1897 Human Embryology 14

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Note - this online text is only at a very early draft stage and contains many errors from the original scanning.

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The Foetal Appendages

III. Origin of the Foetal Appendages

Under this head we have to consider the origins of the chorion, yolk-sac, allantois, proamnion and amnion, but as wo have already considered the development of the yolk-sac, p. to, the allantois, p, 357, and the proamnion, p, 150, we shall recur to them now incidentally only, and concern ourBelvesprinciiially with chorion and amnion.

Extension of the Extra-Embryonic Coelom

The distance to which the coelom can extend around the ovum dei)end« upon the extension of the mesoderm, for of course the cavity cannot go farther than the layer within which it is develope<l. Now, as we have seen, the mesoderm expands gradually and a little niuif slowly than theferminal area. This gradual expansion occurs ia all vertebrates, n the primitive type (Petromyzon and amphibians) the mesoderm and the coelom both grow completely around the yolk ; and tiiis was imdoubtedly the primitive condition, but in the lower amniota the growth of the mesoderm has to be much greater in order to cover the enormous yolk mass; hence in amniota the spread of the mesoderm is slow and long continued, and the embryo advances far in its development before the yolk is inclosed. In mammals the expansion of the mesoderm over the yolk-sac is also slow, and in rabbits (and probably in their allies) the mesoderm never extends over the whole yolk-sac, but in man, on the contrar}% the co'lom as well as the mesoderm are developed completely around the yolk-sac very early. No explanation of these differences among mammalia can be offered at present.

In the lampreys and amphibians the appearance of the coelom around the yolk merely completes the separation of the body- wall or somatopleure of the embryo. In the amniota it also separates the somatopleure from the splanchnic mesoderm around the yolk, but owing to the division of the developing ovum into embryo proper and yolk-sac, only a small part of the somatopleure shares in the formation of the embryo, while the rest acts as a covering membrane of the yolk. This membrane in the mammalia is imiversidly known as the chorion, and I shall apply this name to it hereafter for all vertebrates.

Primitive Chorion

The chorion has been defined by Minot (Buck's '^Handb.," II., 143) to be the whole of ihat portion of the extra-embryonic somatopleure which is not concerned in the formation of the amnion. The term primitive chorion may be employed for the whole extra-embryonic somatopleure before the differentiation of the amnion from it, and the term chorion or true chorion be still used, as defined, for what remains of the membrane after the separation of the amnion.

The somatopleure consists of two layers — the ect<xlerm and somatic mesoderm. The ectoderm consists of a single layer of epithelial colls. The mesoderm consists of a laver of mesothelium next the coelom, and a thicker layer of mesenchyma between the mesothelium and ectixlerm. The exact appearances of these layers are descrilxjd with the aid of figures in the si)ecial chapters on the amnion and chorion.

Origin of the Amnion

The amnion is develoi^ed out of that j>art of the extra-emhrj^onic 8omatoi)leure which immediately surrounds the embryo and the proamniotic area, or in other words, the amniotic region of the germinal area is part of the area pellucida, and jx^rhaps includes the whole of the j^ellucida. The amnion owes its development to the expansion of the coelom. In the 8auropsida the pnx'oss is about the same in all forms, but in mammals there are several modifications of the development knowTi ; hence we consider first the sauropsidan, then the mainmalian tyi)es.

In the Sduropsida the formation of the amnion l)egins with the ai)pearance of the amnio-cardial vesicles, p. IDS, which form c»onspicuous dilatations on either side of the neck. Fig. 117; the vesicles steadily enlarge and spread laterally and forward so as to inclose the proamniotic area, and finally fuse in front of it. The dilatation takes place in siicli a manner thiit the splanchnopleiire is beQt down slightly, while the soniatopleure is h<"nt upward to an extreme degree, forming a sort of dome. Transverse sections of a chick at thi» stage, at the level of the heart, show, Fig. Ill, the amnio-cardial vesicle of each side fused with its fellow in the median line below the heart, Hi; the somatopleui-e, Soiii, of the embryo makes a sharp turn outward and upward, Am, and then bends away again, Cho, from the embryo and finally joins the splanchnopleure of the yolk, Spl. As the upbending of the somatopleure goes on around the entire head of the embrj-o, it follows that tiie cephalic end of the embryo lies in a depression the sides of which are formed by a part. Am, of the extra embr\ on csomatope re W e h s is going on, the head of t e embryo bends or and he ho head gradually rolls over ventralward and h a s f o ced o the yolk but since the pro amnion

nlnn; Snm. »]mato|ileure or

D Kh heR mp Habb F iSplpaiibp re£ l«d ot c a ea es j st 1 ere, t s aginated along with the head, and consequently the head seems covered by a proamniotic membrane, which is known as the cephalic cap (Kopfkappe, capiichon c4phaliqne). Tliis cjip is ver>- noticeable in young chicks, for the head is hidden in it, while the rest of the embryo is uiicoverod. The actual relations are still further complicated by the singidar fact that the edge of the cap is extended backward by the growth of the octoderm alone, as shown by Duval ; the backward growth of the eotwlerm occurs also in turtle embryos, and to a much greater distance than in birds before the mesoderm follows it (K. Mitsukuri, 80.1). Sooner or later the mestxlerm penetrates the ?ctodermal fold, and the coelom appears in it as a forward extension of the cavities of the amnio-cardial vesicles.

the cephalic end of the emhrj'o now soon l>oconies completely covered over by the extra-embryonic wmiatopleiire ; this is due to the expansion of the ctt'loni on idl sides.

The changes in the extra-endirytmic somato])leui'e around the anterior lialf of the embryo, are Bimilivr in a penornl vray to those mimiitl till! anterior half, but the dilatation of the cu-loni is confined to the extra-em bryonii; region, hence the pictures obtaiiietl from cross sections of the two jiurts of the embryo present certain essential

differences. Fig, lUl is n sections , here we see that the dilatatii.n

of the cosloni causes the soniatopleure form a longitudinal fold along ea of the embryo ; oath ■S's fold, ixtKising back "whrI, joins its below behind the embryo that th

may Iw describ«l tg

coiijt)iutly a!j tl o =0

(ail-fold (Schirait miHiale). Thetal ^S fold is devflojx 1 -^ considorably later than the head-fold, " but aa one gro\ ' "^ forwanl and tl otiier grows Ixick they finally meet and constitute tl o complete immiot c f<)ld around the e tin* embrj'ii. Tl o tail-fold gratluallj closes over the ei bryo; the proce s may lie imderstixxl from the acc'oi panying fignretFig. lim represent a cnw.s-section of a rabbit end>iy.). The soniiitopleure, Sin)), of the eiubryo liends over as the amnion, Aiit, so as to citvcr the embryo alxivo which it again winds outward as the chorion, Cho; we can alretuly di.stinguish the ('inbriionic', anniiotic, and chorionic jK)rtion3 of the somatopleure fniiii one another: where the amuiota- jx)rtion joinn the chorionic, the etlgo is prolonged by a thickening of (he ectoderm, which reminds US of the similar thickening at the etlge of the cephalic cap; the Iwo wigfs liuvi- almost met over the back of the embryo; the asymmetry of the folds exists in all amniota and, as shown in the figure, is very marked in the rabbit, but is much less marked in the Sauropsida. In the next stage, Fig. 101, the folds have actually met; their edges grow together by their ectodermal thickening; for some time the thickened ectoderm persists and offers in sections a characteristic feature ; after a time the mesoderm grows across, and the ectoderm of the amnion is entirely separated from that of the chorion ; still later the cavity of the chorion also penetrates and completes the final separation of the amnion from the chorion. Fig. 19. The process of sejmration is essentially the same in the case of the cephalic amnion. The separation of the amnion from the chorion progresses most rapidly at the head end ; at the tail end it begins later and progresses forward ; hence the portion of the amnion over the middle of the rump is the last to he formed, as can at once be seen if the fresh ovum 's examined.

In surface views the gradual closure of the amniotic folds over the embryo can be beautifully followed; for example, in the hen's ovum incubate^l about sixty hours, we find the anterior half of the embryo entirely hidden by the cephalic cap, while the posterior third of the rump is also covered by the tail-fold, and at the sides of the rump the amniotic folds have partially arched over the embryo. These arrangements leave a small longitudinal oval opening through which we can look down upon the embryo. The opening gradually diminishes as the edges of the folds advance, and is finally closed by the meeting of the edges from all sides. As the e<lges have their ectoderm thickened, their final meeting is marked by a local thickening of the ectoderm, which persists for some time after the actual closure. In ruminants the connection between the amnion and chorion at the point of fuial closure is retained for a long time by means of tissue, which grows out into a long thread, the so-calUnl amniotic cord (Am ni oust rang) , A somewhat similar structure occurs in the opossum, Selenka, 87.1, Taf. XXV., Fig. 2.

After the amniotic folds have closed, the embrj^o is surrounded by two membranes, both derivtyd from the extra-embryonic somatopleurc. Of these the outer is the true chorion, Fig. 10, (7//o, the inner the amnion ; from the manner of their formation the former has its ectodermal layer external, the latter its ectoderm internal or facing the embryo. The amnion. Fig. 10, Am, is the direct prolongation of the somatopleure of the embryo ; the space })etween th(» amnion and the embrvo is called the am)hiotic cavitii: it is lined throughout by ectoderm.

In mammals the development of the amnion was prosnmal)ly at first like that in the Sauropsida, for not only do we find many traces of it still preserved, Imt also Selenka, 86.1, 130, has shown that in the opossum the sanropsidan stage is pjissed through, although somewhat mcxlified by the excessive develojnnc^it of the proanniion. The increased importance of the proamnion can ])e seen also in the rabbit (Van Beneden and Julin), and is jK)ssibly characteristic of mammals as a class. In the two animals mentioneil, more than half of the embryo is covered by the proamnion at the time the amnion closes, and hence* the amnio-cardial vesicles cannot attain the size or imjiortance they have in birds, and they are imable, in the opossum, to extend around the proamniotic area ; hence in front of that area there ia no coelom developed, the three germ-layers remaining in close contact and forming, as it were, a single membrane; in the rabbit the coelom does appear in front, as in birds.

In ruminants the amnion appears very early, the folds being well advanced before the medullary groove appears. The formation of the amnion is induced by the preccx^ious development of the extraembryonic coelom, which, as Bonnet's researches on the sheep, 89. 1, have proven, extend very early around the embryo in a wide oval ring, which, by raising the somatopleure, forms an annular amniotic fold, before the embryo can be said to be differentiated ; these folds close over the anlage of the embryo, and by their union produce the two foetal membranes, amnion and chorion, in the same manner as in birds; as already mentioned, there is formed at the point of closure a long cord of tissue {funiculus amnion by which the two membranes remain united for a considerable period.

In the rodents with so-called inversion of the germ-layers (e, gr., guinea-pigs, rats, mice, etc.)i the development of the amnion is extremely modified from the original tjrpe. The cavity of the Trager Fig. 87, a, becomes in part the cavity of the amnion. The manner in which this takes place and the way in which the process may ho deduced from the primitive mode of development are both well illustrated bv Selenka's diagrams, 84. i, Taf. XVI.

The human a tn u ion in the (earliest stages yet known has l)een found completely closed ov(T the enibrj'o, so that nothing is known iis to its development by direct observation. The earliest known disj)08iti<)U was first descrilx^d bv W. His, whose account has boon conlinned by subsecjnent oliservors. The embrvo is from 2.5 to \.0 mm. long; its relations to the rest of the ovum are indicated by the diagram, Fig. UV2, B; it rests uinm the lart^e yolk-sac, T, and is connect(»d by a sliort stalk, y, with the j,,^, ,«,._pia^„,« to illustrate nu' theory of chorion, ( n, Ihe amnion thoOnjrinofthfHuinan Ainni<.ii: a, Firstsmp'-.B,

staps y|;;i, AiiiDitm: J/, nllantoic-stalk or

arises under the head at tll(? HamliKti.'l; C7<.ch(.rion,thevllli of which an^lrawn

iunction of tlu* (MllbrVO and **'""1 ^'^ ami more uimu*n«m than in naturt* ; r, yolk

yolk-sac, and from the sides of the embryo ami from the allantoic-stalk, and arching over the dorsal side of tin* embryo completely incloses it. To explain this disj>ositi()ii His has a«lvance<l the following hypothesis as to the course of developnient. The embrj"© arises upon the surface of the blastodermic vesicle in the usual manner; its somatopleure passes over into the primitive chorion, which is, at an extremely early age, completely separated from the yolk-sac; the chorion now forms a fold, as shown in Fig. 102, A, which arches backward over the head of the embryo; while the tail end of the embr>-o, retaining its direct connection with the chorion, becomes the allantoidean stalk, AL The head- fold, of which the inner leaf is the amnion. Am, the outer loaf a ])art of the true chorion, grows backward over the embryo as indicated by the dotted line, Ani'y until it finally reaches the allantois-stalk, Al^ and thus completely covers in the embryo. This hypothesis is probably correct, but it is pi>ssible that the amnion is preceded by a true proamnion, which Ijecomes obliterated very early by the precocious development of the mesoilemi and the coelom in the human ovum. If Graf Spee's j)lausible suggestion, 89.1. 170, that thei-e is a so-called inversion of the germ-layers in the human embryo be verified, then we shall probably find that the human amnion is doveloi)ed according to the rodent type mentioned above.

The True Chorion is that portion of the extra-embryonic somatopleure which remains around the ovum after the separation of the amnion; it consists of an outer layer of ectoderm and an inner laj^er of mesoderm ; the cavity within it is part of the coelom. By the closure of the amniotic folds the chorion becomes a membrane surrounding all the other parts of the ovum, and makes a complete bag, which is termed the chorionic vesicle. The chorion is the outermost of the foetal envelopes. It is sometimes termed the serous membrane or envelope {memhrana serosa^ serose Hiille)^ espec;ially in writings on sauroj^idan embryologj". Its relations may be rendered clear by the help of diagrams. Figs. 20 and 19.

IV. Known Human Ova of the Second and Third Weeks

As no synopsis has ever been made of our knowledge of the early stages of niiui, I have attempted to collate all the descriptions of embryos not over tliree weeks. A summary of the descriptions is given, p. 308.

Classification by Stages

Any attempt to divide embryos into stages must necessarily est^iblish artificial groups, for in nature there is no demarcjition. Division into stages is for convenience, and ouglit, therefore, to be made l)y natural and obvious characteristics. After much deliberation I have chosen eight stages, which seem to me natural and convenient, and I have classified the thirty-eight embryos reviewed in the preceding pages, placing them acjcording to my l)est judgment in their res[>ective stages; when the assigmnent is doubtful 1 have indicateil it by an int(.»rrogation mark.

First Stage: Appearance of the primitive streak.

1. Reichert's.

2. Breus'.

3. Wharton Jones'.

4. Ahlfeld's.

5. Boigel and Lowe's.

T. Kollmann's a. 8. "

6.

0. Schwabe's. Second Stage: Appearance of the mediillarv plate.

10. W\ His'XLIV. (Bff).

11. Keibel's. VI, Spee's.

Third Stage: Appearance of the mediillary groove.

13. W. His' E.

14. Allen Thomson's No. I.

15. W. His' SR.

16. Allen Thomson's No. II. Fourth Stage: Formation of the heart and medullary canal.

17. Spee's second embryo.

18. Kolhnann's embryo of 2.2 mm.

10. Von Baer's youngest ovum. Fifth Stage: First external gill-cleft.

None. Sixth Stage: Two external gill-clefts.

20. Minot's No. 195.

21. " No. 143.

22. W. His'LXVIII. (Lg).

23. " " LXVI. (Sch. I.).

24. " " L. 24A. Janosik's.

25. Coste's.

26. Schroeder van der Kolk's. ?

27. Hennig's. ?? (0. Schwabe's.) V 28. Remv's. Seventh Stage: Three external gill-clefts.

28 A. Chiarugi's.

20. W. His', Rf.

30. " M.

31. " BB.

32. " Lr.

33. Allen Thompson's No. III. ? 35. Ecker's.

? 30. Hecker's. ? 5. Beigel's (abnormal). JRighth Stage: Four external gill-clefts. ? 34. Von Baer's. ?

37. Johannes Miiller's. 38. R. Wagner's.

Descriptions of the Known Ova.

1. — Reichert's ovum, 73.1, was thought by him to l)e twelve or thirteen days old, and probably correctly so, as it was obtained at a post-mortem examination of a young German girl under circumstances which render the estimate of the age quite trustworthy. The ovimi itself was very imperfectly examined by Reichert, whose verv lengthy memoir deals largely with cognate subje(*ts and contains much speculative matter. The actual description of the ovum is brief (pp. 25-28) ; but as far an he wmit Rcichtrt worki'il with exejiiplary accuracy, which gives vahie to his nftesrcli. the nviiin in ijueKtion was a flattened spiiere with a short iHameterof :{.:! mm., imd mi etpiatorial diiuneterof 5.5 mm. ; smooth around both poles, and with a mai^nal or equatorial zone of villi separating the two smooth areas. The smaller iind flatter of these two areas faced the nterine wall and bore on its inner surface (/.('., within the ovum) a small accumulation of roundeil cells. The opposite area whs more convex. The villi were short (U.2mm.) thick cylinders with rounde<l ends and no branches. The walls of the vesicle consistetl only of epithelium, which also formed the simple hollow villi. The contents of the vesicle were; 1, The inner cell-mass lying, an before mentioned, at one pole; 2, A network of threads, apparently thG result of crmgulation of the contained fluid, for no nuclei were found in it. Kollmaim, 79. 1, i'.'i, thinks that Reichert 'a ovum must have had really two layers fonning the vesicular walls — an inner one mesodemi (young connective tissue) and an outer one of true epithelium ; further, that the true ejiithelitim had l)eeii lost, and that only the connective tissue remained, whicii Reichci't mistook for epithelium. Tiiis KUpi»>isition is, I think, not prol)ahlc. Reichert's i>vuni is presumably younger than any other hitherto descriljeil. and may have lieen in tho stage Ix^foit' the mesoderm had grown over the clKirion. Tile villi jiri' dewcril>e<l as hollow by Reichert — a statement not oiini)alii)lo with the sumxwition that he mistook a solid core of uii'sodtTin for the Imlluw shell of the ectoderm; we know now that yiimig villi usiially CDniain no mesoderm at first.

Z. lirfus" ovum, 77. 1. must l>e considered further advanced than lificlici-fs, altliuugh tho autluT fixes its age as piesnmably ten days. Till' totid diitnu'tcr of tho nvum incltiiling the villi was only 5 mm,, and as the villi wei-o tilxnit 1 mm. lung, the diiimeterof the elumonic vcsii-lf must have l>eeii alwiut -i nmi. The villi, some brunchcd, but umstly witiiimtbi-.mchc'S, were thick set, hut left one siKit lifdd, agreeing,- in this with Jimes' ovum (sea l)elow). The chorion was smooth on its iiiiiiT sin-fare, and consisted of (l) an outiT ciiithelial laj'er, and {i) an inner (inineetive- tissue layer which sent out extensions TKirlly tilling flic villi. The ovtnn containetl a thivady mass which lin-iis thinks was pi\)l«ihlv a product of OFit^ulati'm, and an inner i-i'll-inassaJMiut I mm. Iinigand o..". mm. wide. The pi-esciKv of villi and the exish'nce of liie inesmieniiiti layor "f the chm-inn, contrasteil with the alwcn<-i' of any cnibiyonic slmcture, led lireus to consider liis ovum alinormal. But it is rather the coiitrarv cimclnsion we must draw, sinrc all mn- kuuwUilge ]«iints to the dcdiictiun that, as conipaiii] with the fiidiryo. the dcvdopnii'iit ol' th.' chnrion is very prcfiK-ious ill maiiiitialia. I deem it, thei-cfore, jirclhalile that Hn'u« ovum was normal, and that the inner cell-mass he descril)e8 was in reality the embryo, compare Keibel's ovum.

3. Wharton Jones, o7. 1, long ago described briefly a human ovum, the chorion of which measures in his figure (said to be natural size) C by 4 mm. The following is all that can be gathered from Jones' description : The ovum was already covered by the decidua, and bore shagg}' villi on the side toward the uterus, while the other side was bald. The whole cavitv of the chorion was filled with a fine gelatinous cellular tissue, imbedded in which, toward one extremity of the ovum, was a small round body ; it was evidently the vesicular blastoderma. On l)eing taken and examined under the microscope, it presented the same friable, globular structure found in the vesicular blastoderma of the rabbit in the preceding observation. There was no vitellary membrane to be seen. " To judge from the minute figure given, the villi were already branched; in Reichert's ovum they were still simple.

4. Ahlf eld's ovum, 78.1, represents jHjrhaps the same age as Jones', but he does not give its diameter, which appeal's from incidental references to have been alx)ut 5 mm. The author's description is not exhaustive by any means, but ho mentions two points of great interest: first, the presence of a layer of connective tissue (mesoderm) underneatli the chorionic epithelium, and extending into but only partially filling the villi of the chorion ; second, the character of the villi, which are slightly branched and are constricted at the base, on)}" their tips touclie<l the surface of the decidua (reflexa and sorotina) . He also states that the epithelium of the villi precedes in its growth the connective tissue. This ovum was supposed to bo fourteen to sixteen days old (?). Owing to an accident, no observations of its internal contents were onade.

6, 6. Beigel's ovum, 78.1, of which he maintains that it is the third smallest known, is, if wo may judge from his plate, certainly abnormal to an extreme degree. I hold it to be a midformed ovum of the fifth or sixth week. The ovum described by Beigel and Lowe, 77.1, is of an even more questionable character. Moreover, their account is considered by Breus and Ahlfeld to be very inaccurate. It is noteworthy that Beigel and Lowe have also noticed the early presence of the mesoderm under the chorionic epithelium. L()we, 79. 1, defends himself against Ahlfeld 's attack, and insists with justice uixm the presence of (connective tissue on the inside of the cliorion in ova of the second and tliird week.

7, 8. Kollmann's memoir, 79. 1, is by far the most valuable which h<\d appeared up to the time of its publication u[X)n the stnicture of very young human ova. He describes two ova, a and />, both preserved in the anatomical collection at Basle. Ovum a had been placed in glycerin and water, which preserved the form of the specimen but ruined it liistologically; nothing was made out as to the contents of the chorionic vesicle. The vesicle itself measured 5.5 by 4.5 mm., and therefore was slightly flattened. This measure does not include the villi, which were from 1 to 1.2 nnn. long, and repeatedly brancluHl. Ovum fc, 5.5 mm. in diameter, was well preserved in alcohol; the villi were somewhat branched; the contents of the ovum were lost. On the other hand, the uterus belonging to this ovum was also preserved, and forms the basis of a very valuable description of the uterus in early pregnancy, to which I hope to recur on another occasion. Kollmann's two ova are both much more advanced than those of Reichert, Breus, and Jones, as is shown by their greater size and the branching of the villi. It is a matter of profound regret that only the chorion was left, but, fortunately, Kollmann has taken good advantage of his opportunity. His paper also gives an excellent critical analysis of nearly all the previous literature. He points out that the two primtive layers of the chorion are probably normally present at this stage. The chorion of his ova, he says, consists of " einer Lage jugendlichen, embryonalen Bindegewebes, das zahlreiche Rund- und Spindelzellen enthalt, und da8 bedeckt wird von einer einfachen Lage platter Zellen" (p. 203) . He then passes the literature in review, and insists strongly upon the fact that the two layers have been distinguished in nearly all the very young human ova known except Reichert's. Kollmann, therefore, as was mentioned above, questions, I think without sufBcient foundation, the accuracy of Reichert's account. Concerning the connective-tissue layer Kollmann says but little. As regards the epithelium, he points out that the nuclei occupy a basal position so that the outer parts of the cells form a granular stratmn, which some authors have considered a distinct membrane. The author supposes this granular stratum to become the cuticula described in later stages. Jassinsky, 67. 1, is the chief defender of the existence of a cuticula, which, however, he designates under the extraordinary name of tunica propria, extraordinary because the term is properly applied to the layer of connective tissue immediately uix)n which an epithelium rests. It is probable in the liglit of our present knowledge that KoUmann saw the outer darker layer found in Spec's oviun, see below, and in many others a little older. This outer layer is nucleated, but the nuclei might lx> overlooked. Finally Kollmann adds (p. 297 ff,) observations on the growth of the villi in ova of the fourth week. The outgrowth of ])ranches is very rapid, and occurs with every degree of participation of the connective tissue. There are two extremes: 1. A bud consisting wholly of epithelium, which may stretch out into a prcwess with a long thin i^Uicle and a thickened end, the whole remaining initil it has become quite large without any connective tissue. 2. A thick bud with a w(*ll<lt»veloped core of connective tissue ; such a bud probably grows out as a nearly cylindrical branch. Betw(.H?n those two extremes every intermediate state (•an be fijund. The various forms of growing branches may lie close together. Probably this complex mode of growth persists in older villi, which would explain the multiplicity of forms in the villous branches.

9. Schwalx>, 79.1, has described an ovum which ho considers thirteen to fifteen days old, but ho is certainly mistaken, sin(»o lK)tb. the data he gives as to the a^o jmd his account of the embryo shows that it is more advanced and belongs distinctly in the third week. In conn(»ction with Kollmann's observations wo must notice those of Ortii, 77.1, who has shown that at all a^os, ovon at full term, the villi of the chorion in the placenta havo i'pithelial buds, which an? at first hollow and arc afterward filknl up with a vascularized ingrowth of connective tissue. Apropos of this observation Orth discusses Boll's theory of growth, making the point that in this case the shaping of the parts depends primarily upon the growth of epitheliiim. Boll had maintained, as a general principle, that in the development of organs the shaping is dependent on the co-operation of the epithelial and connective tissues.

10. His' embryo, XLIV. (Bflf), described in his " Anat. menschlicher Embryonen," Heft II., pp. 32 and 87, belonged to a chorionic vesicle measuring 7 by 8 mm. ; the vesicle was somewhat flattened, and on one part had fewer villi than elsewhere; the villi were branched. Closely attached to the inner surface was a small body 1.4 mm. long in its greatest diameter; the body consisted apparently of a yolk-sac and closed amnion ; of the embryo no further description has yet been published.

11. The ovum described by Keibel, 90.1, consisted of a somewhat flattened chorionic vesicle more than half covered with little villi and containing a somewhat distorted embryo. The vesicle measured 8.5 by 7.75 by 6.0 mm. The villi were arranged in a band or zone leaving the two flattened poles of the ovum smooth as in Reichert's ovum ; the smooth areas were of very unequal size, at the edge of the smaller one the embryo was attached by means of its allantois-stalk to the inner surface of the chorion. The embryo, about 1 mm. long, was found twisted at it« hind end, which was continued as an allantois-stalk attached to the chorion ; the stalk was nearly or cjuite as large as the embryo proper ; the yolk was broadly attached along nearly the whole length of the embrj^o, and opposite the embryo the yolk-sac was attached to the chorion as if the coelom had not completely developed. Sections showed that there was no medullary groove yet formed, but the amnion was alreatly closed over the embryo. Keibel places his embryo as intermediate between His' embryo, XLIV., and Spec's embyo. Keibel's ovum resembled externally those of Reichert and Wharton Jones, and as it contained an embr\'(), ho suggests that it is probable that the ova of Reichert and Jones also contained an embrv'o without medullary groove, but with an allantoic-stalk nearly as large as the embryo, ut it seems tome that since KeiWs ovum is nearly twice as large, it cannot be of the same stage ; the presence of the e^iuatorial zone of villi is explainable as an instance of retarded development. The excessive variability of embrj^os is well known.

12. Sjx^e's embryo, 89.1, was contained in a chorionic vesicle measuring, including the villi, 8.5 by 10 by 0.5 mm. The tips of the villi were attached to the surface of the decidual capsule. The embr}'o was attached by a ver>' short allantoic-stiilk to the chorion, and was closely investoil by the amnion ; the attachment of the yolksac occupied nearly the entire length of the embryo, for the head-end had scarcely l)egun to ])roject; the embryo was 1.54 mm. long; its dorsal surface was occupied by the very broad medullary plate of thickened t^ctoclerm; as seen from above the plate seemed somewhat constricted in the middle of the embryo, owing to the arching of the body at that region; the centre of the plate showed a narrow longitudinal furrow, Fig. 1(J4, /; at the caudal end this furrow widened out and disappeared; just behind it Wiis the open and relatively

lari neurenteric canal behind which the short remnant of the primitive Htreak. The embryo was cut into transverse sections, of which there were about ISO, counting the allantois-stalk (Baitcksfiel) ;

section 81, counted from the head, is represented in Fig. 164; the ectoderm, ek\ is very much tliickened to constitute the medullary plate, Jfd,- the narrow central longitudinal furrow, /, mentioned above is ven- noticeable; outside of the embryo the ectoderm is reflected on to the amnion, rf, over the back of the enabrj-o. The entodenn, en, is a thin layer of cells in the centre of which the notochordal band can be distinguished ; in sections nearer

no. iM.—cmM-Secii'iD of 8p#r*H EmtHTu. ExpUcK- the neurenteric canal the UI.X1. bund is bettermavked, being there much thicker than the remaining enttxierm. The mesoderm,

»MR, in a distinct layer, although, as other sections show, it is fused in the median lino of the primitive streak behind the neurenteric canal with botli ectoderm and entoderm. The embryonic oelom has only just be^ui

to ap[>eur ti^f ii small fissure, p, but the extraembryonic ('(floin in com)ileted, so that outside the embrjo the me*5oderm is completely divided into a wjmatic leaf, ct, wliich heii)s fonn the Jimnion and chorion, and a splanchnic leaf, df, which forms one layer 'if tlio wall, of tlio yolk-sac. The sections througli the hca(l-en<l sliow that the lied ha<l grown forward far enough tif lead the

sei)arati(m of the very shoil vordcrdarm;

sections through the allantois--itfilk showed that the allantoic diverticulum extended a.s

a smsill canal through the great accumulation of mrMirKlenn; thnmghout the rest of

its extent the invhentenni is uiiwhciv differ Mitiatctl fn)m the yolk-sac. Fig. I iiri is a

section passing through the neurenteri

<-anal, which leads thniugh theci'ntreof the

meclullan' jiltitc' into the wide yi)lk-s;ic; the

]Mu-t of the s!i(T farthest from the emhrj-fi hiis

its mesiHlenn thickened and vaKCularized,

the ves.i(Js containing yttung bhuxl-cells

often in some stage of division. The chorion of S])C<f's embryo hail a layer of mew^

denn, with <-ells of a well-mnrke<l iu4.'S*(nchy mal tyi)e, and an miter layer of (H-tmlemi i

i<lilanclinn|ileur«:

a/i.t Ornf Siw.

«i sting i

■ity .If J

L thinner

outer layer darkly stained, without distinct cell iMjundaries. Imtwith

Small nuclei and an inner lighter layer of distinct cells with larger nuclei ; the ect<xlerm appeared somewhat as if ciliated. Unfortunately Spee gives no account of the villi beyond a few words to say that they resembleii those of later stages.

13. Wo come now to the embryos with a well-developed medullary groove* ; tlio number of these is four. Their probable age is about fourteen days. The least advanced is His' embryo E (" Anat. mensch. embryonen, " I. , Heft I. , p. 1 4o) , of which only His' sketches are available, the attempt to microtome the specimen not having been fortunate. The oviun was presumably normal; it measured 8.5 by 5.5 mm., and was entirely covered by short branching villi. For the convenience of the reader I have constructed from the author's sketches and descriptions the accompanying diagram. His states that the chorionic vesicle bore at one point a thick stalk, Al^ which ran to the i)osterior end of the i

Fia. l»r>.— Diaj^jam of His' Embryo E: Aee

I length about 2.3 mm. The

embryo is not fepratented in quite its natural attitude iMirate; the the proportion of the parts is not aclUi of the chorion and the Tessols on the yolk are purely dioicntmmatic as to their numl)er and siiape. Emh, Embryo; Al^ supIMise<l stalk of the allantoic.

embryo; the length of the embryo from the anterior extremity to the base of the stalk was 2.G mm. The head-end of the embryo was somewhat thickened, and api>arently showed the medullary groove still open. The small, round yolksac had a broad connection with fourteen (?) days; l^gthobout 2. S" mm.

the ventral surface of the embrvo.

The amnion sprang from the allantois and passed over the head of the embryo. The disposition of the caudal extremity was not made out. There were no limbs, gill clefts, nor organs of any kind discernible — not even a protuberance between the head and yolk-sac, such as marks the position of the heart in older embrvos.

14. Allen Thomson, 39. 1, published an excellent article on young human ova in IH'V.). He gives a very go<Ml critical review of what pn^vious autliors ha<l written, and descrilx*s himself three embryos, which have become classical, for the figures and descriptions given of them by Thomson have be<*n copieil again and again. They are especially known by th(; reprcxluctioiis in Kolliker's " Embr3-ologies," and in Quain's "Anatomy." Two uf these embryos (numbered I. and II. by Thomson) lx?long in the group we are now considering. I cannot, howc^ver, admit at present that either of them is certainly fullv nonnal, though |)erh{*j)S thev are onlv slightlv malformed. In nuiiil)er 1., (see KrJliker's *'Grundriss," 1884, Fig. 11^2, and **Entwickelungsgeschichte/' 1870, Fig. ^^o) the yolk-sac was abnormally dilattMl and the characteristics of the embryo were not ascertained. His ("Anat. Kmb." Heft II., pp. 35-3n) has showoi that the embryo pr()])er was not ob.served, and that what Thomson called the embryo was really only the amnion, springing fnmi the allantois-stalk and pa.ssing over the embryo. Kolliker questions the accuracy of this interpretation, but uixm what ground is not evident, for, so far as I can see, it accords perfectly with our present knowledge. The embryo in question was presumably little iidvanced beyond His' embryo E, Fig. 105, but had an abnormally hyj^ertrophied yolk-sac. As no sufficient description of the embryo exists, and as it is quite certain that the specimen was mure or less abnormal, it cannot ]ye longer regarded as a fair representative of a young ovum.

16. The third embryo of this group. His' SR (I., Heft I., 140-144) measured 2. '3 mm. in longtli, and was probably fourteen days old. The chorion was bv 8 mm. in diameter. It shows considerable advance of development lx?yond the three embryos above considered. The neck of the yolk-sac is already somewhat contracted, or, in other words, the connec»tion between the embryo and the yolk-siic is no longer so broad and long as it was. The head is considerably enlarged ; between it and the anterior wall of the yolk-sac is a large thickening corresponding to the heart. From the under side of the caudal extremity runs off the stalk of the allantois, which is still short and thick; the amnitm lies quite close to the embryo; the medullary ridges are still separated by an oi^en, though deep, and relatively narrow groove; myotomes (protovertebrsB, auct,) are j)resent, but their numlx^r was not ascertained. When the embryo is viewed in profile, the middle of the back shows a marked concavity which has bt^en noticeil in other older embryos, and is probably an artificial dist<jrtion. We shall have to return to this matter. Small ojieniiigs were visible on the inner surface of the chorion. These I take to 1k> the oj^enings to the still hollow villi, such as have been seen in both younger and older ova. His attempted to obtain sections of his si>ecimen, but when cut the sections fell into fragments. 16. Much more valuable is the account of Thomson's second ovum, which ho had Ix^tter opportunities of studying. The original description has been supplemented by His, '" Anat. Embry./' II., p. 34, who examined Thomson's original drawings, and called attention to an important error in the engraving in Thomson's plate. KoUiker, however, still reproduces the incorrect figure in the second edition of his '*Grundriss," Fig. 1 14. An erroneous figure is also reproduced in Ecker's "Icones," Taf. XXV., Fig. 3. The chorionic vesicle measured O.OO by 0.45 of an inch, and was covered with branching villi . The contained embr^^o was very small ; according to KoUiker, only '2.5 mm. The embrvo rested upon the round yolk-sac of 2.2 mm. The embryo consisted of two thick longitudinal ridges. Fig. UJT, A, which curved round in front so as to become continuous with one another, and were broken off post eriorlii — an imj)ortant fact noted by His (c/. sup.) . These ridges are presumably the medullary folds. At the hind-end of the embryo was a tc^ar, making a hole into the hollow yolk-sac. As His suggests, this is ]>r(>bably where the allantois was inserted and broken off. No amnion was observed.

Fig. Ifir.- Thoinsfiu's S^'oond Ovum. A, Embryo from alM>ve; B, embryo from behiuil.

• It must be reinem*)erp(l that tho term protij vertebra* is an entire misnomer, and is inherited from the time when the primitive muscular se^rmeuts (myotomes) were mistaken fur the oomuieucemeuts of the vertebra.

and the nature of the connection of the embryo with the chorion was not ascertained. What we leam from this embryo is simiething more definite than is afforded by His' observations as to the size and disposition of the medullary ridges and the hollowness of the yolk-sac. The apparent hypertrophy of the chorion enforces caution as to accepting the embryo as normal ; but it is not rare to find in abortions a small typical embryo with an enonnously dilated chorion, so that it is not impossible that the embryo in the present case was quite normal.

17. Spee has briefly described a second ovum, but his account is not now accessible to me. According to the notes given by Fr. Keibel, 90.1, 261, the chorionic vesicle measured 15x14x10 mm., the yolk-siic 3.5 mm. The embryo had seven myotomes, and its age in maximo was thirteen days.

18. J. Kollmanu, 89.1, 108-121, deacrilx>s an embryo of alxmt 2.2 mm. ; the yolk-Siic was attacheil to the embryo, Fig. los, for a distance of 1.5 mm., leaving the head to project 0.58 mm., the tail to }>roject 0.3 mm. The head is alre^idy somewhat enlarged and slightly bent over ventralward; it forms at least a third of the whole embryo; there were thirteen* primitive segments which marked themselves externally ; the segmented region of the lx>dy is l)ent so that its dorsal outline is concave; the mcnlullar}' groove is o[)en throughout the anterior two-thirds of its length, but the caudal third is closed ; the tail is slightly curled over, and is connected on its under side with a thick, short allantois-stalk, or Bauchstiel, by which the embryo is attached to the chorion ; there are no visceral or branchial arches, although the gill pouches may have begim forming in the pliarvmx ; no anlage of the eye or ear cr>uld l)e distinguished; the oral invagination has formed, but the oral plate (Rachenhaut) is still intact; the heart is not straight but an alread}' much l)ent tube, which receives at its hind end the two veins from the yolk-sac, which consists of vjiscularized mestxlerm and an ent(Hlennal lining. The amnion was a thin, transparent membrane springing from the Ixxly of the embryo close around the yolk-sac, and enveloj)ing the embryo very closely. The chorion formed a vesicle covered externally by branching villi; its diameter including the villi was 18. cm. Although the data were not very satisfactory^ Kollmann estimated the age of this specimen to be thirteen to fourteen days.

Fig. 1(W.— Human Eiiihrvo of Thirt«vu to Fourteen Days. -4 »i. Amnion: S. 7, fleveul f, medullary i^nxtve, still ui)cu; 7if, heart; Yk.9^ yolk-sac; .4/, allantois-Rtalk

KoUmann fleveuth Regmnent ; After J.

• The ili^re shows fifteen Hegmenta.

19. The description of the ovimi of thirteen to fourteen days by Von Baer, 88.1, was drawn up over sixty years ago. The ovum measured a little over three hues, and was covered with villi ; the embryo was about two-thirds of a lino long ; Von Baer appears to have recognized the amnion and yolk-sac and to have seen tiie allantoic-stalk (his Hamsack)^ though he did not observe its connection with the chorion ; as he states that the back was already formed, it is probable that the medullary groove was closed. It is with much hesitation that I place the embryo here in the series.

There has been, so far as I am aware, no human embryo with one gill-cleft described, imless, indeed, Coste's embryo described below was such. But several with two clefts markeil externally have been described, most of them by His. Those of them which can be assumed to be normal present a remarkable bend in the back or dorsal fiexure by which their shape is so much altered from that of the slightly younger stage, and so unlike that of the next older stage, that the embrj^os with the dorsal flexure might be considered abnormal had we not positive reasons to the contrary. Indeed it seems probable that embryos in this stage may have been, because assumed to be abnormal, discarded. His' embryo L, described below, and perhaps Coste's, p. 300, both probably belong in this stage and were artificially straightene<l out. Nothing similar to the dorsal flexure of the human embr>'o hiis been observed in any other vertebrate, though it may occur in apes and monkey's.

20, 21. Two specimens in my collection are in this stage. The younger of these is represented in Fig. 1 01), and is ver}' near the embryo desig^nated as L,g bj- His; just behind the heart the whole body bends downwartl and then bends abruptly upward, so that the caudal end of the embryo runs nearly at right angles to the pharyngeal region; from the under side of the tail end nms off the thick allantoic-stalk by which the embryo was attiiched to the chorion. The other features observed are shown in the figures. Sections showed that the specimen was imperfectly preserved, and I cannot he sure that it was entirely normal in shape, though it differs but little from the certainly normal embryos of His. My second specimen (Coll. No. 14:1) is a little older, I think, hut as it is somewhat distorted, it is hardly worth figuring and describing separately.

Fig. 100.— Embryo of the Beirinning of Third Week (Minot Coll., No. 196)

All^ Allantois; Am, amnion; 5r, branchial region; //, fore-brain; Hr, heart; Ffc, yolk.

23, 23. Far better preserved are the two embryos of His, which he has studied with such splendid thoroughness. He designates them as Lg (orLXVIII.) and Sch. 1. (or LXVI.), Fig. 17, p. 39. They resemble one another very closely, the most marked differences l>eing that in Sch the heart is more exposed and the neck of the yolksac more constricted than in Lg. Lg measured 2.15 mm. ; Sch, 2.20 mm. The differences noted indicate that the latter is slightly more advanced. The following description applies especially to Lg. In external form the embryo is very similar to Minot's Fig. 109, but no trace of a third gill-cleft was visible externally, and the amnion w^ attached along nearly the entire length of the allantois- stalk (His' BauchstieJ) . The anatomy can be understood from the accompanying Fig. 170. The head bend-being well mnrke<l, the central r system makes at the mid-brain, a bend at nearly a right angle, so that the fore brain is brought verj' near the heart which lies in the large pericardial sac, which protrudes conspicuously between the head of the embyro and the yolk sac. Between the head and the pen cardial sac is situated the oral invagi nation or future mouth cavity, separated from the vorderdarm hy an intact oral Plate (Rachenhaut) o.pl. As regard-the arehenteron we find the vorderdarm above the heart, Ht, with two giil }K>uche9 formed at its head-end and itw lower endwideneil; out of this wider part the lungs and the stomach are t > be differentiated in later stjiges the vorderdarm is compresseii dorsi> \cii trally but widel j' expanded transverseh the middle portion of the arehenteron oi)ens widely into the yolk-sac; where the vorderdarm joins this middle division is foimd the outgrowth of the liver, hi, extending toward the heart; in the posterior region of the embryo the arehenteron has also become distinct from the yolk-siic and ends with a dilatation (His" burita) in the tail of the embryo ; from the under side of the bursa runs out the allantoic diverticulum, AU, which extends as a narrow tube of entoderm through the allantoic stalk to the level of the chorion where it ends blindly. The central nen,-ou3 system forms in bulk a ver>- large part of the embrj-o ; from the f<ire-brain the optic vesicles. Op, have grown nut ; the mid-brain is only slightly dilatetl ; the hindbrain is as long as the mid- and fore-brain together, and is nearly as long as the vorderdarm, which it overlies ; near the centre of the hindbrain lies the open ectxxiemial invagination, Of, destined to form the auditory vesicle or otocyst ; the remainder of the medullary canal corresponds to the future spinal cord and gradually tapers tail ward ; alongside it His was able to distinguish in Lg twenty-nine myotomes. The heart, Hfj is very largely and asymmetrically bent ; the heart at this stage and for some time later may be described as consisting of two tubes, a small inner one, Hf, formed of endothelial cells, and a larger outer one formed chiefly of contractile elements, which are graducdly differentiated into the striated muscles of the adult heart. The way in which the heart is bent can be best seen in front views ; the great veins enter the heart in the median line just above the liver; the heart tube runs toward the head and the left side, making the auricular limbs ; then the tute bends to the ventral side and runs obliquely backward to the right side, making the ventricular limb, and finally tkkes a curving course as indicated in the figures to the median line, and ends close behind the mouth; this third part is the aortic limb. The endothelial heart tube is continued l)eyon(l the pericardial cavity as the aorta, which soon divides into two branches on each side, which pass up around the pharynx, one branch in front of each gillcleft; the front branch curves over, and, passing tailward, joins the second branch; the branches which pfiss aroinid the pharynx are known as the aortic arches ; the united vessels run toward the tail on the dorsal side of the phamyx ; they are called the dorsal aortse, and by uniting in the median line form the single dorsal aorta, which runs away back nearly to the tail of the embryo, where it forks, and its branches, passing one on eacli side of the intestinal canal, enter the allantois-stalk and run to the chorion, where they branch out. The veins of the embryo are tht jugular, which comes from the head and meets cardinal vein from the rump about at the level of the liver; these two veins unite as a short stem, which runs transversely toward the venous end of the heart and is termed the ductus Cuvieri; the ductus is joined, as in adult fishes, by the omphalomesaraic vein, Ofn coming on the same side from the 3'olk-sac, and the umbilical vein, it.r, coming from the allantois; the four united veins meet their fellows from the opposite side and form with them the median shius reimie)is, which communicates d i recti v with the heart ; the course of the umbilical vein is curious, as it takes a short cut from the allantois through the somatopleure along the base of the amnion to the heart ; how this course is possible can l)e understood by comparing figures 17 and 100.

Fig. iro.— Ilunmn RtoIhto of £.19 mm ; Analomy Rpt'tiiiittnH't'r'd frcimthe SFutintw. 1^. Ojitk'voik'lB: v.pl. oral IiIsm: 111. ••wlolbrilal heart 1 Li, livi>r; Oil. omplialo-i

24. We pass now to His' embr^-o L, and Coste's youngest embrvo. It must be seriouslv doubted whether either of these embrvos represent the normal sha|x^. The former had two gill-slits and parts of it were torn awav, so that we mav surmise that it had had the dorsal flexure but was artificially straightened . Concerning Coste's embryo see the next paragraph. His' embryo L is described in his ** Anat. menschl. Embryonen," Heft I., pp. \^!^>-\'M). It measured 2.4 mm. in length, and was obtained from a chorionic vesica* of 8 to ^J nun. diameter. The specimen had been considerably injurcnl, and no exact knowledge could l)e obtained in regard to the heart or the disposition of the allantois or the amnion. Precisely these three points are elucidated by CoBte, while His liaB worked out the internal anatomy o£ his specimen ; in short, the two descriptions complement one another ill a remarkable manner. Nearly all that His ascertained is represented in the accompanying illustrations. Fig. 171. A gives a side view showing the thickening of the head-end and the upward curving of Hxe tail, and the two gill-slits in the cervical region ; the moutb, Jtf, is very large; between it and the first gill-stit intervenes the thick ridge, Mx, of the first gill arch (branchial or visceral arch, auct.), which becomes tlio mandible; between the two slits is the second or hyoidean arch, in connection with which the hyoid bone afterward arises. A lai^e body cavity is present, C, Coe; the walls of the body (soniatopleures) pass over along an extended line into the amnion; tiie i-onnoction between the embryo and the yolk-sac is ^ready much restricted comparetl with Coste's embryo. Fig. 1T3; at the side of the head a line and shadow inark the position of the optic vesicle. B is a ventral view; it shows the large wide month, M, which, according to His, was apparently in communication with the intestinal canal, which is nothing but a straight tube with a great pharyngeal dilatation, and a wide open union with the yolk-sac ; the median light band shou-n at the back of the mouth is the central nervous system shining through the covering tissue. C is intended to show the digestive tract, and is partly a horizontal section. Esjieciatly to be noticed is the enormous size of the pharynx (the region of the branchial arches), the straight, short intestine, and on each side of the latter the distinct Ixxly-cavity, Coe; there are indicationa of four visceral arches, 3!x, -, J, and 4; in front of the pharynx is shown the ventral surface of the fore-brain or first cerebral vesicle, with its lateral diverticula, the optic vesicles. D is a dorsal view of the brain and medullary c€uial which is still open at a. The brain and spinal cord are already differentiated by the dilatation of the former. The brain subdivides very early in all vertebrate embryos into three dilatations or primary vesicles; but in this embryo the two anterior dilatations are not yet clearly separated from one another, hence there is only one widening of the brain in front ; the front end is seen to bend downward and give off the conspicuous optic vesicles, Op^ which, therefore, arise before there is any trace of the cerebral hemisphere — an important fact ; the posterior and larger dilatation is the primitive medulla oblongata; no trace of the cerebellum has appeared. The whole nervous system is a tube the walls of which are of nearly uniform thickness, except that the dorsal wall of the third vesicle (the cavity of which becomes the foxirth ventricle of the adult) is very thin. This thin wall is persistent in the adult and never develops into nervous substance. On each side of the medulla lies a roimd cyst, the auditory sac, AUy the beginning of the adult membranous labj^rinth. Three other points not shown in the figures remain to be noticed. 1. In the tissue at the back of each bodycavity, CoSj was found a single longitudinal epithelial canal, the Wolffian duct, the first part of the urogenital apparatus to be developed. 2. Close below the nervous system lay a median rod of cells with a small central cavity ; this rod is the notochord or chorda dorsaliSj the primitive embryonic axis around which the vertebrae are formed later. 3. All the tissues are still embryonic — that is, the cells are not yet differentiated into tissues. Unfortunately, the number and disposition of the myotomes were not ascertained.

, hyold «rch: I'd, tot derd«rm; K, Bpliknchrtopteura of thn jolk-fuc; V. S, and 4, k^W nrchn: COf, ciptom or prinil. (Irt> body-cavjiy : Op, o^ilic Teslclei ^u, auditory vvsk-lu (olocyst); u. iwlnc vhere this meilulIwy (troove has not yet cloned.

24 A. Janosik, 87.1, describes an embryo with two gill pouches and three aortic arches, giving a few anatomical details.

26. Coste's embryo has been beautifully figured in his great work, 47.1- It is possible that it really belongs to an older stage with the dorsal bend, compare Fig. 169, and that it was stretched out by Coste ; the difficulty of assigning it its place is due to the entire uncertainty as to its actual dimensions. Coste's private collection is, I believe, now in the College of France, but upon search this particular specimen could not be found, so that His' inquiries to ascertain its actual length were resultless. KoUiker states that it was 4.4 mm. long, but his authority for the statement is not given; the measure was probably taken from Coste's figure, *'^ grandeur naturelle,^^ Since embryos of this length are far more developed than Coste's, it is probable that Coste's data as to the magnification of his figures are inaccurate. If we assume the embrv'O to liave l^een really about 2.5 mm., it will then agr(»o, except as to the great length of the rump, very closely with what we know otherwise of such young embryos. I give the accompanying figures, which are careful copies from the original plates published by Coste (4 "Espece humaine," PI. II.), whose illustrations, made by his assistant, Oerlie, have never been surpassed for beauty and life-like accuracy. The embryo in question was inclosed in a villous chorion. Fig. 172, and was provided with a large vitelline sac, T7, having a very broad connection with the embrvo and covered with a network of vessels, in which was a fluid nut yot rt*<l. A thick nlliiiitois-sbilk, Al, can l>e m-eii rumiing inun tile under fide of the embryo's tail t<> t!ie churion; fi\ini ihe anterior side of tiie stalk sprin^^ the amnion. Am, completely inclos

mti e 11 it 111 this I-, in htill

nil n IS tsi ntialh the biimo

hilt, if ittathmtnt f tht un

a ud iruiiiid tlic (.nibi ^ o iil>out

on a line with the top of the yolk. As r^ards the emhryo, it is drawn slightly canted on to its left side; its back is concETe; the head-end is thickest and shows three gill-arches, hence there were

probable two branchial clefts; behind and beuw the gill-clefts cau be seen the heart, already' a bent tube, shinini; through ; behind the arches again, but on the dorsal side, the light-looking cesophagus is distinguishable; in ttie figure a wedge-shaped shadow intervenes between the straight oesophagus and the bent heart; the heart causes a conspicuous bulging of the body l)etween the head and Iho yolk-sac; the caudal extremity i ^ thick and rounded, and curves upward. Fig. ITS is a ventral view of the same embryo after most of the j'olk-sac has been cut off; its walls, Spl (splanchnopleure) , are seen to pass over without any break into those of the intestinalcavity. Inthecentral line thechorda dorsalis, s, can be percoivixl through the translucent dorsal wall of the intestinal cavity; it is flanked on each side by the row of square muscular segments (myotomes). We see the large allantois, Al, behind, and in front the tubular heart, i7(, with a decided flexure to the right of the embryo; the anterior end of the heart makes an opposite bend separating off a limb, which becomes the bulbus I Blip- aortce. The chorion consisted of two membranes one of which passes con

mmea. Am. Amm"^ III, tinuously o\er the inner

^toJiiif'^""^?SndT£5*^'^l 8»rfice of the chorion

brjotu fonn the yoit-nac; while the other outer mem

Sw'^mT^'S^ii'on'Sch brane alone forms the hoi

•id*. ji,»iaikofth^aii«.- low villi. Figs 1 1 and 170; hence, m looking at

the inside of the chorion, we see numerous

round openings which do not penetrate the m

ner membrane. Fortunately we le im from

Kollikor {" Entwickelnngsgeschichte IS" I p

30ft) who had an opportunity in ISf 1 to e\am

ine the chorion, that the outer mtmbran©Ma.s

epithelial with cells of the same character as in !

the epithelium of older vascularizeil villi,* and

that the inner layer consisteil of developing flSr.i'^i' w)i,','i'y"[e'i.'itb^

connective tissue, and carrieti fine blood-vessels. "'"■

It thus appears that Coste was the first to obser^'e the vCAv of thu epithelium in the growth of the villi.

26, 37, 28. It will ho as well to mention here, rather than later,

• TlWln-iZclfttPi'li-ll.das'i^i^Zotl'-numl dtp sr«lr.ni.-i'n.l-lrniili:nTi/ unci car niiM.'pfttiplam

three descriptions of young embryos, which either l)elong in this stage or are a little older. Of these descriptions Remy 's alone brings much of any positive information, but the size and age of his embryo can only be guessed at. The first of the embryos is Schroder van der Kolk's (51.1, p. 100 jf., with figures on PI. II.). Kolk's figures are not very clear. He states that his specimen had two gill-clefts and measured 1.8 mm. in length; one can but ask, Was it not really larger? Kolk's figure suggests that the specimen was doubled up; if this was the case^ the embryo, when straightened out, Avould agree fairly well with His' embryo L, above described. Professor His, for reasons not clear to me, considers Kolk's specimen as somewhat older, but to this opinion I am unwilling to accede. The second embryo is that of Hennig, whose description, 73.1, leaves very much, and whose figures leave everything to be desired. From this paper we can gather very little, except confirmation of Coste's statements in regard to, (1) the disposition of the amnion and its connection with the stalk of the allantois; ('2) the absence of a yolk-stalk. Schwabe's, 79. 1, embryo, to which reference has already been made, and which he assumes to be thirteen to fifteen days old, was probably sixteen to twent}' days old, as shoAvn botn by his own data and by the description of the ovum. Very likely it was a little younger than Coste's embryo, v, sup. There were a well-developed yolk-sac and an amnion closely investing the embryo, which was connected with the chorion by a short allantoic stem. The chorionic villi were considerably branched and entirely filled with mesoderm ; their tips had little thickenings of the epithelium bj- whi(;h they were attached to the decidua • this was the onlv connection between the foetal and maternal tissues. This last fact is an interesting confirmation of the observations of Ahlfeld and Langhans. Remy's embryo, 80. 1, was also a young one, but its exact age is not stateti, nor are the measures of its length given except in the title, where it is called " long d'un centimetre." From the stage of development, and from the statement in the text that the chorionic cavity measured 20x10 mm., it seems impossible that the embryo was so large ; we should rather expect an embryo of 3 mm. Remy's figure is too inexact for one to make out the form of the embryo. If he gives the length correctly, the specimen must have l)een a month old. As to its stnicure, Remy gives the following detiiils: The medullary canal was still united with the ectoderm at its lower end, and extensively so over the fourth ventricle, which was entirelv closed. The heart alreadj' had muscular strije. The epid(*rmis had two layers of cells, the outer somewhat flattened, the inner culx)idal. The cutis was not differentiated. The epithelium of the (chorion he describes as maternal — a common error. He also distinguished t\w inner membrane of the (*horioii, the allantoic. He has also seen, apparently, what is known as Langhans' cellular layer, but has taken it for a deep portion of the epitli(»lium, wlnrli he ac(*ordingly calls many-layered. The sta^(> with thrc^e gill-clc^fts is known through five embryos, four of which have b(^on studied by His, and In^long to the end of tliis st^ige, since in all, except one (Rf), of which we have nodetaile<l (les(»ription, the fourtli gill-pouch of the pharynx was partly formed, and in all there were five aortic arches. The fifth embrvo is described by Chiarugi, and had three gill-clefts and three aortic arches; it therefore belongs to the beginning of this stage.

28 A. Chiarugi's embr}^o, 88. 1, had a very marked dorsal Qezure {insenatwva Jorsale); its greatest length was 2.6 mm.; its chorionic vesicle measured 15x12x8 mm. ; the villi were much longer (1.5 mm. ) than upon the other. The embryo had three gill-clefts showing" externally, and unlike the two embryos of His, BB, Lr, only three internal gill-pouches and three aortic arches; the otocyst was closed but still comiected with the ectoderm ; the yolk-sac had a broad connection with embryo, and measured in vertical diameter 1.0 nun. ; in transverse, 1.8 mm.; in antero-posterior, 1.0 mm. These points show that the embryo Avas intermediate between His' L and M. In Chiarugi's S|)ecimen the WolflBan bodies had become protuberant; the cephalic and spinal ganglia were present, but the spinal motor roots Avere not developed ; the notochord measured 30/^ in transverse, 24/4 in dorso- ventral diameter, and its ciiudal termination was indistinct. Chianigi gives a full and admirable description of all the parts, but as in the respects not specitUly mentioned above, the structure is very similar to that of other embryos witli three gillclefts, further details may l)e omitted.

29—32. The four embryos with three gill-clefts described by His have been designated by him as Rf ; M, Fig. 175 — BB, and Lr, Fig. 16 — they being named in the presumable order of development, il and Lr are probabl}^ the most perfect ; Rf is somewhat rolled up; BB has a distinct dorsal flexure, but, as His himself remarks, this was probiibly due to a media n i c a 1 strain and is artificial; hence wo may assume that in all embryos of this stage the doi*sal flexure has disappeared and the back has bec o ni e (Convex. The four ombrvos are doscriln^d and flgured in His' **Anatomie menschlicluT Embrvoneii/' Heft I. -111." Of ^l a systematic anatomical description is given (Heft I., 1 Od-l :i4) , and additional detiiils concerning BB and Lr are scattered through Heft II I. The lengths are : M, 2.0 mm. ; BB, :{.*2mm. ; Lr, 4.'^ mm.; Rf Inking rolled up could not Ik? measured satisfac^torilv. The chorionic vesicle of M measured 7.")xH.() mm. ; of BB, 1 IxU mm. From the data given by His, the age of BB may be estimated at probably twenty to twenty-one days.

Fig. 175. -His" finl.ryn M.

The head is bent down, the back very convex, and the caudal extremity is rolled up and turned toward the right — in Lr, however, to the left — while the lieiid is twisted slightly toward the left ; the long axis of the body, therefore, describes a large segment of a spiral revolution; the sniral foi-m is more marked in embryos a little older ; it is, of course, produced by the more rapid growth of one side; in view of the differences between right and left in the adult, it is very interesting to find differences between symmetrical parta showing so very early in the heart of the embryo niid the twisting of the body. The caudal end of the body has grown very much; the allantois stalk baa presumably lengtheneil ; the neck of the yolk-sac is much constricted ; the gillclefts can be distinguished externally; the otocyst. Fig. 178, of, bus become somewhat pear-shaped. The neural canal is completely closed; the mid-brain and fore-brain have become jwrfectly distinct, and the latter has h^un tu form the hemisitbores in fn>nt. The mouth is 1 ii^e and at its upper comer the protuberance of the maxillary process i

IS marketl the mandibular process is very prominent Fig 1 r, a geometrical reconstruction from the sections, shows the anatomy of the entodermic canal The phar\ nx bounded on eacli side by four branchial arches, IS still ver\ large uid tapers down posteriorly; the intestine in turned to the left and ojiens into the broad canal, J Yks, of the yolk-sa<!; just in front of the yolk-sac there is a small ventral diverticuhmi, L,i., the commencement of the liver; behind the yolk-sac the cylindrical intestine runs over into the tail, where it expands w ^ ^ d f into the bursa of His, and gives off a cylin £^ *C/ } drical canal, which has vorj' thick connective-tiasue walls, and is the allantoic-stalk,

Al, which carries the two allantoic veins and the two large allantoic arteries, Fig. IT**. Fig. IT" gives a view of the anterior wall of tlio ])harynx of BB ; in front is the largo opening of the moutli, 3/, the oral plato between the mouth cavity and the vorderdarm having disappeared ; the wide

[)harj-nx sliowa foxir gill-pouclics, and at its _ ower end gradually c^jntracts and passes

owihaKiuTor "aii'niaru c into the narrow cesophagus. The aortic "* " vessels aiv indicated by dotted lines; the

t irdiaL aorta readies the pharynx betwi-en the bases of the second and thir<l gill arches ind livides intotwo l)ranclies<ineacliside; the aot«ri 1 1 rinth f rks and luns tbrougli the fii-st and second arches; the

Fo I Mbr r Wall of thn

Pharrux u( IIU Liiil -> BB t ma luDK 1-4, TlwKllanli>v ■«ch uonUlnlnjc an lu n at si nn by HoUnA 1 a -k (If b

posterior branch forks, one fork going to the third, and the ot^r lifter again forkingsnpplim the fourth and fifth arches; this arrangement of the aorta is typical. Between the bfvues of the first and second arches is a email protuberance which is the nnlage of the tongue, and is named by His the fuberculum inpar. the bodycavity of the abdomen lias on each side of its dorsal surface a longitudinal ridge, the commencement of the Wolffian body; the ridge already cimtains traces of the canals of the Wolffian body. Of special interest is the arrangement of the circulatorj' apparatus. Fig, IT**. In the figure the arteries are °?' shade*] dark. The heart is an Sshapeiltubc, the venous end is convex toward the bead, the artcriid end convex toward the tail ; when viewed from in fi'ont the vencnis portion is si-en on the left, Fig. liO, the arterial jxirtion on the right of the embryo. The heart in continued forward by the lai^ aurtii, Ao, which gives off five branches on each side of the neck; these branches unite agiiiit nn the dorsal side and run backward to iinito with the fellow -stt-m, and so form the single median dorsjil aorta, Ao, Mhich runs way back and terminates in two large branches. Fig. 17 S, An, which curving nnmd pass out through the nllantois-stalk. The five hriuii-hes in the neck are known as the aortic arches, and the column of tiswut> nii)und each branch constitutes a so-calle<l branchial oi' visceral arch; l)t>tween the five arches are four spaces, in each of which a gill-cleft is ultimately fonne<I. the i-eoonstruction of Lr in a siilc view, Fig. 180, affords further information concerning the disposition of the heart and lai^ b]o<)d- vessels. the veins, as is there shown, are, 1, the jugular, J, and ciirdinal, car; which unite and f^irin ti single transverse stem, the ductus Cuvieri. D. C; the ciirdmal veins receive chiefly the blood from the WolfRan bodies and atrophy later with those bodies; 2, the large umbilical veins which pass up, Al.v. from the allantois and also open into the ducti Cuvieri, but nearer the heart than the jugulars and cardinals; 3, the omphalo-mesaraic veins, Om, which come up from the yolk-sac. More precise details of the course of the veins through the r^on of the liver will be found in Chapter XXIX. The conformation of the body-cavity

(splanchnoctrle) can bo better considered in connection with the history of the septum fransversum, Chapter XXII.

33-36. Of other embryos about the stag© of those described in the preceding pages several are known. [is has referred the following to this stage :

1. Allen Thomson's o\'um III. (2), 89.1.

2. C. E. von Baer'a described in his " Entwickolungsgeschichte," B<1. II., 3(il-3i;:i, Taf. VI., Figs. 15-19; alsoin VonSiebold's Journal far Geburtshulfe (1834), XIV., 40n.

3. Schroeder van der Kolk's (6), 51.1.

4. Alexander Ecker's (0) 73.1.

5. Prof. Heckcr's {vide infra) . G. Beigel's {/(Je ih/z-u). 7. Bruch's (in). Of these Thomson's embro, I

the figure of which reduced in scale i may be found in His ("' Anat. J menschi. Embrvonen," Heft H., \ Fig. It*, p. 33, marked A. T.l!), is the only one deserving much attention. Thomson's embryo resembles His's M (see liolow) quite closely, not only in genera] form but also in the possession of distinct gill-cIefts and the great prominence of the heart. Its length is given by Thomson at oneeighth of an inch, about 3 mm. Von Baer's embrjo, on the contrary, was only 2 mm. long; it was surrounded by an amnion of about 4.5 mm. diameter, which is abnormally large; Von Baer obacrvwl four o|)en gill-slits; the hind end of the body was partially atrophied, which accounts for the short length. Van der Kolk's embryo, as I have already stated, I refer not to this but to the ppeviotis stage, |»erhaiis mistakenly, but I think not. In Ecker's ovum the chorion measureil 13 by n mm., and the embryo only 2 mm; the author's description is very meagre and his figures not distinct; Ecker expressly compares it with an ovum of Wagner's, figured in Wagner's "Tconcs Physiologicse," and again in Ecker's "Icones Physiologica?," Taf. XXV., Fig. V. ; but the compariwm apparentlyrefers only to the chorion, for Wagner's embryo was evidently older, being 4.5 mm. long and having external traces of limbs. Hecker's ovum (5) I know only through Prof. His' reference, Avhich leaves the impression that Hecker's description is so unsatisfactory as to render it a matter of surmise exactly what stage of development the specimen had reached. In regard to Beigel's ovum I have already expressed, p. 280, my opinion that it is a much older and abnonnal embryo; I do not differ from Prof. His as to the slight value attaching to Beigel's description. Bnich's embryo (Abh.Senck.Ges. VI.,Taf. X. [40] ) appears to me from his description and plate to have been verj' abnormal. Of these seven embryos Kolk's and Beigel's do not belong to this stage; Von Baer's and Bruch's were abnormal ; Hecker's is questionable, Eck(T's somewhat uncertain, and Thomson's the only satisfactory one. Of Thomson's only the general appearance is described, but that confirms Avhat we learn from His' observations on this stiige.

37, 38. Of embryos with four gill-clefts Ave possess no satisfactorydescriptions, unless, indet^d, Ave regard His' embryo Lr, descril^ aboA^e, as l)elonging to this stage, since the fourth pharjTigeal gillpouch is found in it. To this stage may perhaps l)0 assigned the embrvo described by Johannes lltiller (" PhA^siologie, " 4to Aufl.,Bd. II., 713, Taf., iiuil in MuUer's Archir, 1834, p. 8, and 1830, p. clxA'ii), and also Wagner's embrA'o (AVagner's ** Icones PhA'siol.,'* Taf. VIII., Figs. 2 and 3, also in Ecker's *' Icones Physiol.^" Taf. XXV., Fig. 5); important critical remarks on these two embryos may bo found in His' ** Anat. menschlicher EmbrA'onen," Heft 1, 102-1(13, and Heft II., 41-43). Miiller's embryo Avas about 5.5 mm. alone, Wagner's 4.5 mm. They both had a marked dorsal flexure resembling that nonnally present in embryos Avitli tAA'o gill-cleft*^ ; but this flexure was probably produced artificially by a strain upon the yolk-sac pulling the back doAvn ; the flexure is to bo regarded as artificial, l)ecauso in embryos Avhicli Avere certainly normal it was not found in the next 3'ounger or the next older stage. How easily the flexure may be produced is shoAvn by His' observation of its occurring in his embrA'o W, Avhile he Avas manipulating it. Neither of the two embryos under consideration fire des(Til)ed or figured AA'ith sufficient accuracy of detail to justify a fuller description of them. As Von Baer states of his embryo, 34, that it had four clefts, it must })e held to Ix^long in this stage proba])ly.

Summary. Known Young Human Ova. — The detailed descriptions of the preceding pages, 287 to 308, are summarized in the folloAving paragraphs.

First Stage: Primitive Streak.— No human oa-uui has been obserA'cd to haA^e a primitiA'e streak, but there are several Avhich are younger than the embrs'os with open medullary grooA'(% and therefore ])resumably are in this stage; unfortunately there is a satisfactory d(^scription of the embrA'onic structiu'es in no one of them. To this group liaA^e \yeen assigned the embryos 1 to I), ])ut of these Beigel's (5) is certainly abnormal, and Sehwabe's ('.») is probably both abnormal and much older. From the preceding review of the remaining seven ova the following conclusions may Ih) drawn : The human ovum by the twelfth or thirteenth day is a rounded, somewhat flattened sac of three to four millimetres in diameter, l)earing an etjuatorial zone of short unbranchod villi; the villi are probably formed by the ectoderm only ; the wall of the Siic is ectoderm, whether underlaid by somatic mesotlerm or not is uncertain; to the inner Avail of the sac over one of the bare i)oles of the ovum is attached a mass of cells, constituting the anhigo of the embryo; as to the arrangement of these cells we possess no knowledge.

• For UofluitiouM of ihe staffs, i-uro p. A*, onh.

In the next stage the villi have spread over the gomninal area and have l^ecome slightly branched; the villi next appe^ir over the opj)osito ix)le of the ovum and rapidly increase their length and ramifications. The germinal area faces the uterine Avail (Jones' oAnmi, 3). By the time villi are present over the Avhole Acsiclo there is probably alAA'ays a layer of connecti\'^e tissue underlying the epithelium (Breus 2, Ahfeld 4, Lchve 5, etc.), but no embryonic structures haA'e been recognized. The ova of twelve to fourteen days are already completely inclosed by tho decudua (reflexa and serotina) ; only the tips of the Anlli adhere to, or are even in contact with, the decidua; this is the only connection between the maternal and fcetal tissue, for neither d(x»s tho uterine mucosa grow in l)t»twet*n the a* illi, nor do the A'illi |)enetrate the cavities of tho uterine glands. The einthelium of tho cliorion and A'illi is only imixurfectly marked Avith boundaries for tho single cells ; its nuclei all occuj)y a biisal j)ositi«m, leaving a distinct outer layer, often mistaken for a separate structure. The epithelium forms buds Avhich Income branches of the A'illi. These buds may grow out to a considerable size Avithout conne(»tiA'e tissue (lioUoAv A^illi), or the connective tissue may penetrate into them from the start (solid A'illi). The human oA'um, then, is ri»markable for its precocious deA'elopment of tho chorion, lx)th as n^gards the A'illi and the connectiA'o tissue or mesodermic LiA'cr, and for its earl a* complete enca|)sulation by tin* decidua. All these CA'^ents (according to tho scanty observations yet made) prectnle the api)eanuice of the embryo. It is also noteworthy that tho villi are first deA'eloi)e<l arovmd the etpiator, next OA't»r the germinal area l)ole, and last (jA'er the area of tho opix)site ix)le.

Second Stage: Medullary Plate. — To this stage I as.sign the einl)r\'os. His' XLIV. or Bfi: (10), KeiU-rs, 11, and Speed's, l->, and I think they Ijolong in the order named. The chorionic A'l^sicle is rounded and somoAvhat flattened ; in its greatest diameter it measin*es 8-10 mm.; it is lx>set Avith short branching villi, Avhich are present oA'cr tin* entire surface except in one case, Avhere they formed an e<iuatorial baiul as in lleichert's OA'uni 1. The chorion had a distinct e<.*todernial and a distinct mesodermal layer; the former, at lt»ast in Sjx.^e's embryo, had tAVo strata of cells, as is characteristic of the chorion. To tho inner surface of the chorion Avas attached a thick allantois-stalk {BanchsHel)^ Avhich, curving slightly, passeil oA'er Avithout anv demarcatit)n into the ombrA'o, AA'hich in Keil)ers oA'um measured about I mm, in Sj)ee's alxnit l.r> mm. From the sides of tho allantois-stalk and of tho embryo sprang the thin amnion, Avhich Avas completely ilosed. Along n(»arly the entire length of the ventral surface of the embryo was attached the yolk-sac, which was of rounded form and al)out equal in diameter to the length of the embryo; in Keibel's ovum the yolk-sac had blood-vessels contiiining nucleated blood-corpuscles, and was a hollow vesicle whose thin walls were composed of a fine lining of entoderm, and a thicker sheet of mesoderm. Spee was able to study his embryo in detail; it had a well-marked medullary plate with a median furrow, Fig. 104 ; at the posterior end of the plate was the primitive streak, and at the anterior end of the primitive streak was an opening (named by Si)eo the neurenteric canal) leading into the entodermal cavity; the head had grown forward sufficiently to indicate the development of the vorderdann ; the notochord was present, as a median band of entodermal cells, running forward from the neurenteric canal; tbe allantoic diverticulum extended as a narroAv tube of entoderm through the allantois-stalk to the chorion ; the coelom had not appc»ared in the embryo proper; the anlage of the heart was not present.

This stage is, therefore, characterized bj' the size of the chorionic vesicle, 8-11) mm., the completed development of the extra-embryonic coelom, and the absence of the embryonic coolom and heart anlage; by the presence of the medullary plate, neurenteric (or blastoporic) CAnal, notochordal band in the ento<lerm, the vascularized yolk-sac, the thick allantois-stalk with the tubular allantoic diverticulum. The general arrangement can be understood from the diagram. Fig. 100.

Third Stage: Medullary Groove

The development of both the embryo and its appendages has advanced. Particularly noteworthy are the large size of the medullary ridges and the precocious differentiation of the chorion and amnion. The youngest embryos of this group an? in the neighl)orho(Kl of 2.3 mm. in length (Thomson gives the length of his embryo I. as 2.5 mm., but the criticisms made al>ove render it plain that this measure probably refers to the length of the amnion plus the allantois-stalk) ; the embryo not seen l>y Thomson was presumably shorter. The embryo has a broad attachment to the yolk-sac, which in diameter nearly equals the length of the eml)ryo and is already furnished with bloc k1- vessels. The most conspicuous character of the embryo is the presence of two very thick dorsal ridges — medullary folds, running the whole length of the (»mbryo and inclosing the medullary groove, central nervous system to be, between them; the cephalic extremity is somewhat thickened ; from the ventral side of the caudal extremity springs the short and thick allantois-stalk, the op|V)site end of which is inserted into the chorion. The amnion completely incloses the embryo, and is attached on the one hand to the allantois-stalk, on the other to the embrvo nearlv parallel to the lunction of the eml)rvo and the yolksac. The next change involves not merely the growth of the embryo, but also the thickening of its cephalic end, the development of the great heart protuberance between the yolk-sac and the head, the concave fiexion of the back, and the (lee|x?ning of the* meiluUary groove, which, how(»ver, still remains o|M}n. The chorioji forms a relatively large vesicle, its average diameter IxMng alH)ut S mm., but the four si)ecimens vary from T).? to 15 mm. The cliorion l)ear8 villi over its whole surface; the villi are considerably branched.

Probably the villi are formed chiefly if not solely by epithelium, and probably, also, there is a layer of connective tissue, very likely idready vascular, which lines the chorion, but does not extend into the villi. There are many still unsolved problems as to the development of man. It will be observed that not a single one of the ova hitherto noticed has been adequately investigated, and that no specimens have yet been studied at all, showing the first appearance of the embryo, the origin of the amnion or of the allantois, or of the yolk-sac; and fiuidly, that of all the ejirlie-st stages our knowleilge is extremely imperfect. It is, therefore, much to be hoped that all who obtain available specimens will carefully preserve them and intrust them to a comi)eteiit investigator. From the above considerations it is also evident that the summary just given can be only tentative.

Fourth Stage: The Heart

In this stage the embr>'o is probably 2.2 to 2.5 mm. long; the head projects in front of the yolk, and on the under side of the cervical region the heart has appeared ; the deep neural groove is partly closed to form the medullary canal, but is open along the cephalic region ; the dorsal outline is slightly concave; the myotomes have appeiireil, the number varj'ing; Spee found seven, Kollmann thirteen ; the caudal end of the embryo also projects beyond the yolk, but less than does the head ; the auditory invagination is prolmbly not yet formed ; there are no gill-clefts showing externally.

Concerning the chorionic vesiclo at this stage, satisfactory data are lacking.

Fifth Stage: One Gill-cleft. — No human embryo with only one gill-cloft showing externally is known.

Sixth Stage: Two Gill-clefts and Dorsal Flexure. — To this stage we must assign not only my two specimens referenced to above, 20 and 21, and His' Lg, 2>, and Sch 1, 2:J, but also His' L, 24, and probably Coste's, although in neither of the latter does the dorsid flexure api^ear. It is possible that Schroeder van der Kolk's ovum, 2(>, Hennig's, 27, Schwabe's, 0, and Remy's, 28, also belong in this stage, but for reasons given above in detiiil the position of these four is ver} doubtful, that of Schwalx?'s especially so. In His' embryo L, and in Coste's, the dorsal fl(?xure Avas prol)ably obliterated artificially, leaving only the four onil)ryos, 20-2' J, iiiK)n which tho following 8ynoi)sis is based, with the addition of some anatomiciil facts derived from Nos. 24 and 25.

the general shai)e of the embryo and its remarkable dorsal flexure can be best understtjod from Fig. IT. the liead l)end is very marke<l an<l the tail end C)f the embryo is also l)ent over ventral ward; the yolk-sac extends from the heart backward to where the Ixxly of the embryo turns to make the dorsal flexure; the heart is large and very protulx?rant; it is lx»nt so that wo can clearly distinguish the auricular, A'entricular, and aortic limbs, and it consists of a smaller inn(»r tulx>, the endothelial heart, or endcK'ardimu (which is continuous at one end with the Avails of the veins, at the other with the walls of th(» aorta), and of an outer largi»r tul)e, the muscular heart or myocardium; In^tween the two heart tul)es is a considenible sjwicc*; there are two gill-clefts and, at least in the youngest specimens, only two aortic arches, one in front of each cleft ; between the head and the h^art the onil invagination has l)een formed but is still separated by the oral plate {Rachenhaut) from the vorderdarm ; above the gill-clefts is the open ectoilermal invagination of the otocyst, which in His' embryo L, 24, had become a closed vesicle. The central nervous system is very large comi)ared with the whole embryo; the brain comprises in length about one-half of the medullary canal ; the optic vesicles are large, and the optic stalks are well differentiated; the head bend takes place in the region of the mid-brain, Avhich is imi)erfectly8epiirated from the fore-brain ; the hind-brain is about equal to the fore and mid brains together in length; there were twenty-nine myotomes in His' embryo Lg, 2'!. The vorderdann is flattened dorso-ventrally ; the liver is developing in the septum transversum ; the middle portion of the hitestine oi)ens into the yolk-sac, the posterior portion is closed and at its caudal termination is dilated to form the bursa of His, and curves over to pass as the narrow tubular allantoic diverticulum through the allantois-stalk to the level of the chorion. The veins show the typical arrangement, the jugulars joined by the cardinals form the duc*ti Cuvieri, and these after receiving the omphalo-mesaraic (or vitelline) and the umbilical (or allantoic) veins unite in the median line as the sinus reimiens; the course of the allantoic veins is i)eculiar and mayl)e described as a short cut tiirough the somatopleure along the line where the body wall of the embryo is detlected back to form the anmion.

Seventh Stage: Three Gill-clefts. — All the accurately known embryos, except one, '^S A, belonging to this stage, belong to the end of it, and one of them, His' Lr, 3"i, is so far adA'^anced that it might almost be classed in the iwxt stage. Five good embr^'os, t><i-:^*5, are to \^ placed here, and four othei-s, 34, 35, 3G, and 5 have been associated with thorn, but the latter are all doubtful cases; the l)est of them Innng Yon Baer's, 31, wliich probably should Ix) put in tluj eiglith stiige. For reasons static! in the section on the dorsal tiexure, p. 31*), the flexure is probably normally absent in embryos at the close of the seventh stage. The descril>ed embryos varv' from 'iJ) to 4.2 mm. in length; His' II, 30, Avas '2J\ mm. long, andits(4iorionic vesicle measured 11 by 14 mm. His' BB, 31, was 3.2 mm. long, and its chorionic vesicle measured 1 1 ])y 1 1 nun : the age of BB was probably twenty to twenty-one days. The bac^k of the embryo is nonnally (or at least usually) convex ; the liead is bent to one side (usually to the rig)it) and the tail to the other, the whole embryo having a spiral twist; there are three gill-clefts showing externally; the tail end has groAvn considerably and the allantoisstalk has It^ngthcned ; the yolk-stalk (neck of the yolk-sac) is both relatively and absolutely smaller than in the pnn'ious stage, but the embryo is larger. The heart has grown very much: in the older specimens the deA'eL)]>ment of the auricular pouclies has begiUL The ot(X'yst is a closed pear-shaped vesicle, its apex pointing toward the dorsal side. The mouth cavity has deepened, the oral plate is ruptured; al)Ove the mouth the maxillary process ran Ix^ distinguished. The pharynx is wide, comjM'essed d<jrso- vent rally, and has in the known specimens four gill-pouches, and on its median ventral floor a small prominence, His' tuterculum impar, the anlage of the tongue; the diverticulum of the liver is well marked in the youngest and enlarged and branching in the oldest specimens ; the Wolffian ridge is distinguishable and contains Wolffian tubules, but as to the number and form of these we possess no exact infonnation. The medullary canal is closed throughout its length ; the mid and fore brains have become clearly separated since the sixth stage. As regards the circulatory system, besides the appearance of the auricles and the genend iidvance of the heart, we have to note that the great veins passing through the septum transversum have begun their transformations int(^ the hepatic system, and that the aorta has five aortic arches, the two first coming from one branch, the remaining tlu-ee from another branch on each side ; no embrvos are known with only four aortic arches.

Eigeth Stage: Four Gill-clefts. — The three embryos, 34, 1^7, 3s, which were apparently in this stage, are so imperfectly known tiiat there is practically nothing clefinite to sfiy in regard to their anatomy. Wagner's si)ecimen, 38, measured 4./) mm.; Miiller's, 37, 5.5 mm.

The Dorsal Flexure

In a number of embrvos with from two to four gill-clefts there has been observeil a deep bend in the rump, which suggests at once the effect of a pull u|K)n the yolk having produced a sharp concavity in the back, compare Fig. 109. In embryos with two gill-clefts this l)end, for which I propose the tenn dorsal Hexure (Riickenkrihnmunq)^ has Ix^en shown by His to be normal. In older embrvos it seems to l)e abnormal, for in one with three clefts and the doi^sal flexure, 31, the tissues in the region of the bend were lacerated, and in a still older spe<nmen (W of His) the bend wa« artificially produce<l while the embryo was luMng manijnilated. The fjK'ts indicate that the l)ack is to<» long for the somatopleure at the side of the Ixxly, and that it finds r(M)m at th(» stage with two gillclefts by l)ecoming concave; later it springs into a new ]K)sition of e<iuilibnum by becoming cumvex; it is possible that the change from the concave to the convex jx)sitit)n is very abrupt, and it is probable that the time of its <K*currence is verv variable, so that \\v niav find hereafter eniln'vos in the seventh and eighth stages, which are ix?rfectly normal though still having the ilorsiil flexure.

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