the one or the other. DIPNOI. — Both of the dipneumonic Lung-fishes — Lepidosiren and Protopterus — have been investigated (Graham Kerr, 1900 and 1909 : I Judge tt, 1901). They closely resemble one another and Lepidosiren will be chosen here for description (Fig. 200). During the early stages of the modelling of the embryonic body (Fig. 200, A) the latter is curved round the egg, occupying about 290° in angular extent. The head-region becomes demarcated as a slight, somewhat lance-shaped protuberance above the general surface of the egg due to the neural rudiment. The branchial region becomes marked at an early stage by a slight elevation of the sin-face which Siion becomes divided by shallow oblique grooves into the sen- branchial arch rudiments. About stage 25 ( Fig. L'uO. B) the tip of the head and the tip of the tail project sharply above the general surface: the external gills (r.//) are now in the form of tour distinct little kno each side, and the cement organ (c.n) has made its appearance as a civscentic structure on the ventral side curving round tin- tip of the VOL. II 1' 1- 434 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. Fia. 200. — Stages in the development of Lepidosiren \. stage 23; B, 25; C, 26; D, 28; B, 31; F, 33; G, 35; H, 86. c.o, cement organ ; E, eye ; e.g, external gill ; M, month; p.f, pectoral fin; pl.f, pelvic tin : >•/../-, spiral valv.- of intestine. (A - F x 3 ; G x 2-5 ; Hx2.) c.a *9 vii KXTKRNAL FEATUBES OF DIPNOI 435 head. Tin- pnsterior part <>f tin- body ix»\v IM-C..HK-.S laterally e..m- pressecl, it grows rapidly in length and tin- lar\a assumes a somewhat t idpnle-like f<>rm — the apparent "tail" being a i first m-nl \vntrally (Fig. 200, C). The anus is situated close to the tip of this port inn of tin- Imdy. theivlmv it is, strictly speaking, not tail but rather posterior trunk-region. About this period hatching takes plan-. The tail-like hinder region now straightens out (Fig. 200, D) and grows rapidly iu length, the growth being at first mainly pie-anal and the true tail-region developing later. As in Cfossoptefjrgiana the tail is throughout protocercal. As in Polypterus again the head- region for a considerable period sliows no active growth in length : it is not until about stage 31 (Fig. 200, E) that its growth becomes active and the head-region begin's to develop the modelling of iis definitive features. The external gills grow actively in length after hatching : each develops a double row of pinnae along its external margin and eventually all four become fused together at their bases. They reach their maximum about stage 35 and thereafter undergo a process of atrophy resulting in their complete disappearance. The limbs make their appearance about stage 31, each as a little knob bearing a striking resemblance to the first stage of an external gill. The een icnt organ increases in size forming a large cushion-like and very conspicuous organ in the larva of stages 32-34 (Fig. 200, F). Eventu- ally it shrivels up and disappears without leaving a trace behind. In Protopterus as already mentioned the general features of development agree very closely with those of Lepidosiren. Of Ceratodus developmental material was obtained by Caldwell in 1884 and by Semon in 1891. Semon's material has formed the basis of a long series of investigations by himself and others which together constitute an important contribution to Vertebrate morphology (Semon, 1893-1913 ; 1901*). While the general modelling of the body shows a general resemblance to that of Lepidosiren and Protopterus there are certain well-marked differences in detail. Perhaps the most striking of these is that the head-region shoots ahead in its development and grows actively in length so as to project freely in front of the yolk at a much earlier period than in the other genera (Fig. 201, A, B). Again the main mass of yolk undergoes a more uniform process of lengthening so that it assumes a somewhat spindle-like form and allows the body as a whole to become slender and " fish "-like, the "tadpole" shape due to the persisting spherical mass of yolk in Polypterus or Lepidosiren being here absent. During the later larval stages the divergence of Ceratodus from the other two Lung- h'shes towards the more typical fish condition becomes marked by the paddle-like form of the limbs and the much greater development of the median fin round the hinder end of the body. It will be noticed also that two conspicuous features of the young L> i>i< . :..juni-ii1 n! ' '• running • fulfil' ri. ( l-Y"iii Sciiion In (In- .\\ttiti-nliiin A, hU^S'J - : l>. -II ; I i.MaKiiiiir.-iii'Hi :il'""H i>\ 'liam.-;. An i-arly Stage oi' Mich ;i ivlalivdy jiriinitivi- mcinbcr ;is lit iv;ilm\vn 1 ^Ij^^r FIG. 202. — Embryos of /////'«'i/''"/'/"'<>' /W/V/M.S-. (After Braner, 1899.) A, B, Cx4 ; DxlJ. (For details of B cf. Fig. 87f < '). /•-', eye ; e.jjr, external t;ill ; olf, olfactory 01 -MM. 438 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. in the fact that here as in the Dipnoi the embryonic body during the early stages of its differentiation has a much greater angular extent, curving round the mass of yolk cells instead of being restricted to a small extent near the apical pole. Another import- ant point to notice is tin- well -marked downward flexure of the head during early stages — a feature which has already been corre- lated (p. 93) with the presence of a large supply of yolk. The active forward growth of the head -region leads to the rounded main mass of yolk being situated well back, just in front of the anal region (Fig. 202, C), instead of anteriorly as in the tadpole- shaped larvae of Lepidosiren or Protopterus or the Ganoid fishes. ELASMOBRANCHII. — Of the isolated groups of Vertebrates char- acterized by having meroblastic eggs the most nearly primitive is that of the Elasmobranch fishes. Unfortunately for purposes of comparison we are not, up to the present, acquainted with any member of the group possessing small eggs poor in yolk. Here as in other groups with typically meroblastic eggs the early rudiment of the body of the embryo — or more correctly of the dorsal portion of the body — extends through a relatively small angular extent, in striking contrast with the 200° or more of a Lung-fish or of one of the Gymnophiona. As the embryo proceeds with its development it grows actively in length, headwards and tail wards, so as to project freely in a tangential direction, remaining in connexion with the main mass of the egg (yolk-sac) by a narrow yolk-stalk. During the forward growth of the head growth-activity is less pronounced on the ventral side so that the gill -clefts are forced into an oblique position and the head undergoes pronounced cerebral flexure. AMNIOTA. — In the non-mammalian Amniota1 the first point to notice is that although the size of the egg and the absolute amount of food -yolk contained within it are relatively enormous yet the degree of telolecithality is less extreme than it is in the case of the meroblastic eggs of fishes. Consequently the segmentation process result i HIT in blastoderm formation spreads in an abapical direction past the level at which the posterior end of the embryonic rudiment will be developed: as a result of this the entire embryonic rudiment ii its first, appearance well within the boundary of the blastoderm, instead of its hinder end being coincident with that boundary as i- the case in Klasmobranchs. Here airain the rmhr\ t' the ultimate li-ssrr degree <•!' development ..!' ihe tail-region correlated with its diminished locum. .inr importance in (he adult. Tli«- I,,,,, i sinking leal lire however is one which may perhaps ly expressed by saying that the closing inlluence of the 1 Tin- « \!' Th.'ll fr.lllIM- 111 l!l> II I 1 1 • Vc 1 1 . 1 1 1 1 1 ( ' II I .1 I' \\cll 1 I I II S 1 |';l t i •< I 1 1 V till' ( If \ f 1 « '] ij II ^ l'iir-1 '-.'•«• HH- tix'in-.s in <'li:ij.. '• vii THK EXTERNAL FEATURES OF THE BODY 439 yolk upon the growth in length of the ventral side of the-body is much more marked than in other Vertebrates. The result is a strong vent nil curvature of the body. The ventral flexure of the head in the mid-1. rain region already seen in the Klasmobranchs and Gynmopliioii.i is hen- still more marked, but in addition the whole liudy is si lonely curved ventrally to such an extent as to form more than OIK- complete turn of a spiral. This curvature is in its incipient stages of great morphological interest as providing a possible explanation of the downward indentation of the blastoderm by the head and tail regions, and their consequent ensheathment in blasto- dermal pockets, which led eventually to the evolution of the amnion. The formation of the amnion and the separation of true amnion from false amnion involve as will be gathered from Chapter VIII. a solution of continuity of the somatopleure or original body-wall and probably this has initiated what is perhaps the most striking- feature of amniote development — the loss, at the time of hatching, of a relatively large proportion of somatopleure together with the allantois. As a matter of minor detail it should be mentioned that in the case of Birds as compared with the lower Amniota the embryo is dis- tinguished during a long period by the relatively enormous size of the head. It seems reasonable to regard this as in the main an anticipa- tion of the great development of the eyes and optic lobes in the adult. In the foregoing short sketch the author has confined himself to the main branches of the Vertebrate stem. He has omitted all reference to three different types near the base of that stem, namely Amphioxus, the Lamprey, and the Myxinoid, which are of great interest in themselves but which are of less importance for enforcing general principles of Vertebrate development. Of the three types the first will be found fully described by MacBride in Vol. I. and it will easily be seen how in the general form of body, as in various other characteristics, the young Amphioxus has diverged widely from the more typical Vertebrates. The Myxinoids, so far as they are "known from Bashford Dean's researches on Bdellostoma (1899), appear also to be highly specialized. The Lampreys on the other hand have diverged to a much less extent from the normal. The most striking features during the early development of the hndy are (1) that the head-region, as was the case in Ceratodus and ffypo- geophis and as is the case also with JBdellostoma, shows a marked activity in its growth in length, the mass of yolk persisting longest posteriorly and (2) that the outgrowth of the tail is delayed until a comparatively late period. A marked negative feature in all three of the types mentioned is the absence of all trace of paired limbs. (2) THE MEDIAN FINS. — The primitive Vertebrate, with its segmented musculature arranged along the two sides of the body and its skeletal axis and central nervous system lying at the mesial plane, is. clearly a creature constructed for swimming by lateral 440 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. Ilex ure after the manner of an Eel. To secure greater efficiency the body, more particularly its purely motor post-anal portion, becomes compressed from side to side, the compression being most marked near the margin of the body where the thin almost membranous median fin is produced. In development the median fin arises as a projecting fold of slightly thickened ectoderm into which later on mesenchyme penetrates. In what thf evidence points to as being the primitive condition this fin rudiment is continuous and extends round the hind end of the body. In such relatively primitive Vertebrates as Crossopterygians, Lung- fishes, and some Elasmobranchs, it extends forwards on the dorsal side practically to the head-region, while on the ventral side it reaches the anus and may even be continued onwards as a pre-anal median fin, though possibly this has originated in phylogeny in- dependently of the main fin-fold. In the Lung -fishes the median fin-fold during the course of development never loses its continuous and practically symmetrical arrangement round the tip of the tail. It retains throughout life the primitive symmetrical (protocercal) form. In the Crossopterygians (apart from the anterior portion of the dorsal fin which becomes divided up into a series of finlets) the same holds until a very late stage in development, the tail of the adult becoming very slightly asym- metrical though the term protocercal is usually and justifiably still applied to it. A similar protocercal tail occurs in the Amphibian larva while the tail of the adult Newt or Crocodile is simply a pro- tocercal tail in which there is no longer a membranous fin-fold present. It is however characteristic of the fishes in general that, in accordance with their high specialization as expert swimmers, the median fin during ontogeny loses its homogeneous character — cer- tain portions of it, probably those portions which are in mechanic- ally the most favourable positions, becoming enlarged while the intervening portions become reduced to the point of complete dis- appearance. The result is that the place of the originally continuous fin -fold is taken by a series of separate fins — one or two dorsal, a caudal or tail fin, and on the ventral side, an anal fin. Of- these the caudal tin — most favourably situated of all the series to serve as a propelling organ — becomes specially enlarged. 1 1 is also characterist i< «f tin- ni< .re eilieient swimmers that the part of the caudal iin lyin^ on the ventral side nf the axis becomes particularly developed. \Ye may probably associate this with the function of rotating the body about n < ]i.n<_r axis as may be seen in a shark \\lien it sei/es its prey. The uDsymmetrioa] condit ion of the tail so produced is termed hetero- cercal. When oorried to \\< extreme the tip of ilie vertebral axis lilted upward- and. so far as external appcaranc. (geCOndari :iuetrica.l condit. ion is arrived at, M is seen in the homocercal tail of the Teleoetoan lishes. it. will be understood that the protocercul. hateroceroal and homocm-al conditions mei'Lie into one an«»tln-r and camml be distinguished by any ripd definition. vii FINS AND LIMBS 441 They ele.-irly represent successive grades in tin- evolution of the tail as a more .mil more perfect organ of propulsion, a process of evolution which has conic about independently in. the various groups of fishes. Thus even tin- Lnn^-fisln-s — the surviving members of which group • •SB the primitive protocercal type of tail — during the geological periods when they most nourished showed numerous forms in \\hie,h ihere was a highly developed helerocercal tail. Again the assumption of a sluggish mode of life, or the simpli- fication of the swimming movements, is frequently correlated with reversion of the tail towards the protocercal condition. This is clearly seen in many teleostean fishes, such as the Eels and .many deep-sea bottom-frequenting fishes. In such cases all trace of the unsymmetrical condition may have disappeared from ontogeny but there is no room for doubt regarding an ancestral heterocercal or homocercal condition — for in their general structure these fishes are highly evolved Teleosts and the group as a whole is characterized by the tail being homocercal. In the case of the surviving Lung-fishes the general archaicism in structure, and more especially the extremely archaic character of the paired tins, are in favour of the protocercal character of the tail being persistent rather than revertiVe — apart from the evidence of tMii bryology which fails to disclose any trace of a pre-existing hetero- cercal phase. (3) THE LIMBS. — One of the characteristic structural features ol the Vertebrata is the presence of the two pairs of limbs, pectoral and pelvic. Two main types of such limbs can be recognized — the fin type for swimming and the pentadactyle or leg type for moving on a solid substratum. As the former is on the whole characteristic of fishes, and as fishes are on the whole more nearly primitive than are terrestrial Vertebrates, the idea has naturally arisen and has now attained perilously near to the position of a dogma that the leg type of limb has been evolved out of the fin ; and elaborate attempts have been made to define the manner in which this has come about. It is necessary at the outset to emphasize the importance of keeping an open mind upon this question : there exists the possibility — which as will be seen later is not lightly to be brushed aside — that penta- dactyle limb and fin are not in the relation of lineal descent at all but that they have been derived from a common ancestral type of limb differing from either. Xo limbs exist in Amphioxus or hi the Cyclostomata. There is however a general tendency in Vertebrates which have assumed an eel -like form of body for the limbs to degenerate and disappear, ami it is well to bear in mind the possibility that this has happened in the case of both of the types mentioned. The limb at its first appearance in embryonic development forms a little projection from the body surface — a core of mesenchy un- enclosed in an ectodermal sheath. In Lepidosiren or Protopterus or a Urodele it is in the form of a rounded knob identical in appearance 442 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. with the rudiment of an external gill, but in a large proportion of Vertebrates it is narrower in a dorsi-ventral than in an antero- posterior direction so as to have the form of a short longitudinally- running ridge. As is usual where active increase of surface is about to take place the projecting limb rudiment is foreshadowed by a thickening of the ectoderm and by a condensation of the underlying mesenchyme. In Torpedo,.one of the Rays — fishes characterized by the great antero-posterior extension of the large pectoral fin — the two limb rudiments which are at first distinct (Rabl, 1893) become for a time joined together by a transitory ectodermal thickening. This phase, in which the two fin rudiments are as it were parts of a continuous ridge, was the earliest stage observed by Balfour (1878) and it afforded him an embryological basis for the lateral-fold view of the phylogenetic origin of the Vertebrate limbs (see p. 445). As the limb rudiment develops it shows in many cases character- istic changes in its position. First it shows a movement of rotation. This is well illustrated by the case of Ceratodus as described by Semon (1898). Here the pectoral limb rudiment becomes rotated in such a way that its originally pre-axial or head ward edge becomes dorsal and its originally lower or ventral surface comes to face in a head ward direction. In other words, if one could observe the developing left pectoral limb from a point away to the animal's left side the limb would be seen to undergo a clockwise rotation. It results from this that when the fully developed limb is folded back alongside the body its outer surface is that which was originally ventral. A rotation similar in direction though varying in angular extent in different forms occurs also during the development of the pectoral fin in Crossopterygians and Actinopterygians. In Tetrapoda, on the other hand, a rotation of the limb rudiment in the opposite direction takes place — the pre-axial edge becoming ventral. Not improbably this may be regarded as a secondary modification fore- shadowing the pronate position of the fore-limb characteristic of terrestrial progression. The pelvic fin in Ceratodus undergoes a similar rotation but in the opposite direct inn t" Mi;it <>i' the pectoral: the left pelvic, tin regarded from a point away on the animal's left would be seen to uuder-jo a counter-clockwise rotation. The result is that the origin- ally dorsal surface comes to face headwards or, when the tin is folded back alongside the body, outwards. The corivspondin- rotation of the pelvic limb in other fishes and in the lower T'-lrapods appears to stand in need of further in vest i •_' at ion. It i.- clear from t IK- facts of Comparative Anatomy that t he paiivd limbs have underpin.- «-\ i.-nsive shiftin-s ahm^ the surl'aee of the \ '•! h-l.rate l.<»dy in adaptation to nil form and its method of mov.-iiH-nt, (H-C h»-|«.\N.p IIS). It is of interest tlmii^l, not necessary bhe fact 01 -nch phyln-vnctic shift in- t-. enquire vii DEVELOPMENT OF LIMI'.s 443 wheiher any record of it occurs in ontogeny. Obvious evidence which at once suggests itself in this connexion is tin- presence of abortive muscle-huds in front of or behind those which hec<, un- incorporated in the definitive limb. The-.- -,-, -m (Clearly to in- dicate thai. MM- part of the body surface superficial to these buds was at one time part of the actual limb. But unfortunately such abortive buds occur both anterior and posterior to the definitive limb and there is no means of fixing definitely the* time in phylo- genetic evolution from which the two sets of abortive buds date. They may date from the same period, in which case they mi^ht merely afford evidence of the process of narrowing of tin- limb base which has undoubtedly taken place during the later evolution of tins; or they may date from different periods, in which case the anterior set might be taken as evidence of a backward movement of the limb, and the hinder set as evidence of a forward movement occurring at a different period — movements which again have un- doubtedly taken place. In view of the impossibility of determining to what extent the evidence in any particular case is to be inter- preted in these two different directions it seems on the whole advis- able to leave this muscle-bud evidence on one side. Other evidence has been adduofc from cases where the actual limb rudiment as a whole (i.e. the projection from the surface of the body) seems to be displaced during development. For example in the figures illustrating the development of the pelvic limb in Spinax (p. 207) it will be seen that the anterior limit of the fin is in successive stages of development opposite myotomes 21, 26, 28 and 31 ; the hinder limit at the same stages opposite myotomes 30, 38, 38 and 39, and the middle of the fin base opposite myotomes 25, 32/33, 33/34 and 35/36. It seems quite allowable in such a case to speak of the limb as having undergone a backward displacement. In other cases, as that of Scy Ilium according to Goodrich (1906), ontogenetic development discloses no evidence of such backward migration. The limb rudiment gradually increases in size and assumes its definitive form and as it does so it becomes equipped with its characteristic skeletal and neuromuscular arrangements in the manner already described. PHYLOGENETIC ORIGIN OF THE LIMBS. — The limbs are organs highly characteristic of the Vertebrata. While they exist typically as two pairs, pectoral and pelvic, one or both pairs readily disappear in groups where they are no longer needed. They are particularly prone to disappear in those Vertebrates which assume an eh MIL form of body and revert to the archaic method of moving by lateral flexure. Thus in Eels the pelvic limb has disappeared, in ^///nbranchus both pairs. In Lepidosiren, the most elongated Lung-fish, \ve both pairs in process of reduction. We see the same in elongated Urodeles such as Amj'hinttnt, while in the (lymimphiona both pairs have vanished. In Eeptiles we see beautifully how the limbs undergo reduction (Chalcides) and complete disappearance (Amphisbaenidae, 444 EMBEYOLOGY OF THE LOWER VERTEBRATES CH. A//;/ ids) in those various groups of Lizards which have developed an elongated snake-like form. So also in Snakes. In cases where the limbs are completely gone in the adult it may be possible to observe them during early stages of development. The young Symbranchus (Fig. 203) has for a time huge pectoral fins which it uses as organs of respiration (Taylor, 1914). In Gymno- phiona and Blind worms (Anguis) l minute limb rudiments have also been observed in the embryo. In other cases no trace of the missing limbs has been found during early development. In view of this general tendency of the limbs to disappear in Vertebrates which have assumed an eel-like or snake-like form of body it is well, as 01 2345 0:0. 2 3 I 'i<;. 203. — Symbranchtta marmoratits. Larvae showing pectoral fins. (After Taylor, 1914.) ».•!, opi-rciilar opi-nin- ; .--./. r. subinti-sliiKil vein ; y, yolk. already indicated, not to assume a dogmatic attitude in regard to sucli Vertebrates as Lampreys or Hag-lishes. The possibility is not excluded that even these Cyclostomes are descended from ancestors in which limbs were present. The interesting question now run -r^es — How did the limbs of the Vertebrate ori-jinai.- in evolution '. Few morphological speculations have excited more interest and more controversy than this. Two main hypoi h606fl ha\ <• been propounded ami each has round supporters amongst the most eminent, niorplmln:jists. Although in the opinion of tin present mil »r it is no Ion--'-!- necessary to fall hack upon eit.h.-r of ill,. inijiler pn-nhdity having presented itself, a, 1 The i'kmi:iit;iry .stmli-nt in.iy !>»• uaimd not to mi.stukr tin- i inliiiK'iil- ut' tin- |..iir.-.| j.-n.-s in Sn.-ik.- i-!iil.ry««.s for llmbf ! vii EVOLUTIONARY ORIGIN OF LTMI'.s 445 short sketch of each and of tin- arguments for and against n may now IM- k'iv»'ii. ^'H- t.wo hypotheses indicated are the "Lateral Fold " hypothesis of I'.ulfmir, Mivart, Thacher and others and the " Gill- septum " hypothesis of ( Jcgeribaur and his school. Each hypothesis concerns itself with the origin of the paired limbs of fishes tin- fin being regarded as the primitive type of limb from which the pentadactylc limb has brrn evolved later on. THE LATERAL-FOLD HYPOTHESIS. — It will be remembered that the mode of development of the median unpaired fins indicates clearly that these fins are simply persisting and exaggerated portions of a once continuous median fin-fold. According to the lateral-fold hypothesis of the origin of the limbs the paired fins of fishes are similarly to be looked on as persisting and enlarged portions of a continuous fin -fold which once extended along each side of the body. Thr hypothesis rests upon a tripod basis of Embryological, Ana- tomical and Palaeontological fact. Balfour in his Development of Elasmobranch fishes (1878) wrote : " Along each side of the body there appears during this stage (G-I) a thickened line of epiblast, which from the first exhibits two special developments : one of these just in front of the anus, and a second and better marked one opposite the front end of the segmental l duct. These two special thickenings are the rudiments of the paired fins, which thus arise as special developments of a continuous ridge on each side, precisely like the ridges of epiblast which form the rudiments of the unpaired fins." "If the account just given of the development of the limbs is an accurate record of what really takes place, it is not possible to deny that some light is thrown by it on the first origin of the vertebrate limbs. The facts can only bear one interpretation, viz. that the limbs are the remnants of continuous lateral fins." Further embryological support to this hypothesis has been pro- vided (1) by the fact that the muscles of the limb are of segmental origin, derived from a number, often a considerable number, of myotomes (see p. 207) and that apparently vestigial muscle-buds have been found both headward and tailward of the series which take part in the muscularization of the definitive fin, and (2) by the fact that the fin type of limb commonly shows a marked narrowing of its base of attachment during the* process of development, - the rudiment having during early stages the form of a longitudinal ridge attached throughout its length to the body. Regarding the evidence upon which the fin-fold hypothesis rests the following criticisms may be expressed. (1) The ectodennal ridge described as connecting the two limb rudiments in Elasmobranchs turns out to be a characteristic not of Elasmobrauchs in general but only of Rays (Torpedo) i.e. of forms in which there is an enormous, admittedly secondary, extension of the pectoral fin along the side of the body. The Elasmobranchs less specialized in this respect— the 1 =Archinephric. 446 EMBRYOLOGY OK TIIK Lo\VKR VERTEBRATES CH. Sharks and Dog-fishes — do not. so Car as is known, develop this ridge. (2) The fact that the myotomes — from which the linili rudiment, like other portions of the body, lias to derive its equipment of voluntary muscles — aiv themselves nietameric and that the skeletal elements necessarily correspond in .position with the muscles seems in render it unnecessary to seek any further evolutionary explana- tion of the tendency on the part of the musculature and skeleton of the limit to exhibit a markedly nietameric appearance during early stages in its development. The occurrence of abortive muscle-buds in front of the definitive limb is taken — quite reasonably — as evidence pointing to a tailward shifting of the anterior margin of the limb having taken place, and similarly the presence of abortive buds behind the definitive limb is taken as evidence of a headward shifting of the hinder margin of the linib. Hut this shifting of the anterior and posterior margins of the limb may have in evolution taken place either synchronously (i.e. together with a narrowing of the base of attachment of the fin) or at different periods as the limb shifted backwards and forwards as a whole in accordance with variations in adaptational requirements. The present writer sees no convincing reason for rejecting either of these possibilities — and if either be possible then the evidence loses its value as support of one view rather than the other. (3) The narrowing of the limb proximally and its expansion distally is a process which would naturally take place as the tin became more efficient as a propelling organ — just as in the evolution of a racing oar or paddle with its broad blade and slender shaft — and, accordingly, too great weight should not be attached to the occurrence of such a process during ontogeny in arguing as to tin- evolutionary origin of the limbs. As regards anatomical evidence stress is laid on the exceedingly ' -lose structural resemblance in skeleton and musculature between the paired and the unpaired fins. On the other hand it is suggested that, seeing that lateral and paired fins are organs similar in function and built up out of similar muscular elements, a close similarity in their anatomical arrangements may quite probably be merely a oaae of that secondary convergence of which so many striking examples are known in the annual kingdom, An ancient fossil fish. Cladoselaclie, is brought in to corroborate the view, it- paired lins having each a broad longitudinally-running base ol' attachment and being apparently supported by separate without any continuous ba>al skeleton. lint it- is pointed out (1) that what BlgM I here a i «• of basal skelel on may he readily interpreted presenting the axis of a tin of tin- ('eralndus type laid hack against tin- side of the i,«,dy I L69 E, p. 353), and (2) that •riielinv of the tail I- of a Vety highly developed Mild powerful Ivpe and that it is most unlikely that a powerful s\\ imnier, such as ih.- hi'jlilv «• \ol\vd tail demonstrates f 'l,i,/,isel,n-h,- to have been, should MI EVOLUTIONARY olMciN or LI M r,s 117 ha\e retained its pain d tins in a relatively priniili\«- and ineflieirnl condition. r'inally fchere are ere*! physiological difficult ir> in tin- way of accept in- tin- lateral -fold hypothesis. There are no more tundaiuental eharaeierist ics of tin- Vertebrate body than the, arrangement ol' it- longitudinal muscles in segmental masses along each side of tin- body, and tin- position of its skeletal axis, its central nervous system and its main arterial trunk in the region of the mesial plane. It is quite «-li -ar that such a creature is built for swimming by waves of lateral Hex ure after tin- manner of an Amphioxus, a Lamprey or a Lung-fish. Any new swimming organ that became evolved in primitive Verte- brates must have had some advantage over, or at least not interfered with, this primitive method of swimming. It is difficult to see how the supposedly ancestral lateral fold could possibly have complied with these conditions. The suggestion* that the lateral fold may have functioned at first as a balancing organ or as a "bilge keel" will not bear examination from the point of view of elementary physics. Rabl suggests that the two lateral folds may have acted primitively as a kind of parachute and that they became muscularized at their anterior and posterior ends, the intermediate portion undergoing atrophy (thus originating the two pairs of limbs). The skeletal elements on tliis view would also develop at the ends of the ridge first, and spread backwards (pectoral fin) or forwards (pelvic fin). Thus would be explained the reversal of the position of the anterior and posterior edges of the two fins in e.g. Ceratodus. Such an explanation however fails entirely to meet the difficulty that there exists not merely an antero-posterior reversal in the structure of the two fins but also a dorsi-ventral one. THE GILL-SEPTUM HYPOTHESIS. — This hypothesis was based by Gegenbaur (1872) on facts of adult anatomy. In some of the Elasmobranchs (Pristis) the central gill ray attached to the branchial arch is enlarged and the rays next to it have come to have their bases of attachment shifted secondarily from the arch on to this enlarged ray, so as to produce an arrangement recalling the biserial archipterygium of Ceratodus with its central axis and lateral rays ; Gegenbaur suggests that the archipterygium with its limb girdle has in fact been evolved out of such an arrangement of ra\s attached to a branchial arch and that the limb itself is serially homologous with the gill septum. In support of this view it is pointed out that branchial arch and limb girdle are each in early stages of development in the form of a continuous curved rod of cartilage; that this becomes usually segmented in the case of the branchial arch but that even in the girdle it also shows traces of segmentation in some ancient fossil forms (Pleuracanthids, Acanthodians) ; that in some cases the peri- chondrium of the pectoral girdle is known to be innervated by that typical branchial nerve the Vagus ; that in the lower forms the trapezius,oue of the muscles associated with the fore-limb,is innervated 448 EMBEYOLOGY OF THE LOWEE VEETEBEATES OH. by the same nerve; and that connected with the ordinary branchial arches there are myotomic muscles as well as splanchnic, so that the basis already exists for a musculari/ation purely myotomic. On the other hand the objection is urged against the Gegen- baur hypothesis that it involves a very great shifting of the pelvic fin backwards from its assumedly original position at the hinder end of the branchial region. This objection need not lie taken seriously in view of the extensive shiftings of the limbs which are definitely known to have taken place. Thus in Eays we commonly find that the pectoral girdle has moved back to a position in relation to the segmentation of the body far posterior to the position which it occupies in Sharks: in Urodele Amphibians the hind-limb has taken up positions, as indicated by the position of the sacrum, varying between the 14th (Triton palmatus) and 63rd vertebra (Ampliiuma means) while in the Anura — where in accord- ance, with the leaping habits it is advantageous to have the attach- ment of the hind-limb far forward — the sacrum has come to be as far forward as the 9th or even (Hymenochirus) the 6th vertebra l : in Plesiosaurs and Birds a still more striking backward migration of the pectoral girdle with its attached limb has taken place (e.g. in the Swan as compared with Archaeopteryx through 14 or 15 segments) : and finally in many Teleostean fishes the pelvic fins have become so shifted forwards along the sides of the body as to attain to an actually jugular position. The fact that the limb girdles are embedded in the somatopleure while the branchial arches lie in the splanchnopleure has again been raised as a difficulty in the way of accepting the Gegenbaur theory. The difficulty is not so serious as it seems at first sight. The chief obstacle in the way of a splanchnopleural organ becoming shifted outwards into the somatopleure is clearly the coelomic cavity — but in the branchial region this tends to be in great part obliterated. As regards blood-vessels, nerves, etc. — these form by no means insuperable barriers to the change in position of skeletal elements. Such skeletal tissue may, as has already been indicated in Chapter V., spread past a blood-vessel or nerve and if it then becomes absorbed behind the obstacle there is brought about a complete transposition of the two structures. The criticism that the musculature of the limbs is myolnmie in origin while that of the branchial arches is splanchnic is provided against by the mixed character of the muscularization of the hranehial arches, taken in r.nnjunet.iim wit h the demonstration that in such a ease replacement of splanchnic muscle h\ myot-omic may lake plae • (]>. 1' 1 7). 11,-ibl considers the iin-lanirrie mi^m of the muscles etc. <>1 the limb to be enough by itx-ll'to undermine Ihe (le^enhaur h\ pot he-is. but it in dillicull to see how tin- musculature could he otherwise than me|:iim-nc in origin seeing thai, it has t<» he derived I'nuu the seginenlally m ynlnines. 1 Gadnw, in (.',nii /'/•///• vii EVOLUTIONARY ORIGIN OK LIMBS 449 The niiisciil irization of the jugular pelvic fin of Teleostean fishes is admittedly secondary: the limb rudiment becomes musculari/cil by i he inyotiMiies to which it happens to be opposite at the time rnuscularization begins: but if this fact be admitted it is not open to us to deny the possibility of a similar process having taken place in the successive positions taken up by the pelvic limb in the cour.-i- of the movements which it has undergone during phylogenetic evolution. A further objection urged against the Gegenbaur hypothesis is that there have not been discovered, up to the present, any examples of the intermediate stages between gill-septum and limb which must have existed if this hypothesis be a true theory. This objection appears to be a valid one. Again it is urged that in those Vertebrates which would appear, in this respect, to have retained most nearly the primitive condition (Cyclostomata, Elasmobranchii) the gill-septa are fixed firmly in position and are therefore not likely to become converted into motor organs, which must necessarily project beyond the surface and he freely movable. This objection like the last appears to be a valid one. It will be apparent from the short sketch which has been given of the two rival views of the evolutionary origin of the limbs of Vertebrates that neither can be regarded as wholly satisfactory. However these hypotheses are old, as the science of Embryology goes. They were designed to fit the data available at the time they were formulated and the great bulk of subsequent work upon this particular problem has consisted in the adducing of new facts which appear conveniently to fit on to those already accumulated by the supporters of one view or the other. In a rapidly advancing science like Embryology however it is advisable to have from time to time a stocktaking of the facts of contemporary knowledge with the object of seeing whether the more extensive body of available facts suggests the same working hypotheses as were suggested by the facts known at earlier periods or, as is always possible, something quite different. The putting this principle into practice is more conducive to progress and more stimulating to. research than the mere accumulation of further facts to support or to confute the working hypotheses of earlier times. THE EXTERNAL (3 ILL HYPOTHESIS. — Applying this principle to the problem of the evolutionary origin of the limbs one finds an important set of data which were not available to Gegenbaur or Balfour. In their day there was no proper appreciation of the importance of the fact that there existed in three of the less specialized groups of Vertebrates — Urodele Amphibians, Lung-fishes and Cross- optcrygians — those organs which have been described in Chapter III. under the name External Gills. The mode of development of these organs is now known in all three of the groups mentioned and the VOL. 11 2 G 450 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. evidence appears to be conclusive that they are truly homologous throughout. It has been shown that there is a tendency for the External Gills to become eliminated — as e.g. in various Anurous Amphibians : it has been shown further that in some of the main groups of Verte- brates in which they do not occur their disappearance may be accounted for by the evolution of a new physiological substitute — the vascular surface of the yolk-sac. Having regard to these facts and to the relatively archaic character of the groups in which they actually occur the conclusion is considered justifiable that such external gills are organs of high antiquity in the Vertebrate stem. Further, from their dis- tribution upon the various arches it is in- ferred that in all probability an external gill was once present upon each visceral arch. But it has also been shown to be probable that the series of visceral clefts — and therefore of visceral arches — was formerly more extensive, extending farther back along the body than it does in exist- e.g.t M ing Vertebrates. It is therefore concluded that in an earlier phase of its evolution the phylum whose modern representatives we call Vertebrates was characterized by FIG. 204. —Left side of head the possession of a series of external gilla ^"l^,^ ^tending Awards beyond the limit .lays pn-viou>iy, :i j»ien- of reached by the branchial region of exist- skin froin the branchial region ing Vertebrates. of another embryo had been r> f anpU ovtarnnl mile QVP r»ntpnHal grafted. ( After Kk.nau, 1913.) \ aie potential organs of support — as shown by the ...1 mils (auto- "balancers" of Urodeles (see Fig. 88, p. 157) sitic); e.fj.t, external gills (paia- , , , . , .. e sitic) which i.av, fevdoped from —and also potential organs oi movement-* the implanted pi.-n- of skin; op, as shown by the well- developed muscula- "I'enull"n- ture by which they can be nicked back- wards. In other words these organs — and these alone among the organs of the. Vertebrata — possess the qualiiieatimis which have to be postulated for the evolutionary forerunner of the Vertebrate limb. In vie\\ of such considerations as those just set forth the present wrili-r brlievi-s the most plausible working hypothesis nf tin- e\ulu- linnary origin o\ 1h<- limbs haxiii-- regard to our present-clay kiu>\\ ledge — to be tliai uhi'-h interprets them as modified external -ills, he- Inji-jiii^r to visceral arches fart hri' back in t he sei'ies t ban iln.se forming the branchial an-lics of exist in-j \'- rtebrates, 'l'ln- limb ^inlle \\ouli(/i>*t /;•!<, breeding male showing apparent reversion of both pectoral and pelvic limbs to the branchial condition. (From a specimen in the Zoological Museum of the University of Glasgow. ) in the case of the pectoral limb attached to its ventral side, in the case of the pelvic to its dorsal side. But it has already been shown that the definitively ventral side of the pectoral limb is homologous with the definitively dorsal side of the pelvic limb — the difference in position being due to the rotation in different directions under- gone by the limb rudiments in the course of their development. This reversed position of the respiratory filaments in the two sets of limbs clearly then fits in exactly with the view that they are ancient morphological characteristics of the limb which have reappeared in the male Lepidosiren. The striking resemblance between the 'pectoral girdle and the branchial arches in some of the more ancient Fishes again finds its explanation in the morphological identity of the two structures. It is now established that the swim-bladder of Fishes is morphologically a lung, and that the lung is to be regarded as at the least an extremely ancient organ in the Vertebrate phylum. This points to the prob- ability that the early Vertebrates were creatures which clambered 1 Reference should also be made to Fig. 88 (p. 157) which brings out clearly the remarkably limb-like character of the Urodele "balan Budgett (1901) mentions the case of an abnormal Protopterus larva which had failed to develop the pinnae upon one of its external gills. " This bare shaft so much resembled the ju-etnral limb that the larva appeared to have two pectoral limbs on one side." 452 EMBKYOLOGY OF THE LOWEE VERTEBRATES CH. about amongst the vegetation of shallow water and we may sup- pose that in this early stage the limb was of a crude styliform shape such as we see exemplified in the metamorphosed external gill of the Urodele balancers, or in the actual limb of the larva of Lepidosiren. On this hypothesis the ancestral styliform limb has pursued two divergent lines of evolution. The one of these is found in those Vertebrates which have developed along the lines of becoming specialized for efficient swimming. Here it has become a fin, an early stage of this evolution being represented by the crude paddle of Ceratodus. That this biserial archipterygial type does actually represent an extremely early type in the evolution of fins seems to be demonstrated by two facts taken in conjunction with one another— 1. That this thick and clumsy organ represents functionally a relatively inefficient type of swimming organ a-s compared with the thin flat fin of mosf existing Fishes, and 2. That palaeontology shows it to have been a widely distributed type of fin in the early days of the evolution of the main groups of Fishes. It was in fact the predominant type of limb amongst ancient Elasmobranchs, Ganoids and Lung-fishes. Evidence is not entirely wanting to show how the Oossopterygian type of fin on the one hand (as seen in the existing Polypterus) and the Actinopterygian type on the other (as seen in Amia and other Ganoids and Teleosts) may have been evolved out of the biserial archipterygial type. This evidence cannot be gone into here but so far as Crossopterygians are concerned the student should note the close resemblance of the pectoral fin of the young Polypterus (Fig. 197, E) and of its supporting skeleton (Fig. 169, F) to the modified archipterygial fin of the ancient Shark Pleuracanthus (Fig. 169, B). Along the other line of evolution the styliform limb has given rise to the pentadactyle leg with its expanded foot and its characteristic jointing. It is of great interest in this connexion to watch the clumsy movements of a Lepidosiren larva and to note that the hind limb by which the creature pushes itself along becomes bent twice upon itself precisely in the way which would, give rise to the ankle and knee-joints of one of the lower Tetrapoda. Occasionally tin- uppearanee is rendered still iimn- suggestive by the tip flattening out sliirhfly into a foot-like expansion. The observer \\atehin^ a Lepidosiivn larva performing sueh nm\ einents finds it difficult to avoid the suspicion that lie is \\itnessin^ something very like what took place in tin- early stages of the evolution of the pnitu- ductyle limb. Should this lie the true history of the origin of t hat typ<- of liiiih it would explain the unsatisfaet ory and wholly nneon- vinei] ihe efforts of comparative anatomists to derive the skeletal • •I'-ments of the pentadactyle limb from those of one or other type ol fin. ORIGIN OF LIMBS AND TAIL 453 nll'i-l's IK) r\]>lanalinn nf the number ol' digits so generally five. The physiological advantage <>t th«- expanded foot being divided up into separate radiating digits is obvious, as is that nl' the doiiMe nature of the adjoining ]>ortion of the limit skeleton to t';icilil;it<- rotation round tin- axis nl' tin- limb. There are also mechanical advantages in there being a central digit with mir <>n each side of it. Possibly the presence of an additional digit outside of these is to be looked on as of the nature of simple reinforcement. The modification of the pectoral limb in the case of Birds I'm- purposes of flight is of great interest, but nothing is known as to the phylogenetic transition from Reptile to Bird in this connexion. To the present writer it seems most probable that the Birds were evolved out of aquatic Reptiles in which the fore-limb was specialized for use in swimming under water, after the manner of existing Penguins, and that the function of aerial flight was evolved directly from such movement under water. On this hypothesis the more or less terres- trial habits of modern Birds would be regarded as a secondary acquirement. (4) EVOLUTIONARY ORIGIN OF THE TAIL REGION. — It is charac- teristic of Vertebrates that the anus loses its practically terminal position and comes to be situated some distance forwards on the ventral side, the overhanging hinder end of the body forming the tail. This opens up a question of much morphological interest — though one to which we are not yet in a position to give any certain answer — as to the phylogenetic origin of the tail. It seems clear that the tail arose in ancient aquatic Vertebrates as an adaptation to swimming and on the whole it seems most probable that it came into existence through the gradual migration forwards of the anus upon the ventral side. Such a shifting forwards of the anal opening from the hinder end of the body is a familiar feature in many groups of invertebrates where it is associated as a rule with a tubicolous habit and has doubtless for its object the getting rid of excretory products which would otherwise be discharged into the depths of the tube, or burrow, or shell. In the Vertebrate the forward shifting of the anal opening has probably its physiological significance in the increasing efficiency of the tail as the main motive organ — the disappearance from it of the alimentary canal, and its surrounding splanchnocoele, being correlated with the conversion of the tissues on each side of the skeletal axis into a solid mass of muscle. Probability is added to this conjecture by the fact that we see what appears to be a continuation of the same process in the most efficient group of modern swimming Vertebrates (Teleostei) where in the most highly developed forms the alimentary canal and splanchnocoele come to be restricted to a relatively small region immediately behind the head, the remaining and main part of the body being entirely " tail." In actual ontogeny the tail region is developed not by the with- drawal from it of gut and splanchnocoele but as an actual outgrowth, 454 EMBKYOLOGY OF THE LOWER VERTEBRATES OH.VH the hind end of the body continuing to sprout out past and dorsal to the anal opening. It is of course conceival »le that in phylogeny the tail arose similarly as an outgrowth of the body dorsal to the anus hut this seems in every way less probable than the method of evolution sketched above. LITERATURE Agar. Anat. Anzeiger, xxxiii, 1908. Balfour. Monograph on the Development of Elasmobranch Fishes. London, 1878. Budgett. Trans. Zool. Soc. Lond., xvi, 1901. Caldwell. Jouru. Proc. Roy. Soc. New South Wales, xviii, 1884. Dean, Bashford. Kupffers Festschrift. Jena, 1899. Ekman. Morph. Jahrb., xlvii, 1913. Gegenbaur. Untersuchungen zur vergl. Anat. der Wirbelthiere, iii. Leipzig, 1872. Goodrich. Quart. Journ. Micr. Sci., 1, 1906. Kerr, Graham. Phil. Trans. Roy. Soc. London, B, cxcii, 1900. Kerr, Graham. The Work of J. S. Budgett. Cambridge, 1907. Kerr, Graham. Keibels Normentafeln, iii. Jena, 1909. Rabl. Morph. Jahrb., xix, 1893. Semon. Zoologische Forschungsreisen, i. Jena, 1893-1913. Semon. Keibels Normentafeln, iii. Jena, 1901*. Taylor. Quart. Journ. Micr. Sci., lix, 1914. CHAPTER VIII ADAPTATION TO ENVIRONMENTAL CONDITIONS DUEING EARLY STAGES OF DEVELOPMENT I. PROTECTIVE ENVELOPES OF THE ZYGOTE. — The Zygote or fer- tilized egg is in the Vertebrata as in other groups provided with protective envelopes. Of such we may recognize three fundament- ally distinct types which are conveniently designated as primary, secondary and tertiary envelopes respectively. A primary envelope is a cuticular covering of the surface of the zygote : it is therefore produced by the living activity of the protoplasm of the macrogamete or zygote itself. A typical example of a primary envelope is the vitelline membrane which is formed on the surface of the Echinoderm egg in response to the act of fertilization. The " vitelline membrane " which covers the surface of the egg of a Bird is commonly looked on as a primary envelope. A secondary envelope is one which is formed by the activity of the surrounding cells while the egg is still contained in its ovarian follicle. It may be cuticle-like in its nature or it may be composed of cells. Finally tertiary envelopes are formed by the excretory activity of the oviducal lining, being deposited upon the surface of the egg as it travels down the oviduct. Of such a nature are the complicated protective envelopes which surround the egg of a Bird or Keptile, or the simpler jelly - like investment found in the case of most Amphibians. Apart from tertiary envelopes the most conspicuous envelope of the Vertebrate egg is usually what is known as the zona radiata or zona pellucida — the former name being given to it from the fact that it is pierced by numerous very fine canals which give it a character- istic radiate appearance when seen in section. These fine canals apparently contain protoplasmic bridges connecting the protoplasm of the egg with that of the follicle-cells which surround it while still in the ovary, and doubtless having for their function the passing in of food-material from the follicle-cells into the egg-cell. The zona radiata is, as a rule, most conspicuous during early intra-ovarian stages while the egg is undergoing active growth during the storing up of yolk. Later it thins out and becomes less con- 455 456 EMBKYOLOGY OF THE LOWER VERTEBRATKS < ,,. spicuous. Tlie zona radiata is usually looked upon as primary in its nature but this is by no means settled and some cnmprtent authorities regard it as secondary. Outside the zona radiata there may often be found a second envelope which does not show the perforations characteristic of the zona radiata: this also in the case of the large heavily yolkrd eggs becomes thinned out during the process of growth. Envelopes of this type are specially conspicuous in those Vertebrates in which there is no great development of tertiary envelopes secreted by the oviducal wall, e.g. Teleostean fishes. In such cases the FIG. 206. — A, cluster of eggs of lidellostoma,' attached together by the interlocking of tln-ir anchoring filaments ; B, apical portion of the egg-shell, showing the anchoring lilanu-nts projecting from tin- middle of the separable "lid." (Figure by lJ:ishf..rd Dean, 1'iom The (Iniiliridy .\lr with ;i -jroup of >\\\\ anrhoriii'_i tilaincnts ending ••;iHi in a lobcd umbrella-shaped expan- sion (Fig. 206). The piece of shell covering the ^rrminal polr is marked off liy a deep incision from the rest so as to form a lid \\liidi is forced offal, the time of hatching. Whatever the true nature of the envelopes under discussion, whether primary or secondary, they already exist round the egg before fertilization takes place, and as the substance of the envelope is, as a rule, impenetrable by spermatozoa there necessarily exist one or more openings or micropyles through which the fertilizing sper- matozoon makes its way into the egg. In the Myxinoids one such micropyle is found in the middle of the lid, surrounded by the con- centric circles of anchoring filaments. The presence of a micropyle in Lampreys and in Lung-fishes is not definitely established. In Lepidosiren it has been observed that the envelope enclosing the coelomic, and therefore unfertilized, egg is thick and gelatinous while a tin fertilization it becomes dense and horny. Possibly therefore during the first-mentioned condition it is penetrable by the sper- matozoa. In Teleostean fishes a micropyle occurs at the apical pole, and so also with Actinopterygian Ganoids except that in the Sturgeons there exist a group of openings (5-13 in the Sterlet, according to Salensky) instead of a single one. Of the more complicated arrangements of tertiary envelopes found in Vertebrates no better example could be taken than those found in the case of the Fowl's egg. These will be found described in Chap. X. In Birds in general the envelopes resemble those of the Fowl, differences occurring in details of relative size, shape, and colour of the shell. The " egg " (i.e. the zygote with its envelopes) appears to he largest relatively in Apteryx where it reaches about a quarter of the weight of the parent. The shape of the shell is impressed upon it by the pressure of the oviducal wall and differences in shape are no doubt due to differences in the peristaltic contraction. Thus the strong contraction of the oviducal muscles which, acting on the headward side of the egg, propels it onward, if combined with comparatively slight contraction on the tailward side of the egg will naturally cause the egg to assume a more or less markedly conical shape — the end of the egg directed towards the cloaca being broader than the other end. In some cases, as that of eggs laid on bare ledges of rock, this conical shape has probably had a definite natural selection value, in caus- ing any rolling movement of the egg to follow a strongly curved path. In other cases where there is less marked inequality of pressure on the two poles of the egg the shape is more regularly ellipsoidal. The eggs of Birds being commonly exposed to light and to view they very often show a characteristic colouring, either throughout the thickness or merely in the outer layer of the shell. In very 458 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. numerous cases the natural selection value of the colouring means of making the egg less conspicuous is obvious. In Reptiles the tertiary envelopes resemble those of Birds though in many cases, as in various Lizards and Turtles, there is no definite rigid shell. On the other hand there may be a certain amount of lime deposited in the outer layers of the shell -membrane. The albumen varies in amount : inSphenodon it forms only a very thin layer (Dendy, 1899). In Elasmobranch fishes the egg is again enclosed in a layer of albumen and this in turn surrounded by a shell. The shell is of a horny consistency and is frequently rectangular and pillow-shaped. Characteristic differences are found in different genera and species. Thus in the Skates (Raia) each angle is prolonged so that the egg Fi<;. 207. — Egg of Scylfiin/i, held in position by its four elastic filaments being wound round a plant. (Figure by Kopsch, from The Cambridge Natural History. } has an outline like that of a hand-barrow. In Scyllium (Fig. 207) the prolongations become long spirally coiled anchoring filaments : in Pristiurus two short prolongations occur at one end while the other end is simply rounded. . II. MODIFICATIONS OF THE ENVELOPES AND OTHER ADAPTIVE MODIFICATIONS OCCURRING DURING THE EARLY DEVELOPMENT OF THE AMPHIBIA. — The Amphibians form a group of Vertebrates which have taken less or more completely to a terrestrial existence in their adult condition. They have not been able to emancipate themselves entirely from the ancestral aquatic habitat, possibly on account «>t the feeble il»-v»-ln|.meiit of the horny outer layer of the epidermis. 'l'li» -y are still as a rule (3 nti rely aquatic during the early st a fcheir development, the eggs being laid in \vater and bhe young animal passing it I ce in the water. In a number of cases, part ieularly in Anura inhabiting tr«>pieal region- with a well-mai -krd dry season, very interesting adaptations are found wheivby tin- \niin- animal is enabled to pass a inmv ,.r :u>/>iis annm^st Amphilu.ms a similar process apparently takes place and in this case Bles (1905) attributes the formation of the ferment not simply to the diffuse activity of the ectoderm cells but to the action of a special " frontal gland." It seems not improbable that the formation of such hatching ferments will be found to occur very generally in aquatic Vertebrates. 460 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. ivtain its funnel shape. The eggs develop within the jelly up till the stage of a tadpole of 9-10 mm. in length. During this process the jelly apparently liquefies, until only a thin nu'inbraimus bag containing watery fluid surrounds each embryo. K \iMitually the remains of the jelly with its contained tadpoles trickles downwards into tin- water. If, as sometimes happens, the margin of the water has retreated from immediately below the leaf the tadpoles may still make their way for a distance of several inches to the pool by active jumping movements, helped it may be by a shower of rain. In the allied Phyllomedusa sauvagii, from the same neighbourhood, a similar mode of 2os. — i>ii !iii>i,\i;i »*i oviposition occurs, though here the nest is female composed of several leaves (Fig. 209). Agar (1909) finds in this case that both at the 1899.) commencement and end of oviposition there are laid a large number of spheres of jelly which contain no egg in their interior.1 The eggs are thus protected both above and below by a thick mass of eggless spheres. During the later stages of development the layer of envelope next the surface of each egg becomes greatly distended by the accumulation of fluid within it, the jelly between the eggs meanwhile diminishing in volume. The larvae with their huge external gills have thus considerable room in which to move freely. Eventually the envelope ruptures and the larva hatches. The nest thus comes to be occupied by a seething mass of tadpoles, floored and roofed in by a thick mass of jelly formed by the empty splniivs. Kv«'iitujilly — in from 12-24 hours after tbe bulk of the larvae have hatched — the jellv l-i^ins to deliquesce n.:. -JO'.).— /v/////. «,/,//,, and the larvae drip down with it into fgS* spawn' (After Agar' the water. Similai habits occur in other tropical Hylids, e.g. Phyllo- 1 In Hi'- roiiilii' ma I, inpni-nrte loi im :1 in 'jii.tutiiy in tin- uvidwi IM-I,, i, . ..mr., IHMTJ,, |(l, in, i ,1908). Interspersed with tint nonii.-il . , nl of Mii'h i-i^lrxs i-a|i>uli-s. Th.-.s.' a to be deposited rmiii'l small solid particles sm-h as fragments „[' shed ejiillieliuiii 'L.-hi-un, 1891). In Cento • riinrtii.iial. cmptv ternary iopeM are ii I in .the left ovidud (Braui, i(.»06). VIII DEVELOPMENTAL ADAPTATIONS 461 medusa ikeringii (von Ihering, 1886), Hyla nebulosa (Goeldi, 1895), Rhac.ophoru* reinwardtii (Siedlecki, 1909). In the last mentioned the eg.us an- deposited in a mass of foam mrlusud in one or several leaves (Fig. 210). At the appropriate time the central portion of the mass liquefies and the colourless tadpoles make their way into this (vnlral fluid — the superficial layer of the mass being hard and dry. Eventually the lower part of the mass softens and the liquid (nntaining the tadpoles trickles out on to the ground where the larvae are able to continue their development in the smallest puddles. In the second type of such adaptations the eggs or young are carried about, away from the water, by one of the parents. In the simplest of such cases no structural modification of the parent's body is involved. Thus in Alytes obstetricans the male draws the strings of eggs out of the cloacal aperture of the female and loops them round his thighs — the portion of oviducal secretion lying between successive eggs becoming highly elastic and gripping the thighs tightly. Oviposition takes place on land and the male pays only occasional'visits to the water. When one of these happens at the appropriate period the young hatch in the form of tadpoles while the male parent resumes his terrestrial habits. In a number of cases the transport of the young by the parent takes place at a later period, when the tadpole stage has been reached, the larvae adhering bo the back of the male parent and so being transported from one 'pool to another (Fig. 211, A). This habit occurs in various species of Dendrobates and Phyllo- bates (Brandes u. Schoenichen, 1901). In the most interesting cases however the transport of the eggs or young by the parent is associated with the making use of some particular structural feature of the latter — either permanent or specially developed for this purpose. In Rhacopliorus reticulatus (Giinther, 1876) the eggs are carried about by the female, adherent to its ventral surface. In Hyla goeldii (Boulenger, 1895) the eggs adhere to the dorsal surface of the female, only in this case the skin of the parent responds to the stimulus afforded by the presence of the eggs and grows up into a slight ledge surrounding them (Fig. 211, B). In Pipa americana (Bartlett, 1896) the cloaca of the female is protruded at the time of Oviposition as a large spout-like FlG. 210. — Rhai-nftlion's wardtii, mass of spawn with hatched tadpoles. (After Sied- lecki, 1909.) 462 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. structure which projects forwards between the dorsal surface of the female and the ventral surface of the male. The eggs pass out one by one through this and are distributed at fairly equal intervals over the dorsal surface of the trunk of the female. The skin now pro- liferates actively, growing up so as to form highly vascular partitions between the eggs, each of the latter coming to be enclosed in a deep pit. The mouth of this becomes closed in by a dark-coloured operculum, possibly formed of hardened epidermal secretion. Each egg is thus enclosed in a little chamber in which it passes through the early stages of its development, including a modified tadpole stage, and issues forth eventually (after about 82 days) as a young Toad. In another set of Anurous Amphibians the eggs undergo their development in a spacious single cavity within the parental body. In Khinoderma darwini (Jimenez de la ^Espada, 1872 ; Plate, 1897) this cavity is the enlarged unpaired croaking sac of the male, into which the eggs, to the number of from 5 to 15, are swallowed and from which the young issue after completing the tadpole stage. In the genus Nototrema the brood cavity is a special large pouch lying beneath the skin of the back, lined by involuted epidermis and opening to the exterior just in front of the cloacal aperture. In different species of the genus there is much difference in the length of time during which the developing embryo is retained within the pouch, the length of this period being apparently correlated with the size of the egg and the amount [.— A, male of Phyii<>- of food-yolk stored within it. Thus in bates f,-;infiiti'.>i carrying tad- JV". marsupiatum there may be as many as potaj; B, female of Hyia 2QO eggs in the pouch, each measuring gaddtt <;uTving eggs. (After , °® r,. /T. ilenger, 1895.) about 5 mm. in diameter (Brandes u. Schoeniohen, 1901), and the young make then way out as typical tadpoles which doubtless lead for a time a free aquatic existence before metamorphosis takes place. In N. oviferum (Weinland, 1854) the eggs are much larger MO mm., and fewer in number (about 15) and in this ease as in the allied N. testudineum and N. ,//**///»•*. which also possess large e^gs, the younu j^-o on developing within the pouch until after the period of metamorphosis. In Nototrema an interesting adaptive feature eharacterizes the • nal ^ills. T! mfi are present upon branchial arches I and Bfi of a I'.i, Lendei !;ilk which pass.^ at its outer end into a thin hi-jhlv vascular membrane formed by the fused and viii DEVELOPMENTAL ADAPTATIONS expanded outer ends of the two external gills. The two membranes so formed, one on each side of the l>n Aii important, point bo be remembered is that l he vitelline nd work though primarily nutritive, in function is necessarily also respiratory gaaeOdl mlcn-han-e lakin- place liei \\een the hlood circulating in Belfl and t he medium \\hich hat lies its surface. fl'h.- vil.-lline network la the primary hreat hin- or^an in the viii DEVELOPMENTAL ADAPTATIONS 465 majority of Vertebrates during early stages of development. In cases where the embryo lies in contact with maternal tissues the respiratory exchange takes placi- ultimately, through the thin intervening layers <>f fluid <>r envelope, between the blood circulating in the vitelline network ami dial circulating in die oviducal lining <»f the mother. In this way all tin- necessary preliminary conditions are provided for the evolution of a placenta, and as will be shown later these conditions are actually taken advantage of in some cases and a simple yolk-sac placenta is formed. In the niniv highly developed types of yolk-sac the splanchnic nit-sot lenu which surrounds the vitelline vessels sprouts inwards, forming irregular vascular septa which project into the yolk-sac. This modification, which brings about a great increase in the assimilatory surface, reaches such a development in Birds that towards the end of incubation these ingrowths form an irregular meshwork of vascular trabeculae traversing the whole of the yolk right to its centre. Eventually the yolk, whether in the form of a yolk-sac or a mass of heavily yolked cells, is enclosed within the ventral wall of the body. In the holoblastic Vertebrates this comes about as already indicated by the simple spreading of the blastoderm over the surface of the yolk so as completely to enclose it. In the Fowl the spreading of the blastoderm, and its derivatives the endoderm and mesoderm, round the yolk is never quite completed, there remaining a small circular patch at which the yolk is separated from the albumen only by the remains of the vitelline membrane (of. Fig. 215, v.m). Further in the Amniota the region of somatopleure bounding the coelomic space in which the yolk-sac lies becomes converted into amnion and serous membrane (cf. Fig. 215, A), and is eventually cast off, playing no part in the formation of the definitive body-wall. The yolk thus lies outside the limits of the definitive body -wall, projecting through the umbilical funnel which is bounded all round by the stalk of the amnion. Eventually, shortly before hatching, the edges of the umbilical opening are drawn over the yolk-sac in a manner which will be described later (see p. 475). In Lacerta vivipara in which the yolk-sac is reduced the remains of it are simply cast off according to Strahl. The most remarkable of the excrescences adaptive to an embryonic existence are the organs known as Amnion and Allantois —portions of the embryonic body which become greatly hyper- trophied and perform important functions during embryonic life but which are eventually, for the most part, shed about the time of birth or hatching and play no part in the formation of the body of the adult. AMNION. — The most nearly primitive subdivision <>t' the Amniota is the group Reptilia. and we accordingly turn to it and more especially to the Chelonia, which have been worked out by Mitsukuri (1891), to provide a foundation for our description. VOL. II 2 H pa. oe a.e. B 1 •_'. ( 'lifloiii.-in hl.i-toilrnns illu-,tr:itin;_: tin- .,lin-tn, • .lulhi \ I.,;.! ; ...... 1. 1, ,.,ii. i,. ..'i with hc:i.| ,.| ciul.l\.i sli,,\\. OH. vm DEVELOPMENT OF AMNION 46*7 In <'li. -Ionia the lirst indical ion <•!' aiiiiiinn | '..nnat ion appears at a stag*- like that represented in Fig. 212, A. The future body of th<- rinlirvn. indiraird by the medullary folds, lies flat on the surface of the egg, extending out all round into the blastoderm. The first si^ii of the amnion is produced by the front end of tin*, medullary plate coming to dip downwards so as to form a df«-j. slit <>i groove IMLJS. I'll' and iU3, c.g) curving tailwards on each side as seen from above. The posterior wall of this slit forms the anterior limit of the head of the embryo \\liilc its anterior wall forms the rudiment of the amnion (Fig. 213, a.e). The portion of blastoderm in front of and to the side of the head of the embryo is as yet two-layered, the mesoderm not yet having spread into it, and it follows that the amniotic rudiment is also two-layered. This region of the blastoderm, ect. FIG. 213. — Sagittal section through the head end of a Cheloniau embryo. (After Mitsukuri, 1891.) iinnintii- «•(!-,• : <.'/. cc] ilialic groove; <-<•(, ectoderm of medullary plate ; end, endoderm. which is still without mesoderm and which in this case forms the amniotic rudiment, is termed the proamnion. As development proceeds the head end of the embryo increases in size and as it does so it dips more and more downwards so as to deepen the cephalic groove or slit in front of it. While this is going on there takes place active growth of the ectoderm along the sharp edge of the amniotic rudiment (Fig. 213, a.e) in such a way that this edge becomes prolonged backwards as a solid flap, covering over the body of the embryo from before backwards. This amniotic flap continues to grow tailwards, its growing edge concave and prolonged backwards on each side (Fig. 212, B, a.e), until it reaches the tail end of the embryo, so that the whole of the latter is covered in by an amniotic roof. Nor does the process stop now : it goes on with the result that there is formed a long tunnel (Fig. 212, C, a.f) continuous in front with the amniotic cavity, i.e. the cavity between 468 EMBKYOLOGY OF THE LOWER VEKTEBKATES < n. the body of the embryo and the amnion, and terminating behind in an opening bounded above by a concave free edge (a.e). An important point to realize is the relation of the amnion to the cell layers. The first rudiment, as has been indicated, is com- posed of the two primary layers ectoderm and endoderm, and this applies also to the lateral prolongations backwards of the free edire. The whole of the amniotic roof however except these marginal parts is formed at first of solid ectoderm and of ectoderm alone (Fig. ect end. FIG. 214. — Diagrammatic transverse sections through Chelonian embryos ( CV< //< //,„, .somatoplriire : xy/. splam-hnoplrm Hplanchnocoele. 214, A, a.f). As development goes on the inesoderni ex tends between ectoderm and endoderm and then splits into somatic ;ind splanchnic layers. The result of this is that the endodcrm. \\itli its covering of splanchnic, iiirsoderm. sinks dm\ n and no longer jmijcct^ upwards on each Hide into the |,;isc of the ainnicm (Fig. 214, B). Tin- somatic mesoderm on ih«- other hand does continue to project into the kise of the ainnion just as did the .-ndodenn previously Figj 214, B). The (,i-i;jiii.ill\ siiiijde rct<»lcriiial roof of the ;iinniot ie cavit.y ;i pi-nci- .,! putting fix>m il-s lateral niargin inwards ftJKJ this split extends towards the mesial plane the amniotie fold of viii DEVELOPMENT OF AMNION 469 somatic raesoderm spreads with it. Except along the middle line the an mi otic roof thus becomes double — the inner roof being formed of ectoderm internally and somatic mesoderm externally, the outer roof of somatic mrsoilt«rm internally and ectoderm externally. Of llirsf two roofs the inner is the amnion (Fig. 214, B, am), the outer is the false amnion or serous membrane (/.a). The portion which n -tains its original condition of being formed of unsplit ectoderm (sa) may be called the amniotic isthmus or the sero-amniotic con- nexion (Mitsukuri). During later stages of development this becomes reduced to a thin vertical partition in which form it persists through- out, except in the region of the head where it disappears entirely so that there is here a continuous coelomic space stretching from side to side between amnion and serous membrane. The posterior tubular prolongation of the amniotic cavity becomes obliterated through part of its extent and in this way the amniotic cavity becomes completely closed. The first-formed part of the amnion, lying in front of the head of the embryo, remains for a time proamniotic in character, i.e. composed of ectoderm and endoderm, but eventually the meso- derui and coelomic space spread in between the two primary layers and the portion of the amnion in question comes to resemble the rest. As the body of the embryo becomes constricted off from the yolk- sac the basal edge of the amnion continuous with that of the em- bryonic somatopleure becomes tucked inwards so that the amnion, which formed in earlier stages a mere roof, conies to form a complete envelope. The amniotic cavity is filled with secreted fluid in which the body of the embryo floats. BIRDS. — The process of amnion formation in the Birds shows con- spicuous differences from that which has Jieen described for the more primitive Reptiles. Two of the chief of these differences seem to be associated with the fact that the amnion develops relatively later in the Bird, at a period when the head and anterior body region of the embryo project prominently above the general level of the blasto- derm and when the mesoderm has already split into splanchnic and somatic layers. Correlated with this fact we find (1) that in the Bird the amniotic rudiment has to grow upwards so as to surround the projecting head and trunk, and (2) that the upgrowth is com- posed of somatopleure only. The amnion may be said to originate as a kind of wall, formed of an upwardly projecting fold of somatopleure, which conies to surround Hi-1 actual body of the developing embryo. This wall is not abso- lutely vertical : it is tilted, or inclined inwards, towards the middle of the embryonic body. With increasing growth it projects more and more over the .body of the embryo, its free edge bounding a gradually diminishing op'-iiin^, through which the body of the embryo is visible when looked down upon from above. Eventually this opening is reduced to vanishing point and the body of the embryo is completely 4*70 EMBRYOLOGY OF THE LOWER VERTEBRATES . 11. covered in by a double roof formed by the amnion and the serous membrane. The amuiotic fold does not develop with equal activity through- out its extent. Its growth is much mor^ active anteriorly than else- where, with the result that the headward portion of the fold becomes* extended rapidly backwards as an amniotic hood over the head and anterior end of the body of the embryo (cf. Figs. 233, 235, 236). The last remnant of the amniotic opening is consequently situated quite near the hind end of the body. Correlated with the later appearance of the amniotic hood — at a time when the coelomic cavities are extensively developed — it is at no period composed throughout, from side to side, of a simple layer of unsplit ectoderm as was the case with the Chelonian. It is of interest to notice however that the sero-amniotic isthmus has not altogether disappeared, although it never has the breadth that it has in early stages in the Chelonian. The details of amnion formation are readily observable in the Fowl and have been fully described by Hirota (1894). The process takes place as follows : The first step consists in the appearance of a crescentic upgrowth of blastoderm just in front of the head of the embryo at about the stage of 14 segments. At this period the meso- derm has spread forwards on each side but has not yet extended into the space immediately in front of the embryonic head (proamnion). Where the mesoderm is present it has split to form the coelome and owing to this being filled with secreted fluid the somatopleure bulges up somewhat so as to be conspicuously marked off from the flat proarnniotic area. The amniotic fold makes its appearance just about the anterior boundary of the proamnion. As it increases in height it overlaps the head of the embryo and grows backwards over it as the amniotic hood (Fig. 233). Into the fold the mesoderm and coelomic cavities have already penetrated. Where the mesoderm from the two sides meet in the mesial plane of the hood the t\v<» portions of coelome do not open freely into one another but remain separated by a septum of mesoderm — the mesodermal sero-amniotic isthmus. At an early period of the backgrowth of the umniotic hood the ectoderm in the middle of its free posterior ed^v is seen to project head wards as a small wedge, the base of which is formed by the growing edge. As this wedge is carried backwards by the continued progress of the amniotic edge it leaves behind it a kind of trail in tin- form of a continuous line, or rather partition, of ectoderm conned im_r the ectoderm on the outer surface of the amniotic fold with that on its inner surface. This is clearly the ectodermal sero-anmiot ie, isthmus of the K'epi ile ]iersistin^ in a much attenuated form: the ;it feiiuat ion hein^ due to the fact that the ooelomic <-nded much nearer to the mesial plane than in the corresponding sia^c of a muion formation in the L'epiile. Ip till about the time when Ihe amniofic hood has completed its backgrowth it- oavity the amniotie codome remains divided viii DEVELOPMENT OF AMNION 471 into two separate halves l»y ;i septum, \vlii(;h in front is purely mesodenual hut throughout tin- rest of it- extent is traversed hy tin- erto.lermal sen i-a in niotic isthmus. The anterior, purely meso- dermal, purl «»f the septum disappears early in tin- fourth day so as to make th.- amniotio coelome continuous fn»m side, to side, hut the, rest of the septum persists throughout the whole period of development although its central ectodermal portion becomes gradually ivdueed and hy the tenth day lias completely disappeared. Towards the end of the second or early in the third day the tail of the embryo begins to project, bending ventrally and dipping downwards as it does so. As it does this the tail comes to be hidden under a projecting amniotic fold precisely as happened at the head end except that here the coelomic cavity is already completely continuous across the mesial plane there being no trace of a septum po-amniotic isthmus. The free edge of this "tail fold" of the a in n ion is, as was that of the "head fold," concave only here the concavity is directed headwards. Early in the fourth day the concave edges of the head and tail folds become continued into one another at about the level of the hind limb rudiment, so that the body of the embryo is now surrounded by a continuous amniotic fold — most highly developed anteriorly where it forms the amniotic hood, less so in the caudal portion and least of all laterally. The more or less elliptical opening bounded by this fold, through which the dorsal surface of the embryo is exposed, gradually shrink the fold grows and eventually, during the first half of the fourth day as a rule, it becomes obliterated and the amniotic cavity closed. The true amnion at first closely ensheaths the head and trunk of the embryo but from about the fifth day onwards watery amniotic fluid is secreted into its interior so as to form an extensive water jacket in which the embryo is suspended (Fig. 215). For a considerable period the embryo is gently rocked to and fro in the fluid by the slow rhythmic contractions of muscle fibres which develop in the somatic mesoderm covering the amnion on its outer surface. The development of the amnion in the Sauropsida in general is adequately illustrated by the two types which have been described. There occur variations in detail. Thus the inequality in the activity of growth between the anterior and posterior portions of the amniotic fold so marked as a rule may be practically absent (Chameleons), or it may reach an extreme limit, the posterior portion of the fold being obsolete and the anterior portion continuing its backgrowth past the tail end of the embryonic body to form an amniotic tunnel, as in the Chelonians above described (Sphenodon, Gannet — Sula, Puffin — Fratercula).1 ALLANTOIS. — The allantois may also be conveniently studied in the T.ird. In the Fowl it makes its first appearance as a little clear 1 Schauinsljind, 1906. EMBRYOLOGY OF THE LOWER VERTEBRATES « n. , projecting from the ventral side of the trunk near its hind all' .... /<* coel sa coel alh spl a.c , Ilii :-,! rating lli.- aiTan^riiH-nt ol .•uuiiioii, allanttii.s. .-(.•.. in tin- Foul. \;i., i.iiiie, 1908.) A, fourth «la\ ; li, ninth «i..\. '/.., ainniot ic caxity; nib, albumen; nil, allaiiton- ca\it\ ; .•!, ,ini> , BOOI mmbnne ; «, Mrotmniofctfl isthmus; >/-/, >]il:iiicliiin|.l' •. :l.-llin.- nn«inl>rain- ; //, \olk; 1, niil.'i \v:dl nf iillant' • u.iii i)t .ii; :: • VIII ALLANTOIS 473 end (Figs. 239, 240), and serving for the reception of the renal s tion. The study of sections shows that the allantois is simply a pocket of the ventral wail of the gut towards its hind end — corre- sponding exactly with the bladder of an Amphibian. It is thus lined with endoderm and covered externally with splanchnic mesoderm. The allantois like the bladder of the Amphibian bulges into the Bplanchnocoele. As development goes on the allantois. di .-tended witli fluid, increases in size, projecting on UK- ri^ht or upper side nf the embryo till it comes in contact with the inner surface of the somatopleure (Fig. 215, A, all), and with still further growth flattens out against the somatopleure taking a somewhat mushroom-like shape. In the case of an independently living animal such as an \-'\<.. 'J15A. — Diagram illustrating the arrangement of ainnion, allantois, etc., in the Fowl. (After Lillie, 1908.) C, twelfth day. nib, albunim ; n.ll, allantoic cavity ; y, yolk. Amphibian the allantoic outgrowth of the gut can only increas. in size within the restricted space of the splanchnocoele which is already occupied by the viscera. In the Bird embryo on the other hand there are available for the growth of the allantois the wide-spreading extensions of the coelome, on the one hand between amnion and serous membrane and on the other over the surface of the yolk. The allantois accordingly spreads out all round towards the limits of this space (Fig. 215, B). As it does so it loses its rounded vesicular form, its proximal (Fig. 215, B, 2) and distal walls (Fig. 215, B, 1) approach- ing one another. The mesoderm covering its outer surface tends to undergo secondary fusion with that of neighbouring structures. Thus about the end of the sixth day it fuses with the adjacent surface of the amnion. Again towards the time of hatching, a similar fusion takes place with part of the yolk-sac. The most important of these fusions 474 EMBRYOLOGY OF THE LOWER VERTEBRATES < 11. however is thai, which commences early in the fifth day, with the inner surface of the serous nu'inlirane. At a comparatively early period (during the fifth day) the ineso- derm covering the allantois becomes vascular and as the organ becomes flattened its proximal or inner and its distal or outer walls become strikingly different as regards their vascularity, the outer wall developing an extremely rich network of capillary 1 >lood-vessels with very small meshes, while the inner wall possesses merely a sparse network together with the large vessels of supply. This difference between the two walls of the allantois becomes conspicuous about the end of the sixth day of incubation in the common Fowl. The difference is associated with the fact that the distal wall of the allantois is destined to become the great respiratory organ, taking over this function from the vascular area of the yolk-sac by which it is performed during the early stages of development. In correlation with the more efficient performance of this function, the albumen, or white, as it gradually shrinks in volume and acquires greater density gravitates down to the lower side of the egg thus bringing the mush- room-shaped allantois close up to the shell membrane on the upper side. The process is still further facilitated by the ectoderm of the serous membrane becoming reduced to a very thin — hardly distin- guishable— layer in the region where it is underlain by, and fused with, the allantois. The capillary network thus comes into very close relation with the shell membrane and the overlying porous shell, and gaseous exchange can readily take place between the blood circulating in the network and the external atmosphere. As development goes on the respiratory needs of the embryo become greater and greater and these are met by the allantois spreading outwards all round its periphery, so as to provide a greater and greater respiratory area. During this spreading outwards of the allantois the three main allantoic vessels are somewhat retarded in their growth with the result that each one causes an indentation of the growing edge of the allantois beyond which the allantois bulges on each side. When the growing edge of allantois comes, after about nine days' incubjit ion. into the neighbourhood of the remaining mass of albumen, a new phenomenon appears inasmuch as the allantoic margin with its covering <>f serous membrane proceeds to grow onwards close under tin- shell membrane as a circular fold recalling the amniotie fold and enclosing the mass of alhumen (Figs. iM."., B,215A, C), 'I he • •etodernial lining of tin- cavity so formed sprouts out into the albu- men in tin- form of im-giilar projections which become vaseiilari/ed from the allantoic mesoderm and no doubt, play a part in absorbing t he last remains of t he alhumen. I '< about the cud of the second week of incubation the shell memb lined throughout, the whole of its extent by the highly lar outer wall of I be allantois. This remains the breathing ; until a day or I wo before hatching the young chick's beak VIM PKVELOPMENTAL ADAPTATIONS penetrates the air-space and pulmonary breathing lupins. The allantoic circulation then gradually becomes sluggish and stops, and eventually by a. ])rocess of autotoniy the allantois is separated from I IK- ho.ly .»!' tin- embryo and is left behind as the vaseular membrane seen lining the fragments of shell from which a ynim^ bird has hatched. ENCLOSURE OF YOLK-SAC WITHIN THE EMBRYONIC BODY.- already indicated the yolk-sac becomes eventually (about a couple of days before hatching in the case of the common Fowl) enclosed \v it hi n the* body-wall. The process by which this is brought about appears to be as follows (H. Virchow). With the uro\uh of the embryo a great increase takes place in the area over which the amnion is fused with the proximal wall of the allantois (cf. Fig. 215 A, C), the compound and highly muscular membrane so formed extending eventually almost completely round the yolk-sac. At its cd^e it is continued onwards by the somatopleure, this latter termin- ating round the circular area where the yolk remains exposed. The yolk-sac is thus contained in a space the wall of which is formed of the following components in sequence starting from the body of the embryo : (1) amnion, (2) amnion fused with proximal wall of allantois, (3) proximal wall of allantois and (4) somatopleure in the region of the distal pole of the yolk-sac. The proximal portion of this wall, being formed of amnion, is necessarily continuous with the body-wall of the embryo at the umbilical opening and further those parts of it formed from amnion and allantois are highly muscular and con- tractile. During the later stages of development this wall slowly contracts and as it does so the yolk-sac is pushed into the umbilical opening which closes after it. EVOLUTIONARY ORIGIN OF THE AMNION. — As regards this question, which has excited much controversy, the following appears to the present writer to be the working hypothesis which fits most easily the facts so far as they are known. (1) The amnion originated as a fold of blastoderm round the body of the embryo (Fig. 216, A, B). As has already been shown the amnion arises in this way in ontogeny in the Keptilia which are generally recognized as being the most primitive Amniotes. The same holds for the Birds and for some of the Mammals. The Mammalia as a -roii]» are admittedly descended from ancestors in which the egg was large and meroblastic as it is in the Keptilia. This is indicated, apart from other convincing evidence, by the fact that they still exhibit in ontogeny a well-developed though yolk less " yolk-sac." It follows then that it is inadmissible to regard facts derived from the study of certain mammals in which the mode of amnion formation during ontogeny is of a different, even though apparently simpler, type as constituting important evidence in regard to the phylogenetic origin of the amnion, as has been done in par- ticular by Hubrecht (1895). Jfc- am. all. '.•mi illustnitin- tli.- i-\i)liitii>n <>l .-minion. etc, • 'Mt> , niimlic i 476 ; //<>/7///m Iwo or three, in /,'/// //*»/«'///* seven or .-i^ht. The embryn \\ilhin the uterus [fi Mil! nourished primarily hy the yolk in its \" Tin primitive mode of nourishment has not. 1 See Gudger, 191'J. v FIG. 217.— Egg-shell of Acanthias pnsent millimetres. VIVIPARITY IN FISH I - 479 yd. been replaced by a pn>ee>s <>f absorption from the uterine wall a- is the ease in the Mammalia. I'.ut the uterine wall already plays a part though a minor tun- ill providing food material for tin- young individual by its glandular activity. The beginnings of this an- in the albuminous ilnid enclosed within the eg^-shrll, and it is possible that tlie elongated gill-filaments of the embryo play a part in absorbing nourishment from this. A further development consists in the secretion of an abundant "uterine milk " which is drawn into the pharynx through the spiracles by precociously occurring move- ments like those of respiration and passed on into the digestive tract. In accordance with its glandu- .^^^^Hra lar activity the lining of the uterus frequently undergoes an increase of area by growing out into villi or trophonemata (Wood - Mason and Alcock, 1891). In the Sting- ily s specially enlarged tropho- nemata may be drawn into the pharyngeal cavity of the embryo through its greatly dilated spiracles so that their secretion reaches the alimentary canal of the young fish directly (Fig. 218). During the later stages of intra- uterine development there usually comes about an intimate relation- ship between the surface of the yolk-sac and that of the uterine lining and in association with this there is found a varying degree of FlG> 21 8.— Portion of uterus of Specialization Of the Uterine lin- plataea micrura slit open to show an ing (Ercolani, 1879 ; Widakowich, embryo with the trophonemata (/r) pro- -i rv/\hX mi • i i. i lecting into its spiracles. (After Wood- 1907). This latter may be smooth JMa80£ :lll(l Alco£t 18910 (Squatina angelus, Notidanus cine- reus), or project into longitudinal folds so as to give increase of surface (Acanthias vulgaris, Scymnus lichia), or grow out into papillae or trophonemata (Torpedo, Pteroplataea). Or finally it may develop folds which interlock with grooves on the surface of the yolk-sac, the uterine and yolk-sac surfaces being in the most intimate contact so as to constitute physiologically a definite yolk- sac placenta (Cuirlmrinx ylaucus, Mustelus laevis, etc.). Amongst the Teleostean fishes viviparity occurs occasionally. in at least half-a-dozen different families; the Cyprinodontidae, Scorpaenidae and Embiotocidae furnishing the greatest numb* cases. They are particularly numerous amongst the Embiotocidae and Scorpaenidae of the western coast of North America (Eigen- maun, 1894). 480 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. The eggs are retained in the ovary, either in the follicle, or in the cavity of the ovary ; more rarely in the dilated oviduct or uterus. The developing embryo may depend for its nourishment upon the yolk (Scorpaenidae) ; it may absoi^ nourishment by the surface of the yolk-sac which grows out into villi (Anableps) ; or the nutritive secretion of the ovarian wall may be taken into the alimentary canal and there digested (Embiotocidae). Among the Amphibia true viviparity is rare. A well-marked case occurs in Salamandra atra (Wiedersheim, 1890). Here a large number (40-60) of eggs pass into the oviduct when breeding is about to take place but of these all except the one (in rare cases as many as four) next the cloacal opening simply break down forming a kind of broth which fills the oviducal cavity. The embryo nourishes itself, after it has used up its own yolk supply, by gulping down and digesting this fluid, which contains not merely the yolky debris of disintegrated eggs, but also large quantities of red blood corpuscles derived from extensive haemorrhages of the uterine wall. Perhaps the most striking feature of the Mammalia is the extreme degree of adaptation which they typically show to an intra-uterine mode of development in which the embryo leads a parasitic existence attached to the uterine lining of the mother. In accordance with this the external ectoderm of the blastocyst becomes modified to form organs of attachment which eventually, in the region of the yolk-sac and more especially in the region of the allantois, become vascularized and elaborated into the complex nutritive and respiratory organs named placentae. This being so, it becomes of much interest to enquire whether amongst those Amniota which are lowest in the scale of evolution — the Reptiles — there are any foreshadowings of the type of adaptation to intra-uterine development found in the Mammalia. Probably numerous such cases exist but at the present time, with our extremely imperfect knowledge of Reptilian development, we are acquainted with only a few. The most interesting of these is that of the Italian Lizard Chalcides tridactylus (Seps chalcides). Giacomini's description of this (1891) may be said to form the foundation of what will one day probably form an important chapter in Vertebrate embryology. The eggs, which measure about 3 mm. in diameter, are first found in the oviducts early in May, while the first young are born towards the end of July, the period of gestation thus being between one and two months. ' The eggfl hecimie spaced out alonu' the oviduct or uterus, so as to ^ive it a iiionililorin appearance, each e^ heing arranged \\iili its apical pole towards the mesomelriuni. At about the middle of gestation the "egg" presents the appearance shown in Fig. 219, A, the who!.- formin- a kind of blastocyst about 7 nun. in diameter. The outer surface; is formed by the ectoderm of the -.•TOMS membrane. Within t he s.-ruiis im-mhrane there can be the allantois \\ith transparent, richly vascular, wall and the yoik- 1 Alimit M.\t y-tivf d;iy> .u-cui.lin^ \,< M in^a. /ini. VIII EMBKYONIC ADAPTATIONS IN REPTILES 481 sac, more opaque than the allantois and already much smaller than the latter as seen in surface view. The edges of the allantois and the mushroom-shaped yolk-sac fit closely together and between them is the body of the embryo contained in the amnion. As in other Saurop- sidans or Prototherian Mammals the yolk-sac lies on the embryo's left, the allautois upon its right — upon the side, in this case, next the mesometrium. As development proceeds the exposed area of yolk-sac I" -mines gradually reduced by the encroachment of the allantois. The latter however remains merely in contact with the edge of the yolk-sac and never comes to surround it. Over the yolk-sac area there remain visible for a long time the remains of the vitelline mem- brane (of. Bird). Both allantoic and yolk-sac regions of the surface develop plaeental ar- rangements, the former being physiologically the more impor- tant of the two. The allantoic placenta is already becoming apparent at the stage shown in Fig. 219, A, in the form of an elliptical area at the mesometrial pole which adheres to the uterine lining by means of numerous little pro- jections which interlock with similar projections on a corre- sponding uterine area. As de- velopment goes on the egg assumes an elongated shape (Fig. 219, B). The whole of the uterine lining in contact with the outer surface of the FIG. 219.—" Egg" of Chalcides tridacfi/ln*. (After Giaoomini, 1891.) A, 7 mm. in diameter, showing yolk-sac (i/.s), allan- " egg " is provided with a rich ™*(:'")> ;ui(i ((." 'f 1>ort;°° of a;iantoic plfacenta (?°; *?o IT- B» 15'16 "n"- '" loll#'st' diameter, seen from apical Capillary lietWOrk lyillg ClOSe pole, showing foetal portion of allantoic placenta (pi). beneath the uterine epithe- lium and here and there insinuating itself between the epithelial cells. Over the allantoic plaeental area the maternal projections now form undulating ribbons attached along one edge and free along the other. On the surface of these ribbons the uterine epi- thelium instead of beiuu flattened as it is elsewhere is columnar and has a glandular appearance. With the ribbon-like projections just mentioned there interlock the somewhat similar projections of the foetus. These are also covered with columnar epithelium close under which lies a rich capillary network. The latter is not confined VOL. II 2 I 482 EMBRYOLOGY OF THE LOWER VERTEBRATES m. to the actual placental projections for even the smooth parts of the surface over the allantoic area are provided with an extraordinarily rich network of capillaries which show an even more marked tendency than those of the uterus to penetrate into the epithelium. Over the smooth area the foetal and maternal surfaces are in intimate contact, so that the two capillary networks lie parallel and close to one another, separated only by two very thin epithelial membranes. In the region where foetal and maternal projections interlock chinks are apparent between the two in which there appear to be traces of a fluid material — probably nutritive and secreted by the maternal epithelium which as already mentioned has in this region a glandular appearance. The yolk-sac placenta is less highly developed. In the region of the centre of the yolk-sac flattened ridge-like projections also appear which interlock with corresponding uterine projections and become vascularized as the mesoderm spreads beneath them. Between the two surfaces is the remnant of vitelline membrane but this, gradually disap- pears so that foetal and maternal surfaces come into intimate contact. Chalcides (Gongylus) ocellatus, another Italian lizard, is also viviparous and in it occur similar though less marked adaptations to viviparity (Giacomini, 1906). Here in the later stages of gestation the general arrangement of the foetal envelopes resembles that in C. tridactylus. The allantoic region of the foetal surface is smooth and possesses a rich capillary network. It lies in immediate contact with the uterine lining, which in this region is covered with very thin flattened epithelium overlying an extremely rich network of maternal capillaries. The portion of uterine lining in relation with the vitelline region of the foetal surface is less richly vascular, is covered with thicker epithelium of vacuolated cells with large nuclei, and is thrown into low folds which interlock with corresponding folds of the foetal surface so as to form an incipient yolk-sac placenta. The foetal epithelium of this region is thickened and in places columnar and appears t«» have an absorbent function. As in C. tridactylus remains of membrane are to be seen for a time between the foetal and maternal surfaces in this region. To sum up, we find in Chalcides ocellatus a less advanced sta^e of adaptation to intra-uterine development than in C. tridactylus 1'robably similar conditions will be found in various other viviparous Li/ards as r.ij. in tin- Australian. Trachysaurus and Tiliqua scincoii/f* (Cyclodu* l,uisi in<_: throughout development and the lortal envelopes and uterine lining ivmainiii'j- practicallx unmodified. Thus in the three -els of LVplilo above mentioned we see three steps in the evolution of \i\iparity: vin EMBRYONIC ADAPTATIONS IN REPTILES 483 (1) the mere retention of the egg within the uterus, the shell still remaining and iu» intimate relations being developed between foetal and maternal tissues (Anguis, Vipera, Coronella), (2) the rupture at an early stage and eventual disappearance of the shell, and the coming into intimate relations of foetal and maternal tissues, both becoming highly vascular and there being an attempt at the formation of a yolk-sac placenta (Chalcides ocellatus), and (3) the development of an allantoic placenta (C. tridactylus). LITERATURE Agar. True. Zool. Soc. Lond., 1909. Bartlett. Proc. Zool. Soc. Lond., 1896. files. Trans. Roy. Soc. Edin., xli, 1905. Boulenger. Trans. Zool. Soc. Lond., xii, 1890. Boulenger. Proc. Zool. Soc. Lond., 1895. Brandes und Schoenichen. Abhandlungen Naturforsch. Gesell. Halle, xxii, 1901. Brauer. Zool. Jahrb. (Anat.), xii, 1899. Braus. SB. kgl. preuss. Akad. Wiss., 1906. Budgett. Quart. Journ. Micr. Sci., xlii, 1899. Dendy. Quart. Journ. Micr. Sci., xlii, 1899. Eigenmann. Bull. U.S. Fish Commission, xii (1892), 1894. Ercolani. Mem. Accad. Bologna (Sez. Sci. Nat.), x, 1879. Espada, Jimenez de la. An. Soc. Espanola de Historia Natural, i, 1872. Giacomini. Monitors zoologico italiano, ii ; and Arch. ital. Biologic, xvi, 1891. Giacomini. Mem. Accad. Bologna (Sez. Sci. Nat.), Ser. 6, iii, 1906. Goeldi. Proc. Zool. Soc. Lond., 1895. Gudger. Proc. Biol. Soc. Washington, xxv, 1912. Giinther. Ann. Mag. Nat. Hist. (4), xvii, 1876. Haacke. Zool. Anz., viii, 1885. Hirota. Journ. Coll. Sci. Tokyo, vi, 1894. Hubrecht. Verh. Akad. Amsterdam, 2de Sect., 1895. von Ihering. Ann. Mag. Nat. Hist. (5), xvii, 1886. Ikeda. Aunot. Zool. Japonenses, i, 1897. Kerr, Graham. Phil. Trans. Roy. Soc., B, cxcii, 1900. Lebrun. La Cellule, vii, 1891. Lillie. The Development of the Chick. New York, 1908. Mitsukuri. Journ. Coll. Sci. Tokyo, iv, 1891. Plate. Verh. Deutsch. Zool. Ges., Kiel, 1897. Schauinsland. Hertwigs Handbuch der Entwicklungslehre, I. Jena, 1906. Siedlecki. Biol. Centralblatt, xxix, 1909. Virchow, H. Internat. Beitrage zur wiss. Medizin, I, 1891. Weinland. Arch. f. Anat. u. Phys., 1854. Wetzel. Arch. Entw. Mech., xxvi, 1908. Widakowich. Zeitschr. wiss. Zool., Ixxxviii, 1907. Wiedersheim. Arch. mikr. Anat., xxxvi, 1890. Wintrebert. C. R. Soc. Biol. Paris, Ixxii, Ixxiii, 1912. Wood-Mason and Alcock. Proc. Roy. Soc. Lond., xlix, 1891. CHAPTER IX SOME OF THE GENERAL CONSIDERATIONS RELATING TO THE EMBRYOLOGY OF THE VERTEBRATA IN the course of the preceding chapters many of the general principles of vertebrate embryology will have made themselves apparent : the present chapter will deal shortly with some others of these principles which seem to require special notice. (1) THE ONTOGENETIC EVOLUTION OF THE ZYGOTE INTO THE COMPLETELY FORMED INDIVIDUAL. — The Vertebrate commences its individual existence as a zygote — a single cell — in which the specific characteristics, derived from the paternal and maternal ancestors, are already present though not recognizable. That this latter statement is accurate is demonstrated by such a fact as the following. The pelagic fertilized eggs of different species of Teleostean fishes show no trace of the specific features which characterize the adults. Such distinguishing features as are present and enable a specialist to identify them are mere differences in size, amount of yolk, colour of oil globule and so on, and have nothing to do with adult characteristics. Yet if a selection of such eggs are allowed to develop together under a homogeneous set of environmental conditions each is found gradually to unfold the complete array of characteristics which distinguish its own kind. As the various zygotes have developed under the same identical set of environmental conditions it follows that the differences which gradually become apparent cannot be due to the moulding influence of ext» -rnal conditions: they must have been already present though in invisible form in the zygote. It follows 1'u rt her that the evolution of the zygote into the adult is in the main not a process of acquiring greater and greater c<» in plexity, in tli> of acquiring new properties, but ratlu-r »if tin- localization — tin- s.-^ rogation — of special peculiarities in particular portions of the, individual, so that these portions assume a specific <•li.nact.-r and become n-eo^ni/;ibl«- as definite, tissues or organs. The peculiarities were ilieiv to ln-jin \\ith. but they we re diffuse and therefore UnieOOgllizable .^Mn.-\\hat in the same way as the various 484 en. ix ONTOGENETIC EVOLUTION 485 colours of the spectrum are present in ordinary " white " light but ;ire invisible until they are sifted out from one another by the action of a prism. The lesson learned from the developing pelagic zygote — that in its case the full equipment of the complete individual is provided from internal sources — is one which should ever be borne in mind. It makes it easier to realize that in other cases, where the developing organism exists in a less homogeneous environment and where it has to fend for itself, characters impressed upon it directly by the environment, however conspicuous, are still superficial as compared with the really fundamental characters already present in the zygote. The course of ontogenetic development from the zygote stage involves two main processes, (1) increase in bulk accompanied by the assumption of a multicellular condition, and (2) differentiation of parts i.e. the segregation, into localized portions of the living substance, of peculiarities which were in the zygote distributed with- out definite arrangement. The topographical differentiation of the developing embryo does not necessarily keep exact pace with the subdivision into cells. Thus in Amphioxus the egg appears to be still homogeneous throughout up to the time when it has segmented into 8 or even 16 blastomeres for even at this stage a blastomere isolated from its neighbours experimentally may go on for a time pursuing the same course of development as it would have done were it a complete zygote. In other cases, as appears to be the rule in the Frog, the first step in segregation — the segregation of characters belonging to the right and left halves of the body into corresponding hemispheres of the egg — would appear to take place in the zygote stage i.e. before the appearance of the first cleavage furrow. The progressive segregation of specific characters in the various parts of the developing individual is beautifully brought out in the case of various invertebrates by the elaborate studies on " Cell lineage," some of which have been fully described in Vol. I. The animal individual lives its life under a particular set of environmental conditions, constituted by the external medium — water or air — with its other living inhabitants : the latter play an important part, it may be by such comparatively simple and direct methods as by affecting the composition of the external medium, it may be by far more complex and obscure influences due to biological inter-relationships. The individual is able to go on living because of its organization and its living activities being fitted, in the most intimate manner, into the particular set of conditions which con- stitute its environment. So also with the various parts of the body — organs, tissues, individual cells — of the young developing individual. Each lives amidst an environment of extreme complexity and of perfectly definite type of complexity, conditioned by the nature of the body of which it forms a portion and by the character of the parts of the 486 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. body which are in proximity to it. Its living substance is every- where bathed by — and no doubt intimately adapted to life in — an internal medium, watery fluid laden with the products of metabolism of the living substance as a whole. The differences in function of the various organs and tissues necessarily involve differences in their metabolic activity and therefore differences in the chemical nature of the contributions whicli they make to the complexity of the internal medium as well as differences in the substances which they withdraw from the internal medium for their own needs. Physiologists recognize that changes in the constitution of the internal medium play an important part in exciting and controlling vital activity.1 In the ordinary life of the animal important examples of such influence are afforded by changes in the activity of the normal function of an organ, as for instance when the pancreas secretes actively in response to the presence in the internal medium of a special substance secreted by the intestinal wall when stimulated by food material. Other examples are afforded by changes in growth- activity — as of the skin in response to a change in the amount of substance secreted by the thyroid, or of the mammary gland in response to the presence of substances produced by the metabolic activity of the foetus. There is no reason to doubt that the living cells and tissues and organs of the embryo are similarly adapted to and influenced by the constitution of the internal medium, and if this be so the influence in question must play an important part in development. A possible example is afforded by the experimental result that the grafting of the developing optic cup of an Amphibia^ embryo into near proximity to ectodermal tissue (such as the pigment-layer of the retina, the wall of the brain, the olfactory epithelium, the external ectoderm of the head or trunk) is apt to induce that ectoderm to develop into a lens (Bell, 1907). Such influence upon one portion of embryonic sub- stance by another portion in its neighbourhood may well be exercised through chemical or other changes produced by the specific meta- bolism of the latter in the internal medium in its neighbourhood. A corollary to the consideration outlined above, which has an important beariii'.: upon much work in experimental embryology, is that it is unwise to place reliance upon the mode of develop- ment of an organ-rudiment U-injr normal, unless its environment is normal. (2) CELLUI.AKI/ATION OF TIIK XVCOTE: CELLULAR CONTINUITY AND DISCONTINUITY. — Protoplasm bein-j a >i»fi semi-fluid substance a particle of ii,as it in< •!• 'i volume during tin* process of Lrrowth which ifi associated willi normal metabolism, would soon reach a m<-chaii]cally unstable condition, in which retention of it s character- istic form, or even coln-mii. \\ould be impossible. In nature a to this is provided by the protoplasm undergoing fisn mi. In the Prolo/oa the products of this li.->M"ii normally break apart ami 1 For their bearing upon rvulutioniu \ change sue Parker (1909). ix ONTOGENETIC EVOLUTION: CELL-DIVISION 487 lead an independent, existence, while in tin- Metazoa the subdivision is less complete and tin- growing mass of living substance continup.s tn exist/ as a mix-lent individual. The ])hysiological advantages of subdivision of tin- individual body into cellular units is apparent. It renders possible, the intercellular deposit of rigid skeletal materials which act as a support to the organism as a whole: it facili localization of t'unetion and enables blocks of units specialized for particular functions to be transferred during ontogeny to the positions in which they will be most useful: it enables other units to move hither and, thither, either by their own activity or by being swept along in a circulating stream of fluid, to wherever they are specially needed in the course of the ordinary vital processes: and it is of enormous importance in relation to attacks upon the organism from without, whether by limiting the area of injury to comparatively small tracts of living substance or by enabling portions of the living substance specialized for defence to be mobilized and ready to concen- trate at the point of attack. Modern science impresses upon us the importance of regarding the individual not merely as an aggregation of cells and organs, but rather as a mass of living substance imperfectly subdivided up into cells and organs : imperfectly because each cell and each organ is inextricably linked up in the living activity of the whole individual. It brings to our ' notice numerous tissues in which the actual living substance of the constituent cells is linked up by intercellular bridges of protoplasm : it tells us of particular cases of developing embryos where similar intercellular continuity is apparent. The question is thus raised : are we correct in our belief that actual complete separa- tion of cells takes place as a general rule when they undergo fission during ontogeny? More especially is it really the case that the individual blastomeres of the segmenting egg become completely separated from one another: is it not rather the case that the apparently complete separation is only apparent, that the individual blastomeres remain continuous through fine protoplasmic bridges : and that cases of intercellular continuity observed in the adult are merely expressions of the fact that such bridges persist throughout t he whole period of development ? That the latter is really the case has been held by various workers and supported particularly strongly by Sedgwick (1895, 1896). It will however have been gathered by the reader from Chapter I. that such a view is in the opinion of the present writer not tenable. The fact that the blastomeres of a segmenting egg tend to take a spherical form, or at least to be bounded by convex surfaces, seems by the ordinary laws of surface tension to indicate that these blastomeres are not continuous \vitb one another. Continuity of substance between the cells of the embryo or adult is therefore when it occurs a secondary and not a primary phenomenon. At the same time the present writer's observations lead him to agree with Sedgwick that such intercellular 488 EMBRYOLOGY OF THE LOWER VERTEBRATES CH, continuity of protoplasm is much more widely spread than is generally recognized. (3) YOLK. — Theoretically the most primitive type of zygote should from the beginning be able to absorb food for itself. As an actual fact however the zygote is provisioned for a shorter or longer period by the highly nutritious fat and proteid, in the form of yolk which is. stored up in its cytoplasm.1 With increasing specialization the amount of this store becomes greater and greater so as to lengthen the period during which the young individual is provisioned and freed from the necessity of working for its own living. A good example of a high degree of such specialization amongst Vertebrates is afforded by the relatively huge egg of the Ostrich. It has of course to be borne in mind that the degree of specializa- tion in this direction is to be estimated not merely by the absolute amount of yolk present but still more by the relative amount of yolk in proportion to protoplasm. Thus two eggs may be described as equally richly yolked although very different in size provided that the proportion of yolk to protoplasm is similar in the two cases. In correlation with this we find that a group characterized by heavily yolked eggs may evolve in the direction of producing more and more numerous, and therefore necessarily smaller, eggs. Good examples of this are seen in the Teleostean fishes where the eggs may be produced in enormous numbers and of very minute size although still retaining a proportionately large supply of yolk. In C. Rabl's discussions of his " Theory of the Mesoderm " (1889) an important place is taken by repeated losses and re -acquisitions of yolk during the phylogeny of the Vertebrata. Rabl arranges Cyclostorues, Elasmobranchs, Ganoids, Amphibians, rneroblastic " Protamniota " and Mammals, in a linear series, and concludes that Ganoids and Amphibians have undergone a diminution of yolk and have therefore reverted to the holoblastic condition; that nn-m- blastic Protamuiota have re-acquired a large amount of yolk ;iml have therefore reverted to the meroblastic condition; and thai finally Mammals have lost their yolk and again become holoblastic. In the opinion of the writer there is no sufficient justification for any one of these assumptions except the last. There is, as is \\ell known, definite evidence to show that Mammals arc descended from lors with large and heavily yolked eggs and that the small si/e and practically yolkless character of their holoblastic eggs are i si^ns in t hose <>t her cases in which Kahl supposes loss of yolk t,i i have taken place. It, must also he rcineiuhered I hat in the Mammal there is an obvious physiological reason for the Loafl of yolk namely that, the fond material needed during the development of the emhryo is provided from the tissues of the mother. 1 Fora details! .i«ount ,,r tin- (l,-vrl(.|.m<-nt nf tin- vdk in tin- t-i:^ of «.m- <>f tin- lower Vertebrate* (Prottu*) wie ,Jm ••_'• -n-.-n i ix YOLK 489 On t h»- n -capitulation hypothesis tin- .^nn'iiiation and other early stages of ontogeny represent ancestral evolutionary stages common to all Vertebrates. The differences to be observed between such slaves in different members of the group are consequently not to be looked on as ancestral but rather as due to the influence of disturbing secondary factors. Of these by far the most important is tin- presence of the particles of yolk, this dead substance • clogging and retarding the living activity of the egg protoplasm. The extent 1<» which it does this in any particular region of the egg is roughly proportional to its relative amount as compared with the living protoplasm in that portion of the egg. The yolk is as a rule of higher specific gravity than the protoplasm. Correlated with this it tends to be in proportionally greater amount in the lower parts of the egg than in the apical part, with the result that the processes of cell division and of development generally are relatively more slowed down in these lower portions — in extreme cases brought to a full stop — by its retarding influence. Typical examples of this are seen in the holoblastic but unequally segmenting eggs of the ordinary Amphibia. In this case it is possible by replacing experimentally the action of gravity by a more potent force (by centrifugalizing the eggs) to concentrate the yolk still more than is natural in the lower hemisphere with the result that the egg is now converted into a meroblastic one (0. Hertwig) the lower hemisphere being unable to segment. On the other hand by inverting the egg and so allowing the yolk granules to settle down towards the apical pole under the influence of gravity it is possible to cause the segmentation furrows to start from the abapical pole and spread towards the apical. The influence of yolk upon the gastrulation process will have been realized from the perusal of Chapter II. : it is well illus- trated by the series Amphioxus (Fig. 18), Petromyzon (Fig. 23), JKana (Fig. 25), Lepidosiren (Fig. 21), Hypogeophis (Fig. 27) and Torpedo (Fig. 28). Put in a single sentence it may be said to consist above all in the gradual subordination of the process of invagination to those of overgrowth and delamination. In the succeeding stage it makes itself apparent more particularly in the modification of the mode of origin of the mesoderin, the outgrowth of hollow enterocoelic pouches being replaced by the delamination of a solid mass or sheet. The storage of yolk carries with it not merely the modifications .just indicated in the processes of segmentation and gastrulation. Its influence becomes retrospective and affects even preceding stages during the growth of the intra-ovarian egg. This is shown more especially by tin- precocious concentration of yolk in that portion of the egg which will later become endodermal. Thus is the telolecithal condition brought about and telolecithality itself is seen to be really a foreshadowing of a particular adaptive feature of later stages of development (p. 183). In examining sections of later stages of Vertebrate embryos in 490 EMBEYOLOGY OF THE LOWEE VEETEBEATES CH. which the eggs are ricli in yolk it is readily seen that there are conspicuous differences between different parts of the embryo's tissues in regard to the yolk contained in their cells, for example endodermal structures are frequently marked out by larger yolk granules which cause them to stain more deeply with yolk-staining dyes. The condition of the yolk in a tissue may indeed give a useful hint as to the cell layer to which it belongs and as a matter of fact evidence of this kind has played a conspicuous part in many embryological discussions. It is important to bear in mind however the physiological significance of the character in question. It appears to be closely related to the metabolic processes in the tissue concerned. As a given tissue in a yolky embryo goes on with its growth and development its yolk is gradually used up, a necessary preliminary being its breaking down into fine particles easily assimilable. Tissues or cells undergoing active growth and multiplication have their yolk in this fine-grained condition : those which are for the time being comparatively inert retain their yolk in a coarse-grained form. Thus a disturbing factor is introduced which has to be care- fully borne in mind when using the character of the yolk as a criterion of the morphological nature of a given cell or tissue. A still further disturbing factor lies in the fact that while yolk is being used up and disappearing from view in one part of the body it may be deposited in cells elsewhere — as for example takes place in eggs during their period of growth within the ovary. Such increase in the amount of yolk however, accompanied commonly by increase in the size of the individual granules, is naturally relatively rare in comparison with the breaking down of yolk which is occurring through the general tissues of the embryo. It follows that on the whole coarsely granular yolk in a cell or tissue affords more reliable evidence as to its nature than does fine-grained yolk — which may be and usually is merely a symptom of active metabolism. (4) RECAPITULATION. — The fascination as well as the philosophical interest of the study of Vertebrate embryology rests in great part upon the recapitulation of phylogenetic evolution during the development of the individual. In the early days of evolutionary embryology this idea was accepted in an unquestioning and uncritical spirit. and it -upposed that all that had to be done to obtain an accuraie ami fairly complete picture of the phylogenetic history of any particular iiniinal was simply to work out its ontogenetic development. The more extensive knowledge which we have regarding embryolo^ieal phenomena to-day serves on the one hand to eimlinn fully the triilh of the general principle and on the other hand to imli« ate h.»w its workimj i- interfered with by various disturbing factors. The main Controlling I'aetnr in onto^my is the diameter of the adult. This ifl the motive power throughout the developmental period. Just as iieeoidinv .'.tun's First Law a moving body tends to in a state of uniform molmii in a straight line. BO in ix YOLK:: RECAPITULATION 401 ontogeny the developing individual Lends lo progress constantly towards the goal of adult structure. Not in this case however necessarily by the straightest and shortest path. The structure of the adult is the expression of the action of Heredity. The earlier stages are not exempt from the same influence. Each step in the development of the ancestor tends to be repeated in the development of the descendant. The descendant then during its ontogeny tends to pursue the same, it may be devious, path as the ancestor. If in the course of generations the adult structure becomes shifted onwards in a process of evolution, this merely means the adding on of a new portion at the latter end of the ontogenetic path. The earlier portions of this path, built up of similar increments representing previous steps in evolutionary progress, are repeated as before, and so the complete process of individual development forms a record or recapitulation of phylogenetic history. It cannot be too constantly borne in mind that the factor just indicated is the supreme factor in ontogenetic development. Other factors may be superficially conspicuous, may have far-reaching influence upon details, but this factor — the tendency to repeat ancestral steps in development up to and including the final char- acters of the adult — is and must always be paramount. Modern advances in knowledge of the facts of embryology, together with the assumption of a properly critical frame of mind, have shown, however, that the picture of past evolution afforded by the phenomena of individual development is at the best but a blurred and imperfect one, and that this must necessarily be so is readily realized when we remember that a large proportion of the characters of any organism are adaptive to its special mode of life. The circumstances under which a developing organism exists are, as a rule, widely different from those under which its ancestors proceeded along the evolutionary path, and in correlation with this its adaptive features are equally distinct. As we study the development of any species of animal we do not then see before us a complete and perfect picture of its evolutionary history, but merely gain fleeting, and it may be misleading, glimpses through the obscuring clouds of adaptive features. A further disturbing factor is indicated by the consideration that in past evolutionary history each stage in evolution was repre- sented by a complete functional organism, all the parts of which were necessarily at correlated stages of development so as to form a functional whole. Many modern animals however develop under conditions in which the different systems of organs are no longer forced to keep accurate step with one another, and the result is that some lag behind while others, particularly organs of great histological complexity in the adult — such as the brain or the eye — are accelerated in their early development, so as to give time for the complicated histogenetic processes that have to be completed before the organ can become functional. It will be realized that this latter type of 492 EMBKYOLOGY OF THE LOWER VERTEBRATES « it. disturbance affects the development of the individual as a whole much more than it does its component organs, the result being that embryology frequently affords a much more perfect picture of the evolution of single organs than it does of the organism as a whole. In reference to the ontogenetic record of phytogeny an interesting question presents itself regarding the reliability or otherwise of the information derived from the study of larval forms. To what extent may a particular type of larva be taken as probably representing a corresponding phase in the evolutionary history of the group: to what extent are its features to be regarded as ancestral, to what extent as mere modern adaptations to the environmental conditions among which the particular creature now pursues its individual development ? In connexion with various groups among the Invertebrata larval forms have played a conspicuous part in phylogenetic speculation — in some cases without due discrimination in interpreting their features as ancestral — the climax perhaps being reached by the view which regards such pelagic larvae as trochospheres or nauplii — precociously developed and free-swimming heads without any trunk — as representing ancestral forms (cf. Graham Kerr, 1911). In considering whether a particular stage of development is to be taken as probably repeating an ancestral stage of the adult special attention should be directed towards its mode of life, with the object of estimating the degree to which it diverges from the probable mode of life of the ancestral stage. If its mode of life is strikingly aberrant, e.g. parasitic where the normal habit of the group is free-living, or pelagic where the normal habit is not pelagic, then we must always keep in mind the possibility or probability that its most conspicuous features are mere modern adaptations and are therefore worthless as evidence of ancestral conditions. Again it should be considered whether in the main features of its organization it agrees with animals which are admittedly allied to it. Larvae occur in the following Vertebrates — Amphioxus, Petro- myzon, Crossopterygians, Ganoids, many Teleosts, Lung-fishes and 1 1n- majority of Amphibians. Applying such criteria as are indicated above \ve should rule out as probably devoid of phylogenetic Hirnilicaiicr the liirvji «»f Amphwxu* on account of its quite al>errant "pleiirnnrciid " asymmetry (seu Vol. I. Chap. XVII.). We should i nil.- out tin- Tel.-ostean larvae on account of their extreme diversity. In Urodele Amphibians and I Jipneumonic Lung-tishi's • MI tin- «'ili.-i- hand \\<- see larvae which appear to be distinctly of a common type. And in < 'rosso; nand Ad inopi.-ry^ian ( iaimids mi find larvae \\hiHi differ IV these in detail rather than in fundamental eharaeleri-t [< • ••jueiil.ly \\e should incline tn\\anls the view that, the type of lar\a in <|iiestion does not. depart \ery \\idely from th.- 00m ii auee-ti-.il l\pe mil <»!' which exist HILT \ ' i evolved. ix EECAPITULATION 493 A^iiiii in considering whether a particular feature of structure is to be regarded ,-is ;mersinil or as a modern adaptation the following questions should In- asked : (1) Is the feature peculiar to one group of Vertebrates or does it occur in several groups, and (2) if it occurs in several groups do the various animals possessing the peculiarity in question undergo their larval stages in similar sets of environ- mental conditions ? If the particular feature occurs in several groups derived from a common ancestral form this obviously increases the probability of the feature itself being ancestral. If however the several groups show similar sets of environmental conditions during their larval stages this introduces the element of doubt whether the similar features may not after all be merely adaptations to these similar sets of conditions. Again it is important to make out whether the particular similarity has to do merely with parts of structure in direct func- tional relationship to external conditions. If there be deep-seated correspondences in structure with no such direct functional relationship to external factors then this gives greatly increased probability to these correspondences being truly ancestral in their nature. The morphologist in trying to decipher the record of evolutionary history from the data of comparative anatomy or embryology is constantly impressed by the potency of nature's economy of living substance. An organ no longer required may be eliminated within a very short period of evolutionary time. Thus in some species of Mackerel (Scomber) so important an organ as the air-bladder has been eliminated : in various Frogs and Toads the external gills have been eliminated from development. Thus negative embryological' evidence is of peculiarly little weight in relation to phylogenetic problems. (5) THE PROTOSTOMA HYPOTHESIS. — This is a working hypothesis which links together and in a sense explains a number of features in the early development of Vertebrates which are otherwise extremely puzzling. The more important of these features may be summarized as follows : In Amphioxus as has already been shown the dorsal side is at first occupied by the widely open gastrular mouth. Later this becomes roofed in by a backgrowth of the gastrular rim anteriorly. A similar process of backgrowth appears to take place in the gastru- lation of lower Vertebrates in general. The roof of the gastrular cavity formed by this process gives rise later not merely to the dorsal wall of the alimentary canal but also to notochord and central nervous system. I. Now occasionally there are appearances which suggest that this archenteric roof consists really of two lateral halves which have become fused together along the sagittal plane. Thus in Protopterus the down-growing dorsnl lip of the l»l;istop'. jurio) in dorsal view (alter K<>|.M-|. I), al>ni>rnial embryo ol I'iki- (/•>'« I in in-) (.-il'in I ,n fUuillrt . ISi'.ii). m.f. niedullai v fold : m.a, nirsndciiii iii«-s.Kl.'nii ; N, nolocliiinl : '/. opi-iiin- li'.'idin- down into micron ; ot, otocv ^t ; i/. mass ot yolk-. -i-ll-. (1892; l.li.-it 1'V i'crtili/iiiM frog's r^s which liad 'becninc .-iihi-r liy iviciiiinii within tin- oviduct or by l»i'in# kept for from one tofourdiiv- in ;i iin.isl chainht-r. In- <>l>lainril a ccrlniii niinil' ahiioruiid «Mnl»ry«»s «»f Ihc i\]»c shown in Ki<^. 220, A, where a lar^o 086 "1 yolk-Cellfl is visil»lr in dorsal \ir\s instead <>!' hcin^ rt>m- plcl.-ly OOVered in ae would In- (,lie case normally. lii IranswTs.' s«?<;tio!i CFi^. 220, H; such an c.mbryu \\.is found l«» lia.\«- I\MI halt' ix 1'KOTOSTOMA THEOKY 495 neural rudiments and two notochords of half the usual si/..-, widely separated by the mass of yolk or endoderni. Similar al-norma lilies have been observed in Teleostean fishes. E.g. Fig. 220, D shows a Pike-embryo which is normal towards its anterior and posterior ends but interrupted for some distance in the mid-dorsal line by a wide cleft in which the yolk is visible. Again in l-'ig. 220, C a similar cleft is seen to traverse the whole length of a Trout-embryo from the hind-brain region tailwards. An important feature of such abnormal embryos of fish and am- phibians is that they frequently proceed with their development, the lips of the fissure closing up and the two sets of half-organs being brought together in the mesial plane, undergoing complete fusion and the individual becoming in fact entirely normal. The import- ance of this return to the normal on the part of such split embryos is that it indicates that the departure from the normal during the split condition is far less fundamental than would appear at first sight. Here then we have to do with two very remarkable phenomena. Firstly there is the abnormality itself — the fact that the dorsal region of the body is for a time in the form of two distinct halves. Abnor- malities of such a definite type as this usually have a definite evolu- tionary or other meaning and it is necessary to search for such a meaning in this particular case. Secondly, there is the fact that an embryo almost completely bisected in this way is frequently able to right itself and become perfectly normal. This again suggests the question whether this power of righting itself has not some special evolutionary meaning. III. In the higher meroblastic Vertebrates we have seen that there exists along the middle line of what corresponds with the archenteric roof of Amphioxus (i.e. the region which becomes con- verted into the dorsal part of , the body, including notochord and medullary plate) the structure known as the primitive streak. We have also seen that in the lowest Vertebrates possessing it, this primitive streak represents the line of fusion of the gastrular lips, and that we are therefore justified in attaching the same significance to the primitive streak in those higher forms in which the actual process of fusion can no longer be observed. That this interpretation is correct is indicated by the occasional occurrence of openings in the line of the primitive streak communicating ventrally with the enteron and dnrsally with the outer surface of the medullary plate, or its derivative the floor of the neural tube (pp. 51, 53). Such neurenteric communications are readily explicable by the view that they repre- sent simply parts of the line of fusion of the gastrular lips where the actual fusion has not been completed. Here again we have a phenomenon which demands explanation — the occurrence of what seems to be the vestige of a slit-like gastrular mouth along the mid- dorsal line. IV. We have another remarkable body of facts associated with the fate of the blastopore or remnant of the gastrular mouth in 496 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. various groups of the animal kingdom. Thus within the limits of the groups Annelida or Mollusca the blastopore in some forms becomes the mouth, in others the anus. No one would doubt for a moment that the mouth opening is homologous throughout these groups yet in one member of the group it can be traced back to the blastopore while in another member it is the anus which can be so traced. In other forms the gastrular mouth simply vanishes away during development and in some of these cases it assumes a curious elongated slit-like form along the mid-neural line before it disappears. It is the merit of the Protostoma theory that it — and it alone— affords an explanation of these four very different but equally puzzling bodies of facts. It falls there- fore to be accepted by the Vertebrate embryologist as one of his working hypotheses. The Protostoma theory is simply a special development of the theory of the evolution of the coelomate Metazoa which is generally accepted by morphologists, namely that the animals in question have passed, during the remoter parts of their evolutionary history, through a Protozoan and later a Coelenterate stage. The peculiarity of the Proto- stoma theoryis that it includes FIG. 221.— view of neural rudiment iu embryo of within the coelenterate period (A) Peripatus (after Sedgwick, 1884) ; and (B) Lepidosiren. In the case of Lepidosiren the embryo is shown as it appears when straight- ened out. xj B a stage corresponding in its main structural features with the Actinians of the present time, characterized by the presence of an elongated slit-like mouth, dilated somewhat at each end and surrounded by a specially concentrated portion of the ecto- dermal nerve plexus. The portion of the surface on which the slit-like mouth was situated was thus the neural surface. Igwick (1884) was led to the idea by his studies on the development of Peripatus. He found in the species investigated 1>\ him a stage (see Fig. 221, A) in which the gastrula-mouth formed a loni: slit Iraversinir the neural surface and surrounded by the ectodermal neural rudiment-. As development went on the gastrular mouth or piotostoma became obliterated, except in its dilated terminal portions. by fusion of il- lips, The terminal parts remained open as mouth and anus respectively. The port ions of nerve rudiment l-etween tin- two openings lieoame the ventral nerve cords while the portions in front of the Iliout h and behind the aim- gave rise respectively to the ix PROTOSTOMA THEORY 497 supra-oesophageal ganglia and the suprarectal commissure. According to Sedgwick this stage in the development of Peripatus repeats the features of an Actinozoori-like Coelenterate ancestor, not merely of Peripatus, and therefore of Arthropods in general, but of such other groups as Annelids, Molluscs and Vertebrates. It will be noted that on this protostoma hypothesis an important physiological distinction has at an early period of evolution marked off the Vertebrates from the other groups mentioned. This distinc- tion came about witli the acquisition of different habits of movement. In the stem which gave rise to Annelids, Arthropods, Molluscs, movement took place with the neural surface next the substratum (.is in those modern Medusae which are able to creep on a solid surface — e.g. Cladonema), while in the Vertebrate stem on the other hand the neural surface was directed away from the solid substratum (as in the modern Actinian when it creeps). This differ- ence in the position of the body in relation to the substratum would naturally lead in time to the different types of dorsiventrality so apparent in the fundamental organization of the two diverging stems. It is frequently stated by critics of the protostoma hypothesis that it involves a reversal of dorsal and ventral sides during the evolution of Vertebrates from their invertebrate ancestors but it will be gathered from what has been said that this criticism rests on a misunderstanding. It will be readily seen that the protostoma hypothesis success- fully explains the four categories of puzzling facts already enumer- ated. The paired appearance of the gastrular roof would be a reminiscence of the fact that originally it was actually paired : the split along the back of the abnormal embryos would mean the temporary re-appearance of the ancestral split or mouth : the primitive streak would be the scar along which the lips of this ancestral mouth or protostoma underwent fusion : and the converting of blastopore now into mouth now into anus would be an imperfect reminiscence of the fact that in phylogeny it gave rise to both. On this hypothesis the various signs of a split along the neural surface of the vertebrate embryo, whether in the form of a dorsal furrow or a primitive streak or an actual opening, are interpre table as reminiscences of the protostoma slit which traversed the neural surface of the Actinozoon-like ancestor.1 It is of interest to notice that in two Vertebrates at least there exist what seem to be obvious traces of neural rudiment extending round behind the anal part of the protostoma precisely as in Peripatus. In Fig. 221, B, is shown an embryo of Lepidosiren spread out in one plane, with the neural rudiment in the form of a ridge which is continuous behind the blastopore or anus. If it be reflected that this opening may be 1 That the primitive streak and primitive groove are closely related to the gastrula mouth was perceived by Rauber (1877) but a clear evolutionary explanation of this relationship was first given by the protostoma theory of Sedgwick (1884) and Hertwig (1892). VOL. II 2 K 498 EMBRYOLOGY OF THE LOWEH VERTEBRATES CH. regarded as being continued forwards by a potential slit, represented e.g. by the primitive streak of other forms, it will be realized how close is the resemblance to the conditions in Peripatus. The pre-anal portions of the neural rudiment in Lepidosiren come together in the mesial plane to form the spinal cord, while the postanal portion flattens out and disappears so that the anal opening comes to lie entirely behind the posterior limit of the central nervous system. It is clear that if the development of the anal opening were delayed until the neural folds had already come together it would make its appearance completely behind the central nervous rudiment and with no obvious connexion with it. This is very possibly the case in Vertebrates other than those mentioned. Although the anal opening of Vertebrates is thus brought into the relations with the nervous system that we should expect on the protostoma hypothesis there is no such definite evidence in the case of the mouth. It is true that in some cases the dorsal furrow has been traced to the neighbourhood of the mouth and that the mouth opening has in some cases at first the form of a sagit tally placed slit, but in no case, up to the present, has the neural rudiment been traced round in front of the mouth. This difficulty however is greatly lessened when we correlate the facts just mentioned in regard to the anal opening in Lepidosiren with the relatively late appearance of the mouth opening of Vertebrates as discussed on p. 193. It may well be that the non-inclusion of the mouth opening within the obvious neural rudiment is due simply to the pre-oral parts of the medullary folds having already flattened out and disappeared before the oral opening makes its appearance. If this is the case it carries with it the interesting consequence that the supra-oesophageal or pre-oral ganglia of Peripatus have disappeared in the Vertebrate and it is therefore waste of energy to discuss what parts of the brain of a Vertebrate are homologous with the supra- oesophageal ganglia of Invertebrates. This Protostoma idea, dealing as it does with extremely remote phases of the Vertebrate phylogeny, must not be looked on as a definitely proved theory, nor can it be expected ever to reach that dignity, but it is a fascinating working hypothesis which serves, and which alone serves, to link together and in a sense explain a considerable body of otherwise mysterious and apparently inexplicable facts of Vertebrate embryology.1 (6) THE VKI;TKI.I;ATE HEAD. — The two phyla of the animal kingdom which have reached the highest stage of evolutionary development — the Arthropoda and the Vertebrata — are alike char- acterized by the possession of a well -developed head. In the 1 In considering the ditliculties in tin- \\:iy of tin- theory allordcd by OAMfl the gastrula becomes roofed in by a process ..f simple backgrowth without any trace of protostoma (e.g. Amjili''-» • , it is \\ell tu bear in mind the parallel case of the ;imnioii of which ;i lar-e Cation may bo formed by simple l.ack^n.wt h, although the sero-anuiiotic i^thmn> and th-- iir_rro\\ th of mr^.dcrm from t lie i- .em to point dearly to a former formation by tin; meeting of t\\o lateral folds. i\ THE VERTEBRATE HEAD 499 evolution of a head \\v may take it that the principal factors involved an- pmU-ihly the following: (1) The habit of active movement in a direction corresponding with the prolongation of the axis of the body, (2) The concentration of organs of special sense towards the end of the body which is in front during movement, (3) The concentration of nerve centres to form a brain in proximity to these organs of sense. In the case of the Vertebrate the brain has reached a comparatively large size and in correlation with this the protecting skeleton has become highly developed and has lost the flexibility which is characteristic of it in the trunk. Further in the Vertebrate the walls of the buccal cavity and pharynx have become highly specialized, particularly in the matter of their skeleton, in relation to the functions of ingestion and mastication of the food on the one hand, and of respiration on the other. Each of these various factors involves structural change, not affecting merely one organ but causing modification of the whole complex arrangements of the head-region. Thus associated with the loss of flexibility we find (1) loss of segmentation of the skeleton, (2) disappearance or great modification of the myotomes, (3) corresponding changes in the nerves supplying these myotomes and (4) disappearance of the coelomic cavities. The full appreciation of the importance of this feature of the Vertebrata makes it, in the present writer's opinion, impossible to doubt that the possession of a definite head is a feature that has come down from the unknown ancestral form from which the Vertebrate stock has evolved. If this be correct it follows that the relatively feeble differentiation of the head end of the body seen in Amphioxus is to be regarded as a secondary condition, correlated with the peculiar mode of life of this animal, and devoid of phylo- genetic significance. It has already been pointed out that organs of great complexity in the adult tend to be laid down at an early stage of individual development, time being thus obtained for the development of their complex detail. It is perhaps in direct relation to this principle that the highly complex head - region of the Vertebrate, which comes to assume control over most of the activities of the in- dividual, develops particularly early in ontogeny — the various developmental processes making themselves as a rule first appar- ent in the head region and spreading thence tailwards along the trunk. This fact is of practical importance to the embryologist for in the case of segmen tally repeated organs it enables him to find a series of developmental stages within the body of a single embryo. Though this precocious cephalization is a marked feature of Vertebrate ontogeny it never goes within this phylum to the length it does amongst certain Invertebrates where the larval stage 500 EMBRYOLOGY OF THE LOWER VERTEBRATES OH, (Nauplius, Trochosphere) is practically a precociously developed and free-living head which has not yet developed a trunk. As will have been gathered, one of the most conspicuous features of the head-region is the loss of segmentation in organs in which it was once present. Metameric segmentation, which first makes its appearance in typical form in the Annelida, is probably to be associated primarily with the coelome and its lining the mesoderm. The coelome is distended with coelomic fluid and the turgidity so caused gives firmness to the body. The physiological advantage of the coelomic cavity being subdivided into successive compartments is obvious. The segmentation of other organs is to be looked on as secondary to that of the mesoderm, and more especially to that of the muscles. Thus the segmented character of the nervous system of an Annelid or Arthropod is due to the ganglion-cells tending to become concentrated at the level of the .masses of muscle which work the parapodia or limbs. So also the segmentation of the skeleton which permits flexure of the body is correlated directly with the segmentation of the musculature which causes that flexure. So, conversely, with the disappearance of segmentation in tin- head of the Vertebrate. Correlated with the loss of flexibility in the brain region the my o tomes which produce the flexure have disappeared, and correlated with this in turn the ensheathing skeleton has lost its segmentation and the segmentally arranged motor nerves have also gone. The process has taken place from before backwards. It has been carried to the greatest extent in front, to the least at the hinder limit of the head. It is definitely established that the head of the Vertebrate has at least in part come into being by the modification of what was once the anterior portion of the trunk. With the gradual evolution and increase in size of the brain — so characteristic of the phylum Vertebrata — this organ has gradually encroached upon the spinal cord, and its protective skeleton the chondrocraniurn has pari passu encroached upon the vertebral column. This is clearly indicated by the fact that included within the limits of the skull are nerves which are serially homologous with those of the trunk. Putting on one side the probability — as many would regard it— that cranial nerves III, IV, V, VI, VII, IX and X are reallv homologous with the spinal nerves, we find behind the Va.mis a series of spino-occipital nerves (Furbringer, 189*7), which although included within l.ln- limits of the skull are yet undoubtedly im-mUTs of the same series as the spinal nerves. The number of those is very different in the different subdivisions <>l tin- Vertebrata as may be gathered from an inspection of Fig. 222. In all probability Miry will U; 1'ound also to show considerable variation in diMeren! individuals of the same species. lie ••volution of the hrad Mien; is some reason to IX THE VERTEBKATE HEAD 501 that its extension backwards has taken place by successive steps. In the most ancient recognizable stage the cranium (Palaeocranium — Fiirbringer) extended no farther back than the vagus nerve. T\\\^ phase is represented — either persistent or revertive — in the i • • • • •. •. • • • • • • • 3 • 3 • 3 o 1 i ] ] 3 3 3 0 UJ nsit spi ted \l I I A) t'i ri n hy wh ( Irt' um ts, tli udi i asmobrch n from u- ertebra olu n as black do ctangular ou \v«lh the conclu yo1 | 5 i I 6 l ranch tlu- l col IJ silt • n 33 if ii MI i ^ J-- 1^.13. J oi-liii -i --• "ffi* 3 JfllHl adult stage of modern Cyclostomes : it is also seen in the young Lepidosiren of stage 34 (see Fig. 154, B, p. 309). The next phase is seen in the adults of such relatively primitive groups as the Elasmobranchs, the dipneumonic Lungfishes and the Amphibians, in which an occipital region has been added on to the palaeocranium. 502 EMBRYOLOGY OF THE LOWER VERTEBRATES OH, Finally the hinder limit in the other Vertebrates has been shifted still farther back — one segment (Pol ypt eras), three segments (Amniota) or as many as five segments (Acipenser). As far forwards as the hinder limit of the palaeocranium there is, as already indicated (p. 317), clear evidence that the cranial wall represents a series of neural arches which have undergone fusion. As indicated on the same page it is difficult to avoid extending this hornology to the mesotic portion of cranial wall lying still farther forwards. As regards the prechordal portion of the cranium there is no definite evidence, but if we regard the trabeculae as primi- tively in continuity with the parachordals we have to grant the possibility of even this part of the cranium being in series with the portions farther back and therefore also originally vertebral in constitution. In conclusion it must be remembered that the series of myo- tomes is also continued into the head-region, and the occurrence of typical myotomes as far forwards as the premandibular or oculo- motor segment (p. 210) may be taken as strong evidence that the segmentation of the mesoderm originally extended throughout the head-region including its pre-chordal portion. (7) EMBRYOLOGY AND THE EVOLUTION OF THE VERTEBRATE. — The special charm and the chief importance of the study of em- bryology reside in the fact that it is one of the main branches of evolutionary science. The greater part of what is ordinarily called evolutionary research deals with the possible inethods and causes of evolutionary change. The data of Embryology on the other hand form a branch of synthetic evolutionary science which deals not with possible causes or methods but with the actual facts of evolutionary change, striving to map out the course along which this has pro- ceeded. In compiling the record of evolutionary progress we are dependent upon Comparative Anatomy and Palaeontology as well as Embryology, and in formulating conclusions care has to be taken that whenever possible they are based on the data of all three sciences. In cases where these data are not in agreement care must be taken to bear in mind the main disturbing factors which are liable to invalidate the conclusions in each case. In reasoning from Hi n bryology and Comparative Anatomy the possibility that particular features are modern adaptations to existence say within a uterus or egg-shell or under any other set of conditions different from those of the anerstor has to be borne in mind. In the case of Palaeon- tology and Comparative Anatomy there exists the same danger of ic proio/oologist when he endeavours to construct a continuous life-hist<.r\ out <»1 a numher of isolated observations on the dead animal — the error of arranpiiL;; <>l nervations in a series which IH mil natural or (,n the other hand, if the sedation he done oorreofcly, «i revei direction. In Palaeontology errors of this type are peculiarly apt to arisr mi neromit. of the extraordinary imperfection of our knowledge. If a series of organisms a, l>, c, «><•«, m<> known from a series of geological deposits A, B, C, I), this alfnrds convincing evidence in most cases that the particular organisms lived at the time the particular deposits were laid down : the conclusion may also be fairly justifiable that not only did they exist but that they were abundant at the period in question. The conclusion Imwcver which is so apt to be drawn that a, b,c, d, actu- ally made; their first appearance in the same order as the deposits A, B, C, D, is quite unreliable. They may have existed in smaller numbers for immense periods of time before the periods corresponding to A, B, C, D, when they were really abundant, and the order of their first appearance may have been d, c, b, a, or any other. Such a geological series is in fact in itself of little value as an index to the order of evolution. In Embryology on the other hand where the evolutionary stages occur as part of a continuous process, each dependent upon its predecessor, we appear to be safe in assuming that the record, however incomplete, is at least arranged in proper sequence. Another principle to be borne in mind, when the attempt is being made to work out the evolutionary history of a particular group, is that conclusions must be based upon broad knowledge of structure as a whole. No implicit reliance must be placed upon evidence relating to one system of organs unless it is corroborated by the evidence of other organs. Failing this precaution the investigator is liable to the pitfall afforded by convergent evolution of organs of similar function. Here again the palaeontologist finds himself much hampered as compared with the embryologist, for as a rule all evidence except that of the skeletal system has passed completely beyond his ken. EVOLUTIONARY ORIGIN OF THE VERTEBRATA. — In the preceding portions of this book it has been shown that Embryology provides us with a record — in at least its main outlines — of the evolutionary changes which the various organ-systems have undergone within the group Vertebra ta. For, amongst others, the reasons stated at the foot of p. 491 the record is less clear regarding the evolutionary history of the complete individual. Even however if we had this record complete for the various types — Fish, Amphibian, Reptile, Bird — we should find ourselves still confronted with the interesting problem of the first origin of the primitive Vertebrate type : — -from whence came these lowly original Vertebrates out of which the various existing types of Vertebrate have been evolved ? This problem of the ancestry of the Vertebrata is naturally a fascinating one and it has attracted much attention and been the theme of voluminous writing. Enthusiasts have at different times endeavoured to demonstrate that the Vertebrates are descended from this phylum or from that. It is perhaps best not to take such attempts very seriously. They have served a useful purpose in arousing interest and stimulating research but they have little claim to a place in the permanent literature of Zoology. > We are naturally unable to get any evidence bearing upon the 504 EMBEYOLOGY OF THE LOWEK VEETEBRATES CH. problem from Palaeontology. The most ancient Vertebrates of which fossil remains are known had probably already evolved to a far greater distance from the original type of Vertebrate than that which separates them from the existing Vertebrates of to-day. And the probability is that the earliest Vertebrates went on existing and evolving through long ages before they developed those complex skeletal structures which are alone adapted for preservation as fossils in the geological record. Comparative Anatomy fails us too — for up to the present no existing type of animal has been discovered which can justifiably be interpreted as an unmodified survivor of the original Vertebrates. It is Embryology alone which yields us examples of Vertebrates in the earliest stages of evolution but the data afforded by that science do not carry us beyond the formulation of a few very broad and general conclusions regarding the prevertebrate phases in the evolutionary history of the phylum. (1) The fact that Vertebrates, like other Metazoa, commence their existence as a unicellular zygote appears to justify us in postu- lating a unicellular i.e. a Protozoan ancestral stage. (2) The fact that there occurs in the admittedly more primitive Vertebrates a gastrula stage appears to justify us in postulating a diploblastic or Coelenterate ancestral stage. (3) The facts which are united together in the Protostoma hypothesis suggest that the coelenterate ancestor evolved along lines somewhat similar to those of the modern Sea-anemones with their elongated slit-like protostorna dilated at each end and surrounded by a concentration of the ectodermal nerve-plexus. (4) The facts that the coelome was probably originally segmented (as indicated by Amphioxus), that the excretory organs are in the form of nephridial tubes, that the vascular system consists funda- mentally of longitudinal vessels on opposite sides of the alimentary canal connected together by vascular arches, the blood passing tailwards in the vessel on the neural side of the alimentary canal — suggest that there intervened between the coelenterate phase and the vertebrate phase a stage which possessed many features in common with those animals whicli are grouped together to-day in the phylum Annelida. We may suppose that this annelid -like creature became evolved from an Anemone in wliich the body had become drawn out, U iii i lit- LI- mis Herpolitha or one of the brain corals, and -which had become actively motile. In the two diverging stems which gave rise to Annelids ami to Vertebrates respectively we may take it that a dillrn-nee existed in tin- normal position of the hody— the former progressing with their neural, the latter with their ahneural. surface underneath. It is eonceivaJ.le that this ditlerenee may have l>ren associated uilh tin- dilt'n.-ncc Let \\ven ;i creeping mode of life in \\hic-.h tin- chi-l enSOiy impressions were related to the solid sub- stratum and a swimmm •„• mode of life in which they rather came from abov.-. ix GERM LAYER THEORY 505 ADDENDUM TO CHAPTER IX. — More than once in the course of this volume reference has been made to the " Theory of Germinal Layers " or the " Germ Layer Theory." This theory, which has played a great part in the development of embryological science in the past and still dominates to a great extent embryological research, had its foundations in observations made by these pioneers of embryological science — Wolff, Pander, von Baer and Remak. Wolff (1768) observed that the alimentary canal in the Bird embryo is developed out of a thin membrane or leaf (" Blatt ") and inferred that the other organs go through a similar stage. Pander (1817) gave the name " blastoderm " to the first membrane-like stage of the embryo as a whole, saw how this became differentiated into the three layers — outer, middle and inner — and traced out the development from these of the main organ -systems. Von Baer (1828) carried on and elaborated Pander's work, recognized that the middle layer was double, and that it was secondary to the two primary layers : the outer and the inner. He also extended his observations to forms other than the Fowl and laid the foundations of Comparative Embry- ology. Remak (1855) finally worked out the germ -layers in terms of the Cell-theory, traced the origin of the coelome to a split in the middle layer, and worked out more precisely the relations of the layers to the definitive organ-systems. One of the most important steps in the development of the Germ Layer Theory was made by Huxley (1859) who as a result of his researches upon the Medusae recognized the two primary cell-layers in these animals (named by Allman " ectoderm " and " endoderm ") and suggested the comparison of them with the two primary layers of the Vertebrate embryo. Embryology, like Morphology in general, first became a real living science as a result of Darwin's demonstration of the fact of evolution. In the Origin of Species (1859) the principle of recapitu- lation is already admitted. " Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals." The idea was further elaborated by Fritz Miiller (1864). Kowalevsky (1871, etc.) and other embryologists had demon- strated the wide-spread occurrence among the Invertebrates of an early stage of development more or less cup-shaped in form and consisting only of the two primary cell-layers, and the important advance was made synchronously by Lankester and Haeckel of perceiving in this two-layered stage a repetition of a common ancestral form. Lankester (1873) recognized amongst the Metazoa two distinct grades of complexity of structure so far as their cell-layers were concerned — the diploblastic grade (represented by the Coelenterate) consisting of the two primary layers, and the triploblastic grade with an interposed middle layer. Further he recognized that each Metazoon — whatever its definitive condition — passes in the course 506 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. of development through a diploblastic stage which he termed the planula. Such a planula stage he regarded as a repetition of a common ancestral stage of evolution. Haeckel (1872) about the same time as Lankester also developed the idea that the diploblastic stage of ontogeny was to be interpreted as the repetition of an ancestral form : Haeckel called this ancestral form Gastraea. The main difference between Haeckel's view and Lankester's was that the former regarded the endoderm as having arisen by a process of invagination — as it actually does arise in ontogeny in the great majority of cases — while Lankester regarded it as having arisen by a process of delamination from the outer layer. As regards the middle germ-layer ideas remained somewhat vague until Agassiz (1864) showed that in the Starfish the mesoderm arose in the form of an outgrowth of the archenteric wall. The same was found to be the case in various other Invertebrates, and in 1877 Kowalevsky showed how in Amphioxus the mesoderm was during an early stage in the form of archenteric pockets. In the same year Lankester developed the generalization that the coelome is to be regarded as uniformly enterocoelic in origin and comparable with the diverticula of the archenteric lining seen in Coelenterata. The separation of such mesodermal cells as are in their early stages free and amoeboid under the common name mesenchyme was first made by O. and R. Hertwig (1882). The later developments of the theory of the mesoderm involved in the Protostoma theory have already been alluded to earlier in this volume and the same applies to what the author regards as the chief qualification of the germ -layer theory indicated by modern work, namely that the boundary between two layers where they are con- tinued into one another must be regarded not as a sharply marked line but as a more or less broad debatable zone. LITERATURE Agassiz. Contributions to the Natural History of the United States of America, v. Boston, 1864. [Vol. v printed as vol. v, pi. 1, of Mem. Mus. Comp. Zoology Harvard.] Baer. Ober die Kntu i< -kelungsgeschichte der Thiere. Beobachtung und Reflexion, i. KniiigshtM-g, 1828. Bell. Arch. Kntuick. Mrchanik, xxiii, 1907. Fiirbringer. (Ii-gcnlKiurs Foi^-ln-il'l. Lrip/i^, 1897. Haeckel. !>!«• Kjuktohwamme. Berlin, 1872. Heriwig, O. Arch. mikr. Anat., xxxix, 1892. Hertwig, 0. and B. .li-uaischi- /i'it.vhrii'1, xv, 1882. Jdrgensen. li. I li-rt \\ij_- 1 . -t-.-hnit. .inui. 1910. Kerr, Graham. Pi<>r. \i',\. l'h\-. Sue. Ivlin.. xviii, 1912. Kopsch. Internal. ICowtiaohr, Anal, u. I'h\s., xvi, 1899. Kowalevsky. M.-m. Anart. .Imiin. Mii-r. Sri., \\ii, 1877. Lereboullet. Am, i, /,„,!.. \x, 1863. ix GERM LAYER THEORY 507 Miiller. Fiir D;ir\vii.. l,«-i|>xitf, 1864. Pander. I'.ritia^- /m- Knt uiekelungsgeschichtc d.-s IIiilm<-ln'iis im K\v. \Viir/- 1817. Parker. Anirn<-an Naturalist. \liii. 1909. Rabl. Mi.rjih. Jahrl... xv. 1889. Rauber. rriiniti\>t n-ifi-n uml Neurula der Wirbelthiere. LcMp/.i^. 1877. Remak. rutfisiu'liunx'-n iiln-r die Kntwiokelung uart. Journ. Micr. Sci., xxxvii and xxxviii, 1895 and 1896. Wolff. Nov. Comment. A<-.-i<|. Sci. St-lV-lcrslioiirx. xii. 1768. CHAPTER X THE PKACTICAL STUDY OF THE EMBRYOLOGY OF THE COMMON FOWL FOR gaining practical experience in the study of embryology there is no type of material so convenient as that of the early stages in the development of the Common Fowl. Freshly laid eggs can be obtained practically anywhere and to obtain the various stages of development all that is necessary l is to keep the eggs at a suitable temperature (about 38° C.) either under a sitting hen, or in one of the incubators which can be purchased, or even in a simple water-jacketed oven such as can be made by any tinsmith. If an incubator be purchased it will be provided with a proper heat regulator for use with elec- tricity, gas or oil, while with the most primitive water-bath it is possible to arrange a lamp so as to give a temperature sufficiently constant as to carry the eggs through at least the first few days of incubation — the most important period for purposes of study. Bird embryos — apart from their use in learning practical embryology- provide admirable material for giving practice in the ordinary methods of section-cutting which are in such constant use in Zoology, Anatomy, Physiology, and Pathology. This chapter will then be devoted to giving an account of the development of the Fowl with directions as to the technique involved in its practical study. In the description which follows the developmental phenomena will be described in their natural sequence but on account of the practical difficulties involved in the extraction and preservation of blastoderms of the first day of incubation it will be found best, in actual laboratory work, after studying the new-laid egg and its envelopes, to proceed to the stage of about 42 hours' incubation and gain some practice in the manipulation of it before attempting the earlier stages. In the follow in;j- technical instructions the sequence is followed which luis IMM-M found to be in practice most convenient 'I BOHNK iL MKKCTIONS2 I. NE\\ LAII. K»;i;. Fill a glass vessel about 4| inches in diameter ;ni«l L' inches in depth with normal salt solution [\v;it<-r 1 Provided the eggs h:i\« l.i-.-n I, nili;, .1. 2 The reader is assum* I i" have .-in • 1< m< nt.uy knowledge of the ordinary of cutting sectionH. See, how. \- i tin A p|..-ii08 « ii.x PRACTICAL EMBEYOLOGY OF THE FOWL 509 100 c.c., common salt '75 gramme] heated to a temperature of about 40° C. Submerge the egg upon its side in the salt solution ,-i ml remove the side of the shell which is uppermost by cutting with a pair of strong scissors and then lifting off the isolated piece of shell with blunt forceps. Take care to keep the point of the scissors or forceps close to the inner surface of the shell so as to avoid risk of injury to the true egg or " yolk." II. Ecu; AITKR 42 HOURS' INCUBATION. — Open the egg as before. On removing the piece of shell the blastoderm will be seen as a circular whitish area on the upper side of the yolk. Excise the blastoderm by making a series of rapid cuts with the large scissors through the vitelline membrane a short distance external to the boundary of the blastoderm. Should the yolk happen to be tilted round so that the blastoderm is not uppermost but rather at one side make the first cut below the blastoderm so that the elasticity of the vitelline membrane will tend to pull it upwards when the cut is made. Otherwise the blastoderm may be lost by its being pulled downwards. Having isolated the circle of vitelline membrane, with its ad- herent blastoderm, slide it off the yolk by pulling gently on one side with tl\e forceps. Remove the remains of the egg from the dish so as to keep the salt solution clean. Take hold of the circle of vitelliiu- membrane at one edge with the forceps and wave it backwards and forwards beneath the surface of the salt solution. The blastoderm will gradually become detached. Should it not do so at once the separation should be started by freeing it from the vitelline mem- brane with a scalpel at one edge. Notice the difference in appear- ance between the vitelline membrane and the blastoderm which has been detached from it. If the blastoderm is yellow from adherent yolk this should be washed off either by waving the blastoderm backwards and forwards in the salt solution or by gently directing jets of salt solution on the yolky surface of the submerged blastoderm by a wide-mouthed pipette. The blastoderm should now be brought near the surface of the salt solution and a watch-glass slipped under it by which it may be lifted from the larger vessel. The blastoderm is so delicate that it must be kept submerged in the fluid : no attempt must be made to lift, it abpve the surface by forceps. A microscope coverslip slightly larger than the blastoderm should now be submerged in the watch-glass and the blastoderm floated over it dorsal side above. The dorsal or upper side of the blastoderm can easily be identified from the fact that the edges of the blasto- derm tend to curl upwards. Having floated the blastoderm over the coverslip the latter should be gently raised to the surface of the fluid with a pair of large forceps. Take care to keep the coverslip abso- lutely horizontal and lift it out of the fluid very carefully so that the blastoderm is stranded on its upper surface, the lower surface of the blastoderm being in contact with the coverslip. The superfluous salt 510 EMBRYOLOGY OF THE LOWEK VERTEBRATES CH. solution should be drawn away \\ith blotting-paper so as to bring the blastoderm into close contact with the glass ; take great care that the blotting-paper does not actually touch the blastoderm as in that event it will be apt to stick to it. Now* take the coverslip between the finger and thumb and with the aid of a pipette place a very small drop of corrosive sublimate solution (or other fixing iluid) upon the centre of the blastoderm. This will cause the blastoderm to adhere to the coverslip. Now invert the coverslip and drop it on to the surface of some fixing fluid in a watch-glass. The blastoderm is then passed through the various operations of staining, dehydrating and clearing, preparatory to mounting whole or conversion into a series of sections as the case may be. The advan- tage of having the blastoderm adherent to a coverslip is that it makes it easier to handle and above all it keeps it from becoming wrinkled or folded. The blastoderm if fixed in corrosive sublimate can usually easily be detached from the coverslip at the stage of clearing' if it has not already become free at some preceding stage. Should it adhere obstinately it should be placed in acidulated alcohol for an hour or more. The examination of the blastoderm should be carried out as follows : 9 1. Study the blastoderm and embryo as a whole under a, prefer- ably binocular, dissecting microscope while it is submerged in the fixing fluid. As the fixing fluid penetrates the embryo the various details in its structure come into view. Continue the examination of the surface relief in the alcohol which is used for getting rid of the excess of the fixing agent. After examining from the dorsal side invert the blastoderm and examine from below. 2. Repeat the examination of the embryo as a whole as a trans- parent object after staining and clearing. If the individual embryo is to be cut into sections a careful drawing should be made at this stage, the outline being preferably drawn by means of the camera lucida. 3. Study serial sections cut transversely to the axis of the embryonic body. [Sagittal and horizontal sections will also be useful for study after the transverse ones.] III. EARLY SECOND-DAY BLASTODERM. — The same method is used as for the 42-hour stage but special care must be taken on account of 1h«- more fragile character of the blastoderm. In all pro) lability the blastoderm will remain adherent to the vitelline membrane in spite of repeated shaking and the process of detach- ini-nl. will have to ho started by gently easing up the edge of the blastoderm on tin- sid«- n«-\l the forceps in which the edge of the circle of vitelline meinbram is held. To get rid of adherent yolk the circle of vitelline membrane should \n- laid on the hnl.inm of llit^ dish of salt solution, blastoderm i;pp»-i in-- -I. A pipi-Ur \\iili a wide mouth should he held yerlieully x TECHNICAL DIRECTIONS 511 a few millimetres above the blastoderm and the india-rubber bulb squeezed rhythmically so as to wash away the particles of yolk bv very gentle currents of salt solution. When the blastoderm is lifted out of the solution stranded upon the coverslip it is very apt to become folded. When this happens, on account of the fragility of the blastoderm, no attempt should be made to stretch it out by the use of needles or forceps. The folds should rather be straightened out by a current of salt solution allowed to flow out from the orifice of a pipette held vertically just over the centre of the blastoderm. IV. EARLY BLASTODERMS. — Open the egg as before. Let the albumen run off until the vitelline membrane over the blastoderm is exposed. Raise the egg until the blastoderm touches the surface of the salt solution and then bring a wide-mouthed pipette of Flem- ming's solution, held vertically, into such a position that its tip just touches the film of fluid over the blastoderm. Let the solution flow down slowly on to the vitelline membrane covering the blastoderm. If there is any albumen overlying the blastoderm this should be carefully stripped off as it coagulates. A small piece of black bristle should be stuck into the vitelline membrane on each side to mark the line joining the chalazae so as to facilitate the orientation of the blastoderm for section-cutting. The fixing fluid should be allowed to act for several minutes and then a circle of vitelline membrane may be excised with the blastoderm adhering to it. Float' out the circle of vitelline membrane on a coverslip with the blastoderm above and submerge in a watch-glass of fixing fluid. If the circle of blasto- derm adheres to the coverslip so much the better : it may be separ- ated in the clearing agent. Instead of a pipette as above indicated being used for the fixing fluid a small rim of cardboard, e.g. the rim of a small pill-box lid, may be placed on the surface of the yolk, raised up slightly out of the salt solution, so as to enclose the blastoderm and then the little tank so formed may be filled with Flemming's solution which will gradu- ally diffuse downwards. Minchin recommends a triangular instead of a circular rim for facilitating subsequent orientation. For fine work it is preferable to embed the whole yolk in celloidin and then after the celloidin has been hardened to cut out the portion in the region of the upper pole for sectioning. This method con- sumes however much more time than does the paraffin method. V. THIRD-DAY EGG. — A. Open the egg as before. B. Study the embryo and blastoderm while still alive and in situ. A large outline drawing should be made. The details of the body of the embryo will be seen better later but the arrangement of the blood-vessels can best be studied now while the circulation is still active. As a rule they can be seen distinctly through the vitelline membrane but if not the latter should be carefully stripped off. A Greenough binocular microscope with its lowest power otyectives is a useful accessory for examining the blood-vessels. C. Excise the embryo with the surrounding portion of blasto- 512 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. derm, float it on a slide, cover with coverslip supported by wax feet at the corners and examine as a transparent object, comparing the various features with those shown in Figs. 235 and 236. D. Excise a second embryo with its surrounding blastoderm. Float it on to a coverslip, embryo above, and submerge it in a watch- glass of fixing fluid. Watch it carefully under the lens or preferably •low-power binocular as the tissues gradually become opaque. The amnion will be seen particularly clearly during this process. A drawing should be made of the embryo enclosed in its amnion- as an opaque object. E. Carefully strip off the anmion with a pair of needles l and study the configuration of the head end of the embryo. F. Stain and mount the embryo. G. Prepare series of sections (a) transverse to trunk region, (6) horizontal through trunk region and therefore approximately sagittal in the region of the head which is lying over on its left side. VI. THE FOURTH DAY. — On placing the egg in the salt solution the broad end will tilt up more decidedly than before owing to the increase in size of the air space. Care should therefore be taken to make the first perforation of the shell close to the broad end so as to allow the air to escape. Care must also be taken not to injure the vascular area as the whole blastoderm is now much closer to the shell than it was in earlier stages. As soon as the egg has been opened a careful drawing should be made while the embryo is still alive and in situ. The main features of the vascular system in par- ticular should be carefully worked out at this stage. If the circula- tion becomes sluggish through cooling a little warm salt solution should be added but care must be taken not to bring about a great and sudden rise of temperature as in this case the greatly accelerated heart-beat is apt to cause rupture of a vessel. The body of the embryo, allantois, etc., are covered over by the thin transparent serous membrane or false amnion as becomes apparent if the attempt is made to push a blunt needle down into the space round the allantois. This membrane should either be cut through with a pair of fine scissors, just outside the boundary of the allantois, or carefully stripped off with fine forceps. When this has been done it is possible to shift the body of the embryo into such a position that it with its blood-vessels can be observed in side view. Until this has been done it is impossible to get a proper view of the body of a well-developed embryo of this age owing to its dipping down out of sight into the yolk-sac. The einbrvM should now In- excised by cutting round outside the boundary of the vascular area and floated into a watch-glass of dean warm salt solution. The embryo mav now be studied as a trans- parent object on the stage of the dissecting microscope. It is better in niiii'l lli.it -(<•'•! ii'M-dlrs must, not lie alluur.l to toiirh roiTO.siv.- -uh limat< "I solutions are convenient li\in^ agents to use I'm- 1) K. \ TECHNICAL DIEECTIONS 513 however in the first attempt t<> proceed at once to h'x the embryo. An essential preliminary is to remove the true ainiiion which el ensheaths the body of the embryo. In doing this it is best to com- mene.- at the region b«-t WITH the heart and tlie tip of the head where a couple of fine needles may be used to tear the amnion. Its anterior portion may then be seized with fine forceps and pulled backwards over the embryo's head. The operation is simplified by carrying it out immediately after submerging the embryo in fixing iluid as the action of the fluid makes the amnion slightly opaque and therefore 11 1<> iv easily visible. If however corrosive sublimate be the fixing fluid fine splinters of coverslip should be used for dissecting off tin- amnion unless this is done prior to immersing in the fixing fluid. The embryo should again be carefully studied during the process of fixation, many details becoming particularly distinct before the creature becomes completely opaque. Finally the embryo should be studied, preferably with the binocular, as an opaque object, and then prepared either for section cutting or for mounting whole. VII. Six DAYS. — Open freely into the air-space. Carefully tear away part of its inner wall so as to expose part of the vascular area, great care being taken not to injure the latter. Notice the direction in which the vessels of the vascular area converge : this will indicate the direction in which the embryo is to be found. Work towards the embryo, picking off the shell piece by piece, using Hunt forceps. Frequently the escape of the air from the air-space allows the vascular area to sink down and leave a wide space between it and the shell membrane. In other cases however it remains in close contact with the shell membrane and in this event the greatest care must be taken not to injure the vascular area as by doing so the very fluid yolk is allowed to escape and the salt solution rendered so opaque that observation of the embryo in situ is made almost impossible. Notice that the allantois has increased much in size, that it has become richly vascular and that it is spreading outwards in a mush- room-like manner underneath the serous membrane. It has already spread so far as to cover the embryo nearly completely. It is best now to remove the shell entirely and to examine its contents as they lie submerged in the warm salt solution (as shown in Fig. 242). With fine sharp scissors cut through the serous membrane just outside the limit of the allantois, commencing on the dorsal side of the embryo where the allantois is not yet closely applied to the yolk- sac. It is easy to do 'this owing to the coelomic cavity having spread outwards well beyond the limits of the allantois. The allantois being now no longer flattened out, by its continuity with the serous mem- brane all round, its vesicular character becomes apparent, as well as the difference in character of the vascular network on its proximal and distal walls. The relations of the vascular allantoic stalk to the vascular yolk-stalk should be noted : also the fact that the amnion is VOL. II 2 L 514 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. now widely separated from the embryonic body by secreted amniotic fluid. If the embryo is a well-advanced one towards the end of the sixth day the amnion, which is now muscular, may exhibit periods of muscular contraction during which the embryo is rocked to and fro in the amniotic fluid. These movements must be distinguished from the occasional contractions of the muscles of the embryonic body which also occur about this time though they are much less conspicuous. After a careful study of the living embryo with the allantois and yolk-sac hanging from its ventral side it may be excised along with a circle of vascular area, floated into a watch-glass and examined alive with a lens or binocular, and then treated with fixing fluid such as Bouin's solution. The latter brings out the surface modelling which should be carefully studied especially in the region of the gill clefts. Dissect off the amnion, add more fixing fluid and after superficial fixation renew the Bouin's solution. It is a good plan to suspend the embryo by the yolk-sac so that the weight of the head causes the neck to become somewhat straightened. After the embryo is sufficiently fixed the neck may be cut through and the lower surface of the head studied for the relations of the olfactory rudiments and mouth. Sagittal sections through the head are particularly instructive at this stage. VIII. SEGMENTATION. — To obtain segmentation stages hens which are regular layers should be chosen. In such cases the egg is laid at a slightly later time on consecutive days. As a rule egg-laying is confined to the forenoon and early afternoon and when an egg is due after the end of this period it is retained within the oviduct and not laid until next day. The retention of an egg in this way inhibits the process of the ovulation so that a new egg is not shed from the ovary until the preceding one has been laid. HISTORY OF THE EGG UP TO THE TIME OF LAYING. — Tin- e-- aris.-s as a single cell of the left ovary1 which grows to a relatively enormous size as yolk is deposited in its cytoplasm. The yolk is of a characteristic yellow colour but in particular tracts the disini lion of its granules into finer particles gives it a white colour. Of thin white yolk a mass occupying the centre of tin- CL^- is continuous through a narrow isthmus with a tract lyin^ immediately hnirath th.- •j'-rminal disc (" Nucleus of Pander ") and this latter is prolonged as a thin superficial layer over UK- surface of tin- e^Lf. I'd ween the supi*rli<-ial layer and tin- central mass arc a number of thin con f.-ntric layers of whit«- yolk. T> .<• ri^'lit 1 1 vary and oviduct \\liii-han- |>P .-rut in Mrly Stag6S UndeigO atrophy, •niiiX' functional Tins i> |. rok-il.lv tu In- iv-;rdrd as .-in adapt i\ .• arran^' in«-iit uliic-h lias IK-, -n \\ith lar^f »-^,Us t<>a\oid tin- dangers wlii<-h \\oiild In- involvi-d in | li«- synchronous passage Oil pair i>f r^-js (>: ,i!lv if ••Miitain.-d in a ii'jid -ln-11. into tin- narrow terminal |ioii:.ni ..f tin- passage t<> i x EGG OF COMMON FOWL 515 As the egg increases in size it bulges out beyond the surface of the ovary, becoming eventually dependent from the ovary by a thin stalk at the md of which it is enclosed within the distended follicle. Tin- wall of this is richly vascular except on the side away from the stalk when- -in elongated patch — the "stigma" — marks the position in which the follide-wall will rupture to set the egg free. When this process (ovulation) is about to take place the thin membranous lips of the oviducal funnel become active, apply them- selves to the follicle containing the ripe egg and grip it tightly. The follicle then ruptures and the egg is as it were swallowed by the ovidueal funnel. Within the funnel fertilization takes place pro- vided that, spermatozoa are present.1 The egg proceeds now to travel slowly down the oviduct, propelled onwards by the peristaltic contraction of the oviducal wall, the entire passage occupying about 22 hours. As it does so the ^=i^__^_ albumen is deposited on its ^^^^fci^^^^ ,s.m. surface by the secretory activity of the oviducal epithelium. The first to l>e deposited is rather denser than that formed sub- VLS-. \ sequently. It forms a sheath immediately outside the vitel- line membrane and extending in tapering spindle-like fashion for some distance up and down Fl<:- 223.— Unincubated egg of the Fowl. the oviducal Cavity: the tWO a.s, air-space; a/6, albumen; <-h, i-halaxa ; >./,;, prolongations are the Chalazae sl'"U membra™-. ^ the centre-at the apical pole- *_,. O , . is seen the germinal disc with the white " Nucleus of ( t Ig. ZZ6, Cll}. Punrlor " showing through it. The envelope of dense albu- men enclosing the egg is not merely propelled onwards; it also undergoes a clockwise rotation about the axis along which it is travelling, caused probably by the cilia present on the oviducal epithelium. Owing to the prolongations of the albumen in front and in rear of the egg not undergoing this rotation the chalazae become twisted upon themselves in opposite directions. Layer after layer of albumen (Fig. 223, alb) is deposited round the egg and chalazae until the full size is reached. The character of the secretion then changes and the shell membrane (Fig. 223, s.?/i) is formed. Finally in the dilated hinder part of the oviduct ("uterus") the secretion is in the form of a thick white fluid which, deposited on the surface of the shell membrane, gradually takes the form of the hard and rigid shell perpetuating the characteristically "oval" form impressed upon the egg envelopes during the passage down the oviduct. In composition the egg-shell consists of calcium salts infiltrating a slight organic basis of keratin-like material. Structur- 1 The spermatozoa remain alive and active within the oviduct for a period of about three weeks. 516 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. x ally the greater part of its thickness consists of calcareous trabeculae forming a tine sponge work. The inner surface of the shell is rough, projecting into minute conical papillae, while the outer surface is covered by a smooth apparently structureless layer perforated by numerous tine pores. SEGMENTATION. — If the egg has been fertilized, it proceeds with its development as it slowly travels down the oviduct. The process of segmentation is accomplished during this period and consequently the obtaining segmentation stages involves the sacrifice of the parent hens. Owing to the difficulties in the way of obtaining a complete series our knowledge remained for long fragmentary but recently (1910) a number of stages have been described and figured by Patterson which give a fairly complete picture of the process (Fig. 224). From these data we may take it that the early phases of segmentation are based on the normal plan where a meridional furrow appears travers- ing, or passing close to, the centre of the germinal disx3 i.e. the apical pole of the egg, and is followed by a second meridional furrow perpendicular to the first. In the third phase there is occasionally a regular set of four vertical furrows but more usually the process now becomes irregular (Fig. 224, C). In the next phase also there may be a fairly regular development of latitudinal furrows demarcating a group of about eight cells round the apical pole but typically there is no such regularity. The initial furrows, which make their appearance as above indi- cated, gradually extend. They eat their way downwards into the thickness of the germinal disc, never however cutting completely through it. They also extend outwards towards the edge of the disc which however again they never quite reach. The apparent segments into which the germinal disc is mapped out by the early furrows are therefore not really isolated from one another — there being still continuity between the segments on the one hand peripherally and on the other on the lower side of the disc next the yolk. Complete blastomeres are first marked off when, about the time the latitudinal furrows appear, division planes make their appearance parallel to the surface, cutting off the small segments in the centre from the underh in^ deep layer of the germinal disc. The later stages of segmentation are quite irregular. Division planes make tln-ir appearance in all directions by \vhic.h the -vrminal >mei completely divided up into small segments except on its lower surface and round its rd^e \shere there remains a svncvtial mass in which the nuel.-i divide without their division I.einu followed by any protnplasmie sf_'m<-niaiion. It is to be noted that the process in. -nt at ion throughout goes on more actively towards the centre of ili.- disc, more slow ! rdl its margin, so that the blastoderm eonn'8 to be composed of -mailer cells towards the centre and larger toward- lli«- penphrM. Tin- n-ull ol tin- e'jmeiitation process is thai ihe original k B KII;. 224. — Views of tlu- l>l;isto,U-rm of tin- Fowl's egg during segmentation. (After Patterson, 1910.) A, 3 hours alter fntilixution ; 15. I51, his. ; C, -} his. ; 1), 4-5 hrs. ; E, about 5 lirs. ; F, 5^ hrs. C. 7 his. ; H, 8 hrs. 518 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. germinal disc comes to be represented by a lenticular blastoderm lying at the apical pole of the egg and corresponding to the mass of rnicrorneres of such a holoblastic egg as that of Lepidosiren. The superficial layer of cells become fitted closely together and form a definite epithelium — which is destined to become the ectoderm. The cells of the lower layers on the other hand are rounded with chinks between them representing the segmentation cavity. The lowest of all have the appearance of being incompletely cut off from what is ordinarily termed the white yolk lying below them but which is really a syncytial layer full of fine granules of yolk and with scattered nuclei. Apparently a few accessory sperm nuclei are usually present in the fertilized eggs and faint traces of abortive segmentation may be visible round them (cf. Elasmobranch, Fig. 8, B*, p. 14). At the time of laying the blastoderm forms a small whitish disc covering the apical pole of the egg. Sections show it to consist of an upper layer of ectoderm and of a lower layer consisting of numerous rounded micromeres lying about in the fluid of the segmentation cavity. These micromeres become larger towards the lower face of the blastoderm and they are more crowded together round the periphery. It must not be supposed that all newly-laid eggs show exactly the same degree of development. As a matter of fact great variation occurs, one of the chief variable factors probably being the length of time occupied in the passage down the oviduct. Where this time is longer, as e.g. towards the end of the laying season, the stage of development of the egg when laid is more advanced. THE FIRST DAY OF INCUBATION. — After the egg has been laid the lowering of the temperature leads to such a slowing of its vital processes that development appears to come to a standstill. If kept at a low temperature it retains its vitality for a considerable period but makes no appreciable advance in development. If the tempera- ture be raided by incubation the developmental processes are at once accelerated and comparatively rapid changes come about. The blastoderm increases in size, its margin spreading outwards, and at the same time there comes about a distinct difference in appearance between its central and marginal parts — the central portion assuming a dark transparent appearance (pellucid area) which contrasts strongly with tin- whiter "opaque area" surrounding it. The examination of sections at once explains I his difference in appearance: UK- more opaque appearance peripherally is seen to be due to the lower layer cells bcinjj- then- closely crowded ((.-ether. An important change soon comes over the lower layer cells, much as those next to the yolk, in the region undcrlyin- the pellucid area, lose their rounded shape, become somewhat Mied and adhere to-jdhei' cd^e t<> rd-e to form a eontimmus membrane — th«- -< < ondary) endoderm. This appears lirst hcneath the posterior poitioii of I he pellucid area: it -raduallv e.\; FOWL— HKST DAY 519 a.p. a.o. 77.5. B Kn;. 'J'.!"'. - Illustrating three stages of the blastoderm of the Fowl during the second half of the first day of incubation. «.", Mpai|iii' area ; <»./>, pellucid an-a ; /'./>, head process; mes, Ixnuulary of sheet of raesodcrni ; m./, medullary fold ; />.;/, primitive groove, ; p.s, primitive streak. forwards and outwards, and eventually is continuous all round with the thickened marginal part of the blastoderm.1 1 This thickening of the posterior edge of the blastoderm presents in sagittal section a striking resemblance to a gastrular lip growing back over the yolk and Patterson (1907) believt-s that an actual process of involution — a reminiscence of gastrulation by inva^inauon — takes place. It must not be forgotten that any explanation of such obscure developmental phenomena in Birds must, to be reliable, :>20 EMBRYOLOGY <>F TJIK L<»\VKI; V K I IT K P. RATES CH. A gradual change takes place in the shape of the pellucid area which, up till now circular, assumes an oval or pear shape (Fig. 225, B) — the long axis perpendicular to the long axis of the egg- shell, and the narrow end being next the observer when the broad end of the egg-shell is to the left, This narrow end may be called posterior from its relations to the rudiment of the embryo which appears later. Together with the gradual change in the shape of the pellucid area there takes place the development of the primitive streak. This makes its appearance usually during the first half of the first day of incubation, as a linear opacity stretching forwards along the long axis of the pellucid area in its posterior third. As the first day of incubation goes on the primitive streak becomes more and more distinct. A longitudinal groove develops along its middle — the primitive groove — while on each side of this it forms a ridge, the primitive fold. If a number of eggs be examined during the first day of incubation <>nd. FIG. 226. — Traiisvt-iM- -.v-tion through primitive streak of the Fowl. ect, ectoderm ; end, endoderm ; ines, mesoderm ; p.g, primitive groove. it will be seen that the primitive streak, as is commonly the case with vestigial organs, shows extreme variability. More especially its hinder end is commonly bent to one side or the other, or even bifurcates into two branches. At its front end one or both halves of the primitive streak swell up into a slight knob while the primitive groove becomes somewhat deeper and wider. The primitive streak is shown by transverse section to originate from a linear trad of ectoderm along wliieh the cells are undergoing rapid proliferation, afl is indicated by tin- relatively numerous mitotic nuclei. Tli«- cells budded <»H' by the ectoderm are aggregated in-r iii a eompaet mass along the course ol' the primitive streak while on each side they In-come loosened out and \\aiidrra\\ay into the space hHwi'.-n eelodrrm and cndoderm to take part in forming the .-in-. -i of tnesoderm. rest on a firm basis of kn«»\vl. -liip •)!' tlit-M- ilf\ rlopiiii-ntul stages of Birds to the corresponding sUp <,i K. •].!.;••. i : ..i in tin- pi> 'Hi writer's opinion adequate to lonn a tj-iist \\ollhy IM i l,,r |]|i;i mi, i p:.-l..li..n. x I -oWL TIRST DAY 521 For a, short distance in the region of its from end the mass of cells forming the primitive streak is continuous not only with the .'(•((..It-nil out with tin- endoderm as well: tin- primitive streak of this region may In- delined as a tract along which there is cellular coniiniiity lu-t ween the ecloderm and the endoderm. During the latter half of the first day what is known as the • Head process M makes its appearance, (Fig. 225, B, hji). In a view of the whole blastoderm this has the appearance of being a somewhat less distinct prolongation forwards of the primitive streak — in front of the knoh which marks its apparent front end. The study of transverse sections shows that the so-called head process is exactly similar in structure to the primitive streak immediately behind it, except that it is separated from the overlying ectoderm by a distinct split and that there are no primitive folds or primitive groove over it. On its lower side there is perfect continuity with the endoderni — as is the case with the anterior part of the obvious primitive streak into which it is continued. During the same period of incubation there appears the first sign of the surface relief of the body of the embryo in the form of what is known as the head fold (Fig. 227, A, h.f). This is formed by the blastoderm bulging upwards and forwards, forming a projection bounded in front by a steep face crescentic in shape as seen from above, the two horns of the crescent directed backwards. The projection increases in prominence : its front edge soon comes to overhang, the blastoderm becoming tucked underneath it both in front and at the sides, the two horns of the crescent which the fold formed at its first appearance giadually extending farther and farther back- wards. The projection is destined to give rise to the head end of the embryo and there are certain important details to be noticed about its structure which can be made out best by the study of sagittal sections. The region of the blastoderm where the head fold develops is composed of the two primary layers, ectoderm and endoderm, the mesoderm not yet having spread into it. It follows that the head rudiment has a double wall, its outer sheath of ectoderm enclos- ing an inner wall, quite similar in shape, composed of endoderm. It will be understood that this inner wall of endoderni is continued at its hind end into the flattened layer of endoderm which lies on the sui lace of the yolk. In other words the endoderm within the head rudiment may be described as forming a very short wide tube, blind anteriorly but opening behind into the yolk. This endodennal tube is the rudiment of the front part of the endodermal lining of the alimentary canal of the adult and is termed the foivgut. Soon after the commencement of the formation of the head fold the ectoderm of the medullary plate becomes raised up into a longi- tudinal ridge (Fig. L'27, A, m.f) upon each side of the median line. Between the two ridges is a groove — the medullary groove: the ridges themselves are the medullary folds : the two medullary folds 13 vr V ^ s i - = 5 'il o = - r.i FOWL— FIKST DAY 523 are continuous anteriorly. The two medullary folds gradually extend bar,k wards and ai. l IK- same time they Ix-come more prominent and arch over towards out- .mother until at about the end of the first day they meet. It is to be noticed (Fig. 227, B) that the first meeting of the medullary folds is some little distance back from their anterior end, in about the position in which the division between niesence- phalon and rhombencephalon will develop later. Towards their anterior end the folds remain less prominent than they are farther hark with the result that the meeting of the two folds is here con- siderably delayed. During these later hours of the first day important advances are taking place in the development of the mesoderm. In the first place it is to be noted that the anterior limit of this layer is gradually extending forwards, encroaching more and more upon the proanmion —the part of the blastoderm in front of the head fold which is still two layered. In the second place the mesoderm becomes considerably thickened and more compact in the region near the median line — adjacent to the head process or notochord. This thickened portion of the mesoderm becomes divided by transverse splits into a series of blocks — the mesoderm segments — lying one behind the other (Fig. 227, A and C, m.s). The first pair of splits to make their appearance are placed obliquely, sloping outwards and backwards : they mark the hind boundary of the first or most anterior segment. A little later a pair of similar splits develop a little farther back forming the hinder limit of the second segment, and so on, segment after segment becoming separated off from the still continuous mesoderm lying farther back. While this portion of the mesoderm is becoming segmented it is at the same time becoming sharply marked off by its greater thickness from the lateral mesoderm lying farther out from the axis. Towards the end of the first day a further important development takes place in the mesoderm in as much as isolated splits appear in it parallel to its surface and these gradually spread and finally become continuous so as to divide the mesoderm into the outer somatic layer next the ectoderm and the inner splanchnic next the endoderm. The cavity which has made its appearance between somatic and splanchnic layers of mesoderm is the coelome. The portion lying within the myotome, which soon becomes filled up by immigrant cells derived from its wall, is the myocoele (Fig. 228, me). The portion lyinir farther out, in the lateral mesoderm, is the splanchnocoele (splc). The two layers lying external to this cavity — the somatic mesoderm and the ectoderm — constitute the somatopleure or body- wall : the corresponding layers lying internal to the cavity — the splanchnic mesoderm and the endo- derm— constitute the splauchnopleure or gut-wall. While the changes above described have been taking place the blastoderm has constantly been increasing in area and by the end of the first day it forms a cap covering an extent of about 90° at the upper pole of the egg. In the opaque area — the part of the bias to- 524 EMBRYOLOGY OF Till- LoWKi; YKh'TKUKATKS CH, derm lyini: outside tlie boundary of the pellucid area — there are present the same layers of cells as in the pellucid area — the ectoderm, which extends farthest peripherally, the endoderm which passes into a thick yolk svn< -vtial layer peripherally (germinal wall), and the n.r my. me. ect. spLc. ***** I sow end ac Fi<.. 228. — Transverse section through the body of a Fowl embryo about the end of the Hist day of incubation. ect, ectoderm ; end, endoderm ; me, myocoele ; my, myotome (mesoderm segment) ; JV, notochord ; n.r, neural rudiment ; sow, somatopleure ; spl, splanchnopleure ; splc, splanchnocoele. mesoderm the outer part of which is still unpenetrated by the coelomic split. The part of the opaque area where mesoderm is present assumes a very characteristic mottled appearance (Fig. 227, C, a.v) caused by the rudiments of blood-vessels and blood : hence the name vascular area which is given to this part of the blasto- derm. When the embryo has reached the stage with about seven mesoderm segments the secretion of tin id (plasma) commences within the blood islands. THE SECOND DAY OF INCUBATION. —The general appearance of an egg opened during the second day of incubation is seen in Fig. 229. Tin1 blastoderm has increased consider- ably in si/o and now covers about 110°. The pellucid area has assumed a somewhat liddlc-like shape. On examining the excised blasto- derm about (he ColimieiieeimMit of lbi> day it is seen thai the formation of tin- head fold lias progressed COD urably and the head rudiment prnjct-i< more OpnSplcUOUSlj ahn\v • •in-ral h-vrl o| i|M- blastoderm, Within tin- head rudiment . refill can he seen and it is noticeable that it stretches farther hark than doeg tin- mil.-r \\all «•!' the head rudiment. In ntlu-r \sortl- the head !'..|d «.!' the eiidndenn hassjirrad lartherbaek ; Fl • -a, with tin- Mi.lim.Mit (,r . FOWL— SECOND DAY i25 than that of the ectoderm. This is hrought out clearly by a sagittal section such as that shown in Fig. 2:50. Such a section also brings out the fact that while the greater part of the portion of blastoderm tucked in beneath the head of the embryo is two-layered (proamnion), there being no mesoderm present, this does not apply to the larih'->i back part of the fold. Here, in the wide space between ectoderm and endoderm, mesodenn has penetrated which will give rise to the p«-ri- cardiac wall and the heart. The medullary folds have met over a considerable extent but still remain separate at their extreme front ends as well as over the whole extent which will later form the spinal cord. Here they bound a deep neural groove. Towards their posterior ends tbe two medullary folds diverge to pass on either side of ji lance-shaped area (rhomboidal sinus) which they enclose by converging towards one another behind it. Along the centre of the ae. pa. end. hf splc. Kit;. 230. — Diagrammatic sagittal section through anterior end of Fowl embryo with 15 segments. a.e, rudiment of amnion ; br, brain ; &•(, ectodfi -in ; cue, endocardium ; end, endoderm of yolk-sac : f.g, Ibregut ; h.f, posterior limit of head fold of ectoderm ; me, myocardium ; N, notochord ; pa, ectoderm of proaiimion : t/ilc, splanchnoeoele. tioor of the rhomboidal sinus the primitive streak is still visible separated by a knob-like elevation from the part of the primitive streak which lies farther back. The mottled appearance characteristic of the vascular area is now seen to be continued inwards, though much more faint, across the pellucid area to the body of the embryo. An embryo with about ten segments is shown in Fig. 231. The pellucid area is still somewhat fiddle-shaped with the body of the embryo lying along its axis. Apart from the increase in number of the mesoderm segments the most conspicuous advances in development are in the central nervous system. The medullary folds have met and fused together to enclose the neural tube except towards their hind ends where they still bound the rhomboidal sinus on each side. The forebrain region is greatly dilated, its projection on each side being the optic rudiment (o.r). It will be noticed that a slight notch in its wall in the mesial plane anteriorly indicates that at this point the two neural folds have even yet not 526 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. completely fused. Posteriorly the neural folds seein to be continuous with the lips of the primitive groove. A faint continuation forwards of the primitive groove may he seen in the floor of the rhomboidal sinus. pa or ms. . ' //JN Fi'.. 'l'-\\. UlastmiiTiii uitli Fowl embryo with al>out 10 or 11 inesodcrni M-L;inriit>. t : I,, lii-ail : |fc/ in.-.liillary f'olil ; ?H.S. iii.->mliTiii s.-iii«Mit : ;it i<- rniliiiu-iit : pa, prDaniiiinn. Iiiijtoriani incsodci in t'ciitun-s an- in l»e noticed. The nmttled appearance of the vascular area pm.lii.-r.l h\ tin- i iidiin.'nis <.!' hlno.l- veisels developing in lh<- -planHmir iiifs..«lcnn is conspicuous. The formerly isolated vascular rudiments (white in the li^-uiv an- now becoming joi in -i i up i., i',,nn a net \\ork ami tlu- net work can be traced 'Hid and «ui a .- ma 1 l«-r >ca If aOIOBa Mn1 pellucid area. At x FOWL— SECOND DAY 527 its anterior and inner corner the network is continuous with a short and wide vessel which slopes obliquely forwards and inwards and disappears beneath the hind end of the foregut (shown more clearly in Fig. 232, v.v). This vessel is the rudiment of the vitelline vein, which drains the blood from the vascular area towards the heart. Another conspicuous vessel rudinn-nt is the terminal sinus — a marginal vessel which bounds the vascular area externally. In front of the head of the embryo is a somewhat rectangular area of the blastoderm distinguished by its being very transparent (Fig. 232, pa). This is the proamnion — its transparency being due to the fact that Lnf pa o.r fg- . N. S/'/C — U.V: m.s. Fi<;. '2'3'2. Head of Fowl einlivyo of same sta^v ;is tliiit shown in Fig. 231, more liighly magnitietl and seen l>y transmitted light. /.'/, t'ore^ut ; 11, heart ; //../', hinder limit . of head fold of ectoderm ; />/, iiifundibulum ; m.s, mesoderm •M Amenta ; AT? notochord ; u.,-. optic rudiment; /»", prtKininion ; s/ilc, patent portion of splanchnocoele containing nieloinie tluid ; /•. r. vitelline vein. the mesoderm has not yet spread into this region of the blastoderm. On each side of the head of the embryo the surface of the blastoderm bulges upwards into a dome-like swelling (Fig. 232, splc). This is due to a precocious splitting of the mesoderm in this region to form a large coelomic space. The bulging appearance is produced by the coeloinic space being tensely filled with fluid. The raising up of this region of somatopleure is preliminary to the formation of the head fold of the amnion. By turning over the excised blastoderm and examining it from below or by staining and then examining it in dorsal view by trans- mitted light (Fig. 232) it will be seen that between the two coelomic spaces there lies a A-shaped structure. The two diverging limbs of 528 EMBRYOLOGY OF THE LOWER VERTEBRATES OH, the A posteriorly are the vitelline veins already alluded to (v.v), while the median portion (H) — a straight tube passing forwards beneath the foregut — is the rudiment of the heart and ventral aorta. It will be noticed that the two vitelline veins when traced backwards from the heart are seen to fit round the tunnel -like opening of the foregut. In the forebrain region is seen the downwardly projecting pocket of its floor — the ini'undibulum (Fig. 232, inf} — and extending back from this in the middle line the notochord (TV). On each side of this posteriorly are seen the meso- derin segments (ra.s). In a slightly more advanced embryo with about fifteen mesoderm segments the tucking in of the blastoderm under the head has proceeded considerably further. The neural tube has become closed in entirely except for the slit-like remnant of the rhomboidal sinus posteriorly. The optic rudiments projecting prominently from the forebrain on each side and beginning to be narrowed slightly at their base give the brain a conspicuous T-shape. 'The wall of the brain in its posterior region shows a series of puckerings one behind the other marking it off into a series of what used to be called brain " vesicles." Of these the anterior one, the largest and most distinct, is destined to become the mesencephalon while those behind it enter into the formation of the rhombencephalon. The latter are often interpreted as vestiges of a once present segmentation of the brain, but are regarded by the author of this volume as being adequately accounted for by the active growth of the brain within its confined space, aided possibly by the varying consistency of the rnesenchyme outside it (see p. 101). On each side of the head region posteriorly, just in front of the first obvious mesoderm segment, the rudiment of the otocyst has made its appearance as a cup-like depression of the ectoderm. The heart, growing in length more rapidly than the neighbouring parts of the body, has been forced into its characteristic bulging outward on the right side. The first traces of haemoglobin are making their appearance in the posterior portion of the vitelline network. An imp' .riant new feature becomes visible about this stage in the form of a whitish line on the bulging roof of the splanchnocoele on each side. The lines in front curve in towards one another, meeting in front, of the proaumion and sweeping back in a wide curve on each side. This line is the first rudiment of the amniotic fold. As the fold becomes more and more prominent it bends .vards and inwards, ardiin- over the head region, and towards the end of the second day (Fig. 233) forming the amniotic, h«><>d which en the head portion nf the embryo, Manv of ili'- important details in the si met nro of the second ,l,iModerm •••'in "iily !»• made out by tin- study "f s. ries of transverse sections. In studying the sia-e m,w under consideration it in advisalile 1" ln^in \\ith a section taken from ah.uit the middle of FOWL— SECOND DAY 529 tlic total length <>!' the, « -ml. ryo such as that represented in l-'i--. 234A. The blasi.,derm sum.- little distance away from the median line of the embryo is seen to consist of the usual two double layers — the somato- pleure (som) composed of ectoderm and somatic mesoderm and the splanchnopleure (spl) composed nf splanchnic, mesoderm and endo- sa. \ n a.v, FIG. 233. — Blastoderm and embryo Fowl with 18 mesoderm segment-. o.e, l>ack#ro\vinK ''J^e ot'ainniotir hood ; o./*, yxillucid area ; tt.r, vascular aroa ; //, heart; -.•ro-ainniotic connexion. derm. In immediate contact with the lower surface of the endoderm in the complete egg there would be the yolk. In the splanchnic mesoderm overlying the endoderm are seen the blood-vessels of the vascular area. When traced inwards towards the mesial plane the two layers of mesoderm are seen to come together to form the narrow proto- vertebral stalk or nephrotome which joins up the lateral mesoderm to VOL. II - M 530 EMBRYOLOGY OF THE LOWE K' V K 1 ; T K li KATES OH, the mesoderm segment. Immediately above the nephrotome, between it and the ectoderm, is seen the rudiment of the archinephric duct — a rod of cells which is gradually extending tailwards. In the centre of the section is the neural tube (s.c) with its thick walls and the solid notochordal rudiment (N) lying immediately s.c. N FIG. 234A. — Transverse section through the middle of a second-day Fowl embryo (15 segments). A, paired dorsal aorta ; a.n.d, archinephric duct ; ect, ectoderm ; end, endoderm ; my, myotome ; N, notochord ; s.c, spinal cord ; xom, somatopleure ; spl, splanchnopleure ; splc, splanchnococlr. below it. The blood-vessel (A) on each side between nephrotome and endoderm is the dorsal aorta which is at this stage double. Working back towards the tail end of the embryo it is seen that subsequent sections show less and less advanced stages of development end.. •!'.'> IB. —Transverse section through a second-day Fowl embiyo just In-hind the hinder limit of tli<- 1'nregut. A, dornal aorta; end, endoderm; my, myotome; N, notoHionl ; 1.0, -pinal i-or.l ; fum. som:itopli'im> ; >/./, splanchnopleure ; xjdr, splani-liiiiicorlf ; ,-, irenwll "I \:i*riil;ir area. in concordance with I In- fact that development pn»eee«ls fnun tin- head end tailwards. 'I'hus the neural tube opens <>ui l.y tin- slit.-like rliomlioidal sinus; tlie arohinephrio duct disappears; the notochord passes back into the andiffexentiated tissue of the ])rimitivc streak. On tin- niht-i liand llieexaniinalK.il «»!' sect i«»ns 1'ai't her l«»r\vanl towards tin- 1 ion l.rin^s into \ie\v vai'i.iiis iinportaiit. lurlher devel' Such a section U 'hat sho\\n in Ki'j. L':1,!!; illustrates FOWL— SECOND DAY 531 clearly ;in early stage in the folding off of the foregut from the cavity • »!' t In- yolk-sac — a fold of splanchnopleure growing inwards on rarli side U'ln\\ what will become the foregut. The large vessels Been in the splanchnopleure external to the fold just mentioned are tributaries of the vitelline veins, and a few sections farther forwards they would In- found to be united together to form the main vitelline vein on each side. As the series of sections is traced forwards the two folds of the splanchnopleure are seen to approach one another and finally to meet and undergo fusion, so that there now exists a foregut cavity shut oft' (as seen in transverse section) from the yolk-sac, the walls of the two structures being still connected by a median vertical partition formed by the fusion of the endoderm from A. ,s.c. so.m sp.m. trmc. FKJ. 234c. — Transverse section of a second-day Fowl embryo passing through the rudiment of the heart. A, dorsal aorta; d.mc, dorsal mesocardium ; vnc, endocardium; end, endoderm;/.^, foregut; me, myocardium ; s.c, spinal cord ; so.?//., somatic mesoderm ; sp.m, splanchnic mesoderm ; splc, splanch- nocoele ; r.mc, ventral mesocardium. the two sides. A little farther forward this partition disappears from the section and the foregut as seen in section (Fig. 234c) is quite isolated from the endoderm of the yolk-sac wall. The vitelline veins have also fused to form the tubular heart. It is seen that the splanchnic mesoderm ensheaths the endothelial wall of the heart (enc) on each side and that where it does so it is somewhat thickened (me) as compared with the same layer in the region overlying the yolk-sac. This localized thickening of the splanchnic mesoderm is destined to give rise to the entire thickness of the heart wall except the lining endothelium. It is seen to be continuous with the extra- cardiac portions of the splanchnic mesoderm by the dorsal (d.mc) and ventral mesocardium (v.mc). Traced forwards through the series of sections the heart is seen to narrow in calibre as it tapers off into the ventral aorta. Towards its front end the latter gives off a large branch on each side which 532 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. passes outwards and upwards round the foregut to become continuous with the dorsal aorta. These two hoop-like vessels which connect up ventral and dorsal aortae are the first pair of aortic arches. Still further forward the region of the forebrain and optic rudiments is reached (Fig. 234D). Owing to the folding off of the head rudiment the section of the head itself appears completely detached from the blastoderm and the latter is beginning to form a depression which will later become more marked and in which the head will lie. In the blastoderm it will l>e noticed how away on each side it shows the normal four layers of cells — ectoderm, somatic mesoderm, splanchnic rnesoderm, endo- derm — while on the other hand in the region underlying the head of the embryo it is only two layered the mesoderm being here absent. o.r FIG. 234D. — Transverse section of a second-day Fowl eml>ry<> passing through the optic rudiments. ect, ectoderm ; tntl, endrxlerm ; ntvs, inescm-liyine ; o.r, optic rudiment ; *///'. tplanchnoooele ; thai, roof of thalamencephalon. . proamnion This two -layered region of blastoderm is the proamuion before alluded to. The head itself is occupied almost entirely by the brain rudiment —the thalamencephalon in the centre (thai) continued outwards «>n each side as tin- optic rudiment (o.r). For the most part the external ectoderm is closely apposed to the surface of the brain but dorsally the former is com UK MIC ing to recede from the latter, the space between the two lu-in- occupied by mesenchvme (mes). TIIK THII:I> I)AV OF Ivri'.ATinx. During the later hours of the second and earlier hours of the third day of incubation there take place a number of important changes which render this period perhaps the mo>t iui.-n-st ing of all to the morphol<,-ist. K,.r ih,> student \\ho is training himself practically in the technique of embryolo'jiral observation there is no liner malt-rial than that afforded by I'.ird embryos of about ih. far lea ruing one of tin- most important parts of that technique namely the interpretation of serial sections. FOWL— SECOND AND THIRD DAYS 533 It is advisable to make a careful study of the anatomy of an embryo of about the stage shown in Fig. 235 or 236. l s.a. ot. H. ~ a.e ita. tf I'l". -;!.">. Third-day Fowl embryo with the vascular area. ".>, edge ofaranlou; }•:, eye; //, heart; ot, otocyst; .-•.-. s,n,-amniotic connexion; g.t, sinus terminahs; /../, tall-fold; -•.•/. vib-llin,- aiteiy; . portion ot splanchnopliMiiv involute.! to forma nvi'ss roun.l th«- hcud of tlie t-mbryo. 1 It is customary to mount transverse sections with the posterior or tailward surface of the section next, the slide: consequently the figures represent the sections as seen from in front and the side of each figure towards the right-hand side of the — corresponds to the left-hand side of the embryo. 534 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. On opening the egg it is at once seen that the blastoderm has increased considerably in size, the outer limit of the opaque area having spread downwards as far as about the equator of the egg. The vascular area has also increased considerably and is still bounded by the conspicuous terminal sinus which anteriorly turns inwards and passes back parallel to the corresponding part of the sinus of the other side to open into the vitellme vein close to its inner end. Of these two veins which run parallel to the long axis of the embryo the right is reduced in size and eventually disappears. The yolk has assumed a more fluid consistency ; the proportion of white yolk has increased ; the albumen has shrunk considerably in volume, and the air space has increased correspondiugly. The free edge of the amniotic hood (Fig. 235, a.e) has f this difference in rate of growth • •en dorsal and ventral sides is that those parts of the embryo which are detached horn the general blastoderm assume a strong re toward- lln- \.-nlral side. This is particularly pronounced in the head region the h. ad \*>\\\£ completely Lent up«.n itself so that FOWL -TlfllM) DAY 535 I In- front end <»f tin- bruin is n:\.-rseil in position, what was its ventral side ha\ in-' conn- to In- dorsal. Tin- nirsodrriii sro'iiirnts have increased in number there heing am. v.c. U.O. a.e. FIG. 236. — Third-day Fowl embryo (No. 47) viewed as a transparent object. a.e, edge of amnion ; am, amnion ; E, eye; //, heart; «i.s, mesodfini se-nients; ot, otocyst ; .--, indications of ] uootic mesoderm segments (?) ; /MI, vitelline aitcry : .<•. visceral cleft II; *, portion of •planohnopleore buljiiuj; dmviiwauls into the yolk, forming a recess in wliidi lies the head of tlie embryo. now al H>ut LI5-30 metotic segments and those towards the anterior end are shewing a considerable amount of dorsiventral growth. In some embryos (Fig. 236) the series of definitive mesoderm seg- ments is continued far into the head region by what appear to 536 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. be the ghostly vestiges of formerly existing segments (see pp. 210, 211). The central nervous system has made important advances in development The brain shows a relatively large increase in size as compared with the spinal cord : thalamencephalon, mesencephalon and rhombencephalon are marked off by definite constrictions — the mesencephalon being particularly prominent at the bend of the head. The greater part of the roof of the rhombencephalon is assuming its definitive thin membranous character. The three great organs of special sense have made their appearance. The eye (E) forms a large conspicuous cup-like structure lying at the side of the fore- brain. Its rim is cleft ventrally by the choroid fissure (Fig. 236). Its mouth is partially blocked by the round lens rudiment. The otocyst (ot) is also conspicuous — a pear-shaped sac, its narrow end dorsal, lying at the side of the hind brain. The olfactory organ is represented by a slight dimple of thickened ectoderm .near the tip of the head. The side walls of the foregut are perforated by visceral clefts. The series of these develop from before backwards and by this stage three have commonly appeared — clefts I, II, and III of the series. It is perhaps the vascular system which shows the most interest- ing features during the third day. The heart is still in the form of a simple tube, but its active growth in length has caused a great increase in the curvature which was already pronounced about the middle of the second day. Its y-like curvature is shown in Fig. 236. At its morphologically front end the heart is continued into the ventral aorta and this at its end gives off a series of vessels, the aortic arches, which pass up round the sides of the foregut between adjacent gill-clefts and open dorsally into the aortic root which lies just dorsal to the clefts. Like the clefts themselves the aortic arches develop in sequence from before backwards and by this stage arches I, II, and III have made their appearance (Fig. 241, A). At its front end the aortic root can be traced for some distance into the head as the dorsal carotid artery (Fig. 241, A, d.c). Posteriorly the two aortic roots become hidden from view by the myotomes but the study of sections shows that they have here united to form thi- unpaired dorsal aorta. Still farther hack this vessel again becomes paired Jtud a little behind the point of bifurcation iMf-h ••!' ill.- l.ranehes Lrives (.IT ;i large vitelline artery (v.«) \\hieh passes outward- a1 ri'jht angles to the axis of the body to supply the vascular area. ()l tin- reuoufl system the most enn-pieuous components are the great vitelline veins I ig. 241, A, v.i?) whieh. ivccivin- numerous l.ianehes from the va.v-ular area, pass |nr\\ards eoiivrruinu 1"\\ards one another t«» form l.y iheir fusion the hind end »»!' the heart, •ion of the vaseular area shows that 1 he I. ranches of the vitelline arteries and of the \ BOmpanv one another in their ramification-. In the livin-j-eoiidition, in which all these arrangements x FOWL—THIRD DAY 537 of the vascular system should be studied, the arteries are seen to be more deeply coloured and more conspicuous than the veins. The two vitellim- u-iiis by their fusion form the hind end of the tubular heart and on traein- iliis forwards a somewhat Y-shaped vessel is seen opening into it laterally. The stalk of the Y which is very short, though showing considerable variability within its limits, is ill.- right duct of < Juvier (Fig. 241, A, d.C). The branches of the Y are the cardinal veins. Of these the posterior (p.c.v), coming from the region of the kidneys, is only visible for a short distance, bciiiLi soon hidden as it is traced backwards beneath the myotomes. The anterior cardinal vein (a.c.v) on the other hand can be traced forwards for a long distance into the head from which it drains the blood back towards the heart. It will be noted that here in the embryonic Bird we find exactly the same arrangement of main veins — duct of Cuvier, anterior cardinal and posterior cardinal — as som. KM,. 'j:37A. --Transverse sections through third-day Fowl embryo. (Partly based on figures by Duval.) A, Through the hinder part of the trunk region. A, dorsal aortue ; am, ainniotic folds ; end, endoderm ; my, myotome ; s.c, spinal cord ; .-•»//(, somatopleure ; s/e recognized — the differences being mainly differences in detail. The most conspicuous of these is caused by the development of the amniotic fold of the somatopleure which rises up on each side, urchin- tnw.mls the median plane over the dorsal side of the embryo (am). Traced forwards through the series the amniotic folds of the two sides are seen to meet and undergo fusion so as to give rise 538 EMBRYOLOGY OF THE LOWER VERTEBRATES Ctt to the inner true amnion and the outer false ai union or serous membrane: the former continuous at its inner edge with the somatopleure of the embryo's body, the latter at its outer edge with that of the blastoderm. It will be readily seen that the space between true and false amnioii is morphologically part of the splanchnocoele. It will also be realized that both true and false amnion being somatopleural in nature are composed of ectoderm and somatic mesoderm but that the relative position of these two layers is reversed in the amnion as compared with the false amnion. Important changes have taken place in the niesoderm. The mesoderm segment is no longer connected with the lateral mesoderm the nephrotome having become* converted into renal structures — the archinephric duct and mesonephric tubules. The relations of these will be understood by referring back to the general description of renal Y FK;. 237fi. — Transverse section just behind the point of union of the two vitelline veins. A, dorsal aorta ; am, amnion ; c, conns arteriosus ; ect, ectoderm ; ?nt, enterun ; /.a, false, amnion or si-rons membrane; my, myotome ; N, notochord ; s.c, spinal cord; «i, seni-amniotic isthmus; smu, somatopleme : «/y tin- sem-amniotic connexion. In the me.soderm of the two folds of splanchnopleure which are approaching one an«th< -r to floor in the alimentary canal (ent) are seen the two large vitelline veins (v.v). The ventricle and the conus are seen cut longitudinally in the wide coelomic space lyiii.^ to the right of the body of the embryo. A section a little farther forward in the series has the appear- ance shown in Fig. 237c. The definitive gut (ent) is completely separated at this level from the yolk-sac, and corresponding with this the two vitelline veins, which in sections farther back lay one on each ent. ani. /am. n.c.u som.< r sbm. Fie,. 237c. — Transverse section a little in front of the hind end of the heart. "///, ainnion ; .1, dorsal aorta ; d.r, ductus venosus ; eit-t, alimentary canal ; ;."w. false ainnion ; /t.l, anterior liver rudiment ; //.-_', posterior ditto ; Ar, notochord ; p.c.v, posterior cardinal vein ; s-nn, soma- Inpleure; .-•/•/, splanchnoplenre ; >•/-/••, s]»lanchnocoele ; V, ventricle. side of the yolk-stalk, are now completely fused into a large median vessel, the ductus venosus (d.v), which is simply the backward prolongation of the heart. The posterior liver rudiment, a blindly ending pocket of the gut-wall projecting forwards ventral to the ductus venosus, is seen in the section figured (li.2), although its com- munication with the gut-wall is no longer visible, lying as it does several sections farther back. At this level however a second pocket- like outgrowth of the gut-wall has made its appearance (/t.l). This is the anterior liver rudiment. It will he noticed that it lies dorsal to the ductus venosus. In the coelomic space ventral to the ductus venosus and liver rudiments, and quite isolated, is the rounded section through the ventricular region of the heart ( V). In the sections studied so far the body-wall of the embryo is widely open on its ventral side — the opening heinir bounded by the recurved edge along which the somatopleure of the body is continuous 540 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. with the non-embryonic region of the somatopleure forming the ainnion. As however the folding off of the embryo progresses the edge alluded to grows inwards and the opening bounded by it becomes reduced in size. It will be gathered readily from Fig. 237D that through the opening in question the splanchnocoele, included within the definitive body of the embryo, is continuous with that part of the coelome which lies outside (extra-embryonic coelome). In the section figured the heart is seen to be cut through in two places. Reference to the figure of the whole embryo (p. 535) will show that the piece of heart which lies towards the left side of the embryo (at) is the atrium, while that on the embryo's right (C) is the ventricle or con us. In the section figured a large blood-vessel (d.G) is seen cut f.nm ' ph. am. som sptc. som. }•']<.. 237D. — Transverse section a short distance behind the front end of the lu-.-nt. .1, dorsal aorta ; am, ainnion ; at, atrium ; (', conns ; h, pharynx; v.c.I, iirst visceral cleft ; r.ca, ventral carotid. Transverse section passing thn.ii-li the eye :IIM! just in IV. mt ..f tin- «>t.u-y>t. "I inal vi-iii ; a., . :i«>i t n1 runt ; ./., ,i. ,[,,1 >al cai . .t is downwards as the rudiment of the allantois (all). The latter is covered with a thick layer of mesoderm and bulges into a dilated portion of the splanchnocoele. Towards the front end of the embryo a still more widely dilated portion of the splanchiiocoele accommodates the cardiac tube. At its anterior (v.A) and posterior ends (at) this is ensheathed in the thick mesoderm on the ventral side of the tore- gut, while its middle portion ( V) hangs free in the cavity. Finally the amniotie ibid of the somatopleure is seen to extend almost completely over the body of the embryo, the. aimiioiic ed^e (a.e) bounding a comparatively small opening near the tail end. Having studied in some detail the features characteristic of an individual third-day embryo it will !»• eonvenirnt. now to -i\c ,i x FOWL— TRIED DAY 545 general sketch of the chief advances in development which take place during this day. At the commencement of the day the body of the embryo lay flat along the surface of the yolk : only at its head end was it clearly demarcated from the surrounding blastoderm and this head region owing to the commencing ventral curvature was beginning to lean over on to its left side. During the course of the third «l;tv the tucking in of the blastoderm under the definitive h«.dv proceeds apace so that the body becomes more and more completely demarcated from the part of the blastoderm forming the yolk-sac wall, and the yolk-stalk becomes correspondingly narrowed. The preponderance of growth activity on the dorsal side which leads to the ventral .curvature is during the early hours of the day especially marked in the region of the mesencephalon but as the day goes on becomes very pronounced about the level of the heart and still later in the tail region. Thus the axis of the body develops strong ventral curvature especially marked at three different levels — inesencephalic, cardiac and caudal. Along with this increasing curvature the whole body of the embryo comes to lie over on its left side so that the .observer looking down upon the egg from above sees the body of the embryo in profile from its right side. During the day the embryo becomes ensheathed in the arnnion in the manner already described. The vitelline network of blood- vessels attains to its highest development, forming as it does the organ for respiration as well as for absorption of the food and its transport into the body of the embryo. Correlated with the lying of the embryonic body over on its left side the paired venous channels which convey the blood from the vitelline network into the heart gradually lose their symmetry, those of the right side dwindling in size while their fellows show a corresponding increase. In the brain the main regions become established : the roof of the thalamencephalon and medulla oblongata assume their thin membranous character while the hemispheres bulge out in front of the thalamencephalon. The central canal of the spinal cord becomes reduced to a vertical slit by the thickening of the side walls. The olfactory rudiment makes its appearance : the auditory rudiment becomes converted into the closed pear-shaped otocyst, still however connected with the ectoderm by a solid strand of cells. In the eye the lens thickening has become involuted and converted into a closed vesicle with its inner wall markedly thickened. The optic cup has been completely formed and the retinal layer differentiated from the thin and degenerate pigment layer. In the latter the first deposition of pigment takes place during the later hours of the day. The definitive alimentary canal is still open towards the yolk-sac over about half its extent but in addition to the foregut there becomes folded off during the course Of the third day a considerable extent of hind-gut, the ventral wall of which commences to bulge out to form the rudiment of the allantois towards the close of the VOL. II 2 N 546 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. day. The hind-gut is still closed posteriorly but the tbregut late in the third or during the fourth day becomes thrown into communi- cation with the stomodaeum by rupture of the velar membrane. The pituitary rudiment makes its appearance. The four gill-pouches are formed and reach the ectoderm, the fourth in the closing hours of the day, and the first or it may be the first two become perforate. The thyroid rudiment makes its appearance and during the latter half of the day becomes closed. The pulmonary rudiment develops and becomes constricted off from the pharynx except at its front end. About the beginning of the day the two liver rudiments appear and during its course the process of anastomosis begins between the branches which sprout out from them. During the latter half of the day the pancreatic rudiments make their appearance — first the dorsal, then the left ventral, then the right ventral. During the course of the day the mesoderm segments increase from about 20 to 25 up to about 40. Early in the day the Wolffian duct becomes tubular and in the latter half of the day it completes its backward growth and reaches the cloaca. The germinal epithelium becomes recognizable. The skeleton remains throughout the day purely notochordal. The heart retains its S-shape and during the latter half of the day the atrial septum begins to develop. The two dorsal aortae begin about the commencement of the third day to undergo their fusion to form the definitive unpaired aorta. In addition to the first one or two aortic arches which are already present the third makes its appearance (Fig. 241, A, III, p. 550), then the fourth, and during the latter half of the day the sixth, while the first becomes obliterated. As regards the venous system the most important feature is the assumption of the same general plan of the main trunks as is characteristic of Fishes. Finally it should be noted that during this day the body of the embryo becomes enclosed within the amnion. It will be realized even from the bare summary that has been given that the third day of incubation of the Fowl's egg is morpho- logically the most important of all and the student will be well advi-'-d to devote a liood deal of time to making a detailed study of embryos of this period. THE FOURTH DAY OF INCUBATION. — By the end of the fourth day of inclination the blastoderm has spread about half-way round tin yolk. The vessels 6f the vascular area are conspicuous, though it is to l>e noticed that th<- terminal sinus is becoming relatively less so than it was during the third day. The folding off <>f the body of the embryo ha- progressed -jTeatly. I'.v the extension haek\\ards of the head fold the r.-^ion of the hc;irl has 1,,-cmne Moored in on its \entral side. Posteriorly the tail fold is deepening in a similar fashion. »ei] hi-;id fold and tail fold tin* soinat o|ileure of the einhryonie body is prolonged vent ralwards into a \<-ry short and \\ide tuhe— the .-••malic .-talk t he \sall of which is retire! ed dorsalu ards as the t rue x I ( )WL— THIKD AND FOUKTH DAYS 547 aninion. The Litter is now complete and closely invests the body of the embryo. Lying loosely within the somatic stalk and of much smaller diameter is the splanchnic or yolk stalk — the continuation of the splanchnopleure in a ventral direction as it passes out into the wall of the yolk-sac. The body of the embryo has undergone a great increase in size. The growth of its tissues has been particularly active in its dorsal region and this has led to a continuation of the flexure towards the ventral side which was already well marked in the third dii y embryo. An important new feature in the fourth day embryo is provided by the two pairs of limb rudiments each in the form of a dorsiventmlly Hatteued ridge with rounded edge and broad base of attachment to the body. The head of the embryo at once attracts attention by its relatively enormous size. This is due to the relatively immense size of the brain and eyes. We have here to do apparently with a case of the precocious growth in size of organs which in the fully developed condition possess extreme complexity of minute structure. The main regions of the brain can be seen very distinctly: the relatively large mesencephalon with its bulging dome-like roof, the thalamencephalon with the pineal rudiment, the rapidly growing rudiments of the hemispheres, and the hind-brain with its relatively thin and membranous roof. The three main special sense organs are all conspicuous — the olfactory organ, the eye with its choroid fissure and lens, the pyriform otocyst. Arranged in a row ventral to the otocysts are the pharyngeal clefts — three or four in number. In the case of cleft I the ventral part of the cleft is becoming much narrowed by the approach of its anterior and posterior walls. The dorsal end of the cleft on the other hand remains dilated : it corresponds to the spiracle of fish-like forms. The heart, which forms a large structure lying between the tip of the head and the region of the fore limbs, is still in the form of a coiled tube but the appearance of localized bulgings of its wall fore- shadows its division into the various chambers characteristic of the adult. Thus the curve of the tube lying posteriorly and on the right is becoming dilated to form the ventricle : the part morphologically in front of this leading towards the ventral aorta is slightly dilated to form the conus arteriosus, while the curve lying anteriorly and on the left side shows a slight bulging on each side foreshadowing the two ,i u rides. Slight constrictions separate these various bulgings — an atrio- ventricular constriction narrowing the cavity to form' the auricular canal, and a less conspicuous one between ventricle and conus. The general arrangement of the peripheral vessels is intermediate between that of tin- third day (Fig. 241, A) and that of the fifth day (Fig. 241, B) and need not be described in detail. Aortic arches I and II undergo in turn a gradual process of obliteration while arches IV and VI make their appearance farther back if they have not already done so. It is also during this day that arch V makes its brief appearance. 548 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. all a.v. The allantoic veins, which at first are merely veins of the body-wall, during the fourth day establish their connexion with the allantois, and in the course of the day the right vein disappears. The allantois itself forms a conspicuous new feature for towards the end of the day it begins to project distinctly from the ventral side of the embryo about the level of the hind limb. Owing to the increasing size and complexity of the embryo the elementary student will not as a rule prepare complete series of sections later than the third day. He will however find it profitable to have transverse sections through the developing sense organs, sagittal sections through the head, and transverse sections through the posterior trunk regitm. From the study of sections the following advances in development during the fourth day may be made out. In the brain the rudiment of the paraphysis makes its appearance and the pineal outgrowth begins to sprout Out into diverticula about the end of the day. The olfactory rudiment becomes connected with the buccal cavity by a slight groove. The rudiments of lagena and recess make their appear- anC6 as slight blllgillgS of the ^^^ wall The cavity of the lens becomes obliterated by the growth of its inner wall : pigment becomes conspicuous in tin- outer wall of the optic cup : the layer of nerve fibres in the retina becomes nizable: mesenchyme begins to invade the cavity of the optic cup and about the end of the day also intrudes between the lens and t ll<- ectoderm. Tin- |«>-t anal LI' ill beeumes reduced to ii solid strand of cells and finally di-intc'jrates. The yolk-stalk becomes narrowed to a fine tubular channel. Tin- -jail-bladder begins to dilate towards the dose of the day: the dm-sal pancreas be-in- h» de\elop oiit^mu I hs : and the rudiment- <>! tin- i-aeea mako their appearance. The nie-ndd in e-iiiciiK increase in i in in her i . . about f>0. Karly in tli' not done so already, the Wnlllian duel opens the el.ue.i The Iliesiillrplirie 'jloliiel Illi be-ill to appear aild I he become eloiiL'al.cd and cnilcd. In the posterior re-ion of the Fi<;. 239. — Fowl's egg opened at the end of the fifth day. The embryo enclosed in its amnion is sunk down in the centre of the vascular area, the allan- tois projecting upwards towards the serous membrane — a transparent mem- brane through which the embryo and allantois are seen. The increasing fluidity of the yolk is shown by the outward bulging of the yolk-sac wall over the broken edge of the shell at the lower side of the figure. The albumen nowlieft completely underneath the yolk -o U to In- invisible ill a view from above. vascular ar«-a ; nil. allantois ; yolk ovt-.i lin.ki-n i-« !;_••• ut ^ln-11. FOWL— FOURTH AM) FIFTH DAYS 549 mesonejthms secondary tubules make their appearance while in the anterior region a process of degeneration becomes apparent. Imriiiir the second halt' of the day the ureter lupins in sprmit out I'mm the WolIHan duct and about the end of the day the rudiments <>t Mul- lerian ducts and of the inetanephric units may become recognizable. In the heart the atrial septum becomes completed about the end of the fourth day and the endothelial cushions lie-in t<> develop. RA. som. FIG. 240. — Chick extracted from the egg at about the middle of the fifth day of incubation. all, allantois ; C.H, cerebral hemisphere ; K, eye ; Hy, operrulum ; .V, mandibular arch ; pin, pineal rudiment faintly visible us slight elevation on roof of thalamencephalon ; Bh, thin roof of rhombencephaloii ; .<»m, edge of somatopleure cut through where it becomes reflerti-il hack ovtr tin' body of the embryo to form the amnion ; t.», roof of mewnoephalon (oj)tic lobe); I". . visceral clefts III and IV : //.-. yolk-sac. FIFTH DAY. — The progress in development during the course of the fifth day is illustrated by Figs. 239-241. The albumen has so shrunk in volume as to be no longer visible in a \ ie\\ of the opened egg from above : the yolk has become extremely fluid : the vascular area has increased considerably in size. The allantois is now a con- spicuous object and the mesoderm covering its surface is beginning to develop blood-vessels. The head of the embryo is, as before, of relatively very large size: the flexure in the region of the mesen- 550 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. cephalon is still more pronounced. The operculuin (Fig. 240, Hy) is conspicuous, growing back from the hyoid arch over the posterior visceral clefts. The limb rudiments now project freely though their form is that of simple nippers without any of the peculiarities -of the leg or wing of the Bird. The body of the embryo is floored in on its ventral side completely but for the rounded opening (som) along whose lips the somatopleure is continued into the amnion and through which emerge the narrowing yolk-stalk and the stalk of the allantois. j}The study of the living embryo in situ shows the general plan of the blood system to be as is shown in Fig. 241, B. The heart still CLC.U a.v: FICJ. 241. — Diagram showing the main parts of the vascular system u embryo during the third day (A) and the fifth day (B). a.a. in ;i Fowl a.a, allantoic artery; a.r.r, anterior cardinal vein; at, atrium; u.v, allantoic vein: Tior vena cava ; i:A, ventral aorta ; v.«, vitelline ai-tery ; r.«, ventral carotid; v.v, vitelline \ein ; I-VI, aortic arches. betrays its tubular origin though the chambers are clearly recogniz- able as dilatations. Three aortic arches (III, IV and VI) are distinctly visible and occasionally the fleeting vestige of the penultimate arch as in the specimen represented in the diagram. In front of the aortic arches the ventral aorta is seen extending forwards as the ventral carotid (v.c): the pulmonary artery (p.a) passes back from the sixth ;ireh. Dorsally the aortic root extends forwards into the head as the dorsal carotid artery (d.c). A little distance behind I In- li\vr I lie \ iti-I- line artery (v.a) loaves the dorsal aorta and far! her l>aek I he ;i 1 Ian t« 'it- artery (a.a) a branch of \\hieh, the iliae artery, ]»asses t<» (he hind linil-. In the venous system the duct of Cuvier is seen, eonl iimous at its dorsal end with the anterior and posterior cardinal \eins. Tin- former (a.c.v) branchei thi-on^li the head: the latter (/).c.?') can he x lo\VL FIFTH AND SIXTH DAYS 551 (raced dimly back into the re-inn of the kidney. The main blood- stream to the heart comes from the vitelline vein (v.vj and is jorhed within the substance of the liver by the blood from the It-it allantoi<- vein («-.v) and the posterior vena cava (p.v.c). Ignoring the vitelline and allantoic vessels \\hich an- clearly adaptations to the peculiar conditions of the developing embryo tin- main plan of the blood system is seen to be clearly the same as is characteristic of Fishes! By cutting off the head after fixing and viewing it from U-l.m (Fig. 245, A) the modelling of the face can be studied. The fronto- nasal process (f.ri) is bounded on each side by the shallow oro-nasal groove connecting it with the buccal cavity. The ridge forming the outer boundary of the olfactory organ is demarcated from the maxillary process by a faint transverse groove passing outwards towards the eye — the lachrymal groove. Posteriorly the stomodaeal opening is bounded by the mandibular ridge with a distinct break in the middle line between the two mandibular arches. Of other developmental features of the fifth day we may note the following. The first indications of turbinals appear on the mesial wall of the olfactory organ, and of semicircular canals in the otocyst. .The optic stalk becomes solid : the rudiments of the ocular muscles become recognizable. The pituitary body begins to form outgrowths. The rudiments of thymus and bursa fabricii make their appearance : the bronchi begin to develop branches. The formation of new mesonephric tubule rudiments comes to an end and the mesonephros begins to show signs of functional activity. The atrial septum develops secondary perforations. The fourth aortic arch on the left side, and the portions of aortic root immediately behind the third arch undergo reduction. The horizontal septum of the ventral aorta begins to extend back into the conus and the anterior portions of the posterior cardinal veins begin to undergo atrophy. SIXTH DAY.— During the sixth day of incubation the body of the embryo increases rapidly in size and in correlation with this it dips down into the very fluid yolk, pushing the splanchnopleure of the yolk-sac wall in front of it, so that it is almost hidden from view when the egg is first opened. The anmion is now raised up from the body of the embryo by a marked accumulation of amniotic fluid (Fig. 242). The allantois has increased greatly in size arid in the natural condition is flattened mushroomwise against the inner surface of the serous membrane. In the embryo excised as directed on p. 513 it will be seen that the somatopleure of the embryonic body is completely closed in ventrally except for a small circular space round which it is reflected outwards in a funnel-like fashion and continued into the thin membranous amnion. Through the funnel-like opening a slender probe can be passed from the extra-embryonic coelomic space beneath the serous membrane into the portion of coelome enclosed within the body of the embryo which will become the definitive splanchnopleure or body -cavity. Through the opening 552 EMBRYOLOGY OF THE LOWER VERTEBRATES < n. am. there pass out the stalks of the yolk-sac and the allantois (Fig. 246, B) each conspicuous owing to its large blood-vessels. The peripheral distribution of the vitelline and allantoic vessels shows a characteristic difference (Fig. 242) — the vitelline network (vascular area) terminating, in the now greatly reduced terminal sinus at a considerable distance from the distal pole of the yolk-sac while on the other hand the allan- toic network is most richly developed on the distal side of the allantois (p. 474). The body of the em- bryo now for the first time begins to show indications of bird -like form, and faint traces of digits and of feather-rudiments may become apparent about the end of the day. In the eye the rudi- ment of the pecten, which first became recognizable during the fourth day, is now conspicuous as an ingrowth of mesenchyme through the choroidal fissure, bounded on each face by the inflected lips of the fissure. The tongue begins to project and the thyroid becomes constricted off from the pharynx. The oesophagus towards the end of the day loses its cavity; the dilatation of the gizzard becomes evi- dent ; the intestine begins to grow actively in length (Fig. 246, B). The three pancreatic rudiments become continuous with one another. The muscles of the body begin to exhibit contractility, tin- trunk occasionally showing twitches of vent ral ilexure. Tin- ureter develops outgrowths to form tin- primary collecting tubes of the melane] lin>s about, tin- I't^innmi! of tin- ,-i\th or the end of the fifth day and the leimiual part of tin- dud of the opisthoiiephros may heeomr incorpor- ated in the cloaca so as to ^ive the ureter its [ndependeirl oju-niu^. About this lime tin; first indications of sexual different iation heeoine recogni/ahl»-. tin- ;jvniial strands lu-^i lining to show siuns of degenera- tion in th«- female a.u alb. Fi<;. 242. — Common Fowl. View of contents of the egg-shell extracted at the end of the sixth day of incubation. The serous membrane has been removed as to allow the allantois to be displaced slightly in order to give a clearer view of the body of the embryo contained within its amnion. a.r, edge of vascular area; ulb, remains of albumrii ; '///', outer wall of all.-mtois ; nil", inner wall of allantois; am, arnnion ; *, portion of vascular area lying, in the natural position, benwath the head of the rmbryo and free from blood- vessels. FOWL— SIXTH TO Kid I ITU DAYS The main purl inns <•!' the skeleton !)<•<•«, m,- laid dn\\n in chondral tissue and, towards tin; end of the day, in cartilage. Tin; In-art brains to assume its definitive external form; the ventricular septum develops and the conus septum begins to do so. The fourth aortic arch becomes obliterated on the left side. SEVENTH DAY (Figs. 243 and 244). — The mushroom--!. allantois is spreading actively all round beneath the serous membrane. The aumion is beginning to show waves of contraction passing along its wall. The brain and eyes and consequently tin- head as a whole are of relatively enormous size. In sections the roof of the fourth ventricle is found to be developing ir- regular folds in which the vessels of the choroid plexus will appear. All three turbinal rudiments are pre- sent in the nose. The crop is beginning to expand. The visceral clefts are all closed. The glands of the stomach are beginning to make their appearance as rudiments. The cavity of the enteron disappears for some distance forwards from the point of origin of the allantois. The FIG 248i_Fowl.8 opencd dnring tlu. sewnth (1;lv. Mullerian ducts Uiay Show The body of the chick is seen dimly through th.- incipient asymmetry. The highly vascular allantois. The vessels of the nOtOChord is beginning tO allantois can he distinguislMMltn,,,, th,,>, of the • it i_ vascular area by their turning back at the edge of DC Constricted by the Verte- the allantois while those of the vascular area pass brae. The first traces of onwards uninterruptedly. The highly fluid char- ossification are making their ^ of th« ^ is shm;u !'y !he •V?lk1;8acf "t11 . „ q bulging outwards over the broken shell at th.- appearance, especially in the point marked *. skeleton of the limbs. H// .,nailtoi.s. The septum of the conus arteriosus is complete and the muscular coat extends into it from each side : the pocket-valves are becoming excavated. The fourth aortic arch on the left side has disappeared while the portion of aortic root between arches III and IV on the right side, and U-hind arch III on the left side, are becoming obliterated. EIGHTH DAY. — The movements of the amnion now reach their highest degree of activity. The fronto-nasal process (Fig. 245, C) is growing out to form the pointed beak while the lower jaw is taking a similar pointed form, the two mandibular arches being now con- tinued into one another ventrally without a break. The rudiments of feathers are beginning to make themselves apparent. In the brain the cerebellum is becoming folded on itself so as to bulge outwards. The oro-nasal grooves are covered in to form the all. 554 EMBRYOLOGY OF THE LOWKU VERTEBRATES < n. tubular communication lu-tueen nose and mouth. The lachrymal groove is no longer visihle : the lachrymal glands are developing as solid ingrowths of ectoderm. Tin- pit uitary body now forms a rounded mass of branched glandular tubes lying between the trabeculae and communicating with the buccal cavity by a narrow tubular duct opening ii u mediately over the glottis. The air-sac rudiments make their appearance on the surface of the lung (Fig. 246, C, a.s). The mesonephric tubules have been growing actively up till now : the metanephric units are making their ap- pearance: theMiiller- ian duct reaches the cloaca if it has not already done so although no actual communication is es- tablished until about six months after hatching. Ossification be- comes conspicuous in the limb -bones and the investing bones of the head. The keel of the sternum forms an ossification distinct from the two lateral rudiments of the body of the sternum. The terminal sinus of the vascular area has disappeared. The septum of the conus is now completely traversed by muscle Fio. 244.— Chick extracted from egg during seventh day S0 .^^ bot]l ''" "'' !(> •towing oprentam ( veaselfl is inaugurated by the appearance <»r a Longitudinal incision along th«i line of attachment of the septum. regards the further progress of development, the I' appm\ imate i imef may '"• mentioned. Altout tlie ninth «lay the (.esoplia^us gradually heeomes patent again. On the tenth day tin- artri-ial aivhrs have praetieally assumed the definitive ( nnd it ion and the m. -tap .dial skeleton is ossified. FOWL-LATER DEVELOPMENT Up to about tin- eleventh day the contractions <»!' tin- unininn remain very active, l»ut thereafter they gradually bcconu- umn- gentle until during the closing days of incubation they stop. Tin- mesoneplmis also attains to its maxiiuuni aetivity and there commences the process of degeneration which will continue till the time of hatching: tubules have developed throughout the length of tin- nietjinephros. By the twelfth day the duct of the pituitary body has becnnn- reduced to a solid cellular strand: the exact time at which this happens is very variable ; it may be as early as the sixth or seventh day. The lachrymal duct, which originated as a solid ingrowth <•!' ectoderm along the line of the lachrymal groove, now becomes tubular. About the twelfth or thirteenth day the cavity reappears over the greater part of the rectum except just at the hinder limit of the occluded portion immediately in front of the allantois. Here the cavity remains blocked till nearly the time of hatching. olf. ^ * A FIG. 245. — View of head of Fowl i-ml.ryo as seen from below. (After Duval, 1889.) A, live days ; 15, six days ; C, eight days, f.n, fronto-nasal process ; injr, maxillary process ; olf, olfac- tory opening ; o.n, oro-na&il groove ; *;>, hyoinandihular cleft ; F, ventricle ; I, II, visceral arches. About the thirteenth day the cartilaginous skeleton is complete ami the rudiments of claws begin to develop. About the fifteenth day the Eustachian valve develops in the heart. By the sixteenth day the albumen has all gone and the yolk-sac wall becomes completed ventrally. About the nineteenth day the yolk-sac becomes enclosed within the body-wall and the partition between mesenteron and proctodaeuni breaks down so that the alimentary canal communicates with the exterior. About the twentieth day the umbilicus closes. The violent struggles of the young bird cause its beak to penetrate the air-space : its lungs are filled with air : its further struggles cause its beak to break the shell and it emerges, leaving behind the broken shell lined with the cast-off allantois and serous membrane. Correlated with the process of hatching important changes take place in the circulation : the gap in the atrial septum (foramen EMBEYOLOGY OF THE LOWER VERTEBRATES CH. ovale) becomes closed so that the blood arriving in the right auricle can only reach the left auricle by the circuitous route through the ra. as Fl<;. 246. — I)i— .-.•ti-.jis from tin- ri-lit .sid<- showing the ^i-m-ial aiTan^fiiinit ul' tlic \i a Fowl <-n.l.i\u at th«- i-ii.l of tin- liith (A . sixth (Hi, and eighth (' ,la\s ..r in.-ul.-iti.»n •!'ilinii;il ;ni -s.'ic ; nil, alliintuis ; ,-.n, >• ,, , . rio-riiiii ; „/_ -i//;ip| ; //, ljv»>r; ii !<•!.•; ,-./. riK'lit lull.-; /' \cntrn-lr; :lik ; f.f, yoU right ventri«-lr ;m«l ]Milni(.n;iry circiilal ion, and llic allant i»ic, vein, duct ul' I'.'il.'illiis, and diic.lu.s \cii(»sus in MM- liv«T I'rroinr nl i|j I era 1 rd. EMBRYOLOGY OF COMMON FOWL 557 LITERATURE Duval. At las d'Kmlirvolo^i.-. Paris, 1889. Foster and Balfour. Th«- Elements of Embryology. Second Edition, edited by A. Scd^'wirk and \V. Hf.-ijif. London, 1883. Eeibel und Abraham. Keibels Normentafeln /ur Entwioklnngagesohlohte d. i Wirbflticiv. II. Jena, 1900. Lillie. Tin- Development of the Chiok. New York, 1908. Marshall. Vertebrate Embryology. London, 1893. Patterson. Hiol. Bulletin, xiii, 1907. Patterson. .lourn. Morplu, xxi, 1910. 'I'lit- ni«i>t i ninplete account of the development of tlie Fowl is tliat by Lillie. It, and Diival'.- Atlas if a «-njiy can be obtained, for it is unfortunately out of print, should form part of the i'«iuipini-iit of every embryological laboratory. CHAPTER XI HINTS REGARDING THE PRACTICAL STUDY OF THE EMBRYOLOGY OF THE VARIOUS TYPES OF LOWER VERTEBRATES A.MPHIOXUS. — The interest and importance of Amphioxus to the student of Vertebrate morphology are due to the fact of its position near the base of the Vertebrate phylum. It is true that in its adult structure Amphioxus is intensely specialized in correlation with its burrowing habit. Further, it is necessary to recognize that a burrowing like a pelagic mode of life, in which the environmental conditions are comparatively uniform, is likely to lead to a kind of fixing of the organization which will be fatal to its adaptability to new sets of conditions and consequently to its capacity for evolving along new lines. We must therefore regard it as improbable that the Vertebrata passed through an ancestral condition of specialization for a burrowing habit and the specialized features of the later stages of the life history of Amphioxus cease on that account to have a phylogenetic interest. The main interest to the Vertebrate morphologist lies therefore in the earlier stages before the specializa- tion of the adult has developed — in such features as segmentation, gastrulation and the origin of the main systems of organs. And the interest of these stages is heightened by the fact that food yolk-- that potent disturbing factor — is present to a far smaller extent in the egg of Amphioxus than in that of any other of the lower Vertebrates. Unfortunately the known localities in which fresh emhrynlo^ieal material of Amphioxus can be obtained in abundance are still lew, and in most laboratories recourse must he had in preserved material purchased from supply stations such as the Naples aquarium. Tim best locality so 1'ar known i'..r obtaining developmental stages of Amphioxun is the pantano or shallow lagoon at l-'aro near Messina. Here tin- spawning takes place ,-ach evening, when condition favourahle, during the summer months from April to .lulv. The eggs pass to tin- exterior through tin- atriopmv. It' in a dish on . a hual, the Bggfl aW liahle l.y its movements to hecome .hutrd i hrou^h the water and t h«-\ axe i hen apt, t<> become dra\\ n hy the inspiratory ciim-nt in amount, tin- hueeal cirri. When the 65fl (ii xi PRACTICAL HINTS 559 becomes inconvenienced by such entangled eggs amongst the cirri it is able suddenly to reverse the respiratory current so as to clear them away, and in this way there is produced a misleading appearance as if the eggs were being laid through the mouth. The first meiotie division lias been completed before oviposition while the second is in tin- spindle sla^e at this period. Fertilization probably takes plan- immediat ,el v, spermatozoa being disseminated through the water. It is best (Cerfontaine, 1906-7) to bring the adults into the la!. oratory and wait until they spawn which operation may be considerably delayed. To a dish of pure sea-water is added a little sea-water containing sperm then the eggs, collected with a pipette as soon as extruded, are added. Batches of eggs are fixed periodically, preferably in strong Flemming's solution or Hermann's solution. After dehydration they are placed in a mixture of 2 parts clove oil and 1 part collodion in which they may be kept indefinitely. For examination whole the egg or embryo is placed on a slide or coverslip in a drop of the clove-oil-collodion. After the specimen has been arranged in the desired position by means of needles a drop of chloroform is applied in order to cause the collodion to solidify. The whole is then cleared with cedar oil and mounted in Canada balsam. For the preparation of sections the procedure is similar, only in this case the slide or coverslip should be coated with paraffin as a preliminary to allow the collodion block to become detached, and the latter should be embedded in paraffin. PKTROMYZON. — The various species of Lamprey make their way up streams to suitable gravelly spots for spawning in the spring or early summer (April, May, in the northern hemisphere). Material for emhryological study is best got by "stripping" the ripe males and females i.e. by passing the hand back along the body with gentle pivssure so as to force out the eggs or sperm. The gametes from the male and female are collected separately in two small dishes : they are then mixed together, stirred gently with a feather, and water added. This " dry " method gives a smaller proportion of unfertilized than when the eggs are received from the fish directly into i (Herfort, 1901). As fixing agent the ordinary corrosive sublimate and acetic acid is quite satisfactory. MYXINOIDS. — The only Myxinoid eggs that have been obtained in any numbers are those of Bdellostoma, which are dredged near Monterey, California, on shelly and gravelly bottom at a mean depth of about 12 fathoms (Bashford Dean, 1899). Much still remains to be done in working out the details of their development but it is el. MI- that this is of a. tighly peculiar and specialized type. ELASMOBRANCHII. — The eggs are fertilized in the upper part of the oviduct. Thev may traverse the oviduct comparatively rapidly and be laid as in Birds at an early stage of development [Ckimacra, Scylliidae, Cestracion, Rain] or thev may remain in the oviduct for a prolonged 560 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. i; B FlO. 247.— Bl:i-t • period and the young born in an advanced stage [Notidanus, Mus- telus, Galeus, Carcharias, Zygaena, Lamna, Alopias, Cetorhinus, Acan- thias, Scymnus, Squatina, Torpedo, Trygonidae, Myliobatidae]. Amongst the viviparous Elasmobranchs pre- served developmental stages of Torpedo (Fig. 247) may be obtained from Naples, and of Acanthias from various marine laboratories. Amongst the oviparous forms certain species of Skate (Eaia) are used as food-fishes and their eggs can frequently be obtained in quantity at trawling centres. In such cases arrangements can be made with local fish-dealers to send on by post the " skate-purses " taken from the oviducts when the fish are cut up.1 The eggs of the different species differ in size and in the characters of the shell — shape, colour, degree of translu- cency (Williamson, 1913). Of the European species R. batis is the most convenient species to use ; the normal period of spawning is from December to April but the retarding effect of the low tempera- ture is so great that December eggs are practically overtaken in their de- velopment by the April eggs. The complete period of development is roughly 20 months, most of the eggs hatching about August. The eggs should be posted in damp seaweed. On arrival the soft sticky marginal zone of tin- shell, which separates off except at one end and serves to anelmr 1,1 u> egg to the sea-bottom, is removed, and I lie tlale is marked in ink \\itli a \\oinleii style upon the llat pnrtinn i.f shell hel \veen 1 he t \\«» horns. OB ol.si-rvv.l out of iiuiny tlmusiii'l.s «>f «gg8 only mi.- r.i.-r «,|' thr <.f i wo eggs within ;i - OIDIIIUH shell. xi PRACTICAL HINTS— ELASMOBRANCHII 561 For hatching boxes ii is convenient to take ordinary fish I >o.\rs freely perforated with auger holes, provided with a cross partition in the centre, and pitched inside, and out to discourage i In- growth of seaweeds. The hatching boxes are moored afloat in pure sea-water within a breakwater or other shelter. About 20 eggs are placed in each compartment. < >M alternate days the boxes are drawn a few times backwards and forwards through the water to dislodge any sediment that may have accumulated. Once a week they are hauled out of the water and eacli e^-shell tested by rubbing the finger over its surface. If a slippery mucus-like layer has developed on its surface the egg is useless and should be got rid of. When the egg has reached the desired period of development it is removed from the water, placed in a horizontal position with the more strongly convex side below and opened by carefully removing the greater part of the less convex side of the shell. The isolated piece of shell must be lifted oft' very carefully as the albumen is very adhesive and the vitelline membrane extremely delicate. In the early stages the embryo is almost invisible in the fresh state so the egg, still held carefully in a horizontal position, is gently submerged in fixing fluid. The blastoderm then comes into view and after a short time may be excised and floated into a watch-glass to complete fixation and the subsequent processes. In later stages (Fig. 248) where the body of the embryo is constricted off from the yolk-sac, it is narcotized by 'submersion in sea- water containing 3% alcohol and then the yolk-stalk is ligatured with thread and the embryo excised for further treatment. Embryological material of the Sharks is to be preferred to that of the Skates or Kays on account of their, less specialized character but mi fortunately it is more difficult to obtain in quantity. Small sharks of the genus Scyllium and allied genera occur commonly round the shores of the various continents and their eggs may be found attached to seaweed at extreme low tides. On the British coasts a well-known spawning ground for Scyllium exists at Careg Dion about 2J miles from Beaumaris on the Anglesea side of the Menai Straits in between 3 and 4 fathoms of water and in spots not exposed to strong tidal currents.1 The eggs are deposited usually in the morning, the shorter stouter pair of filaments which issue first from the cloacal opening being trailed about amongst tufts of the seaweed Halidrys siliquosa until they become entangled when the fish swims round so as to wind the elastic filaments firmly amongst the seaweed. The eggs can only be obtained at very low and specially favourable spring tides and as White finds at one time embryos of all stages of development it would appear that oviposition is not limited to any definite season. Scyllium not infrequently deposits its eggs in aquaria and at the 1 For thr details in regard to this locality I have to thank Professor -Philip J. White of VOL. II 2 0 562 EMBRYOLOGY OF THE LOWER VERTEBRATES < 11. Berlin Aquarium it has been observed that pairs of eggs were deposited at intervals of about ten days. The methods of technique mentioned in connexion with the Skate are also applicable to the eggs of Scyllium. It should not be forgotten that, as mentioned earlier in this ot ucV Ki';. 248. — Raia batis, embryos. nt, at rial portion of In-art ; E, eye ; c, conus ; f.g, foregut ; //, heart ; /, lens ; /i, liver ; ot, ot<-oyst ; l-lu. piiH-al or^an ; il>, thin roof of fourth venti iolr ; P.O. I, i-tc., visi-eral clefts ; I/.N. yolk-stalk ; \ , Nil, VIII. cianial !, volume, one of the greatest desiderata in Vertebrate rinl.ryolo-y. is ;in oviparous shark with rggs of small si/c. TKLEOSTOMI. — The most archaic ;m<• ol'taim-il from tii.- Wood*. Ilolr l.al-oiaini v "i from Mr. J. C. Stcphenson, \\'a-luii--ton l!ni\ /;.,. !..,„„ n, Bolwtj i • Domfl Sootlmnd, XI PEACTICAL HINTS— FISHES 565 n umbers in the tow-net I nit these are not so con- venient for investigation on account of their re- duced size. As there is little doubt that the Tele- ostei ha\v been evolved out of ancestral forms with large eggs investiga- tions are particularly de- sirable on those teleosts, mostly freshwater forms inhabit inu- warm climates, in which the large size of I he i-u- has been retained. Then- is an important tield for investigation in the embryology of tropi- cal freshwater fishes. Of individual families the Siluridae, Characinidae and Gymnotidae - call especially for investi- gation. DIPNOI.— The Lung- fishes form a group of much importance to the Vertebrate morphologist on account of, on the one hand, their great an- tiquity and the retention of many archaic features in their organization and, on the other hand, of the fact that they present to us foreahadowinga of vari- ous features which become prominent characteristics in the tetrapoda or terrestrial animals. A knowledge of their ern- bryology consequently ber.ame one of the great desiderata of Vertebrate Embryology. The first discovered of the three surviving representatives of the group — Lepido- at Pf Kic. 'J'tO. — Blastoderms and embryos of Trout (Salmo fario). (After Kopsch, 1898.) iye; ot, otocyst ; p.f, pectoral tin ; rh, rlioinbenc. j hai-n \pusivl surface of yolk. 566 EMBRYOLOGY OF THE LOWER VERTEBRATES m. siren — remained unknown so far as its development was concerned until 1896 when Graham Kerr succeeded in obtaining abundant embryological material in the Gran Chaco of South America. The developmental stages of Protopterus, the next representative of the group to become known to science, were first obtained on the (lainbia River by Budgett who had taken part in the Lepidosiren expedition a few years earlier. Ceratodus, the last of the surviving genera to become known in the adult condition, was the first to be made known embryologically by Caldwell and Semon as already mentioned (p. 435). The Lung-fishes like other animals living under similar conditions breed at the commencement of the rainy season (Protopterus, Gambia, August ; Lepidosiren, Chaco, November but incidence of rainy season irregular and may be delayed — till e.g. June — or omitted altogether ; Ceratodus, September to December). In the case of Ceratodus the eggs are scattered loosely about amongst the water plants, while in Protopterus and Lepidosiren they are deposited in a special burrow at the bottom of the swamp where they are guarded by the male parent. Dipnoans live well in captivity and there is little doubt that it will be found easy to induce them to breed by using similar methods to those described under the heading Amphibia. It is particularly desirable that this should be done in the case of Lepidosiren on account of the large size of its histological elements which make it a peculiarly suitable type for the investigation of various problems of histogenesis. The eggs of Dipnoi, especially of Lepidosiren, are of large size and this makes it especially advisable to use celloidin in addition to paraffin methods of embedding. When paraffin is used it is necessary to remove the egg envelope by slitting it up with fine scissors, care being taken to keep the point of the scissors close to the envelope so as to avoid injury to the surface of the egg. Corrosive sublimate and acetic acid is a good stock fixing agent. For stages before hatching 10^/ formalin is convenient. AMPHIBIA. — The most easily obtained embryological material is that of the common Frogs of the genus Eana the masses of spawn of which are familiar objects in pools during the early weeks of spring in temperate climates. The exact time differs with climate and also with species, some species such as E. esculenta in Europe and .//. catesbiana in North America lagging several weeks behind the others, pawn, fertilized as deposited in the early morning, may con- veniently he kept during its development in earthenware pans. The water should In; It-It stagnant and unchanged during the period prior to hatching as under these circumstances tin- spawn is less liable to be attay the varying size of the yolk-plug. in these it may be taken as a general rule that breeding takes place at the commence- ment of the rainy season, or in other words when environmental conditions become favourable after a prolonged period during which they have been unfavourable. By bear- fjc ing this principle in mind such tropical amphibians may usually be induced to breed in captivity. Bles in his excellent f account of the life -history of Xenopus (1905) describes a method which will be found to be of general use. The pair of ^ animals were kept in a Budgett tropical aquarium consisting of a glass bell-jar 20 inches in diameter dipping into a galvanized iron water-tank heated by a I small Bunsen burner and oxygenated by plants of Vallisneria. During summer the temperature of the water in the bell- jar was kept at about 25° C. The water was not changed. The frogs were fed daily with small earthworms or thin strips of raw calf's liver until they would ft/ eat no more: In December the tempera- ture was allowed to fall to 15°-16° during i h, -j.vj. Embryo of PI, nil, medusa the day and as low as 5°-8° during the oTitmf' attened out innight> As the temperature rose with the onset of spring the frogs became more b.c, buccal cavity ; W, blastopore ; ,. i • & , 6n .1 i ^i me,, mesoderm Ngmente! active, waking up out of the lethargic condition induced by the winter's cold. Breeding was induced by simulating the natural conditions of the rainy season. The temperature was raised to about 22° C. Each morning and evening about two gallons of the water was drawn off, allowed to cool for twelve hours and then returned to the aquarium in the form of a fountain of spray from the upturned 568 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. end of a glass siphon drawn out to a fine point so as to produce the effect of a shower of rain. Within a week or two breeding took place. The chief difficulty in the way of cutting sections of Frog's e is due to the presence of the jelly-like envelope. This may be got rid of by prolonged soaking, six months or more, in -5% formalin (Ogushi, 1908), or by fixing in Zenker's fluid and leaving the e- this fluid renewing it after 2 to 3 days and continuing the treatment <*. e.g.L of. E op. B. D. KM;. 253. — Stages in the development of Phyllomedusa /•.'. <•>•(• : i-.//, i-xti-rnal gill ; o]>, t>i>erculuin ; of, otocyst. for 8 to 14 days or longer, shaking gently so as to remove the envelopes (Kallius, 1908). For cutting sections paraffin is commonly used but it should U- supplemented by celloidin e.g. the clove-oil im-tlmd mentioned under Amphioxus. In the Urodeles the eggs are commonly laid singly in water and attached to water plants (Triton) or other solid objects such as logs or stones (Proteus, Necturux). In Cryptobranckus and A-tnph'minn they iniiii a MILC, adjacmt envelopes being connected together 1)\ a narrow isthmus. I • 1 1 ili/ai inn is rarely external (Cryptolranchus — Smitli, M)12). In the Newtfi th«- i. male (ak(?s up a spermatophore into the cloaca. XI i'KACTICAL HINTS— AMPHIBIA 569 Such internal fertilization leads up to the condition in the Salamanders where fertilization takes place in the upper part of the oviduct and the developing embryo is retained for a less or more prolonged period within the body of the parent. In Salamandra maculosa larvae about an inch in length are born in May resulting from fertilization during the preceding summer. As in the Anura wide differences exist in the richness of yolk and run sequent size of the egg — the latter varying from under 2 mm. in the Newts to 6 mm. (Necturus) or 7mm. in diameter (Cryptobranchus japonicus): so that here again though not to. the same 'extent as in Fi';. '2i> 1.— Tadpole of unknown Frog from Tropical -Africa. A. side vie\\ ; H, ventral view. //.c, l.uccal ca'vity ; c.o, cement or-an ; a, anus; E, eye; e.g, external -ill ; «//, oll'actory or^an ; •///, operculum. the Anura there is an excellent field for investiga- tion into the influence of yolk upon developmental processes. The eggs of Urodeles are commonly collected under natural conditions and kept in earthenware 'dishes. Or the adults just about to breed may be brought into the laboratory and allowed to deposit their eggs in a suitable aquarium. The Urodela form one of the relatively primi- tive groups of Vertebrates and their embryology deserves much greater attention than it has hitherto received. Most of the older literature deals with special details in the development of the Newts but comprehensive monographs, including " normal plates " on the development of such genera as Proteus, Siren and Amphiuma are much wanted. A general account of the development of the American species of Cryptobranchus has been given by Smith (1912), while the Japanese species has been dealt with by Ishikawa (1918), De Bussy (1915) and Dan. de Lange, Jr. (1916). Of Necturus normal plates with accompanying tables have been worked out by Eycleshymer and Wilson (1910). The Gymnophiona — though an aberrant group of Amphibians highly specialized for a burrowing existence — are of much embryo- logical interest and have provided the material for work of great morphological importance, such as that of Brauer upon the excretory organs. A general account of the development of Ichthyophis 570 EMBKYOLOGY OF THE LOWKK VlvIITKP.K'ATKS « n. will be found in Sarasin (1887-90) and of Hypogeophis in Brauer (1897). The eggs, fertilized internally, are normally deposited in the soil and the embryologist has, as a rule, to depend upon such scanty material as can be obtained by digging in the damp soil of localities where Gymnophiona are abundant. TypJilonectes in South America and Dermophis in West Africa are viviparous. Of the group in general it may be said that a comprehensive monograph on the development of each genus beyond Ichthyophis and Hypogeophis is a great desideratum. As standard fixing agents for Amphibia corrosive sublimate and acetic acid, and for the later larval stages strong Flemining's solution, may be used. For the early stages (segmentation and gastrulation) quite good results are obtainable from eggs that have been preserved alive in 10% formalin : in this case it is well to treat the egg before dehydration for an hour or two with corrosive sublimate solution as without this precaution the formalin -preserved eggs are difficult to stain well. When any other fixing agent than formalin is used it is necessary, as a preliminary, to remove the egg envelopes. In the case of the larger eggs of the Urodela and Gymnophiona this can be accomplished with the aid of fine scissors and forceps. REPTILIA. — For gaining practical knowledge of Reptilian development the student will find the group Chelonia most con- venient as it is possible to obtain l excellently preserved series of developmental stages of Terrapins (Chrysemys) and Snapping Turtles (Chelydra). In particular localities especially in warm climates he may have opportunities of obtaining the eggs of Lizards, Snakes or Crocodilians. In all cases the same technique may be used as in the case of the Fowl. AVES. — The Birds, although showing conspicuous differences in external appearance and in minute details of structure, form a very compact evolutionary group and there is little likelihood of important differences in principle existing in their development. Interesting difft-rences in detail however are to be found — such as the presence or absence of neurenteric canals. Groups which there is any reason to suspect of being particularly archaic — such as Divers, Grebes, uins — are worthy of careful scrutiny for possible persistence of Reptilian features. LITERATURE files. Trans. Roy. Soc. Edin.^ xli, 190B. Brauer. ZooL Jfthrbttoher (Aimt. 1897. de Bussy fde Lange,, L. P. Ki-rst.- out \vikkeling8stadini \.m nutt Si-lilcj.M-1. Am-t«T- v its main object is to gain information concerning the lines along wbioh tin- structure of existing groups of animals has evolved. In the phylum Yertrltrata there is an immense amount of work still to be done and it is important that the would-be researcher should be guided by certain general principles as to the technique of the subject, otherwise he is apt to achieve no more than the addition of relatively unimportant details to the vast accumulation of details which during the past few decades has tended to hide away general principles and incidentally to smother interest in the subject. The incompetent or inexperienced investigator frequently betrays him- self by his choice of subject : he chooses a problem of relatively minor interest when there lie ready at his hand others which are of real importance, or he chooses a subject really important but of such difficulty that the probabilities are heavily against the feasibility of its solution under existing conditions. The beginner then should see that he has the aid of some competent adviser before he decides upon his line of research. Having chosen his particular problem he has. next to decide regarding the particular animals upon which his research is to be carried out. The earlier workers were guided mainly by the accessibility of the material. Fowls and Rabbits — of which embryos were easily obtained and easily investigated — provided the material for the great pioneers of vertebrate embryology and the embryology of to-day suffers much from the difficulty of getting rid of general ideas founded on such narrow bases. Now that embryology has taken its place as a branch of evolutionary science we recogni/.e the importance of basing our general ideas upon the phenomena of development as displayed by the more primitive existing groups. In attempting any important problem of vertebrate morphology evidence must be got from Elasmobranchs, Crossopterygians, Lung-fishes, Urodeles, before \ve can feel completely confident as to general principles: in other words we must go to groups which are admittedly archaic. Apart from directly adaptive features an animal which is archaic in its adult structure may be expected to show primitive features in its development. Naturally we should not look for this in cases where development takes place under peculiar conditions, for these necessarily involve adaptive modification. A pitfall into which investigators frequently stumble is that, starting from 573 574V EMBRYOLOGY OF THE LOWER VERTEBRATES APP. some particular group — say Amphioocus, or the Mammalia — with whose structure they happen to be thoroughly familiar, they assume its general organization to be primitive. As a matter of fact it may be assumed with considerable probability that every existing vertebrate is to a certain I'xti'iit a mixture of primitive features and specialized. It is only by careful comparative study that it can be decided which features an probably primitive and it is quite certain that these will not be found all within one group. Consequently speculations based upon the intensive study of one particular group are to be distrusted, though there is always less ground for distrust if the group is one which is recognized for reasons other than embryological, as being on the whole archaic. When minute histological details are concerned another qualification which should be possessed by the animal chosen for investigation is large size of its cell units. The material should be abundant. Not only should there be a con- tinuous series of stages but there should be numerous specimens of each stage. There is no such thing as an absolutely normal individual : the conception " normal " is an abstraction based upon the observation of numerous individuals. Only by observing numerous individuals can we therefore arrive at a knowledge of normal development. Work carried out on a few specimens may of course provide isolated observations of much interest and value but it is inadequate to serve as a basis for general conclusions. In all descriptive embryology it is necessary to have some method of specifying the stage of development of individual embryos. Unfortunately there has been a great lack of uniformity as to the particular method of doing this. One of the most frequently used is that of specifying the period of time during which development has been going on as for example a "chick embryo of 40 hours' incubation." This method is quite un- satisfactory, owing to the fact that the actual stage of development of any individual embryo is a function of other factors in addition to mere time, such as temperature and individual idiosyncrasy. Thus in many tropical freshwater animals a statement of the age of the embryo is practically worthless unless accompanied by a record of the temperature, and even then there remains the unknown element of individual peculiarity such as is for example illustrated by Fig. 251 where a number of sister eggs of a Frog are seen to have " lost step " with one another to a marked extent even at a comparatively early stage of development. In other words or embryos of the same age are liable to vary greatly in their de-ree of development, and a statement of their age is not adequate as a precise indication of tin- stage of development. The want of precision varies in dillerent cases: it is less for example in a Kul herian mammal where development take- |)l;iee ;it a fa i rl y < I' .'li 1 1 i t e t einj irrat lire t han it is in a Kish or Amphibian inhabiting a tropical pool or swamp where the tern pera lure is li;ihle to -ji-.-.-it variation. It i- nere^.iry then m referring to particular stages of development t<> define them by structural features. Here however a new difficulty presents in the I'aet that ! he relative rate of development of ditlerent « Of i- not the lame in dill'erent individuals. It follows that if a number nf indi viduals he grouped together as hem- at tin- same st:. judged b\ a particular orpin A it will he found that other AIM-. MKTIloDS OK BMBBYOLOGICAL EESBAECH 575 organ* I'., C, ''if. 9X6 n«»t. exaetly ;ii i! »pment — some are less developed some mure in the various individuals. Still for pra< purposes this is a u>H'ul way of indicating roughly the stage of develop- ment, l-'or example early stages in the development of Vertebrates may be defined by giving the number of mesoderm segments which have developed — these being fairly conspicuous structures and definable by a number. A much better system, however, is to use numbered stages denned l»v tin- general external form — the first structural feature met with in the examination of an embryo. Keibel has published "normal plates" of the development of various Vertebrate types in which standard stages in development an- defined by accurate figures. Unfortunately some of the normal plates an- incomplete as regards the earlier stages during segmenta- tion and gastrulation, but wherever the plates extend over the whole period of development they should be made use of by the working embryologist as his standard stages. Where no normal plates exist the embryologist should make it his first business to construct one by carefully working over the external features of development and defining by careful drawing and description a series of stages which he judges to be roughly equidistant. The embryology of any animal is an account of the observable changes which take place in its structure from the zygote stage up to the adult. Logically the investigation of its embryology should proceed similarly from zygote to adult but in actual practice it is better to work in the opposite direction — to commence by getting a clear idea of the adult organization and then to work back from the known to the unknown of earlier stages. An embryological investigation should commence with a careful study of the entire embryos or larvae at the various stages. Each stage should be examine! 1 first alive by transmitted and reflected light, careful note being taken of any movements due to muscular contraction, ciliary action etc. Particular attention should be paid to the arrangement of the blood-vessels, the time of commencement of heart movements, of circulation of the blood and of the appearance of haemoglobin in the corpuscles. The appearance of chromatophores should be noted : the seat of their first appearance and their reactions — whether by changes of form, movement of pigment granules in their protoplasm, or by actual migration — in response to changes in direction or intensity of light. During this phase of the work constant use should be made of the binocular microscope and rough ^ketches should be made. Embryos of each stage should be submitted to the action of various tixing agents and it is important to watch the embryo during the process of fixing, for the fluid as it gradually penetrates the tissues often makes special structures stand out distinctly for a short space of time — to dis- appear again with further penetration. The fully fixed embryo should be subjected to further careful scrutiny by reflected light under the Greenough binocular. To detect small inequalities of the surface it will be found necessary to arrange the lighting carefully. The light from an in- candescent gas-mantle may he concentrated by a large condenser and caused to illuminate the embryonic surface in a tangential direction. It is often well to cover the specimen with a little house of opaque cardboard or metal resting on the stage of the microscope and possessing two apertures one in its roof through which the observation is made and one at the side through which light is admitted. The embryo must of course be 576 EMBRYOLOGY OF THE LOWER VERTEBRATES APP. completely submerged in fluid and is preferably contained in a round glass dish with a layer of pitch or black wax on the bottom in which, if necessary, small excavations can be made in which the embryo can rest securely in the desired position. The glass vessel should be rotated slowly during the observations so as to allow of the incidence of the light from different directions. It is important to observe a number, preferably a considerable number, of embryos of the same stage, as owing to individual variation particular features may be much more distinct in some than in others. A number of thoroughly typical specimens of each stage should be picked out for further investigation and these should be carefully drawn under the camera lucida, a piece of millimeter scale being placed by the side of the embryo and drawn at the same time so as to form a reliable record as to dimensions. At this stage the normal plates should be constructed if not already in existence and the embryos classified in accordance with them. For the study of internal structure the great method is that of cutting the embryo into serial sections l but a much older method, that of dissection, should by no means be ignored. Careful • dissections made under the Greenough binocular are often extraordinarily instructive. It is advisable to experiment with embryos fixed according to various methods as different methods give different degrees of consistency, opacity etc. Van Beneden and Neyt's fluid will be found in many cases to give very good results. In section - cutting a fetish to beware of is excessive thinness of sections. The expert section cutter is liable to become so interested in his feats in accomplishing the preparation of sections of an extraordinary degree of thinness that he is apt to forget that the criterion of good sections is not simply their degree of tenuity but the relation which their thickness bears to the size of the cell-elements of the particular embryo. Thus while in some cases it is of advantage to have sections so thin as 1 /*a or even *5 ft, in other cases, such as segmentation and gastrulation stages of some of the large heavily-yolked holoblastic eggs, the sections should reach as much as 80 p or 100 //, in thickness. Before an embryo is cut into sections its soft protoplasm has to be supported by infiltration with some suitable embedding mass. For this purpose the two substances used at the present time are paraffin of high melting-point and celloidin. Of these the first is used frequently alone but the student should realize from the beginning that if he is to obtain reliable results, especially where yolk is present in the embryonic tissues, he must use both methods and control and check tin- results obtained from one by those obtained from the other. The process of infiltrating the embryo with paratlin is usually carried out in a hot-water oven heated by oil, gas or electricity and kept at a temperature just ;il>ov»- tin- melting-point of the paraffin by a thermostat. The melted paratlin may be contained in small copper pans preferably plated infill'' \\ith silver or nickel. An essential preliminary is a very thorough dehydration followed by a very thorough soaking in tin- clearm-. :ej« -lit. To get the best results it is well to take tin- eml.ryo through 1 A useful guide for beginner "///.-/ l>y ''• .l;iniirs..n in i>ir|..-iiatiuii. For those who already possess an elementary knowledge of the subject an ••xrrllent \voj i l.nlli'.s \s-<'> Mit for tin- iiiiiiiiiiiini time (which will have to be determined by experiment1) and be carried out at l he ininiiiiiini temperature. It may IK- rem-'inbered that the complicated and bulky water-bath with its thermoMat is iii no \\ay necessary for the embedding process. A very simple apparatus which is perfectly efficient consists of a small metal trough (copper, or tinplate) resting upon a metal table kept heated at one end by a small flame. By sliding the trough lengthwise along the tal>le a position can be found such that the entire thickness of paraffin is fluid at the end next the flame and solid towards the other end. I.etween these two points stretches an inclined plane of solid paraffin upon the surface of which the embryo rests without any risk of the temperature rising appreciably above melting-point. A simple embedding trough of the kind indicated is of great use in the field as there is no method of storing and transporting embryos so free from danger of accident or of bistological deterioration as having them embedded in solid paraffin. To get a block of paraffin in good condition for section-cutting the embryo should be transferred to a bath of fresh paraffin as soon as it is infiltrated. With certain clearing agents, e.g. cedar oil, it is well to give two or three changes of paraffin. The vessel containing the embryo in a considerable volume of paraffin should now be floated on cold water to give a homogeneous translucent block of solid paraffin. On no account should the vessel be actually submerged in the cold water for in this event the contraction of the inner paraffin as it cools within the already rigid outer layers will lead to the formation of cavities into which the water penetrates. For the actual process of section-cutting it is necessary to use a mechanical microtome. The Cambridge Rocking microtome is one of the most convenient for ordinary embryological work while the Reinhold- (Jiltay microtome is a most excellent instrument both as regards accuracy and rapidity of working. The paraffin block containing the embryo is trimmed dowii so as to be rectangular in section and is then fixed by the interposition of a hot spatula to the paraffined surface of the microtome carrier in such a position as may lie necessary to uive the required direction of sections. Where the object is a "difficult" one, e.g. containing much yolk, it is advisable to have it surrounded by a paraffin block of considerable size. A considerable mass of paraffin above the specimen makes it cut better, while a considerable mass to the side causes successive sections, with their long edges, to ad li ere better together and form a continuous ribbon. The embryo should be near one of the lower corners of the block to facilitate exact orientation. Kor thorough investigation of the structure of embryos it is advisable to have specimens cut into sections in the three sets of planes — transverse, sagitial (.r longitudinal vertical, and coronal or longitudinal horizontal. To obtain these it is necosary to have the embryo orientated exactly on the microtome. In most cases this can be accomplished with a sufficiently close approximation to accuracy when fixing the paraffin block on to the 1 k'.'t. for a Chiek at about tin- niiiMlr «>!' the second day about 20 minutes will be found to be sufficient. VOL. II 2 P 578 EMBRYOLOGY OF THE LOWKIi YKKTEBRATES APR carrier, especially if care has been taken to trim the surfaces of the block parallel to the three chief planes of tin- embryo. \Vhere greater accuracy is needed, as in the case of very small embryos, they should be arranged in position in the melted paraffin with warm needles under the prism binocular microscope. This may be done by placing the watch-glass or other vessel on the top of a small flat copper ci>tern full of water, provided with inlet and outflow tubes, and heated up by contact with the top of the. water-bath or hot stage. In the bottom of the embedding vessel is placed a small plate of glass on the upper surface of which are engraved parallel lines intersecting one another at right angles. When the embryos have been accurately orientated with regard to the engraved lines a stream of cold water is allowed to run through the cistern and this causes the paraffin rapidly to solidify. When the block is quite hard the glass plate is picked off' and the ridges formed by its engraved lines serve as accurate guides to the position of the embryo. Still greater accuracy is obtainable by arranging that the melted paraffin in which the embryo is being orientated is already in its definitive position on the holder of the microtome, the paraffin being kept melted as long as necessary by an electric current passing through a loop of high resist-mce wire.1 For the actual cutting care must be taken that the razor (solid ground) or other knife has a very fine edge which does not show irregularities when examined under the low power of the microscope. The blade should be thoroughly cleaned with pure spirit before commencing work. If very thin sections, e.g. of 1 /x in thickness, are required it is well to commence with sections of 5 p, then without stopping to change to 4 /x, then to 3 /x, then to 2 /x, then to 1 /x — cutting a continuous ribbon throughout and going ahead rapidly when the 1 /x sections are cutting properly. The celloidin method should be constantly used as a check on the paraffin method. Where yolky eggs or embryos are being cut the celloidin method gives the only trustworthy sections as by it the yolk granules are held in position and prevented from sticking on the edge of the knife, ploughing through the tissues and destroying much of the line detail, as is always liable to happen if paraffin alone is used under such circumstances. In cases where there is no need for specially thin sections (say under L'.~) /i.) a convenient method is that in which the celloidin block is hardened by exposure to chloroform vapour and then cleared by immersion in .)• wood oil. The block of celloidin is usually fixed to a block of wood which is -ripped by tin- holder of the microtome. Care should be taken that such wooden Nocks are baked for Several days so as to ensure their being absolutely dry. Otherwise moisture will dilluse out and produce a milky opacity in the crlloidin which ought to lie absolutely clear and transparent. Sometime* it \\ill be found thai the I. lock becomes too hard and will not cut properly. it> edges frilling or breaking. This is sometimes due to the pretence Of a trace Of chloroform in the cedar oil used for clearing. When this is the OM6 the cut lurfaoe of the block should ha\e perfectly pun- cedar oil Applied to it with a brush just before each section is cut. 1 A MJH-ci.'ll ;i|i| ;il.llll I'. I (lii.S |.III|.O.SC i.s lll;ieni-ii. .17 /,-/-. Set., xlv, 1902. - Trims. Zo.if. Soc. London, xvi, 1't. 7, 1902. 580 EMBRYOLOGY OF THE LOWER VERTEBRATES AFP. plates are fitted on in a similar manner until the particular organ stands out like a solid model in the mass of plates. The same set of drawings may be used for different organs : the clove oil is removed by treating with strong spirit, and the water colour by holding under the tap, and then, after drying, a new organ can be coloured in. By colouring merely the cavity of an organ the relations of the cavity can be displayed as by an injection. When finally done with the drawii are removed by scrubbing with " Monkey brand" soap. By this method, after a little practice, reconstructions can be made with great rapidity and accuracy. Though less accurate and much more tedious the older method of reconstructing with plates of wax is useful for building up a permanent model. Its use is also indicated where only a single specimen is available. Instead of wax plasticine may be used l which allows of a kind of dissection being made, in as much as particular parts of the model may be bent out of the way to display structures which would otherwise be hidden. In investigating the development of the skeleton the cartilage is often found to pass by imperceptible gradations into unmocfified mesenchyme. The absence of sharply denned surfaces in such cases makes. the recon- struction method unreliable and it is advisable to supplement it by subjecting the embryo to treatment with a specific stain wrhich picks out the cartilage while leaving the other tissues uncoloured so that the cleared and transparent specimen may be studied as a whole under the binocular microscope. An excellent stain for this purpose is v. Wijhe's Methylene Blue.- The embryo is fixed preferably in '5% watery solution of corrosive sublimate, with 10% formalin added just before use, and preserved in alcohol. When about to be stained it should be treated for a day or two with alcohol containing J% hydrochloric acid — care being taken to renew this so long as it develops any yellowness due to traces of iodine. The stain consists of a solution of J% methylene blue in 70% alcohol to which 1% hydrochloric acid has been added some time before use. The embryo is stained for a week and is then treated with 70% alcohol containing ]% hydrochloric acid and renewed several times the first day and thereafter once daily until no more colour comes away. The embryo is now dehydrated, cleared gradually in xylol, passed through stronger and stronger solutions of Canada halsam in xylol, and preserved eventually in balsam so thick as to be solid at ordinary temperatures though liquid at 60° C. An excellent method of cleaning small cartilaginous skeletons is to place them amongst I'Yog tadpoles \\hich remove the muscle etc. from the ace of the cartilage by means of their oral com I is In re-ard to the gi-neral principles of embryological iVM>aivh it need hardly be .-aid that, as in other branche- of science, accuracy of observation be lir.-t place. And yet, curiously, accuracy may become a f;tult. In tho>e I, ranches of science \\hich are more etlectively under the control of mathematics it is well ive<»gni/ed that in any type of invest i-a t.ion then- i> a limit of probable error of observation due to instrumental . imperfection or to disturbing factors of one kind or another 1 MJIIIIHT, /'t-i-nl,i-inir/u,i ,y' SHH«j,i A'.81 beyond which it is mere waste of time to push observation. In all biological observation tin1 limit <»F probable error is particularly high yet this fact is jM-culiarly apt to be ignored and it is no unusual tiling to find dimensions or other numerical data stated to three or four places of decimals when anything beyond the first place is worthless for the reason indicated. To secure accuracy of observation not merely training and experience in the art of observing is needed but also a proper psychological outlook: the observer must be able to take a completely detached point of view and must ever be on the \\atch to guard against some particular hypothesis or preconceived idea causing actual error instead of fulfilling its proper function of keeping the powers of observation tuned up to the highest pilch of alert iii The whole spirit and aim of scientific investigation is directed towards the serial ion of facts and the devising of general expressions or formulae which unite1 them together. In this it contrasts with the more primitive state of mental development which observes isolated phenomena, noting the ditl'erences between them but blind to the common features which link them together. In embryology as in other departments of "knowledge the able investigator sees the general principles which run through and organ i/e the masses of detail: he interests himself in discovering the likeness which is hidden under superficial difference; he is constructive not destructive. In this volume embryology is treated as a branch of morphology but it must be borne in mind that morphology and physiology are inseparably intertwined. The living body whether of an embryo or an adult is above all a piece of exquisite mechanism fitted to live and move and have its being, and to ignore this is to make morphology as sterile and as misleading as would be the study of machinery apart from the movements and functions of its various parts. More particularly in attempting to delineate tin evolutionary past of an organ, or set of organs, speculation must always be rigidly controlled by the reflexion that at each phase in evolution it must have been able to function. When at length the stage is reached of putting results into form for publication the first thing to aim at is absolute clearness of expression. It must be remembered that clearness of language and clearness of thought are closely interdependent. Sloppy obscure language means sloppy obscure thought. The greatest care should be taken in the correct and precise use (,f technical terms. Argumentation in regard to scientific and other matters is, when the disputants are equally well informed, due as a rule to some word or expression being used in slightly different senses. Klegant literary style, however desirable, must always be subordinate to clarity and precision of language. Indeed actual harm is sometimes done to scientific progress by the writer whose literary skill carries away not merely himself but others of uncritical and impressionable mind. Scientific problems are eventually settled not by skill in dialectic but by increase of knowledge. As a rule the proper presentment of an embryological thesis involves pictorial illustration. In this the elaborate coloured lithographs of former days may conveniently be replaced to a vjreat extent by simple line or half-tone drawings in India ink or process black which can be reproduced EMBKYOLOGY OF THE LOAVKII YKRTEBKATES APP. photographically and inserted in the text in contiguity with the passage which they illustrate. Their function is to render more clear the statements of the author: they represent as accurately as possible phenomena as observed by the skilled and trained eye with a brain behind it. Actual photographs, which represent merely details lying in one particular plane and as seen by the untrained photographic lens, should be avoided. Apart from the imperfections indicated they are so blurred by the ordinary processes of reproduction as to be liable to misinterpretation and in these days of skilful manipulation they are of course useless as guarantees of truth. INDEX (References to figures in Clarendon type) . 1 Abdominal pores, 251 Acantt brain, S»2, 93 rlioinlrorranium, 313 egg-shell, 478 In-art, 374 liver, 187 muscle-buds, UOS spinal iranglia, 12? spim>. 822 . 1 • •//*••/, - aivualia, 295 cement-organs, 180 egg, *'• -ill-lamellae, origin, 160 segmentation, 23, 24 Acroehordal cartilage, 315 A.ctinopterygii LruMnilatiou, 46 nares, 125 pylorie caeca, 191 segmentation. 19 Adrenal, 282 Air- bladder — evolution of, 17:i of Teleosts, 166, 167 Air-sacs, 164, 165 Allantoie vein, 423 All.mtois, -Jl'2, 471, 476, 549 . I////V.S- oviposit ion, 461 Anififi/fifoiiKi ( — Sii-i'i/nn = Axolotl) — arcualia, 296 eye, 1 11 gornid, 269, 272 vertebral centra. 300 vomer with teeth, 332 Amic — air-bladder, 168 cement- organ s, 178, 181 segmentation, 24 vertebrae, 297, 339 Anmion, 465 evolutionary origin, 47T>, 476 Amniota — buccal cavity, 148 Amniota (continued)— external gills, physiological replacement of, 157 gastrulation, 47 haemal arches, 297 nares, 126, 151 sclerotome, 205 Amniotic isthmus, 469 Amphibia — adaptations in early development, 458 egg, 2, 27 endolymphatic duct, 132 gastrulation, 38 gonad, 268 heart, 389 mesoderm, origin, 60 opisthonephros, 251 pharyngeal clefts, 159 practical hints, 566 pronephros, 235 segmentation, 27 testicular network, 279 vertebral centra, 299 .1 nifiliiuXUS — brain, 96 egg, 2, 3 gastrulation, 30, 31 gonad, 267 liver, 186 rnesoderm, origin, 57 practical hints, 558 sclerotome, 285, 286 segmentation, 7, 8, 9 Ampulla, 130 Ancestry of Vtrtebrata, 503 Anura — cement-organs, 80 horny jaws, 75, 76 oral combs\ 75, 76 Anns, 51, 144, 192, 194, 497 Aortic arches, 394, 395, 396, 397, 398, 401 Apical, 4 Archencephalon, 85, 92 Archenteron, 30 Archinephric duct, 221, 225, 238, 239, 240, 241 Archiuephros, 221 583 584 EMBRYOLOGY OF THE LoWKK VEBTEBRATES Archipallium, $1 Arcualia, •_. . il system. 393 Auditory organ, 129, 130 >ke"leton, 343 Axial niesoderni, 63 Axon, 84 Balancers, 156, 157 Hasilav plate, 310 " ostoma — egg-envelope, 456 segmentation, 18 tongue, 151 Bilateral segmentation, 9 Birds. See also under Fowl allantois, 471 amnion, 469 auditory organ, 129, 130 Mood and vessels, 368 bursa fabricii, 192 carotid arteries, 404 chondrocranimn, 314, 316 embryo with amnion, etc., 472, 473 endoderm, 51 flight, origin, 453 heart, 367, 383, 384, 385 liver, 188 lung, 162, 163, 165 mesoderm, origin, 64 mesonephros, 253, 254 metanephros, 256 neonychia, 74 neural crest, 122 neurenteric canal, 53 otocyst, 129, 130 pancreas, 191 pecten, 140, 552 pelvis, 356 pleural cavity, 202 practical study, 508-557 primitive streak, 53, 519, 520 pronephros, 237 ureter, 257 venous system, 425 BlastuL-oele, 7 Blastoderm, 7 name, 505 Blastomere, 5 Blastopore, 31, 496 Blastula, 7 , 365 islands. 368 :'.68 in.tl tfills, 156 ImiK, 169 .< reaa, 191 with grafted limb, 115 Bone, 821, • •volution. BoneK — • l.-nlal, 33'.! Investment Bones (c.•!.. me, 59, 197 ('olleetin- tubes, renal, 2-17. '2^': -J.'.7 Oommi anterior, 87 posterior, 89 superior, 89 t 'onneeiive tissue, 291 Cornea, 139 ('..ilex, 91 Orooodilia— blood-vessels, 401 carotid arteries, 404 lu-art, 391 neonyrhia, 75 tOOth-Mleeession, 329 • t erygii ( I'oii/pterus) — segmentation, 19 ( kyptobrancfau filiation of external gills, 156 Cuticle, 70 Cyclostoniata — genital pores, 246 horny teeth, 77, 78 olfactory organ, 128 pancreas, 189 Delamination, 34 n.-im-rsiil, 20 Dentine, 322, 325 Dennis, 69 Diaphragm of Birds, 164 Diplospondyly, 341 Dipnoi (Long-flab) — practical hints, 565 rminal, 4 Dorsal lip of blastopore, 32 |)..rsal sac, 89 Duetus venosus, 539 Iv-to.lerm, 30 name, 505 Egg- envelopes, 455, 515 size, 2 tooth, 326 •iobranc.hii (Sharks, Skates, etc.) — aortic arches, 398 archinephric duct, 240 arcualia, 295 blastoderms, 560 brain, 92, 93 chondrocranium, 312 chromophile organs, 284 external irills physiological replace- ment of, 157 tin rudiments, 445 gastrulation, 45 iiill-tilanients, 160 heart, 366, 373, 374 interrenals, 283 lens, 138, 139 Klasin obranch ii (am I tuned)— limb-skeleton, 351 liver, 187 mesoderm of head, 208 motor nerve-trunk, 111 Mullerian dm is, •_>):;, 244 muscle-buds, 207 neural crest, 122 opisthonephros, 247, 248 origin of mesoderm, 63 otocyst, 132 ovary, 276 pancreas, 189 peritoneal funnels, 251 pharyngeal clefts, 158 practical hints, 559 pronephros, 236, 237 segmentation, 12, 14, 15. 29 sympathetic, 124 urinogenital sinus, 242 vasa efferentia, 279 venous system, 413 viviparity, 478 Electrical organs, 212 Embryonic development, 464 Enamel, 322, 325 Endoderm,-30 name, 505 secondary, 49 Endolymphatic duct, 130, 132 End-plate, motor, 204 Enteron, 58, 144, 146 development of openings, 193 muscular sheath, 217 temporary occlusion, 192 Epidermis, 70 Equatorial, 5 Eye, 133, 134, 136 evolutionary origin, 141 muscles, 210 Face, 555 Feathers, 71, 72 Fin-rays, 346 Fins, median, 439 skeleton, 346 paired, 441 skeleton, 849, 353 Flask -glands, 79 Foregut, 144 solid, 148, 521 Fowl, common — blood-vessels, 650 early blastoderms, 519 egg, 514, 515 embryos — first day, 522 second day, 524-532 third day, 532-546 fourth day, 546-549 fifth day, '548-551 sixth day, 551, 552 >eventh day, 553, 554 sagittal sections, 525. 543 586 EMBRYOLOGY OF THE LOWER VERTEBRATES Fowl (continued)— segmentation, 516, 517 Fronto-nasal process, 147, 555 Gamete, 1 Ganoids, actinopterygian (Acipenser, Amia, Lepidosteus) — cement-organs, 180 gastrulation, 47 opisthonephros, 259 ovary, 277 pancreas, 189 practical hints, 563 pronephros, 234 segmentation, 23, 24 Ganoine, 326, 336 Gastraea, 506 Gastrula, 30 Gastrulation, 30 Gecko — endolymphatic duct, 132 gastrulation, 48, 50 Genital fold, 268 pores, 246 ridge, 267 Geotria — pineal eyes, 100 Germ cavity, 16 layer theory, Historical note, 505 layers, delimitation, 150, 181 wall, 17 Germinal epithelium, 268 Gills- evolutionary history, 160, 449 external, 154, 155, 157, 450 as forerunners of limbs, 449 internal, 159 Glomerulus, 223 Glottis, 161 Gonad, 266 Gonocytes, 266 Grafted limb of Toad, 115 Gymnarchus — egg, 20 external form, 432 gill-filaments, 160 nophiona — .-xternal gills, 155, 156 gastrulation, 42, 43 pronephros, 223 segmentation, 28 Hali-'iinlar commissure, 89 ganglia. :ial arches, 292, 296 process, 297 spine., 297 Haj.loi.l, 17 n.-.-i.i ioM. . general morphology, 498 ).r«.. -hH, 53, 621 n "I, 208, 601 < r;il iiioj pholo^y, o70, 391 Hemispheres, cerebral, 86, 91, 95 Hippocampus, 91 Holoblastic, 7 Holouephros ( = archinephros). 221 ffyla— extemal gills, 156 with eggs, 462 Hyostylic skull, 320 Hypobranchial, hypoglossal, muscles, 212, 213 Hypochord, 290 Hypogeophis — chromophile organs, 284 external form, 437 gills, 155 gastrulation, 42, 43 interrenals, 284 opisthonephros, 246 pharyngeal clefts, 159 pronephros, 224-227 thymus, 177 Hypophysis cerebri, 143 Ichthyophis — intestine in section, 186 modelling of intestine, 183 segmentation, 28 Iguana, pineal eye, 97 Infundibular gland, 90 Inftmdibulum, 86 Intermediate cell-mass, 253 Interrenal, 283 Intestine, 145, 183, 184, 185, 186 Invagination, 32 Iris, 136 Isolecithal, 4 Jacobson's organ, 128 Kidneys, evolutionary origin, 217, 221 Labial cartilages, 307 Lacerta — archenteron, 51 heart, 380 neural arches, 296 pancreas, 191 pineal eye, 97, 98, 99 tympanic cavity, 343 venous system, 422, 424 yolk-plug, 50 Lachrymal groove, 551 a, 129 Lamina teniiinalis, 90 Larvae, evolutionary significance, 492 Lateral line organs, 13'J Latitudinal, 6 Leu, i:w, 138, 139 Lepidosiren — brain, 85, 86, 87, 88 lnieeal cavitN, 149 rcinrnt oi-ans, 78, 79 rlioiitlril'uMtinii ol imtorlionlal >lieatli, 293 , .'H)S, 309, 311 INDEX 587 1., /, /,/w/v,/ (contii comiiouinl n.'Hiin- n| pliarvngeal con- strictor niiis.-l,-. 'J17 egg, -"•• external form, 434 external gills. 154 inilation, 34, 35, 36 heart, 375, 379 Inn/, Iti'J, 170, 171 modelling of intestine. 184, 185 motor nerve-trunks, 10(5,108, 109, 110 my'tome, 201, 202, 203 neural rudiment, 496 olfactory nerve, 113 optic rudiment, 136 origin of niesoderm, 58 otocyst, 131 pancreas, 190 1'inkus's organ, 133 pnmephros, 231, 232; 233 respiratory limbs, 451 rods, 137 selerotome, 285 • -illation, 26 skull and niyotomes, 213 teeth, 324, 331 thynms, 177 thyroid, 175 venous system, 407, 409 bone and scales, 334 cement-organs, 181 external form, 431 muscle-buds, 206 ovary, 277 segmentation, 24, 25 Limbs, 441 evolutionary origin, 443 skeleton, 349 varying situation, 448 Liver. 186 Lung, 161 LriiLr-lish (fjepidosiren, Protopterus, Cera- f <>i tits)— cement-organs, 78, 79 cloacal caecum, 242 egg, 25 eye muscles, 211 hypobranchial muscles, 212, 213 opisthonephros, 260 otocyst, 131 practical hints, 565 segmentation, 25, 26 vasa efferentia, 279 Lymphatic system, 426 M.-KToinere, 6 Mulpighian body, 227, 246, 260 JNIan.libular arch, 319 ridge, 145 Maxillary process, 127 ridge, 14.5 Medullary folds, groove, plate, 83 sheath, 85 Meridional, f» Meroblastic, 7 Merocyte, 14, 17, 18 Mesencephalon, 87 MUM-II. thyme, 66, 286, 506 Mesenteric arteries, 407 Mesenteron, 144 Mesentery, 199 Mesoderm, axial and peripheral, 63 general remarks on, 54, 506 lateral, 59 origin of, 65 segments, 59 of head, 208 Mesonephros, 221, 253, 254 Mesothelium, 67 Metanephros, 221, 256 Methods, embryological, 573 Micromere, 6 Micropyle, 457 Mouth, 498 Miillerian duct (oviduct), 242 Muscle-buds, 205, 206, 207, 208 Muscles, 205 of median tins, 205, 206 of paired fins, 206, 207 Myoblast, 201, 202, 203, 204 Myocoele, 202 Myoepithelial cells, 202, 204 Myosepta, 202 Myotomes, 198, 201, 203 Myxinoids — pericardiac cavity, 200 segmentation, 18 spawning locality, 559 Nares, 125 external, temporary obliteration of, 129 internal, Anmiota, 151 Nasal process, median, 127 Necturus — rib, 304 Neonychium, 74, 76 Neopallium, 91 Nephridia, 217 Nephrocoele, 198, 226 Nephrostome, 222 Nephrotome, 198 Nephrotomes, secondai'y, 247 Nerve development, general discussion, 113, 345 optic, 140 pineal, 97 Nerves as landmarks, 345 cranial, 122 sensory, 112 Nerve-trunks, motor, 102, 104, 105, 106. 108, 109, 110, 111, 204 Nervous system, 82 evolutionary origin, 118 Neural arches, 292, 294 crest, 121 spine, 296 tube, 83 588 EMBRYOLOGY OF THE LOWER VERTEBRATES :teric canal, 41. f>l. 52, 53 Ntnirite, 84 Neurobiotaxis, 121 Neuroblasts, 84 Ncuroiibrils, 85, 112, 11!' >glia, 84 Neuromasts, 132 Neuromery, 101 Nt-urone, 84 theory, 104 " Normal plates," 575 Notochord, 147, 289 Sototremn, 463 Nuclear division, synchronism of, 16, 17, 22 Odontoblasts, 322 Olfactory bulb, 91 organ, 125 tract or peduncle, 94 tubercle, 91 Ontogeny, general principles, 484 Opaque area, 518 Operculum, 158 Opisthonephros, 221, 246 Optic cup, 134 nerve, 140 38, 90 thalamus, 89 Otocyst, 129 Ovary, 274 Overgrowth, 32 Oviduct (Mullerian duct), 242 Palaeontology and Evolution, 502 Paludicola, larval jaws, 76 Pancreas, 189 evolutionary origin, 191 Pander, nucleus of, 514 Pauizza, foramen of, 391 Parachordal cartilages, 307 — Paraphysis, 87 Parapineal body, 97 Pecten, 552 Pectoral girdle, 354, 357 Pellucid area, 518 Pelvic girdle, 355, 358 Periblast, 23 Pericardiac cavity, 200, 210 nlio- peritoneal canal, 200 Periderm, 70 >'lerm, 63 -.rial, 226 fiiuin-1, 22»i, 227, '251 fun; -i-nl, 2f.l, :• Peritoneum. 217 Petromyzrm (Lamprey) — i. ilia, 294, 296, 299 .Isilioii, 37, 38 * homy tcrtli, 77 Itor, 187 mesodt-nn • lii-Mil. -Jl I |M-iirart in in tuli:n', *_' I". ntation, 14, 15 Proftmnion, !'i7 laeiuD, l 1 1 Pronephric chamber, secondary, u:;i. I'ronephros, 221, 222-237 in adult T. I'rotoiiephrMium, 218 I'l-nti'iit, , nti — buccal cavity, 149 choiiilrocranium, :50X, 309, 311 larva, renal organs, 230 lung, 169. 190 olfactory organ, 125, 126 otoeyst, 131 pancreas. 190 Pinkus's organ, 133 l'n>t<»stoina, 31 theory, 493 tylic skull, 320 1'rotovertcbral stalk, 198 Pseudobranch, 159 Pulmonary artery, 402 Pyloric caeca, 191 lobe, 91 liadial s.'-iiifiitation, 9 Raw skate)— electrical or-an, 213, 214, 216 eniltryos. 562 practical hints, 560 buccal cavity, 148 calcareous bodies, 1 -'!'J cement -organs, 80 ciliaiion of ectoderm, 70 envelopes, -159, 460 140, 141 ! niation, 38, 40 heart rudiment* 361 larval teeth, 76 origin of mesoderm, 61, 62 renal organs, 222, 223 lentation, 10. 11, 12, 13, 28 iiitnlation. I'.Mt. ;,u;, '.'byiiuth), 129 1 caecum, 192 lu-nal ducU. evolutionary origin, 264 us, general. -J17, L'tll Ueptilia aortic arches. 395, 396, 397 archeiiteron, '>] •tooth, 326 embryological material. ~>7^ ilar lip, 49 Ileptil: •'/) gastrulation, 17 opisthonephros, 260 rni, 64 pronephros, 238 vertebral centra, 301 viviparity, 480 Kctina, 135, 137 Rhacophorufi, 461 It/It', lni/,-,-1,111, 462 /;/i,,(/<>nx, air-bladder, 167 Hl'onibencephalon, 85, 89 * Rhomboidal sinus, 525 Ribs, 302 Saccule, 130 Salamandra (Salamander) — heart, 389 opisthonephros, 252 rib, 304 tongue, 150 venous system, 421 vertebral centra, 300 viviparity, 480 air-bladder, 168 egg, 20 gonad, 267 heart, 389 lepiddtrichia, 347 opisthonephros, 259 ovary, 277 prouephros, 234 pseudobrauch, 159 segmentation, 21, 22 Sarcoplasm, sarcolemma, 203 Sauropsida — lung, 162 segmentation, 29 Scales, cycloid, 336 ganoid, 336 of Birds, 73 placoid, 321, 322 Reptilian, 71 Sdiiieiderian folds, 128 Sclerotic, 139 Sclerotome, 67, 198, 205, 285, 286, 538 Scyllium — chromophile organs, 284 elongated buccal opening, 147, 148 interrenal, 283 practical hints, 561 segmentation, 14, 15 vertebral centra, 298 Segmentation, 5 abortive, 14, 29 ,-y, 7 metameric, 500 ofcoelome, 197, 199 of head region, f»0<>, 501 Semicircular canal-. I'J'.i Seio-ainiiiotic connexion, ioit 590 EMBRYOLOGY OF .THE LOWER VERTEBRATES Serous membrane, 469 Serranus, segmentation. 21 sheath, medullary, 35. ii-_> of notochord, chondritication, 293 primitive, gray, 105, 110, 112 Shield, embryonic, 48 blystoma, Axolotl), lens, 139 Skin. 69 Skull, 306, 341 Soma, 266 Somatopleure, 144 Somites, mesobla*tic, 59 - pineal eye, 97, 99 . ribs, 305 vertebrae, ossification, 338 vertebral centra, 302 Spinal cord. 83 ganglia, 121 Spirvax — muscle-buds, 207 occipital myotomes, 209 visceral arch skeleton, 321 Spiracle, 158, 344 Spiral segmentation, 9 valve of intestine, 183, 184, 185 Splanchnic mesoderm, 144 Splanchnocoele, 198 Splanchnopleure, 144 Spleen, 427 Spongioblasts, 84 Sternum, 305 Stomodaenm, 144 Sutolavian artery, 405 Subgerminal cavity, 49 Subnotochordal rod, 290 Supraperieardial body, 178 Suprarenal, 282 Symbrandius — % external form, 444 opercular opening. 158 Symmetry of egg, 8, 11 Sympathetic, 124 Syncytium, yolk, 16, 22 Tail, autotomy in Li/.ai. evolutionary origin, 453 form, 440 Technique of Kmbryology — Amphibia, 566 . 508 gei. in optimum, 89 327 Teleost.-i loi.lmai \ Fishes)— air-bladder, ICO, 167 I .rain, Hf>, 96 -•ans, 182 egg, 20 lal.-iforui process, 140 Teleostei (continued] — gastrulation, 46 lepidotrichia, 347 male genital duct, 281 opisthonephros, 259 ovary, 276, 277 oviduct, 276, 278 i pancreas, 190 practical hints, 564 pronephros, 234 segmentation, 19, 21, 22 separation of yolk-sac, 464 urinary bladder, 242 viviparity, 479 Telolecithal, 4 Terminal sinus, 527, 533 Testicular network, evolution, 279 Testis, 274 Thalamencephalon. 87 Thalamus, 89 Thymus, 177 Thyroid, 175 evolutionary origin, 176 Tongue, 150 Torpedo (Electric Kay) — buccal opening, 147 fin rudiments, 445 gastrulation, 45 heart, 366 mesoderm segments in head, 209 muscle-buds, 208 ostium tubae, 243 pronephros, 237 segmentation, 14, 15 vertebral centra, 299 Trabeculae cranii, 307 Trachea, 161 Trifnit (Newt) — heart, origin, 360, 361 larvae, 157 opisthonepliros, 252 origin of mesoderm, 62 rib, 304 tongue, 150 vertebral centra, 300 Trophouemata, 479 Tropibasic, 307 Tropiiluiiiihi* (Kuropean Grass Snake) — origin of mcso.lerm, 64 Turbinals, 128 Tympanic cavity, 344 membrane, 344 I'mbiliral vein, 423 Ureter, 257 I 'r«..lt-la, segmentation. '_".' I'trir], Valvula rnvbelli, 95 Vuni'lliis, primiti\ Meivntia. 272. lii nambnuM, I i."> • MI, 90 INDEX 591 \Yn.-i ca\a anterior (superior posterior (inferior), 41" \Yiiuii- >\ -tun, 407 Ventricle, lateral, i«i i i, r.-inpoiu-nts of, 339 i:ic. os-iiirat inn, :;:;7 ••ral rriitra, 'J'.W, 'J(.i7 N'citical furrow, 6 Viper-, poison-fan^, :5-t>- 329 Visceral aivlit-s, -k1-li-toii, 318. 321 delta, 158 Vit.-lliiio artt-ry, 'J:57, 550 Vitn-mis bu.lv. i:3f), 140 Viviparity, 478 Wolffian du- Yolk, 2, 488 colour, 3 -plug, 33 -sac, 464 retraction into body, 475 -stalk, 186 -syncytiuni, 16, 22 Zygote, 1 envelopes, 455 END OF VOL. II Printed by R. & R. CI.AKK, LIMITED, o f I BINDIN* LIST University of Toronto Library DO NOT REMOVE THE CARD FROM THIS POCKET Acme Library Card Pocket LOWE-MARTIN CO. LIMITED