1897 Human Embryology 16

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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

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
Note - this online text is only at a very early draft stage and contains many errors from the original scanning.
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Chapter XVI. - The Yolk-Sac, Allantois, and Umbilical Cord

The three structures mentioned in the heading of this chapter have Buch intimate relations with one another in the human embrj-o that it iu convenient to study them together; but it ahould not be forgotten that morphologically the yolk-sac and allantois are absolutely distinct origins, as their development in the embryo clearly demonstrates.

I. The Yolk-Sac

General Morphology. — Ah the yolk-sac is the container of the nutritive yolk, destined to bo assimilated by the embryo, the evolution of the yolk-sac must have depended primarily upon the accumulation of yolk in the egg-cell In the primitive form of vertebrate development (Petromyzon, „ n jj Gmoul Amphibia), we

\ / hnd this material comes to lie in the walls of the digestive tract betw een the heart and the all mtois,und chiefly on the ventral side of tho j mal the yolk of the embryo m BitnateI, in other nord in the region of the abdominal cavity. When

I the liver appears it separates the heart from the

niites (f \olk in the entoh ' ^ou*™^ derm \'< so()n ivi the metsoII ans dtnn lsdt^(.l^lledcomI>lete' h ar( und thi, ovum it of course separates the yolk and the ectotlorm. and us s(m>ii as the cw^lom is deveIope<l in tho abtlomintd region there is a hiyer of mi'sotlemi enclosing tho yi>lk; now as tho yolk is oiitodermal it f<.>ll()H's that the yolk, together with the mesodenn layer ai-uiind, Jir<; inorphnlogically part of the splaiichnopleurt;. Thi« splaiichiii>jilfuricl«ig isthc honiologuoof the yolk-sac. In thomerolla«ticaiiamniota* (elaMinnhrimchs) there is a septu'ation of the yolk-sa<r from tho embiyo. and it hangs down from the intestinal cimal of the embryo by a small stalk; but it is covered by the somatopleure .inst as in the mi)re jirimitive types, so that thotnio yolk-sac is iiiclose<l in a second membrano. The same arrant»einent oxi.sts in the amniota; there is an inner or true yolk-sac formed by tho vitelline ent(xlenn and splanchnic mcsi^lcrm, and an outer somatopleiiric sac, homologous with the external membrane of the elasmobranch, but commonly known as the mewbrana serosTi in Sauropsida, and an the primitive chorion in mammals. The term yolk-sac, as applie<i to the elasmobranchs, includes both the inner or true yolk-Biic and the outer somatopleuric covering, homologous with the chorion; but as applied fai amniota, it cymmonly refers only to the inner sac, to the exclusion of the chorion.

Yolk-Sac of Sauropsida. — The manner in which the embryonic archentcron is separated from the yolk-sac has already been descrilied, p. 255, and we saw that the peripheral part of the area pellutnda, the whole of the area opoca, of tho so-ealleKl germinal wall and of the yolk-mass are included in tiie yolk-sac; all the parts mentioned! constituting collectively the entodermal lining of the yolksac. The whole of tho vitelline ento<lenn tends to ti^sume a distinctly epithelial structure; the change Ix^ns in the region of the embryo and thence spreads gradually in all directions; in the region of the area i)ellucida the vitelline epithelium {IJottersackfplfhel) has thin wide colls ; in the region of the area opaca the cells are high cylinder cells, Fig. 1!W, c, of somewhat irregular shape, containing a loose network <if gmnular prutoitlassni; the lower ends of the Pells are rouu<led and projecting Jind havo a woU-marked Imnler of dense prtttoplasm; the nuclei are variable in size, but for the most i>art lai^, often three or four times greatt-r in diameter than tho neighboring niesodennic nuclei : they have iisually one, stimetimes two. conspicuous nucleoli ; tho inndfi idways lie on the upixT or iMisal ends of the cells, generaliy near tino side — a ]ynnt Ix'st made iiut in surface views; thtt ceils further contain yolk grains, which apiHiar to be undergoing resorjitioii; near tho area peltucida the cells are smaller, the network of protoplasm closer, tuid the yolk grains either absent altogether, or, if present, small insi^x; and few in number; thetninsition to the thin <'ntoderm of the area iK'lhicida is tinite abrupt, according to H, Viivhow, 75,1, but I havo found in some cases a gradual change. Tuward the jwripliory of the area ojMica the entiKlcnnal cells bcoonie larger and richer in yolk-gndns and pass gradually into the germiiwil wall. The cylinder cells of the ojiaca entoderm stand at various inclinations, sti that they are cut obliquely Uw the most part ; conseiiuently only here and there can wc recognize them clearly, as ill Fig. 198, c. The germinal wall is the connecting link between the epithelium on the dorsal side of the cavity of the yolk-sac and the yolk forming the tioor on the ventral side of the cavity. The structure and metamorphoses of the germinal wall have been the subjects of much discussion, leading to very little result, for many authors have sought in the germinal wall the origin of mesodermal and even of ectodermal cells; that all such views are erroneous was demonstrated by H. Virchow, 75.1; it would have saved a great deal of confusion if his admirable little paper had received the attention it deserves. H. Virchow has since confirmed and amplified his results in two valuable memoirs, 91.1, 92.1.* The germinal wall is the transition from the cellular opaca entoderm to the non-cellular yolk, hence it consists of protoplasm charged with yolk grains and having numerous nuclei, which toward the embryo become situated in discTete cells, which, as we jmss to the ojmc^, g^radually take on a more and more epithelial character; the non-cellular yolk has nuclei also, but they are further apart than those of the germ wall; these nuclei are the so-called jMirablastic nuclei (see p. 352). As development proceeds we see the area pellucida encroach upon the opaca, the area opaca uiK)n the germinal wall, and the germinal wall upon the yolk proijer; the whole series of changes may be describeil as a centrifugal metamorpliosis.

The mes(xlenn of the yolk-sac is a thin layer which gradually spreads over the yolk, and so slowly that, according to M. Duval (** Atlas," Fig. 052), it does not completely enclose the yolk until the seventeenth day in the chick. The early appearance of bloodvessels in it marks out the area vasculosa, which is a part of the yolk-sac ; the expansion of the vascular area has already been describoti, p. 27G. A further peculiarity of the mescxlerm is that it sends down partitions into the mass of yolk, carrying along the blood-vessels, and thus increasing the absorbent suriFace; the partitions in the chick l)egin to appear during the sixth day, and continue multiplying and growing for at least ten days.

As the yolk-sac contains the nutritive material for the embryo, it diminishes in size as the latter grows ; the shrinkage causes the sac to l)ecome, the sixth or seventh day in the chick, flaccid and somewhat irregular in shape, two peculiarities which l)ecome more and more marked as development progresses. By the eighteenth day the sac is very much smaller; by the nin('tot»nth the reduction is still more striking and the sac l)egins to be withdrawn within the body of the chick, and Ix^fore hatching the embryo takes in the yolk-sac comj)letely through the umbilical opening; during its retraction the sac has a characteristic hour-glass shajx?, owing to the narrowness of the umbilicus.

Concerning the structure of the yolk-mass during the n»sorption of the yolk material we know very little, and of the ])hysi()logy of the assimilation of the yolk, almost nothing. Von Baer pointed out, 28.3, that the yolk becomesmore fluid in the (»hick, and H. Rathke, 39.1, 113, that in the snake the separate yolk-granules disapj)ear, and the yolk becomesa greenish-yellow homogeneous translucent fluid. H. Strahl, 87. 1, gives an important account of the yolk-sac in the lizard, showing that the dissepiments of mesoderm are covered with large yolk-cells — the whole yolk apparently becoming cellular in later stages ; the cavity of the sac is very distinct ; the sac itself becomes flattened ; and it is only on the inferior side that the dissepiments acquire a considerable development, and on this lower side the cellular structure is perhaps never fully attained. The regular form of the yolk-sac persists in the lizard, but in the snake, H. Bathke, 39.1, 183-184, it becomes flaccid and irregular.

  • that thetie memuirs cauic to my hauds too lute to eiiahlt* luv io inforp<irate Virchow*8 results in the text.

Yolk-Sac of Mammal B. — In order to understand clearly the development of the mammalian yolk-sac, it is best to start with the two-layered blastodermic vesicle, with a small embryonic area in which there is mesodenn ; the inner layer of the vesicle is the homologue of the yolk-mass of Sauropsida, and is able to assume the cellular structure owing to the loss of yolk, which is undoubtedly also the cause of the large size of the cavity of the vesicle — this cavity being, as we have seen, the vitelline cavity ; the inner vesicle then is the homologue of the ent(xlermal part of the yolk-sac. The extraembryonic mesoderm and ccjelom are extremely variable in extent in the mammalian ovum; in man, as we have seen, the mesoderm is ver}' early developed completely around the yolk-vesicle, and so is the coelom, so that in the earliest accurately known of human stages the yolk-sac and chorion are completely differentiateti. In the sheep, and prolmbly in all ruminants, there is a similar early separation of the yolk-sac and chorion. In the rabbit the mesoderm never extends over more than about half of the blaHtodermic vesicle, but the coelom extends nearly to the i)eriphery of the sheet of mesoderm ; hence we have a half-way separation of the yolk-sac and chorion. In the <)ix)ssum the mesoderm extends about half-way over the blastodermic vesicle, but the ccflom is developed only around the allantois, so that there is only a very partial separation of the yolk-sac and the chorion. In lx)th rabbit and opossum the lower half of the yolk-vesicle is in direct contact with the ectoderm, preserving to this extent the condition of the stage of the two-layered blastodermic vesicles.

That the partial extension of the mesoderm represents a modified condition is evident, since in all non-mammalian vertebrates both mesodenn an<l coeli)m extend completely around the yolk. Hence, the comj)lete separation of the yolk-sac in man and the sheep is nearer the ancestral tyix> than the relations of the extra-embryonic germlayers to one another in the rabbit and the ojx)ssum. The question as to what was the primitive mammalian arrangement must l>e left o])on; we cannot say whether the opossum or man most nearly represents th(» ancestral tyjH\

Man. — the human yolk-sac is an apjx^ndage of the digestive canal formed by the extra-embryonic somatopleure. At the Ix^ginning of the third week the diameter of the yolk-sac is about e<iual to the length of the embryo. By the middle of the third week the sac has become distinctly y)ear-shaixKl and is attached by its pointeil end to th(» intf^stinal canal of the embryo, Fig. 17. The sac continues growing up to the end of th(» fourth week, after which it enlarges very slightly, if at all: its diameter is from T-1 1 mm. It is then a Ixvir-shapiMl vesicle attached by a long stalk to the intestine, the stalk having been formed by the lengthening of the neck of the yolksac, Fig. 1(59. Sections show that the sac is hollow, with a lining of entodermal cells, and a thicker layer of mesoderm, containing blood-vessels ; the network of vessels imparts a characteristic appearance to the external or mesodermic surface of the yolk-sac, compare Fig. 175. The accompanying Fi^. 199 represents a section of the yolk-sac of an embryo of about 1 mm., after Keibel. The cavity of the yolk-sac extends at first through the stalk to the intestine, but it early becomes obliterated in the stalk. The entoderm disappears altogether and quite early in the yolk-stalk ; thus in an embryo of 12.5 mm.. His ('*Anat. menschl. Embryonen," III., '^()) found only remnants of it in the stalk. In the vesicle itself the entoderm consisted in a very young ovum of a single laver of cuboidal cells (Graf Si>ee, 90.1, 103), but is said to become fatty and to change into a i>avement epithelium, which is also thrown up into vascular villi (Kolliker). In regard to the further contents of the yolk-sac, Von Baer states, 37. 1, 272, that in young ova (at six to seven weeks) the contents are sometimes as thick and yellow as the yolk of a bird's egg; in ova of this |)eriod the thinner the contents the more rounded and fully distended is the yolk-siic. A little later the contents are alwavs liuid, but at the end of pregnancy, according to B. S. Schultze, 61.1, when the si)c has shrunk to -4-7 mm. in diameter, it contains variable (luantities of fatt}' substances and cartonates. It thus appears that during the first montli, at least, the yolk-sac does contain more or less true yolk — an idea which is confirmed by Raulx?r's observations on the rabbit's ovum. It seems indetM.1 probable that the rudimentary yolk-sac of man still })erfornis for a short jx^'icxl the function of a t'cHxl reservoir for the embryo, amnion, and the chorion. (B. S. Schultze, 61.1). Shekp. — The two-lavered blastodennic vesicle, with an enibrvonic shield, has an elongated form; the mt^sodorni sjavads out gradually l)etwoen the ect^xlerm and entoderm (yolk-vesicle) starting from the shield; the ctt?lom is develoi>ed in it as it spreads, so that l»v the thirteenth day (R. Bonnet, 89.1, Taf. VI., Fig. :\) about a third of the ovum is furnished with mosiKlerm, and in this third the splanchnopleuro of the yolk-sac is completely sei)arated from the chorionic somato])leure, while elsewhere the yolk (?ntoderm is still directly in contact with the ecrtoderm; this stage, st^ Fig. 'iOO, is as far as development progresses in the rabl)it. In the sheep the development of the mesodenn and the coelom proceeds, until al>out tln^ s(n'(»ntet nth day the yolk-sac is completely seimrated from the chorion; the yolkstiilk remains short, but the yolk-sac projier l>ecomes drawn out and twisted, following in its growth the I'haracteristic elongation of the ruminant chorionic vesicle.

Fig. 199.— Section of the Yolk-Sac of a Human Embryo (No. 11, p. 29D. Ent^ Ento<lemi; mes^ niesoilerni; V^ bloodvr.*s»el8. Aftor Fr. Keibel.

Rabbit. — The development of the extra-embryonic mesoderm and coelom is entirely arrested at about the stage reached by the sheep on the thirteenth day, so that the yolk-sac and chorion are never differentiated over more than half the ovirni, the inferior hemisphere of which remains in the stage of a two-layered blastodermic vesicle, and is said by Duval to degenerate and be resorbed. The accompanying diagram will suffice to render the disposition clear ; it will be seen at once that the chorion, C/io, exists only part- way round the ovum. I introduce here Fig. 201 of a section through the wall of the yolk-sac of a rabbit embryo of thirteen days; the structure closely resembles that of the area

Fig. 200.— Dlagram of the Embryo and Yolk-Sac of a Rabbit, coe^ Oceloni: Clko, chorion; Yk, yolk-ftac; me<, mefloderm, v.f, vena tenninalis; Ent^ entodi'rm ; Ec^ ectoderm.

Fig. 201.— Vertical Section of the Wall of the Yolk-8ac of a Rabbit Embryo of Thirteen Day*. Blood-vet»el8 ; 6/, bloud-cellu; meM, mesoderm.

opaca of the bird's yolk-sac, Fig. 108, except that the entodermal cylinder epithelium is composed of much smaller cells in the rabbit, owing to the absence of

An ^ r* yolk.

Opossum. — Our knowledge rests mainly upon the ol>ser\'ati(m8 of Selenka, Phxam ®7. 1, whose diagram I have copied. Fig. 2()'2; the embryo, Kmb^ is ahiiost entirely covered by the pn>amnion, Pro. am, the amnion. Am, lieing very much reduced; the allantois, All, projects also into the yolksac cavitj% Vk; owing to the development of the proanmion and allantois the cavit}', Yk, of the yolk-sac acquires a very complicated

Fig. 9.— Diagram of an Oi>u«itnmi Embr>'0 and ita Appc*ndaf?es. ^f. Sinus terminal Ik: C%o, chorion ; ^m. amnion: Er, ect«Klerm; men. Pro. am. iinvamnion; Ent yolk-sac: .4//. allantois; Coe

, mesoderm: embryo: form; the extra-em})r\'onic :;ri^I""VfJ*E.*il'ii' coelom, Coe, is hardly "im.re

than a space around the allantois, and consequently the true chorion is reduced to an insignificant area, Cho; the extra-embrj'-onic niepoderm, mes^ extends over nearly half the ovum, from st to st, but contains — except around the allantois — no coelom; in this sheet of mesoderm the blood-vessels of the area vasculosa are developed ; and as there is no coelom over the area, the vessels are almost as closely related to the ectoderm, J?c, as to the entoderm, Ent. Here, then, we have the mesoderm spreading out as in the rabbit, but the development of the coelom is arrested. Although the opossum stands low in the mammalian scale, its foetal membranes show many changes from the sauropsidan type and are probably modified in an aberrant manner, differently from mammals of other classes. For the peculiar relations of the yolk-sac to the allantois, see the description of the latter organ.

The So-called Parablastic Nuclei of the Yolk. — In meroblastic vertebrate ova, after the embryo is formed, there appear in the yolk near its surface underneath the extra-embryonic blastoderm peculiar large nuclei, which are commonly designated as the parablastic nuclei. The following description applies to Pristiurus.* The extra-embryonic ectoderm is a thin, much-flattened epitheliiun lying close to the yolk ; below the ectoderm is the superficial layer of the yolk, a broail stratum of protoplasm with scattered small yolk granules ; a little deeper down a row of irregular vascular spaces, and again a little deeper a layer of very big nuclei, each with a distinct intranuclear network and several deeply-stained nucleoli ; the nuclei vary in size, being from two to five times the diameter of the nuclei in the embryo. The uj)i)er part of the protoplasmatic stratum contains numerous small and a few larger yolk-grains, and contains near and under the embr\'o small nuclei; the middle part of the stratum contains the viicuolos, the big nuclei, and but few yolk grains ; the deei>est jmrt coiitiiins larger granules and merges gradally into the yolk proper. (See also His, 82.1, 75, and Riickert, 86. 1.) Ruckert designates these nuclei as ^* Merocytefikernej'^ and the cells which they represent as ' Merocyten,^ H. E. Ziegler, 87.1, states that the i»arabhustic nuclei of teleosts multiply up to a certiiin stage by indirect division, but later they assume a peculiar habitus and multiply by indirect division, and iissmne various shai>es. Those cliaiig(?s are perhaps connected with the death of the nuclei, their active functions Ijeing completed. The special function of the protoplasmic biyer appears to be the assimilation of the nutritive yolk. Riickeii; also maintains, but without proper evidence, it seems to me, that merocytes become cells, some of which join the ectoderm, some the entoilerin, and yet others the mesenchyma. In the Saurojisida we find similar nuclei and similar relations of the nucleated layer, but in them the protoj>lasmic layer Ix^comes the epithelium of the yolk — see esj)ecially H. Strahl, 87.1 — and I consider it prol>able that these parablast nuclei in all meroblastic ova belong to the vitelline entoderm. J. Riickert, 92.3, claims that some of tlic "parablast nuclei" are derived from spermatxizoa, which enter the yolk but do not unite with the female pronucleus; it is doubtful whether or not any of these spermatozoa nuclei share in the production of embryonic tissue.

  • From sections iu the collection of Professor His, which he generously iKTiiiittixl me to irtudy.

In holoblastic mammalian ova the vitelline entoderm is cellular, and no nuclei are known similar to large " parablastic " nuclei of meroblastic ova.

II. The Allantois.

The origin of the allantois we have already described, p. 257. It arises as an entodermal evagination behind or below the blastopore and anus, and extending into the anterior end of the primitive streak.

Allantois of Sauropsida. — The allantois becomes rapidly distended by the enlargement of its entodermal cavity, and hence comes to project freely into the coelom as a vesicle, attached by a i)edicle to the anal end of the intestinal canal. This vesicle is of course formed by the splanclmopleure, and therefore line<l by entoderm, with an outer layer of mt^oderm. In the chick the vesicle is about as large as the eye by the middle of the fourth day, and after that grows very rapidly, becoming bent so as to project on the right side of the embryo; by the end of the fourth day it is already about as large as the mid-brain at that stiige (c/. Duval, "Atlas," Fig. 122), During this expansion its mesoblastic walls, which are at first very thick, become thinner, and at the same time the allantoic blood circulation lK?comos important. The blooil is supplied directly from the dorsal aorta, which terminates in a fork, of which each branch is an allantoic arterv, and the blood is returned bv two allantoic veins, which run along in the body walls. By the third day in the chick they are found, after having united into a single trunk, to open into the vit€»lline vein, close behind the liver. The allantois continues enlarging, and pushes its way very rai)idly into the extra-embryonic coelom, l)etween the amnion and chorion (serosal or false amnion). Curving up around the right side of the embryo the vesicle comes to lie on the dorsal side, al)ove the amnion, and separated from the shell by nothing more than the thin chorion. In this position its rapid growth continues ; it forms a tiatteneil bag, covering the right side of the embryo, and rapidly sprefuling out in all directions. It is filled with fluid, so that in spite of its ttatteneil form its opposite walls are distinctly separated from one another. The expimsion steadily continu(»s, so that bv the ninth dav the allantoic vesicle nenrlv surrounds the yolk; during the eleventh day the outer wall of the allantois lx»<j:ins to grow, together with the chorion; hence in opening an egg during the later stng(?s of incubation, there is much danger of tearing the allantois when the shell membrane is removed. The embryo niav now 1h^ said to Ix) surroundeil bv two new membranes — the outer and inner walls of the allantois. Alx)ut the sixteenth dav the allantoic sac completely envelops the ovum, and by the seventeenth its (mIixcs fuse. T?he closure, according to Duval, 84.2, takes place in sucli a manner that there is formed a sac of ectcxlerm, inclosing the remnant of white at the pointe<l end of the ovinn ; this sac, as well as tin* yolk, is inclosed bv the allantois.

Allantois — Concerning the tissues of the allantois we possess very little information; the entodermal lining appears, at least in advanced stages, as a low cuboidal epithelium, while the mesoderm is thicker and consists of more or less widely sepitrated mesenchymal cells, covereil by a thin mesothelium; the mesoderm contains bloodvessels ; and since contractile pulsations have been observed in the allantois of the chick toward the close of inculcation, it is probable that some of the mest>nch\Tnal cells assume the form of smotcth muscle fibres. Where the allantois fuses with the chorion (membrana st>rosa) the mesothelium of both layers disiippears, and there is no demarcation or difference l)etween the allantoic and chorionic mesenchyma — compare Duval, 84.2, ]Pl. IX., Fig. 8.

Allantois in Mammals. — The alUmtois is very variously developed in the mhmmalia, l>eing a distinct vesicle in most fonns, but never growing around the embryo aiul j'olk, as in birds. In the ojxt.s.sfiin, Seleiika, 87.1, HI, the allantois does not even come in contact with the chorion, but invaginates the wall of the yolk-sac, as shown by the diagram. Fig. '^O'^ ; the wall of the yolk-sac forms a IHX^ket in which the allantoic vesicle is lodgcnl : the walls of the two organs do not unite; the poc-ket in the yolk-sac has ciu'ious relations to the main blood-vessols running from the embryo to the area vasculosti, for the two omphalo-mesaraic veins run sti-aight l^ack from the embryo along the edg(.»s of the mouth of the i.K>cket, while the single omphalo-mesaraic artery runs in a great arch in the median plane round thc^ lx)ttom of the i^x^ket. These features are beautifully illustrated by Selenka, 87.1, Taf. XXVII., Figs. 1-4. The allantoic wall is figured by him, Fig. -A, Taf. XXV., as consisting, in an embryo six days old, of an iimer layer of entoilermal culx)idal epithelium, a thin outer laver of mesothelium, and a thicker laver of vascular mes(.>nchyma. In thes rabbit (and ju-obably all ro<lent«) the allantois iMX'omes a moderate-sized vesicae. Fig. 190, .4//, which grows out until it reaches the placenta chorion, with which it unites to co-oi>erate in the development of the placenta. In insect iiora the allantois sin^nis to resemble that of the nnlents, though it acquires greater size; exact investigations are nmch needed. In ruminants the allantois expands very early, growing out transvei*S(^ly and continuing to enlarge with extraordinary rajndity until it takes up most of the chorionic vesicle, thus lx.*coming, relatively to the embryo, of enormous size. For f mother details S(H» R. Bonnet, 89.1, *^(.>-l5f>, and Bi.schoff, 64.1. A few histological facts may be gleaneil from the very verbose article on the allantois l)v A. Dastre, 76.1, 17-44.

Man. — The allantois in man and other primates is essentially different from that of any other known amniiotes. It never lxH*omes a free vesicle, but alwavs remains a narrow tubular diverticiUum. In man the eml)rvo, when the amnion is formed, becomesevervwhere separated from tlu» chorion, except at the hind end, where the aivinnulation of m(»S(Klerinal c<»lls into which the allantoic* diverticulum extends, sei» Figs. 17o, ISO, and 'Vl'l. constitutes a thick st-alk. This stalk has l)een ntimed the Bfturhstiel hv W. His: it mav be regarded as a din^'t ])rolongation of the bcKly of the embryo; it permanently conniH'ts the (»mbrvo ])roper with the ch<)rion. The amnion siu'ings from the sides iA' the Bauchstiel in the same manner as from the body of tho embryo. In man, therefore, there is no free allantois, and the history i>f tho organ may be «iid to be reduced t«> that of the entodermal allantoif diverticulimi. The entodermal aUantoie is a small, long, epithelial tube extending, us we have seen, to the chorion, p. iSi., Fig. 1*0. The tube increases very little in diameter, after the second month; conn>are A and B, Fig. 444. It is very persistent, aiipeariug usually even in the cord at full term, at least in the proximal end, according to KoUiker ("' Eutwickelungsgescii.," 2te AuH., p. ^14). After the second mouth it is a small group of epithelioid cells, with distinct walls, irregularly graunlar coutents, and round nuclei ; around the cells. Fig. So;J, eiit. which mayor may not show a remnant of the central cavity, there is a slight condensation, mes, of the connective tissue to form, as it were, an enveloj)©. This structure has been regarded by Aldfeld and others as the persistent yolk-sac. I think tho correct iuteri>retutiou was first suggested by KoUiker.

It has been supposed by some writers that tho human tdlantois grew out as a fi-ee vesicle. Haeckel even went so far as to prophesy that when a human embrj-o of tho right stage should be obtiiine<l, this condition would be found. Shortly after this ^V, Krauae pid>lished a description, 76.1, of an embno, which he said was human and had a free allantois. mistaken, tho former tliniiigh hasty

latter thn>ugh an i-rror as to tho identity of his embryo. W. His ItaH shown that it Wiis certainly not human, pnibably not even mammalian, but avian. 80.1, i. Krause still maintained that it was human. The discussion as to this specimen was a long and animated one, but has now little interest except historically. See Krause, 80.1, 81.1, 2, KoUiker. "Entwickehmgsgescli.," ISTO, 30(5, 1013, Ahlfeld (Chi. fiir Gymik., lS*"(i, No. 2r.), Krause, ibid., 81.1, and F.cker in His" Arch. f. Anat., lt*80, 405.

Allantoic Fluid. — The fluid contents of the allantois cannot be well studied in man, owing t<) tho minute size of the cavity tif the organ ; but when the allantois l>ecomes n lai^ sac, as in the cow and jiig, the fluid is readily collected. There are many obser^■ation8 rocordetl tH)nceniing the chemical composition of the fluid, but the In-st work known to me is that of Dikierlein, 90.1, on the fluid in cow (embryos. His results may be sunimiirized us follows;

Both Haeckel and Krause were unfounded siKJCulation, tho

Tho allantoir; fluid dittV-rs marke^lly fn^m the amniotic — compare tluj Uililf^ afKivo with thosf? on \t, X'M — and shows in its composititm thai it is mi excnjtory prrxltict of the f<fctus, coming from the Wolffian }Ki<lirrH and thf? kidnfjvs. In the chick, bv the sixteenth dav, deposits r>f water iKji'ome abundant in the fluid (Foster and Balfour's '* Eleineiits," HfMrond edition, 'ZHd),

Notices of s<?veral of the earlier investif^ations on the allantoic thiid may Jx- fotnid in A. iMstre, 76.1, 45-^11, together with some results of liis riwn.

III. Thk Umbilical Cord

Bauchstiel. — As the* Hauchsti<»l is the anhige of the human um])ilical cnrd, wo inust consider its structure and relations. As we have already seen, it is the prolongation of the tail of the embryo. Fig. I'W;, .4/, i-uiming to the chorion and containing the tubular allantoic div(Tticuluni, Fig. ITO, -1/; it consists mainly of mesoderm, and from its side sju'ings the amnion. Prof. I lis ('* Anat. menschl. Emluyonen," Heft III., t>'^>:>-:>->ii) has made a special comparison which shows that the fundamental mnri)liological relations are the same in the human Hauchsticl as in the embryo j)roi)er, and that there are even traces of a rudinumtary medullary groove. The rcs(»niblance is illustrated bv the accumpanying Fig. -2(4. The amnion, .Im, arches over the dorsal sidt*, which is covtM'i^d over by thickened lu'tiMlcrin, ///(/, which His regards as the rudimi^nt of the nuMlullary groove; the archeiiteron is re])resented by the allantoic diverticulum, .!//, lined by tluMMitod(»rmal t»pitbeliuni : in the mesoderm run the two large allantoic veins, F, T', and the two smaller arteries, -4, A; the space around the cord is of course part of the embryonic coelom, coe; the amnion represents the somatopleure, the walls of the allantois the closed splanchnopleure.

To convert the '* Bauchstiel " into the umbilical cord, the somatopleure bends down on each side, and finally closing on the ventral side below the allantois, shutting in a j)ortion of the coelom, and becomes separated from the amnion. The amnion separates from the embryo first, then from the embryonic end of the Bauchstiel, and last of all from the distal end of the Bauchstiel ; hence, when the closure of the somatopleure is completed the amnion arises no longer from the embryo, but only from the end of the cord, where it joins the chorion. The closure of the Bauchstiel forms a long tube running from the embrj- o to the chorion ; the cavity of this tube is part of the coelom ; the whole tube is known as the umbilical cord.

When the coelom of the cord is shut oflf, it is shut oflf in such a way that the long, narrow stalk of the yolk-sac, or the so-called vitelline duct is included, compare Fig. 222, v.s. This is possible owing to the rolling up of the embryo, which, as shown in Figs. 10i», 172, 175, and 222, brings the Bauchstiel into close proximity with the neck of the yolk-sac.

The development of the cord shows that it is never covered by the amnion, which, on the contrary, is always separate from the cord proper. This point is important to note, because in most textbooks the cord is erroneously described as covered by the amnion — compare Minot, 98, 380.

Development of the Cord. — The origin of the cord from the Bauchstiel has been described in the preceding section. The

Fig. . - Sti'ti«nis of Human UmWiliral 0»nlH. A, Kinhryo uf 21 nun: B. iMnhryo of sixtvfour an«l sixty-niii«* ilays; r, left umbilical vfiu; Jr, arteries; All^ allantoiH; C*tK',*oa.'lom; Fn, yolk-stalk or vitfllinf auirt.

Structure and growth of the cord may be best studied in croas-sectioiis. Fig. 'ii)i). The ca4om, Coe, is a large cavity and contains tilt' yolk-stalk, T'.s, with its two vessels, and its entodennic cavity entirelv ul)literate(l: near the enibrvo the coelom mav bi^come much enlarged, and is often found during the second month, and even later to contain a few coils of the intestine; above the iKxly-favity is the duct of the allantois, ,4//, Hnwl by entodermal epithelinm; and in this region are situated the two arteries and single vein ; the section is bounded iiy ectoderm.* Thp further development of the cord de Eends upon throe factors: 1, the growth of the connective tissue and lood - vessels ; 2, the abortion of the coelom yolk-stalk and allantoiB duct, in the order named ; 3, differentiation of the connective tissue and of the ectoderm.

The growth and differentiation of the mcwxlenn proccetl rapidly, encroaching u]Hm the ccel m which is bliterated {e irh in the fourth month). At first the connective tissue Fij, *0 is con pt seil merely of miinerous cells embedded in a clejir sut)staiice; the cells fonii ii complex in'tw<irk, of which the filaments and nicwhcs mv extivmcly variable in aizf, the nuclei art! oval, gi-anulur, and do not have always acTumiiliititms of prutuplajjm alvmt them, tonning main ci'llboditw.

I notice also a few cells, which T supiwisc to 1h> lciK'<H'vtcs, hut see no other structures. By the end of the tliini mouth the cells have assumed nearly thi'ir definite form; the prutophism has inen-asod in amount and fnrnis Ji large cell-lmdy aiinmd eaeii niiclens. l-'ig. 'JOT. The network luis liceonie siniiiler and oiiiii>!er, the niestu'S bigger, anil the tilameuts fewer and thicker; in the matrix are nrunerous connective-tissue fibrils, not yet disposed in bundles except here and there; as they curl in all directions many of them are cut transversely, and tiierefore appear as dots. In older cords there is jm obvious increase in the number of tilirils, and they form wavy bundles. In the cord at term the matrix contains mucin, and may Ih) stained by alum ha^matoxylin ; at what period the reaction is first develope<l I have not ascertained. I have observed nothing to indicate the presence of special hTnph-channels in the cord at any period, but I have not investigated the iK)int. Tait's lymph-channels are merely the intercellular spaces.

Vui. ;S'T. Connective Tissue of the Umbilical Cord of a Human Embryo of about three .'MiKitlis.

X 511 (liaiiieterH. Stained with alum, cochineal, and etMiiu.

The ectoderm is at first a single layer of cells, a condition which is |x>rmanent over the amnion ; in an embryo of three nuniths I find tlu» two-layered stage. Fig. i3()8. The outer layer is granular, and in SOUK* pnrts oixch cell i)rotrudes like a dome;* the inner layer consists of larger, clearer cells. By the fifth month the stratification of the cpitheliuin becomesmore evident and cornification begins. The ectoderm (Er), therefore, develops like tlu* epidermi proper, although much more slowly, so that it never gets beyond the stage which the true epidermis reaches by })erhai)s the ftnirth month; on the other hand it differs entirely fn^m the amnioticr epithelium.

  • Fn»rn the investi^rations of T>r. J. T. Bow»»n on tin* ilevelopment. of the enideriiiis. which Ik' has Imn'ij ciirrvinir on in the histolo^ieal hilM»ratory <»f the Harvanl Metlical Soh<>«il, It Heeiiw i«> UMi prohaldf that this external layer iK homologous with epitrichium.

the blood -vessels steadily eiilai^ and afqiiire thick muscular walls. In the cord of an embryo of il mm.. Fig. ■JO.'i, the arterial niiincHlaris is well marke<l, the venous inwwM/«/'(> just beginuiug to show. At sixty-three days I tind the coat thickened on all the vessels; there is a (gradual passHf^e from the muscle cells to the surrounding connective tissue, so thjit one wins (he iniprci-sitin that the connective-tissue cells are being directly metamorjihoeed into muscular fells. By the tifth month the demarcation of the muscular coats is quite sharp, and it is probable that the further growth of the layer deitends ujwn the growth of the elements it already contains and not ui)on the accretion of now ones; that the muscle- cells do actually become bigger is easily ascertaineil by dirwt oliserviitiou.*

The obliteration of the ca-loni goes on rapidly during the second and third months, and by the beginning of thefnurtli is nearly or quite complot«l. The vitelliue duct persists longer, biit soems to (tisappear by the si.^th month; for a time it is distinguishable as a shrunken icmnant in tlio midst ijf the ciamectivc-tissntj cells of the i-ord. The allantoic duct occupies usually a [wsition ln'tween the two arteries; it attains its maxtnnim diametc^r alxnit the fifth week, when it is a small epithelial tube. Fig. 2o;i, of irregular width, as wliich it remains for some time without noticejxblo altcmtion ; during the third month it loses this character and beconn.'S s«ilid. by the enlargement of its epithelial cells ; the duct iwi-sists up to birth in this fonn, though losing, according ti» Kiilliker, its <'oinplcte continuity; after it becomes solid there is a uliglit (iiudeusatiim of tissue aromid it.

The Human Umbilical Cord at Birth.— The humau cjnl is a long twisted rejie of tissue, wliitish in color, and attjiched by one end to the navel of the embryo, bv the otluT tci the surface of the placenta. Ttw dintcnsions are extremi'ly variiible at all periixis; at birth it is usually al)ont fifty-five iH'ntinietres long and twelve milli^ metres thick; it is said that c<irds only fifteen I'cntiniettvs longas one extreme, and over one hundnHl and sixty irentiuietvi's long as the other extreme, have been observed. Its surface is smooth and glistening, except at the constrictcnl foetal end, where the ei)idermi8 stretches about one centimetre on to the conl. The placentid end expands to fuse with the chorionic membrane. The placenUil insertion is generally eccentric, that is, the cord joins the placenta at a point between the centre and margin of the latter orgiin ; usually the eccentricity is well marked, and not infreciuently is so great that insertion becomes marginal ; in still rarer cases the cord joins the chorion outside the region of the placenta {itisertio relanientosa'j. Occasionally the cord forks l)efore joining the chorion insertion f areata).

The twisting of the cord is always well markwl externally at the time of birth by the spiral ridges within each of which a largo bloodvessel runs. I have observed the number of spirals to varj' from thrt»e to thirty-two; the turns, beginning at the embryo, go usually from left to right, but ' sometimes from right to left. The cause of the twisting, which begins about the middle of the second month, has been much and very unprotitably discussed. Of the many theories on the subje(!t which have becm advanceil, there is not one, so far as I know, having the slightest claim t<3 acceptiince. These vagaries have l)een collated by Hyrtl, 70.1, and also less fully by Lawson Tait, 76. 1, who adds to them. All we <*an say is that the vessels grow faster in length than the cord as a wIkjIc, and therefore assume the spiral disposition; the cause of this inequality is as completely unknown to us as the causes of all the other inecjualities of growth which o<*cur .in the embryo. One jK)int must be sixK'ially mentioned, namely, that there is no reason to suppose that the cord as a whole a(?tually twists any moi'e than the spiral intc»stine of a shark is fonnod b} twisting; many writers have falsely jissumed the occurrence of this twisting motion, and have dissertiitcnl at no little length on the revolutions of the embryo in u ten), Tliore is nc > evidence that such revolutions occur, nor have we any ground for assuming that the twisted appearance of the cord is dut» to an actual twisting like that of a r()})o; if a long rul)lx.»r tulxj forms a coil within a short glass (cylinder, it does not indicate that the cylinder has l)een twistinl.

Tlio cord is covered by a layer of epithelium which is continuous at the distal end with the epithelium of the anniion. Its int(»rior consists of a pt^'uliar embryonic connective tissue known as Wharton's jelly, whifli is described Ih.»1ow; in this jelly an* found at birth thrcM' Jargt* bl<:M>d-vesst»ls, and usually a few degenerated remnants of the e])itheliuin of the allantois. There are no capillaries except dose to the nav(»l, and, in spite of the opinion of some writers, it appears safe to say that there are no lymj)!!- vessels,* and no nerves in the distal part of tin* cord. Si'liott, 36.1, claims to have followed branches of th«» he])atic pl(»xus along the vein three or four centiin<*tres into tiie ('(jnl, and branches of the pl(»xus of the colon and uterus an cnjual distance along the arteries. Val(»ntin has found n(Tvrst'v<'n further, s-ll ctm. from thr navel. As KoUiker remarks in his lar^or t«xt-book, 79.2, ii. 347, the uliseiico o£ nenes iu the diHtiil portion of tbt; ord mid in the chorion is of no little physiolt^ical interest, since the blood-vessels arc so contnictile. In a cross section, Fig. ^i»i(, as usually obtained, the vessels an; found contriwtwl, tlio nrturies, AA, with their c;ivities ^' -^ - ^^ almost obliterated. The vessels have thick walls comiwsed of a muscular a)at tiiid rudimentary intima, but without »ny sjwciul cxtenial cunnective- tissue layer. The vt'».sels differ fi'oin adult vessels of similar caliltre in many res])ec'ts; thoit) is no ehiatic tissue so far iLs I have observed in any iwrt; the muscle-cellH ai-e short, fusiform, loosely aminged, iind run in various directions ; next the intima the fibivs are longitiidiFio w»-«*«ws«.ii)dof no umw- nal in trend ; in the rest of the cimt they w™ ^1 [temM™of the'nii«ntoi^'"r! «n3 {i^upod in l"miiiie, which have the ™lS?!.?iJr™'™'*' ™'"' '^^ ■ """ fibres obliquely in one diivction or another, thus (jiving rise to the appearance of alternating spiral <rofits, noticed bv J^awson Tait. 76.1 (p. 434 and Plate XIII., Figs. 17 and IS)". the muscular emit passes over without any sharp demarcation intt) the surrounding tissue, known as Whartwn's jelly, which consists of scattered anastomosing cells, compare Fig. 2U7, ami a muciimmus matrix with Terj- numerous connective-tissue fibres. The cells and fibres tend to arrange themselves in concentric lines around the bliKxi- vessels and parallel to the surface of the cord. Fig. ■■ii'li, so that we may speak of four systems; within each system the cells t<'nd to an elongated form, but where the systems apjiroach one another the cells become more triangular, as seen in sectittn. Fig. 3(i!i. and show three or four main processes. These triangular cells form, of coiu-si'. long columns which are more or less distinct from the tissue eiu-om Imssing the vessels; to these columns the name of clionlii' Jiuiicilii' lus lieeu tipplied by HyrtI; they are said to have been iiotictKl b\Woortwyck over a centurv ago. The external covering of the cord is a stratified epitheliiun, of which the outer layer is distinctly i-orneous; sometimes thon> are spai-es without cells, which Imvc Ixi'U i-eganiwl as true lymph stomata {Kiister auil also Tait) ; the middle layer is composed of clear cells, and the basal layer of granular cul>oidal cells; in section the apj)earances aix- ckiscly mniiiarabli' to those of the embiyonic epidermis fi-om i>arts where there an- no hairs, and at the time when the homy layer Ix-gins to ap)>ear. As there is no differentiateil conm^tive-f issue layer iK'neatli the cpithehum, the ci)vering of the coiil is liest de-scriU'd as endiryonic skin. According to current descriptions the cord is said to l>e covered by the anmion. but this is obviously an ermr, as shown by His' oljservations uikmi the divelopment, and my own uin>n the histology of the cord.

  • Wninh'rinir rflls «M'fiir in thn iiircrcflhilar •<prtr«»N nf Wlmriotrs .i«'lly. and it is |H»ssilili* thnt ther»' jin* l.viiipli ' //'f/Kir/N ill tin* matrix. t)ii>ii;rli no v»*sst»ls. Coinpuiv i»iirtii'iilaiiy K«ist»*r'H pa|M'r

there is usually to lie seen in sections of the rord at term, according to Kiilliker, 79.2, p. :i44. esjiecially in sections from thcjii-oxiinal end and middle i-cgion. a small giiKi]) of ejiilhelioid cells, with distinct walls, irregular granular contents, and roundcnl nuclei ; around the cells. Fig. 203, there is a slight condensation of the connective tissue to form, as it were, an envelope. This structure has been regarded by some writers as the persistent yolk-stalk, as, for example, by Alilfeld (Arch, fiir Gifndk., VIII., 303). Kolliker, 79;2, p. 344, considered it to he the remnant of the allantoic cavity — a supposition which my own observations confirm.

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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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