1897 Human Embryology 12

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

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History

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Chapter XII. Early Development Of The Archenteron

The archenteron is, strictly speaking, the entire cavity lined by the entoderm, a fact which comes out very clearly in the primitive type of vertebrate development as preserved in Petromyzon, ganoids, and amphibians. In these forms the yolk is the ventral floor of the archenteron and consists of a thick mass of cells, all charged with yolk granules (deutoplasm), see Fig. 70, Vi, In all amniota, on the contrary, the first step is the division of the primitive archenteron into the primitive alimentary canal of the embryo and the yolk-sac. Before this occurs the mesoderm has appeared and the coelom is present in it, so that splanchnic mesoderm is already differentiated and united with the entoderm to form the splanchnopleure; henceforth the two germ-layers develop in close correlation with one another, and the history of the archenteron becomes the history of the splanchnopleure, except that in the cervical region the entodermal canal is developed without the coelom appearing.


We have to consider the following stages: 1, Separation of the archenteron proper from the yolk-sac ; 2, origin of the allantois ; 3, origin of the pharynx.


Entodermal Cells

In the primitive type of vertebrate development (marsipobrauchs, ganoids, and amphibia) the archenteric cavity is of small diameter and is bounded. Fig. 102, Enty by cells which differ from those of the surrounding yolks very slightly at first, although they early assume an epithelioid arrangement. In the amniota, on the other hand, the archenteric cavity undergoes great lateral expansion at a very early stage ; this has been accompanied by certain modifications in the entoderm, which becomes divided into a cellular part on the dorsal side of the cavity and a multinucleate vitelline part on the under side ; moreover the cellular part becomes divided into two regions ; one, known as that of the area pellucida^ includes and surrounds the meroblastic embr\^o proper, and is characterized by the cells becoming very much flattened and expanded, compare Figs. 144 and 161, and the other, known as that of the area opaca^ ourroundb the first and is characterized by having high cylinder cells, with more or less of remnants of yolk grains in them. The cylinder cells of the opaca entoderm pass toward the embryo into the thin cells of the area pellucida, and in the opposite direction when they reach the periphery of the expanded archenteron they pass into the yolk ; the transition constitutes the so-called germinal wall. As development progresses we see that the region of the opaca belongs to the yolk-sac and not to the embryo, hence the cylinder cells represent part of the entodermal lining of the yolk-sac, and in the higher mammals the whole yolk-sac is lined by cylinder cells, which represent lx)th the opaca cells and the yolk mass of Sauropsida, there being no non-cellular yolk in placental mammals. For further details see Chapter XVL, on the yolk-sac.


As soon as the embryonic archenteron begins to separate off in amniote embryos the thin entoderm grows thicker, until in young embryos it resembles a cylinder epithelium, but in the area pellucida around the embryo the entoderm remains thin, the thickening being strictly confined to the embrj- onic portion.


1. Separation of the Achenteron proper from the Yolk sac

The following diagrams, Figs. 14i2 and 143, may help to render the process clear as it occurs in the Sauropsida ; both figures are supposed to represent a hen's ovum with the embr^'o ifi situ and cut transversely ; the embr\'o is much too large in proportion, and the coelom is much too wide. In Fig. 142 the archenteron, Aeh^ is a wide cavity bounded by the cellular entoderm above, and the yolk-mass below ; the cellular entoderm in the axial region corresponding to the aren i^ellucida is thin, and thickens at each side corresponding to the area opaca. The mesoderm is divided into its two layers by the coelomatic space, Coe, but it is to be noted that in reality the extension of the coelom around the yolk is gradual, and at this stage of the embryo is never completed; the splanchnic mesoblast is thus laid against the cellular entoderm on the dorsal side of the archenteric cavity, forming the splanchnopleure, Spl; that portion of the splanchnopleure which lies within the area pellucida, and is therefore lined by the thin region of the entiKlerm, is alone destined to form the intestinal canal or embr\'onic archenteron. This is effected, as indicated bj' Fig. 20, p. 35, by a simple folding-down of the splanchnopleure, by which the ar(»henteron is divided into an upjKjr embryonic portion, 7/i, the anlago of the intestinal canal, and a lower portion, the walls of which are constituted chiefly by the (^normous mass of yolk. In the next stage. Fig. 10, the separation of the yolk-siic. Yolk, from the embr^'onic archenteron is still more marked than Ix^fore; there remains, however, a narrow connection l)etwo<^n the intestinal canal, 7/i, and the yolk-sac, yolk\ making, as it wero, a hollow jx^dicle; the pedicle is known as the yolk-stalk or vitelline stalk.


If we view the embryo in longitudinal section we find the relations are similar and that the head and the tail end of the embryo become fre<^ from the yolk, and as the embryo grows its head and tail project more and more. This is often descrilied as a separation of the embryo by the folding-in of the germ-layers, but this traditional d(»scription is incorrect, for the seixaration of the embryo is really due to the expansion of the embryo, not to the constriction of ite connection with the yolk, compare Chapter XIII. The accompanying diagrams. Fig. 143, A, B, and C, show at a glance how the original width of the communication is retained, while the intestinal canal, or embryonic archenteron, extends forward and backward. In A, the archenteron is open to the yolk throughout its entire extent ; in B, the head has b^un to be free, and with it the archenteric cavity has begun to extend forward and forms a distinct cephalic portion, which is entirely within the embryo and is not open directly to the yolk; in C, the tail has also grown forth from the yolk and the archenteron with it, so that now we have a caudal embryonic archenteron. By further development the erabrj'o fflilarges more and more, hut the opening into the yolk-sac remains nearly the same absolute size. I know of no exact data as to the dimensions. The proportion between the yolk-»tatk and the embryo steadily changes, the stalk becoming relatively smaller.


Fig. 142. — DiafiTammatic S(>ctioD of thei Ye1h»w of a Hen's Esg at an Early Stapre to 8how the Relations of the Archenteron, Ach^ to the Yolk-sac. Som, Soniatopleure ; Spl. splanchnopleure; In. intestinal portion of archenten)n: vi, vitelline jwrtion of archenteron ; C'ae, cu'loni ; Yolk, yolk mass.


I believe that the entire separation of the archenteron from the yolk-sac is due to the primitive connection being retained with little or no change of size while the embryo is growing, and that the bending of the layers, instead of being the cause of the closure of the archenteron, is the result of the arrested growth of the splanchnopleure where it passes from the embryo proper to the yolk-sac.


The development in mammals proceeds in the same manner as here described for the Sauropsida, but the appearances are modified principally in two ways: first, by the loss of the yolk material, and second, by the retarded development of the mesoderm. The loss of yolk leaves the yolk-sac a hollow body lined by well-developed epithelium (entoderm) , a condition reached by the Sauropsida only after most of the yolk matter (deutoplasm) has been resort^, see Chapter XVI. The delaye<l expansion of the mesoderm is verj- marked in the rabbit, and probably in other mammals; in the rabbit the embryo becomes separated from the yolk-sac and far advanced in development, while the mesoderm extends only over the embryonic ' eoftheovum; hence in the rabbit the yolk-sac is complete only over half its surface; the complete sac of the entodenn ia present, but only the einbrjonic half of the entodemial aac is covereii by raesoderm and separated from the ectodenn by a coelom ; the inferior half of the sac lias its entoderm lying directly against the ectoderm, and this half, moreover, degenerates and entirely disappears, as discovered by Duval, 90.1.


In mail the mesoderm extends completely around the erabrj-o at a very early stage, and so also does the coelom; in the earliest known human embryos botli of these processes had been finished and the yolk-sac lay entirely free, the fmidamental morphological relations agreeing with the diagram, Fig. KiG, For details see the descriptions of young human embryos in Chapter XIII.


The actual appearance of a rabbit embryo as seen in cross sections is indicated by Fig, 144, while a similar stage in the chick is shown by Fig. I'il ; in Iwth tho embryo only i.s represented, jind the space Ix'low the splaiichnopleiire, Spl, is the part of the arrhenteric ca^-ity underlaid by yolk in the natural condition.


The portion of the embryonic arohenteron. Fig. 14:1, C, in front of the yolk-sac, gives rise to tho pharynx, oesophagus, stomach, anterior iKirt of tho intestine proix;r, and all their appen<1»ges. The portion l>ehind the yolk-stalk jiriHluces most of tho small, and the whole (if the large intestine and ils appendages. The first development of the anterior division is tho differentiation of the pharynx, of the i>osf erior tho formation of tho alliintois ; as tho latter organ always appears earlier than the former we must consider it first.


Origin of the Allantois

The first indication of the allanbois in all amniota is a considerable accumulation of mesenchyma around and below the posterior extremity of the embryonic archent«ron. This accumulation is proltably a remnant of the primitive streak. A longitudinal section, Fig. 14-1, shows the relations clearly, as found in the sheep at about sixteen days; the corresponding stage occurs in the chick at the end of the second day ; the three germlayers are all fused in the primitive streak, pi\ s; the anal membrane, a.m, is well marked by the absence of mesoderm ; the amnion, arises close behind; the archenteric cavity forms a wide diverticulum behind the anal membrane, and this diverticulum, is the commencement

g allantfiis; it is lined by the entoderm, En, and has an external layer of thickened mesoderm, "** mes. The development of the

Fi(i. 145. -I.«>ii(ritu(linal Section of tholVwtepior «ii _^j.^:. :,^ xu^ ^vr:^u u«« u^^..

EiHi of a Sheep Embryo of Sixteen Duys. AfuT alhuitois lu the chick has been

l,r. ^. primitive Ktreak ; En. entoderm; Arh VJaSSer, Hrehenteric cavity, or aix'henten>n ; ^/^ allan- 74.1, and bv ". VOn DobrjTlin, toic niverticulmn ; 3 mesodenn.

membrane is formed as a small |X)Uc*h extending upward into the hind end of the primitive streak; the tip of this pouch lies just l>ehind the lH)ttoni of the furrow, which marks off the caudal extremitv of the embrvo; tlio bottom of this furrow is the site of the anal plate; the p<.)uch gradually enlarges and tussumes the dipper sha}>e, very much as in the sheep. Fig. J 45, .1//; in the chick, however, this stiige is readied relatively later than in mammals, for in the chick we tind the* t<iil already far advanciMl, so that it not only projects free^ly but has begun to curl over downward so as to bring the allantois and anus on to the ventral side of the bcnly as well as to cause the formation of thi» enddarm, which is a short extension of the archent(Ton into th<» caudal <»xtrenntv. The whole series of metamorphoses is admirably illustrate<l by (iasser, /. c, Taf. I. In mammals the fomiation of the tail is t^newhat retarded, but in them also it n^sults in curling ovi»r and so bringing lK)th the allantois and the anal plate on to the ventral side, with the further result that the allantois now comes to lie head ward of the anal plate, although l)efore the curling over it lay beliind it.


It is important to note that the amnion arises betwcM.oi the anal plate and the allantois, and, as shown in Fig. 1 4r>. fuses with the wall of the allantois.


The allantois is charaeterized by the rapid development of itH mescxlerm, which scvms to l)e derived from the middle cells of the hind end of the primitivt* streak. The aniouut of the allantoic maso<lerm is subject to much variation during the (sirly stages of the organ, l)eing much gn^ater in mammals than in birds, so far as observtnl. the mesoderm is particularly conspicuous in n^dents; in the rabbit it makes a di.stinct mound, compare^ Fig. 190 ; in the guinea-pig (E. Selenka, 84.1, Taf. XI.) it aecjuires an excessive size, becoming larger than all the rest of the embrvo; in Mus the preciX'ious <levelopment is almost equally marked ; it is into this nuiss of mesoderm that the allantoic diverticulum of the archenteron grows. In the opossum (Selenka, 87.1, loD) the amount of mesoderm is more nearlv as in the rabbit. The mescKlenn is also characterized in rodents, and i)erhaps in other mammals, by its pi-ecocious vascularization, which has been expressly emphasized for the rabbit by G. Rabl, 89.2, 152, Taf. IX., Fig. 14; the vessels give the tissue a spongy character. The protuberance caused b^^ the allantoic mesoderm is termed Allantoishocker by recent German writers, the Allantoiswnht by Kolliker.


The earliest stages of the human allantois are not known. There has been some discussion as to whether there is a free allantois, but no proof that such a stage occurs has been brought. The matter is discussed in the chapter on the youngest known hmnan embrvos, and in that on the umbilical cord, compare also Fr. Keibel, 91.4.


Primitive Anus

The terminal portion of the intestinsd canal receives in early stages the urogenital ducts, a condition which is pennanont in the Sauropsida; the portion of the archenteron common to these ducts is known as the cloaca. The ectoderm in amniota forms very earlj^ a small anal invagination (proctodaeum) which grows in toward the cloaca until the ectoderm and entoderm come into contact; the membrane formed by the two epithelia breaks through and the cloaca thereby acquires an opening to the exterior; this opening subsequently divides into two: 1, the urogenital opening ; 2, the permanent anus ; in distinction from the latter the cloacal oi>ening may be called i\\oj)rimitive anus.


In anmiotci the anal membrane arises in the anterior region of the primitive streak some distance l)ehind the neurenteric or chorda canal. It has been studied in birds by Bomhaupt, 67.1, and moi'e fully by Gasser, 80. 1. It has been noticed in Lacerta by H. Strahl, 86.2, lOG, who states that it appears in that type at a much later stage than in birds or mammals. In mammals it was mentioned first. I think, by KoUiker, 83.1, and has been since then studied by H. Strahl, 86.2, F. Keibel, 88.2, 410, R. Bonnet, 89.1, 90, Ketterer, 90.2, Toumeux, 90.3, and especially by C. Giacomini, 88.1, most of all these observations having been made on the rabbit. In rabbit embryos witli five pairs of myotomes, the anal membrane can be distinctly recognized near the rear of the primitive streak, comi)are Strahl, /. c, Taf. IV., Fig. 6; it begins as a slight depression of the ectoderm ; behind it are situated the amniotic fold and the allantois ; the depression rapidly deepens, pushing awa^' the mesodermic! cells until the ectoderm comes into contact with the entoderm, wliich at this spot becomes, meanwhile, thickened into a cylinder epithelium ; when the contact takes place a slight entodermic depression appears. The two layers soon become indistinguishable, and by the proliferation of th(*ir cells produce a cord of cells; a similar (H)rd has been found in the sheep by R. Bonnet, 88. 1, and in the guinea-pig by F. Keibel, 88.2; the latter states that the cord is connecttnl only with the ectoderm; the cord is completely surrounded by typical ])riniitive-streak tissue; according to Giacomini, 88.1, 287j the cord develops very soon a transient lumen, which he calls the **anal canal.-' While during the further development of the enibrvo the caudal extreniitv is rolled over ventralward, the cord changes in character, becoming a membrane, and at the same time it is brought on to the ventral side of the body and comes to lie behind, instead of in front, of the amnion as it did before the rolling up of the embryo. The change just referreil to consists in rendering found in the sheep at about sixteen days ; the corresponding stage occurs in the chick at the end of the second day ; the three germlayers are all fused in the primitive streak, pr, s; the anal membrane, a.m, is well marked by the absence of mesoderm ; the amnion, arises close behind; the archenteric cavity forms a wide diverticulum behind the anal membrane, and this diverticulum. Ally is the commencement of the allantois ; it is lined by the entoderm, En^ and has an external layer of thickened mesoderm, mes. The development of the membrane is formed as a small pouch extending upward into the hind end of the primitive streak; the tip of this pouch lies just behind the bottom of the furrow, which marks off the caudal extremity of the embryo; the bottom of this fun*ow is the site of the anal plate; the pouch gradually enlarges and assumes the dipper shape, very much as in the sheep. Fig. 145, All: in the chick, however, this stage is reached relatively later than in mammals, for in the chick we find the tail alreadv far advanced, so that it not onlv projects freely but has begiui to curl over downward so as to bring the allantois and anus on to the ventral side of the boiiy as well as to cause the formation of the enddarm, which is a short extension of the archenteron into the caudal extremitv. The whole series of metamorphoses is admirably illustrated by Gasser, /. (*., Taf. I. In mammals the formation of the tail is somewhat retarded, but in them also it results in curling over and so bringing both the allantois and the anal plate on to the ventral side, with the further result that the allantois now comes to lie headward of the anal plate, although before the curling over it lay behind it.

Fig. 145.— Titudinal Section of the Posterior EmI of a Sheep Embryo of Sixteen Days. After allautOlS in the ChlCK UaS been

RBonnet. ^mn. Amnion; a. »i, anal membrane; cfurliArl in flof«il hv TT. dacoi^j*

pr. ft, primitive streak; Eii, entoderm; Ach, Ijasfeer,

an^henteric cavity, or archenteron ; All, allan- 74.1, and bv P. VOn Dobr^nim,

toic diverticulum; A/*-*, mesodenn. 711 Tt l)PQ'ins beforft thft Anal


It is important to note that the amnion arises Ix^tween the anal plate and the allantois, and, as shown in Fig. 145, fuses with the wall of the allantois.


The allantois is characterized by the rapid development of its mesoderm, which seems to be derived from the middle cells of the hind end of the primitive streak. The amount of the allantoic mesoderm is subject to much variation during the early stages of the organ, l)eing much greater in mammals than in birds, so far as observe<l. The mesoderm is particularly conspicuous in rodents ; in the rabbit it makes a distinct mound, compare Fig. 196; in the guinea-pig (E. Selenka, 84.1, Taf. XL) it acquires an excessive size, becoming larger than all the rest of the embryo ; in Mus the precocious development is almost equally marked ; it is into this mass of mesoderm that the allantoic di\?erticulum of the archenteron grows. In the opossum (Selenka, 87.1, 139) the amount of mesoderm is more nearly as in the rabbit. The mes(xlerm is also characterize<l in rodents, and perhaps in other mammals, by its precocious vasciilarizatiou, which has been expressly emphasized for the rabbit by 0. Rabl, 89.2, 152, Taf. IX., Fig. 14; the vessels give the tissue a spongy character. The protuberance causeil by the allantoic mesoderm is termed Allantoishocker by recent German writers, the Allantoisnmlst by Kolliker.


The earliest stiiges of the human allantois are not known. There has been some discussion as to whether there is a free allantois, but no proof that such a stage occurs has been brought. The matter is discussed in the chapter on the youngest known hmnan embrvos, and in that on the umbilical cord, compiire also Fr. Keibel, 91.4.


Primitive Anus

The terminal portion of the intestinal canal receives in early stages the urogenital ducts, a condition which is permanent in the Sauropsida ; the portion of the archenteron common to these ducts is known as the cloaca. The ectoderm in amniota forms very early a small anal invagination (proctodseum) which grows in toward the cloaca until the ectoderm and entoderm come into contact; the membrane formed by the two epithelia breaks through and the cloaca thereby acquires mi opening to the exterior; this opening subsequently divides into two: 1, the urogenitid opening; 2, the permanent anus; in distinction from the latter the cloacal opening may be called tho primitive anus.


In amniota the anal membrane arises in the anterior region of the primitive streak some distance l)ehind the neurenteric or chorda canal. It luis been studied in birds by Bomhaupt, 67.1, and more fully by Gasser, 80. 1. It luts been noticeil in Lacerta by H. Strahl, 86.2, 1()G, who states that it appears in that type at a much later stage than in birds or mammals. In mammals it was mentioned first, I think, by KoUiker, 83.1, and has been since then studied by H. Strahl, 86.2, F. Keibel, 88.2, 410, R. Bonnet, 89.1, 90, Ketterer, 90.2, Toumeux, 90.3, and especially by C. Giacomini, 88.1, most of all these observations having been made on the rabbit. In rabbit embryos with five pairs of myotomes, the anal membrane can be distinctly recognized near the rear of the primitive streak, comjwire Strahl, /. c, Taf. IV., Fig. G; it begins as a slight depression of the ecto<lerm ; behind it are situated the amniotic fold and the allantois; the depression rapidly deepens, pushing away the mesodermic cells until the ectoderm comes into contact with the ent(xierm, which at this spot becomes, meanwhile, thickened into a cylinder epithelium ; when the contact takes place a slight entodermic depression appears. The two layers soon become indistinguishable, and by the proliferation of their colls produce a cord of cells; a similar c»ord has lx?en found in the s1kk:»p by R. Bonnet, 88. 1, and in the guinoa-pig by F. Keibel, 88.2; the latter states that the cord is connected only with the ectoilerm ; the cord is completely surrounded by typical primitive-streak tissue; acconling to Giacomini, 88.1, 2S7j the cord di»velops very soon a transient lumen, which he calls the **anal canal. While during the further development of the embryo the caudal extremity is rolled over ventralward, the cord changes in character, becoming a membrane, and at the same time it is brought on to tlio ventral side of the body and comes to lie behind, instead of in front, of the amnion as it did before the rolling up of the embryo. The change just referre<l to consists in rendering the two epithelia distinct again and converting each into a single cell-layer, making a double epithelial membrane from which the mesodenn is entirely excluded, and which has been appropriately named the anal membrane by Strahl. The membrane lies at the bottom of a shallow pit, which is conmionly viewed as an ectodermal invagination, and has been called the Afterdarni by German, proctodiBum by some English writers. It is to be regarded as the rudimentary homologiie of the well-develoi)ed invagination of annelids and other invertebrates, which forms in them a considerable portion of the digestive canal; the anal invagination results in invertebrates in the formation of the so-called Hinterdann (hind-gut of Foster and Balfour), which must not be confused with the vertebrate Hintenlarm, which is derived from the archenteron.


The rupture of the mial membrane is said to occur in the rabbit about the twelfth (Kolliker, '^Grundriss," :35y) or thirteenth day (Strahl, 86.2, 165). I know of no exact description of the process in mammals. In the chick the epithelial cord arises and becomes jKjrforated, according to Gasser, without passing into the stage of anal wembrane observed in mammals; irregular cavities appear in the cord (Gasser, /. c*., Taf. XIII., Figs. 6a, 7a); these cavities enlarge and fuse, the cells of the cord or plate meanwhile undergoing degenerative changes; the rupture is completeil about the fifteenth day of incubation. The anal ectodermal invagination is somewhat more marked than in mammals and gives rise on its dorsal side to a considerable diverticulum, the bursa Fabricii, which is found in birds but not in mammals or reptiles.


The anus of the lower vertebrates arises, as has already been shown, in intimate relation with the blastopore. This fact wjis first discovered by Max Schultze, 66.1, in Petroniyzon, ascertained in alytes by Gasser, 82.3, in the newt by Alice Johnson, 84.1, in Rima bv Durham, 86. 1. The nature of this relation was first elucidated by Schanz, 87.1, and has since been worked out for various amphibia, as described, p. 189.

The Endoderm

The prolongation of the archenteron into the tail of amniote emlnyos is generally known as the Euddarni, the German name most in use; it is also called Schwunzdann, tftil-giit, aiiil 2)o,sf-((nal gut. It results from the differentiation and rolling over of the tail. The tail is produce<l by the growth of the tissue of the primitive streak betw(»en the anal membrane and the blast o]X)re or neurenteric carnal, compare Chapter XIII. ; the p^i'owth occurs in such a way that the tissue curls downward, and folds off the region of the archenteron underlying the piimitive streak, and the disposition becomes as show-n in Fig. \i)'l oi" Kolliker's "Grundiiss," "Zie Aufi., the enddarm extending into the tail lx»hind the ventrally situated anal membrane.


I consider the endoderm to Im? distinct from the neiirent'.M*ic canal, with which Balfour ("Conip. Enibryol./' II., 11»3, Fig. 1'24) brings it into relcition. (). Heriwig a])parently agrcvs with Balfour, since he copies the latter's diagi'am ("Entwickelungsges.," ^ite Aiitl., Fig. 126). It seems to l)e confined to earlv embrvonie life, but there are a few data as to its ultimate fate*. Preuant, 91.2, *2'M, studyiiig the rabbit found the i)ost-anal gut to be a short wide pouch before the tail develops ; as the tail develops, the gut extends into it and becomes long and narrow, and its posterior extremity merges with the fused anlages of the medullary tube and notochord. In still older embryos it degenerates.


Origin of the Vorderdarm. — As is well known the first part of the embryo in vertebrates to project from the yolk is the head end. In the same measure as the head and neck become free the portion of the archenteron which pertains to them becomes closed below and shut off from the yolk. A longitudinal section of a chick in which the head has just become free is shown in Fig. 14G. In consequence of the head end, H, having grown forward above the proamnion, pro. a, which overlies the extra embryonic archenteric cavity, it has become free on all sides, and at the same time the archenteron has been carried forward with the head, making the so-called Vonlerdarm, ]"(/, of German authors. The term /ore-f7M< has been proix)sed by Foster and Balfour as an equivalent English term, but has not come into general use, so I have prefereil to use the German tt»rm. Vorderdarm is also used in invertel)rate embryology", but in a different sense, for it designates the oral inva^^ination of the ectoderm, whereas the vertebrate vorderdarm is the ceplialic ix)rtion of the archenteron.

Fig. 146.— LoDgituclinal Mwlian Swtion of Yoiinj? Chick Kmbryo. H, Heatl: Trf, vupilenlarm: m^ff, iiieso lenn : /o, fovea cardiaea; p, pericardial cavity; jiro. a, pruamniuu; vlcA, aivlien* teric cavity; iV. *. primitive Htrvak.


Even at the stage of Fig. 14r», the vorderdarm has begun to be differentiated into an anterior division and a posterior, which at this time are distinguished chiefly by the coelom, jii, being present onl^in the mesodenn below the ix)sterior division. The anterior division fonns the pharynx proix^r. Tlio distincti(m l)etween the two ))arts of the vorderdarm h<as long been recognized (see, for example, Goette's ol)servations on Bombinator, 75.1, 221), but its morphological signifi(*ance has Ix^on overlooked. The vorderdarm is a short canal under the anterior end of the medullary gr(X)ve ; it ends blindly in front, but ot)ens widely l)ehind into the general archenteric cavit}' ; this o|)ening is termed the forea cardiacd {rordere Darmpforfe of. Kolliker), having lx»en so named by C. F. Wolflf. The fovea is easily seen, when the chick embryo is removed from the yolk in the usual manner, and viewed from the imder side; its curving edge marks the end of the dosetl archenteron behind which the archenteric cavity of the embryo oix»ns directly into the yolk-sac. In transverse sections. Fig. 147, the vorderdarm appears widely expanded sideways, but com}>ressed dorso-ventrally, and also l)ent, the concavity being upward ; it is, of course, completely' lined by ent<Hlenn, the cells of which form a vers' thin laver on the dorsal side and a much thicker layer on the ventral side ; moreover, on the ventral side the entoderm is thickened towanl the median line. These features are highly characteristic, but their significance is (juite unknown. Are they ancestral in origin?


In the explanations usually given, the development of the vordertlann is not attributed to the forward growth of the head, but to the down-folding of the splanchnopleures. Indeed if aectione of Buccettsivo stages be compared the idea appears justified , for at first the cephalic archenteron opens widely into the yolk-sac, then as the head of the embryo begins to rise up and project forward fn>m the yolk it seeni.s as if the sides of the head were being tucked under ; but if it lie rememberetl that the head is growing and that the opening between the archenteron proper and the yolk enlarges very little, it will be cleai- that the growth of the head is the real cause of tbe formation of the vorderdarm.


In mammals the process is the same as in birds, but the vorderdann is less expanded laterally and less compressed dorso-ventrally than in the chick, hence the appearance in cross-section is somewhat different. In the opossum, however, there is a marked resemblance to the avian type in the shape of the vorderdarm, see Selenka, 86. 1, Taf. XXII., Figs. 9-U), and it is probable that more careful study ^vi\\ show that the mammalian vorderdann jiasses through the flattened form before assiuning its more familiar shape.


The Oral Plate

The fact that the anterior end of the vorderdarm lies against tfee ectoderm has long been known for advanced embrjds. The two germ-layers, entoderm and ectodenii are soldered together with no mesoderm lietween them, thus forming a double epithelial plate (as shown in Figs. 10(i and 170, o.pl), which separates the buccal from the archenteric cavity. The plate, which may be called the oral plate (meinbrana fauces, Rachenhaut, Mundrachenhaut), by its subse<iuent niptui-e brings tho mouth into communication with the pliar\nix.


Fr. Carius, 88.1, i-i, lias shown that the oral plate is present in the rabbit at a very early stage, the spot where the entoderm and ectoderm come into contad being distinguishable before the head is neparatKl from the yolk. This spot lies just in front of the interior end of the medullary groove and ()f the chorda, the end of which fuses with the entoderm of the membrane. As the head of the embryo grows forward and bends downward toward the yolk the oral ]ilate is roUed over so as to lie on the ventral side of the embryo, and to constitute i>art of the venti'al floor of the vorderdarm as shown in Fig. 100.


Origin of the Pharynx. — From what has been said in the preceding section it appears that the vorderdarm very early divides into an anterior part without any splanchnocoele in the surrounding mesoderm and a posterior part, underneath which lies the pericardicQ division of the coelom. The anterior division becomes the pliarynx proper and is remarkable for its rapid enlargement during the earliest embryonic periods of amniota ; the large size of the pharynx is characteristic of the lower vertebrates, hence we have in the pharynx another illustration of the appearance in the embryo of a higher form of features characteristic of the adult lower forms. The posterior or epicardial division of the vorderdarm undergoes differentiation later than the pharynx, but ultimately gives rise to the oesophagus and stomach ; as the lungs arise near the junction of the two divisions, it is not quite certain, at present, whether they make part of the anterior or posterior division.


The pharynx then is the anterior portion of the vorderdarm, and is further character! z(h1 by never having a continuous coelomatic cavity d(?vcloiKHl in the mesoderm surrounding it.


The relations of the pharyngeal entodemi to the ectoderm are extremely im|)ortant to the morphologist, since they result in the formation, I, of the oral plate and conse(iuently of the mouth cavity; *^, of the gill-clefts, which in their turn determine to a large extent the complex morphology of the head.


The Branchial Clefts, or gill-clefts, are permanent structures in the fishes and tailed ampliibia, larval structun\s in anoura, and embryonic structures in amniota. They arise as a series of paire«l })0uches from the sides of the pharynx. They are called Schfund-, Kietnen- or Viticeral'.Hpalten in (Jerman; fenfes branchiah*s in French.


The nnnthrriy{ gill-clefts varies in the different clasws of vertebrates. In mammals and birds there are four; in reptiles, taile<l junpliibians, and most fishes, five; among the selachians, however, the number is variable, there being often six and in the Notidanidie eight, it is said. In the lamprey there are eight during larval life, but the first aborts when the larva (Ammoca^tes) changes into tho adult (P(^tn)myzon). In Amphioxus the pharynx has eighty to one hundred ojjenings and even more. These facts have led to the general conclusion that within the vertebrate series the numl)er of gill-clefts has IxMMi gradually reduced — a liyix)thesis of great importance, from its l>earing u}>on tho solution of the morphology of the head.


In all bircfs and mammals there ar(» four pairs of gill pouches <levelopcHl, all in essentially the same manner. The anterior jwiir apiK»ars first, the others in succession l)ehind it. The entoderm of the pharj-nx forms a small outgn.»wth on each side, making a xxmch, which expands until it reaches the ectcnlenn. Soon a second pair of outgrowths ai)i)ear behind the first, and a third and a fourth. For a lontf time it was l)elieved that the membrane formed bv the entoderm and ectoderm at the end of each pouch ruptureil and converted each pouch into an actual cleft or opening by which a free passage was established through the side of the neck into the pharynx, as occurs in all Ichthyopsida. W. His pointed out, 81.1, 319, that this was open to question, and later showed that the membrane is not ruptured in birds and mammals — a conclusion which has since been confirmed by Bom, 83.1, 275, Kolliker, "Grundriss," p. 77, and Piersol, and which is, I think, probably correct, for those who have called it in question (De Meuron, Kastschenko, and Liessner) seem to me to haveoflfered insufficient evidence. Piersol, 88.1, 162, studied the question with great care in the rabbit, and finds no satisfactory evidence of the closing membrane being ruptured in any of the branchial clefts at any time.


The shape of the pharynx and its four pairs of branchial pouches has been carefully studied in the rabbit by G. A. Piersol, 8o. 1, b^' means of mcxlels of the cjivity at various ages, constructed in wax by Born's method. Two views of the model or Citst of the pharyngeal cavity at eleven days are given in Fig. 148. As the oral plate is already ruptured at this ago, the buccal and pharyngeal cavities have fused, and the models show also the oral evagination of the hypophysis,. The figures sufficiently indicate the complex configuration of the ixmches with their wing-like expansions and ascending dorsal i)oints, as well as the progressive diminution in size from the first to the fourth iK)uch.


Fig. 118.— Two VI»»WR of a Wax MtKlel of the (^avity of tht» Pharynx of a Rabbit Kuibrvo «>f Eleven Days. A, Showing the la(<'rul and ventral Kui*fac'e: B, showing dorsal and lateral surfact' After Piersol.


It must be borne in mind that while the gill-slits are developing the hetul is growing, and therefore lengthening, so that the phar^-ngeal portion of the vorderdarm elongates. At the time the first gillcleft is formed there is not nxmi for the remaining clefts, but tlw* gro^vth of the pharynx provides the needed room soon. Thus in the chick there is at first only a very small distance between the region of the pericardium (and heart) and the anterior extremity of the embryo (see Fig. 140), but by the end of the third day there is a considerable interval l)etween the anterior end of the heart and the actual head. This interval constitutes the embrvonic neck, and corresponds to the pharyngeal region, and is characterized by two principal features: 1, the absence of a splanchnocoele ; 2, the presence of the gill pouches.


As soon as the pharyngeal evaginations reach the ectoderm they become attached to it, first on the dorsal side and then downward until the attachment is completed thnnighout the whole area of contact (A. Goette, 75.1, 222). It seems now as if the ectoderm were actually held down where resting upon the entoderm, for we see as the next phase that the germ-layers grow freely in front and behind each gill pouch, thus producing columns, which are placed at the side of the pharj^nx and are separated from one another by the gillclefts. As there are four gill-clefts it follows that there are five columns. These coliunns are known as the branchial arches, also as the i^ill or visceral arches (Kteme)ibogen^ Visceralbogeny arcs branch laux) . Each arch is marked out by projecting into the pharynx and upon the outside, and consequently soon after the gill pouches are developed the arches become easily distinguishable upon the exterior, and the depressions between them show the positions of the pouches. The depressions become part of the gill-clefts when the membrane (ectoderm and entoderm) breaks through; hence, when the clefts become, as in the lower vertebrates, open passages, their lining is partly of entodermic, partly- of ectodermic origin, but as the epithelia fuse perfectly, the line of demarcation cannot be distinguished in the oi>en clefts.


As to the time at which the gill-clefts a}>i)ear, we nee<l more exact information. C. Ral)l, 89.2, 216, gives tlu5 following data for selachian embryos (Pristiurus) :

Embryos with 18 myotomes show tlio first gill jx)uch.

    • 23-24 " the second iK)uch l)eginning.

2<*>-27 ** the second pouch well formed.

31-32 " the third pouch well advanceil.

38-40 " the fourth pouch beginning.

" 4r)-ir. '* the fourth pouch completed, and the

second breaking through. :»4-r)'j " the fifth pouch begim, and the fii-st

and third breaking through. Cttj-t)>< " the first, second, and third pouches

are clefts, the fourth is breaking through, fl " till* sixth f)ouch is forming, the first four are oi^n, the fifth opening. In tlio chick the gill-clefts begin to appear with third day, the fourth Ix^ing present at the end of that da}'. In the rabl)it the first IK)uch is seen the ninth day, and the fourth the tenth day. In man the ixmches are develoi^eil during the beginning of the third week.


Tlio pharynx expands rapidly in all directions during the development of the branchial clefts, and there is a corresponding enlargement of the cerv'ical region, wherel>y the fonn of the embryo is affected. The external features resulting from the development of the pharynx are described in Chapter XIII., to which the reiider is referred. It may, however, help to make the fundamental relations of the pharynx clear, to inst^rt hero the figure of a longitudinal horizontal section of a dog-fish embryo. The pharynx is a very wide cavity, Ffi, the sides of which are bounded by the five gill-arches; the gill-cle£ts behind each of the arches are already open through; the space in front of the first arch, J, is part of the opening of the mouth, which came into communication with the pharynx at a much earlier stfige than that represented in the figure. The size of the pharj-nx forms a striking contrast with that of the intestinal canal, In; each branchial arch consists uf amassof connectivetissue bounded by a layer of epithelium derived partly from the entoderm of the phar>"nx, partly frtim the ectoderm.


The shapes and positions of the gillslita are remarkably uniform in all vertebrates. They iire clongatetl doi-so-ventrallj- and narrow in the direction of the longitudinal axis of the embryo, p^ig, 150. The first is the largest and the remaining ones gradually diminisli from in front Iwckward, Viewed from the outside they ai'p seen not to Iw strictly parallel, but to convei^e somewhat towanl the ventral side, the nngle l>etween the first flud !-0(i>nd c-tefts Ix'ing the larges^t. It is also noteworthy that the lower edges of the clefts recede further and further from the median ventral line from the first to the last cleft. Fig. 151; the first clefts nearly meet on the ventral side, while the fourth and fifth clefts are far apart. The observation o£ this peculiarity has led to the supposition that the moutii may have been evolved by the meeting of two gill-clefta which have fused into one opening on the median line; this hypothesis ia discussed in the section on the evolution of the mouth.


The Branchial Arches

These are structures of great morphological importance, which undergo modifications of increasing complexity as we ascend the vertebrate series. They are also termed gillarches and visceral arches {Kiemenbogen, Viaceralhogen). In their earliest form they are merely the columns of tissue bordering the gill-clefts; in a horizontal section of the pharynx of an erabrj-o they are cut transversely and are then seen to consist merely of a core of mesenchyma, eui-rounded by a layer of cylinder epithelium, derived in part from the ectoderm, in part from the entoderm, as explained a above. In those cases where, as in the amniota. j! the gill-clefts do not become open, of course the ecto- h..™'"- ■"'■) ■"^"': derm from one arch passes across to the next, and the !,V!'h«u/; i^"yoi^ entoderm likewise, but not the mesoderm, compare *'""^Fig. SriS. As previously stated the inner and outer layers t<^ther form a niembrano ( I 'erschlussphttte) . which closes the gill-cleft.


Ill more advanced stages a<l(litional jiarts are gradually dlfferentiiiled in cath gill-arch. Typically there ai-e four principal structures dcveloix?(], an aortic vesst'l, a do^vugrowtli of the myotome overlying the dorsjU end of the arch, two branches of nen-es, and a rod of cartilage — and they appear in the order named. The aortic vessels arise verj- i-arly and cstabliBli a direct communication between the ventral and dorsid aortce, and are callctl the ,11 aortic arches. Their arrangement

luid metamorphoses are discussed in .';"' Chapter XXlV. Fig. 152 shows the

aortic arch, ,1, in a cnws section of

.. ii gili-arch. The jwirts have a typical

, primitive arrangement from which

"-" \ all miwlifications aro derived. The

y c " " details Jire discusswl in subsequent

,13, oliaptcrs.

. \ iewed externally the gill-arches ^ri. present the following j>ecuiiaritie8 in

amniote embrj-os at the stage when the gill-arches have their maximum typical dovolopment. The first arch di\-ide8 the mouth from the first branchial <'left, and has its lower end enlarged and somewhat knob-like; the second arch has a similar knob, but a little smaller; at first (he four knobs are quite <listinct, but they soon fuse and become more or less indistinct; the third and fourth arches, on the contrary, simply thin out and melt into the general ventral sunfaee. The anterior (cranial) border of the mouth, after the buccal cavity has formed, is also thickened and its upper end joins the dorsal end of the first branchial arch and hence is sometimes called the maxillary process (Oberkieferfortsatz) of the first arch. Additional data and figures of the external appearances are given in Chapter XXVI. Seessel's Pocket. — This term is applied to a small diverticulum which api»ear8 in birds and mtmimals on the dorsiil side of tho pharynx. It was first described by Seessel, 78.1, and has been noted since by various observers, Piersol, 88. 1 , et al.


Origin of the Liver

The liver ia the primitive type of development, as preserved in Petromyzon and amphibia, appears exceedii^ly early. Fig. 153 (compare also A. Goette's figures 76. 1, Taf. II., Figs. 34-38). It is a diverticulum of the archcnteron. Fig. 15:t, Li, near its anterior extremity, and projecting on the ventral sitle downward into the mas« of j-olk-cellH. The short stretch of the archenteron in front of the hepatic evagination is the homolt^ue of the vordeninrm which shows however in this tyjw of development ^^.^-—^^ no t a e, as yot, of its sub nch sequent division into pha. al and epicardial regions When, however, £ e art appears the two

gi 8 of the vorderdarm jecom distinguishable, and I e 1 'cr diverticulum is

I «n lie immediately bcI c ixtsterior or venous

b I ity of the heart. It

1 Mihlo from these facts hat le liver is an older g- in the ancestral histrS" } n. "■ "■ H«i^ "^ ap «. *» ' vertebrates than the vai u ^ M tMw> hr\nxor even the heart. (nation of the liver auses t Kc to 1 1 n, s bsetiueiitly developed ojif-f he k ath-Ht are e»i»ecially developed an rt kn wn e omphalo-niesaniie veins.

The further development, to be described later, brings the liver into peculiar intimate relations with the venous circulation.


In elasmobranchs (Balfour, "Works," I., 45i>) the liver arises during stage I (/. c, three gill-pouches iK'gun, but the first not opfnyet) jisH ventral outgrowth at the hind end of the vordenlarni and immediately in fi-ont of the union of the yolk-sjic with the jin-hentei-on, or in other wonis just in front of the yolk -duet or innbiliod canid, thun bringing the liver into pi-oximity with the vitelline veins entering the heart. As tho gill-pouches are present the ]»h!irj-nx is already differentiated, and, therefore, the liver arises relatively later than in Petromyzon and the amphibians. " Almost ns soon as it is fonne«l this outgrowth develops two lateral diverticula, o|>ening into a median canal. The twc) diverticula are the rudimentaiy lol)es of the liver, and the median duct is the rudiment of the common bile duct (ductus choledochus) and ^1 bladder. By stage K the hepatic diverticula have begun to bud out a number of small hollow knobs."


In teleosts the liver arises quite late, e. gr., in trout the twentyfifth day — as a solid outgrowth from the arclienteron close behind the heart — thus offering one of the many instances of a solid growth in the embryo replacing a hollow growth. (Mcintosh and Prince, 80.1, 774, give their own and cite some previous observations.)


In amniota the anlage of the liver arises in the same position as in the anamnia, but has the peculiarity of showing its bifurcation almost, if not quite, from the start, at least in birds and mammals. The two forks embrace between them the omphalo-mesaraic or vitelline veins just before they empty into the sinus venosus. In the chick the anlage appears between the fifty-fifth and sixtieth hour (Foster and Balfour, "Elements," 178, 179), the right fork being in all cases of greater length but less diameter than the left. In the rabbit (Uskow, 83.2, 220) the anlage appears during the tenth day, and on the eleventh sends out branches; according to Kolliker ("Qrundriss," 372) only the left branch appears on the tenth day, the right on the day following. In man the anlage is well marked in embrvos of three millimetres (His, 81.1, Taf. XL, fig. 7-8, also " Anat. Monschl. Eml)r>^," Heft III., ir,-17). Hishasshown, 81. 1, 322-323, that the liver anlage is a long strip on the ventral side of the vorderdarm, and that when the vorderdarm is separated oflf from the yolk-sac the most ventral part of the entoderm of the vorderdarm already shows traces of the hepatic differentiation. In front of and al)ove the heart the vorderdarm is completely shut off from the rest of the arclienteron (c^ivity of the future 3'olk-sjic), but immediately behind the h(*art the entoderm, as it passes from the vorderdarm around the edge of the fovea cardiaca, and so out on to the extra-embryonic region, is caught, so to sjwak, and forms the anlage of the liver, so that the liver is initiate<l not so much by a local growth of the ent<Klonn as by retention of the downward extension of the layer, which results from the manner by which the embryo is separated from the yolk. The ix)int is important as an illustration of the comparatively simple meclianical factors of development.


Relation of the Liver to the Septum Transversum

The tissue through which the vitelline veins pass to enter the heart forms a transverse partition, whicli divides the pericardial coelom from the aUlominal c(elom. This partition is the rudiment of the diaphragm, and has l)een named the septum transversum by W. His. It lies just l>ehind the he^irt, and forms the ventral edge of the fovea cardiaca, or o]x»ning of the vordenlarm into the general archenteron; it is overlaid in the nunlian line by the hind end of the vorderdarm, and (*onse(iuontly the anlage of the liver is situated in the dorsal median ])orti()n of the aei>tum. As the great veins also pass through tlu? sei)tiim to reach the heart, the iH^pitic^ anlage comes into immediate? contact with the veins; in their further development the veins and entcKlermal liver are closely connectf^l, with the result of complex modifications in lK>th parts.


Comparison of Mammalian and Amphibian Archenteron

For the conveni(»nce of students I have inserted the accompanying diagrams, Fig. 154, A and B. They are extremely conventionalized and may be considered especially inaccurate in that they fail to show the way in which the head {and with it the vorderdarm) projects forward, iwd in that the heart and liver are omitted. £mb is the axis of the embryo represented in nature by the medullary tube and notochord ; bl is the blastopore or nourenteric (anal, behind which the anal opening or anal

A plate should be added

SSi. were the diagram to be

be completed. All is the All infra-blastoporie diverticulum or allantois; Ent

is the cavity of the archenteroii — the letters being placed where the archenteronof the embryo proper passes into that of the yolk-sac; br indicates the four gill-slits. Tiie yolksac, IV, is represented as enveloped in mesoderm, indicated by a shaded layer and lined bj' enfop derm which is indicated ' by a broad black line; it must be remembered that in amphibians, A, the cavitj" is rt'ally filled with yolk-cclls, which are represented in mammals, B, only by a layer of epithelial cells. CIt is the chorion, consisting of a layer of ectoderm indicated 1 ly the outside black line, and a layer of mesoderm, indicated by shading. Between the chorion and the yolk-sac lies a space which is the extraembryonic cadom. In amphibia this jMirt of the coelom develops gradually ; in man it is devel()i)ed very early completely around the yolk-sac; in rabbits it nevei' extends more than half-way round, and other variations occur in other mammals; to surest these differences in mammals the lower half of the yolk-sac in B is ilrawn with a dotted line only; rt. is the vena terminalis.


These diagrams suffice to show that the closest homologies exist between the two tyjies, however much the tictual proiM)rtions may differ. The primitive homologies of the archenteron hold true of all vertebrates.


Fig. 154.— DiBrains tloDHorthe ArclicDl'-n For eiplanation i>( llie



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Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History



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