1897 Human Embryology 17

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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 XVII. The Placenta

For convenience the placenta may be considered as an organ by itself rather than as a derivative of the chorion and of the decidua, which it must be considered from a strictly morphological standpoint. I give as full an account of the human placenta as possible.

I. The Human Placenta

Placenta at Full Term. — The human placenta {Mutterkucheu) is a disc of tissue to which the umbiliciil cord of the child is attached by its distal end. As a result of normal labor the amnion and chorion, by which the foetus in utero is surrounded, are ruptured ; the child is then expelled, but by means of the long umbilical cord remains attached to the uterus ; after an interval the placenta with which the cord retains its connection is loosened from the uterine wall and expelled, together with the foetal envelopes and portions of the decidual membrane (uterine mucosa) of the mother; the parts thus thrown oflf secondarily constitute the so-called afterbirth of obstetricians.

The placenta at full term, as thus obtained by natural expulsion, is a moist mass, containing a great deal of blood, spongy in texture, about seven inches in diameter, but very variable in size, being roughU^ pro]X)rtionate to the bulk of the child ; usually oval, sometimes round, but not infrequently irregular in shape. One surfa(*e is smooth and covered by a pellucid membrane (the amnion), and reddish-gray in color; to this surface the umbilical cord is attached, and it shows the arteries and veins branching out irregularly from the cord over the surface of the placenta. Fig. 210. The opiK)site surface is rough, lacerated, and is usually covered irregularly with more or less blood, which is often dark and clotted. When the bIcKKl is removed the surface is seen to l)e crossed by a system of grooves which divide the placental tissue into irreguhir aretis, each perhaps an inch or so in diameter: these areas are called cotyledons. The placenta is about twenty-five or thirty millimetres thick, but tliins out rai)idly at the edges, and its tissue passes over without a break into thin f(jetal membranes, which accordingly spring, fis it were, from the margin of the placenta.

When in situ the placenta is fastened to the walls of the uterus by its rough or cotyledonar>* sui'face; its smooth, amniotic surface faces the cavity in which the f«etus lies.

A more detailed examination of the gross apjiearances of a placenta dischargeil at term leads to the following additional observations: The (*olor is a reddish or purplish gray, varying in tint according to the (Condition of the bkxHl, and mottled between the divaricating blood-vessels by patches and networks of pale yellowish or flesh color. The light pattern is produced by the tissue of the villi shining through the membrane of the chorion. These appearances are less distinct when the placenta, as is usually the case, is covered by the thin amnion. The amnion, however, is very easily detached as far as the insertion of the umbiUcal cord, but from the latter it cannot be pulled off. The blood-v^sels run out in all directions from the end of the cord; each vessel produces a ridge upon the placental surface

Fig. am.— Plaivutu ut full T.Tm. rtouhly Inii-^'ini liy Dr. TI, V. yului^y to stow the Dlslriliutlon

so that its course is readily followed. Tlio iirtcvies and veins are more ea.«ily distinguishe*! after double injection, as is sluiwn in Fig.

The two kinds of vessels do not run together : the ai-teries lie nearer the surface, the veins deeper; the arteries fork ropeiitedly, until they are reprtwented only by small branchi.>s and fine ve»sels;'somo of the small branches disapiwar tjuite suddenly by dipping down into the deeper-lying tissue in onler to i»nss nilo the villi. The veins. Fig. 210, are considerably larger than the arteries; they branch in a similar manner, but some of the tnmks disappear from the surface more abruptly than is the case with the arteries. There is the greatest possible variability in the vessels of the jJacenta; I have never seen two placentae with vessels alike. So far as I have observed, the insertion of the cord is always obviously eccentric; the degree of eccentricitv is variable and is easilv seen to be relate<l to the distribution of the vessels.

The insertion of the cord may even ])e entirely outside the placenta, which yet, as B. S. Schultze has shown, may otherwise be normally developetl. Such insertions are called velamentous. The usual type is shown in Fig. '210. The arteries come down together from the cord ; they usually, but not always, anastomose by a short transverse vessel, which lies about half an inch above the surfa(*e of the placenta; it could not be shown in Fig. 210.

I have never noticeil any arterial or venous anastomoses on the surface of the placenta. The arteries there spread out in a manner which may te described as roughly symmetrical. The veins partially foUows the course of the arteries. When the cord is inserted near the margin the symmetry of the placental vessels is greater; when the insertion is nearer the centre the symmetrv is less than in Fig. 210.

The reverse or uterine surface of the placenta is rough and divided into numerous rounded oval or angular portions termed IoIhjs or cotyledons,* as stated above. These vary from half an inch to an inch and a half in diameter. The whole of this surface consists of a thin, soft, somewhat leathery investment of decidual membrane, which dips down in various parts to form the grooves that sej)arate the cotyledons from each other. This layer is a portion of the decidua serotina, which, as long as the parts are /// .s////, constitutes the boundary between the placenta and the muscular sul)stance of the uterus, but which at the time of labor l)econies split asunder, so that while a portion is carried off along with the jJacenta and constitutes its external membrane, the rest remains attac^hed to the inner surface of the uterus. If a placenta is cut through it is found to consist of a si)ongy mass containing a large quantity of blood and b(^unded by two membranes, each less than a millimetre thick ; the up^KT one is the chorion covered by the still thinner amnion, and greatly thickened where the vessels liv^» in it; the lower one is the decidual tissue together with the ends of the villi imljedded in it (r/. esi)ecially p. 17 and Fig. 211); it represents only a portion of the decidua, the other portion has remained ujxjn the uterine wall. The sjjongA' mass is foimd ui)on examination to consist of an immense number of tufts of fine ro<ls of tissue, which are irregularly cylindrical in shape. Further examination shows that they are twigs. Fig. 1S:J, with rounded ends and springing from little branchlets, which in their turn arise from branches, and so on, until a large main stem is found, which starts from the chorion. This branching system is richly supplied with blood from the fcetal vessels on the sinface of the placenta. The villi are interwoven so that the twigs of one branch are interlaced with those of another, and apparently separate twigs may grow together and their vessels anastomose; but on this point 1 tun unable to 8{)eak positively.

• the division of the plat.'eiita into cotyledons is not priuuvry, but, on the coutrarj , is not developed until the fourth or fifth month.

The villous twigs next the surface of tho deciilua [rthetrate that tissue a slight distance.

The intervillous spaces are filled, or nearly so, with blo<Hl ; they form a complex system <jf clmnnels. The intervillous blood, as we know from tho researches of Farre, Turner, and Waldeyer. is matemul. Farreeays, in his article in To<M's"Cycl(ipBe<lia,'" V. Huppl,, p. 71*1, in reference to the placental I'ceidutiL "Niimerous valve- like apertures are observe<l upon all jKirts of the surface; they are the

orificeH of tho veins, wlmh hiuc Wt-n toin tt fn m the nt<rui \ pnilw (Kissed into Jiny out of the se after t ikmg an ol li |uc. direction enters at ouci' into tin placental sul)st uu Small arteries ^l)out half an incli in length ire dsoeMr\«h(iL ol>spt\ed nnht-ddal m this hiyer. After making '«,^(lal sliar]i spii d tnrns the\ likmise suddenly ojx-ii into the placfnti and on p "Hlie adds These viinons iiritices oeonjiy thitf sitiiatKins The first and most numer <ius are S(^tt(?n'd over tin inntr sidt. of thi gincial li\ci of decidua which eonstitutes the upjir iMiniliiv jf the ])1 it cnt i the second form openings ujiim thi s|l<s (f H d Tidu d pn.)l nigatnms or dissepiments which separate the lobes (cotyledons) from each other; while the third lead directly into the interrupted channel in the margin, termed the circular sinns." The circular sinus (Fig. 211) is merely a space at the edge of the placenta which is left comparatively free from the villi. It is not a continuous channel, but is interrupted here and there. Surtbequent writers have gone but Tittle beyond Farrc's account, which has been entirely overlooked by most recent Gennan investigators, who accordingly amiounce facts known to Farreas new discoveries. Underthese circumstances itseems no more than just to direct renewetl attention to Farre'e masterly article. To study the histology of the placenta sections are best mode after imliedding the organ in celloidin. Fig. 211 rcpi-esents parts of a section of a placenta at term from which the amnion was removed. Fig. 211, A, repi-esenta the placentiU margin magnified thirteen diameters; E, a jjortion of the decidua near b in A, but more highly niagnifie<l. The chorion, C'Ao, and decidua, D, are in immediate contact at the left of the figure, that is, outside the placenta, though remnants of a few iihorted villi, c/, are still plainly recognizable; but they are found only close about the placentti. At the margin of the placenta and in its neighborhood the chorion and decidua are not clearly delimited, hut, on the cimtrarj', the decidual cells find hen; an opixjrtunity to penetrate the chi)rionic membrane. The placental chorion exhibits its characteristic stratitication a short distance within the margin. I have found, however, that the distinctness of that stratificatiim varies considerably, not only in different placenta-, but also iu different parts of the same phicenta. The dcwidua, J), outsidethe placenta is L ^ l^ very thick, but at the

edge it b^ns to thin . out, and, as it {uisses over the under side of the placenta, rai>idly becoinesBO much re<luce<l as to bo even less in thickness than the chorion, Cho. The decidua is char'. acterized by an ini'■ '" mensc numix'r of large decidual cells,

not si'attered alxnit  as in Fig. 1(\ but denselvpackeil. Fit;, 211, li. the «-lls an^

Fm. 212.- MiwiK-hyiiuii Tisim- uf a VJihis. r,,.u. - I'luwi.ia t.f irregularly oval in

anrr mmh-imrk fmiii nMir a capillary.

somewhat gratuilar, and have usually a single nucleus; a few are larger, more gi-mnilar and multinucleate.

At the wlge of the jilacenta the chorion and decidua separate; where they first jMirt there are verj' few villi, Fig. •111. SV, but elsewhere the HM-ni lietween them is occupied by innumerable branches of villi, Fi, Vi, with narrow spaces between for the blood ; the sections of the villi are of all sizes and shapes; they all contain blood-vessels, but only the larger ones can be distinguished with the magnification of Fig. 211, A, where they have been made as distinct as possible by being drawn black. The spaces between the villi have been left white, the blood which partially filled them not being represented.

Placenta in Situ. — The placenta in its natural position in the uterus follows the curvature of the uterine walls, hence its free or amniotic surface is slightly concave, its decidual surface is strongly convex; it is thickest in its centre and thins out gradually toward its edge. There is no definite boundary between the portion of the decidua serotina which is to be torn off with the placenta, and the part which is to remain in the uterus after delivery.

Vertical sections through the uterus with the placenta in place are very instructive. Fig. 213 represents such a section through a placenta of about seven months. The thin amnion, Am, clothes the inner surface of the chorionic membrane, Cho; this membrane is separated from the decidua, Z), by a dense forest of villi; in the younger specimens the distance between the chorion is conbiderably less than the thickness of the uterine wall, Z>, Mc, but in the present specimen. Fig. 213, it is much greater; in younger stages the villi are much less numerous, and much smaller than in the older one ; these differences correspond to the growth of the placenta and to the changes in shape of the chorionic villi already described, p. 319. The ends of some of the viUi touch and are imbedded in the decidual tissue; these imbedded ends are without. covering epithelium, but their connective tissue is immediately surrounded by hyaline substance, which is probably the degenerated epithelium. The decidua is plainly divided into an upper compact, and a lower cavernous layer, see p. 8. The section passes through a wide arterial vessel, Ve,

Foetal Circnlation of the Placenta. — The following paragraph refers to the placenta during the later months of pregnancy ; it is copied almost without change from my article on the placenta in Buck's "Ref. Handb. Med. Sci.," V., 69G-G97.

To follow the course of the foetal blood-vessels within the placenta, the best method is by corrosion injections. These may be made either with fusible metal, wax, or celloidin. The first is specially suited for the study of the large trunks; the latter for that of the smaller vessels also. I have a very beautiful celloidin injection by Dr. S. J. Mixter, which, with others of wax and metals, has sensed as the basis of the following description : The veins leave the surface somewhat more abruptly than do the arteries, which gives off more small branches to the surface than do the veins. Fig. 210. Both kinds of vessels leave the surface by curving downward for a shoi-t distance into the trunk of a villus; the vessels then divide, and their branches again take a more horizontal course; the branches then curl over downward, and after a second short descent toward the decidua, again send out horizontal branches. The result of this arrangement is a terrace-like appearance in the course of the vessels ; they approach the uterine side of the placenta in this very characteristic manner. The mmiber of terraces is variable ; usually there are two or three, but sometimes there is only one, or they may number four or even five. Arrived at the end of its terraces the main Teasel takes a more neariy perpendicular coume, and rapidly subdivides. Immediately after entering the villi, the arteries and veins give oflf but few capillaries, but after a short course in the main stalk of the villus the vessels ^ve rise to many branchlete, and gradually the character of the circulation changes until in the smallest villous twigs there are capillaries only. Fig. 314. The vascular trunks have a marked tendency to dichotomous division which is maintained within the villi to a certain extent ; the arterioles and veinlets in the mature placenta go from their trunks at wide angles for t£e most part, and subdivide in the same manner, so that they spread out through the whole substance of the placenta. The vessels next

the deqidua take a more horizontal trend, like the top branches of a wind-swept tree. As the vessels run in the villi, of course the way in which the latter branch out determine the paths of the former; hence, by following the distribution of the vessels we inform ourselves RB to the ramifications of the villi. Thus the horizontal course of the ves.sols on the uterine side of the placenta corresponds to the wellknown fact that the ends of the villi attached to tne uterus become bent and adhere by their sides to the decidual surface.

The capillaries of the villi are remarkable for their large size, and on this account have been described as arteries or veins by E. H. Weber, Goodsir, and other writers. Their calibre is often sufficient for from four to six blood-discs abreast. They are very variable in diameter, and also peculiar in exhibiting sudden constrictions and dilatations, Fig. 314. In the short bud-like branches there is often only a single capillary loop, but as the branch becomes larger the iramFjer td Wfpft increaj^H, and tbev form anastonioees. In branches lar^e enoufdi Vj fserre af« a fftem, ftcme one or two of the Teaeiels may tie enUkTu^ful, an maj be seen in Fig. ^14 ; in the branches large enough V} airJmit of it, there are two (or fsometimes only one) longitudinal i^tmiTHl vewtelM, an artc^rr and rein, and a superficial network of r«j/iIlarieH, Fig. 215. Good»ir and other early writers laid great HtrtiHH tm the formation of the capillary loops, but this feature is a fiffminou one in ttie development of the foetal vascular system, as is tt\Hff the width of the capillarieH, In my opinion these peculiarities ar»; cliaracU*riHtic rather of thefcetns than specificaUyof the placenta. In wnn*9 of the older writers (Goodsir, Farre, et al.) it is asserted tlmt the true ^^pillary systems disappear toward the end of gestation, I am unable t^> r:^^nfirm this, but find instead that in the slender t^nrrninal villi of the placenta at term there is often only a single, wm\t<\tt\itt^ long, r^anillary loop; the capillary is very wide, and its width is proUibly the reason of its having been held' formerly to be a vein or an artery.

Maternal Circulation of the Placenta. — The course of the mat«;rnal bUxxI in the placenta has been the subject of nearly conHt;irit detmt45 for a centur}' past, and the problem has received its final answer only within the last few years. The discovery of the fa^rts bc^longs to w) many authors that it seems not worth while to aiUiinfit to r-it^) the authorities for each detail, accordingly I give a Hunirnary of what is known, and in an historical note refer to the prinripnl investigations.

The arteries and veins bfjth open upon the surface of the decidual Hffrotina, at legist during the later half of pregnancy; concerning the cinrulation during the first half of pregnancy we possess no positive inforiuation, although the fundamental arrangements are presumably the muno. The blfKxl, which is poured out from the arteries, cir(ruliit4*H in the intervillous spaces, which act as maternal blood eliHnn<*lH.

Both arteries and veins change the character of their walls as thny i\,\)\}Ti mc\\ the Hurf ace of the decidua ; when they enter the decidua thny are nearly or quite without muscular walls, and can, therefore, 1m» r(H'ogniz(Ml as art(»ries or veins, not by their histological structure, but only by their size and their continuity with undoubted arteries and v««inH in the musc^ularis; during their passiige through tl>e dev\A\\\i thi'ir walls gnidually become reduced to the endothelial layer; but tho arieri(»H have, what the veins do not have, a thin clear layer just outside thr (»ndotheliinn ; this layer colors readily with carmine, contains a f(»w s<»att<^rcMl nucrlei, and is probably the result of degon<«ralion; it ceaw^s In^foro the arter>' actually reaches the surffice. The <MMlothelial nuclei of the veins project distinctly into the lumen of ihn vosH(»l. Wald«\yer, 80.1, 33, summarizes the differences betw<H»n tlu^ arteries and veins as follows: The arfer/V^s are smaller; th<\v take* a spiral course and nm within special columns of fibrous cHMUMM'tivt* tissue; they make numerous turns within the decidua, and H(* in the bnwul ridgt'is of the membrane; they usually do not bran(*h but terminatt* with a single opening, which generally lies in tlu* upiHT or lat^Tal jwirt of a decidual ridge; the opening is narrow and the villi do not project into it at all or but slightly; the terminal piece of the artery is round in cross-section ; the artery in the decidua has a special layer outside the endothelium, to within a short distance of the opening. The veins are, generally speaking, wider; they have no special sheaths, and do not run in spiral, but in direct courses, more or less parallel to the surface ; their openings lie, for the most part, between the ridges (septa) and never at the summits of the ridges; from the border vein {Grenzvene, Waldeyer) run out terminal branches which open on the surface and are usually numerous ; the chorionic villi project into the mouths of the veins and reach down even into the " Grenzvenen ;" the mouths of the veins are irregularly shaped, and the veins themselves are irregular in cross-section, never circular. The position of the vascular openings is such that the arterial blood flows out from the septa, while the venous blood flows off through the surface between the septa ; hence, as pointed out by Bumm, 80.1, each cotyledon represents a more or less distinct circulatory region, the blood entering at the sides and leaving at the bottom.

Historical Note, — The long prevalent erroneous view that there is a direct communication between the maternal and fcetal circulations originated I believe with Haller ("Elementa Physiologise," VIII.). It was revived again by Flourens, 36.1, and though long since entirely disproved is still encountered from time to time. The first impoilimt evidence of the circulation of the blood in the intervillous spaces was brought by F.. H. Weber, whose investigations were published in Hildebrandt's " Handbuch der Anatomic des Menschen," 4to Auflage, IV., 41>0. Weber's doctrine was adopted by most subsequent investigators. The most important additions to his observations were made by Farro, 68.1, and Turner, 73.1, 76.1, 76.3, 77. 1, 77.2, 88. 1, until we come to the recent researches of Langhans, 77.1, 82.1, and his pupils, Nitabuch, 87.1, Rohr, 89.1, etc.; of Waldeyer, 87.1, 90.1, of Bumm, 90.1, Minot, 98, BlcK»h, 89.1, and others, which have finally settled the problem. That the intervillous spaces normally contain blood was seriously questioned by Braxton Hicks, 72.1, whoso doubts were again brought prominently forward by C. Ruge, 86.1. Ruge's position I wiis inclined at first to adopt (see Minot, Anat, Anzeiger, II., 1 U) , but I have since become entirely convinced of the correctness of Wel)er's doctrine as established by Langhans, Waldeyer, etc. A thorough and very valuable critical review of the whole subject is giv(*n by Waldeyer, 90.1, upon whose citations this note is based, but I have referred only to a few of the numerous authorities quoted by Waldeyer.

Nutrition of the Foetus. — The mechanism of the transfer of nourishment from the uterus to the child is not well understood. It is evident that the supply must come from the maternal blood and reach the fcetus through the veins of the imibilical cord ; although the amniotic fluid ma}' be a source of supply, {iS some have maintained, yet at most its role can be only secondary and the main transfer of material must take place through the placenta. Our present knowledge of the structure of the organ renders it unnecessary to discuss the old theory recently revived by Currie, of a direct communication between the maternal and foetal vessels, for we know positively that no such communication exists. This theory has been put forward a^ain with the modification that the vascular walls will et small solid particles through. Thus Koubassoflf, on the basis of some inconclusive experiments, sought to maintain that microbes, and ergo other solid particles, could pass from mother to embryo (see Comptes Rendus Acad. Paris, t. CI., 508-510). More careful tests by Marie Miropolsky failed to confirm this (Arch, de Physiol, y n. etp.y 1885, 101-108). A second theory, at present the best accredited, is that of diffusion, which finds its chief basis in the elaborate arrangements found in all placental types for bringing the foetal and maternal blood into immediate proximity. A fA/ra theory is that Rauber, 79, who attributes the chief role in the nutrition of the embryo to the immigration, by way of the placenta, of maternal leucocytes. A fourth theory attributes an active part to the utricular glands, which are suppose<l to pour out a nutrient secretion into the intervillous spaces, where it is taken up by the chorionic villi. It is impossible at present to form a final judgment upon these theories. As we have seen, the intervillous spaces are probably maternal blood-channels at all periods, so that, from a very early stage on, the conditions for the transfer of material, either by a migration of leucocytes or by simple transfusion, are established. Rauber's leucocyte theory has not commended itself to me, and I incline to accept the transfusion theory. That the uterine milk exists in man has not been proven, and the occurrence of such a secretion is not compatible with the degeneration of the glandular epithelium observed by Minot, see p. 10.

II. Theory of the Placenta

Attachment of the Embryo. — That the rabbit embryo is attached to the surface of the uterus by a thickened region (area placentalis) of the ectoderm of the germinative area was first shown by Van Beneden and Julin, 84. 1 ; this discovery has since been confirmed by Minot, 98, Masius, 89.1, Duval, 89.1, and others. That a similar method of attachment exists in other mammals has been shown by Strahl, 89. 1, 4, 90. 1 ; in the dog it has been recorded by G. Heinricius, 89.1. In all these cases the thickened ectoderm is found to be closely adherent to the uterine surface, upon which it is apt to remain when the ovum is forcibly removed ; it fits exactly to the surface of the maternal epithelium ; there is no visible layer of cement, and we do not know how the adJierence is made so close.

It is probable that we have here the primitive form of attachment, and that therefore the evolution of the placenta be^an with the differentiation of the ectoderm of the area placentalis.

There is another type of attachment found in the hedgehog and in. rodent ova with inversion of the germ-layers, characterized by the ovimci being so closely invested by the uterine mucosa that the whole surface of the ovum comes in contact with the maternal tisues (see E. Selenka, 84,1, and Hubrecht's superb monograph on the placenta of the hedgehog, 89. 1).

  • Reference is made especially to the true chorionic placenta.

Degeneration of Uterine Tissues. — Over the region of the placental attachment, which varies in different animals as to position, there occurs an extensive degeneration of the tissues of the uterine mucosa, affecting both the covering epithelium, the glands, and the connective tissue. The degeneration takes place most rapidly in the epithelium and glands, while the connective tissue undergoes an extensive hypertrophic metamorphosis, usually in the form of the development of decidual cells, before the degenerative change acquires the upper hand. The nature and extent of the degenerative changes have become known for various types by investigations published since 1888, several of which appeared during 1889 (Minot, 89, 98, Masius, 89.1, Heinricius, 89.1, Duval, 89.1, Hubrecht, 89.1, Strahl, 89.1, 4, etc.), and represent simultaneous and independent researches. In view of what we now know it must be considered probable that in all placental mammals, or at least in the orders of the unguiculate series, the uterine degeneration is an invariable factor in the development of the placenta.

The form of degeneration is not fixed, but varies greatly. This is illustrated by the history of the decidua in man and in the rabbit. Other modifications occur in the dog, the heilgehog, the mole, and doubtless in other animals.

The result of the degeneration is: firsts to bring the chorionic ectoderm of the embryo into direct contact with the connective tissue of the mucosa uteri in consequence of the degeneration anecomes extremely complicated. Their form is highly characteristic of the various orders ; it is known exactly only in man, but is certainly very different in various animals.

The villi occupy only a part of the mucosa, there being always a considerable layer of decidual membrane left between the end of the villi and the muscularis.

The villi, as here described, consist of a core of mesoderm covered by foetal ectoderm, and are essentially different from the ectodermal outgrowths assumed by Duval * to exist in the rabbit.

Union of the Allantois with the Chorion. — We know two principal modifications of the union of the allantois with the chorion : 1. The allantois joins the chorion early, and serves as the stalk, connecting the embryo with chorion ; in this type the allantois brings the blood-vessels to the chorion and the vessels then nimify over the chorion itself, which has therefore its own circulation, though dependent upon the allantois; this modification is characteristic of the unguiculate series of manmials. 2. The allantois grows out into a large vesicle, which has for some time no connection with the chorion but maintains a well-developed circulation of its own ; its expansion brings it ultimately into contact with the chorion, and its outer or mesodermic layer grows together with the inner or mesodermic layer of the chorion (Bonnet, 89. 1) which thus becomes indirectly vascularized ; this modification is characteristic of the ungulate series of mammals. How far other modifications, distinct from these, may exist, we cannot say at present.

We have then two types : 1 , the chorion has its own vessels (unguiculates) ; 2, the chorion acquires vessels by growing together with the vascular walls of the allantoic vesicle (ungiilates) .

In both cases the chorion is the part of the foetus and the only part in direct contact with the uterine wall, and therefore in both cases it is the essential part of the foetal placenta. In unguiculates the chorion, after it receives its blood-vessels, has its own blood supply and circulation, and therefore suffices to develop the placenta, n ungulates the circulation is entirely allantoic, and the walls of the allantois are essential to maintain the fcetal circulation of the placenta; the chorion, therefore, does not suffice to develop the fwtal placenta. While we recognize that the chorion is always the means of union between the mother and the offspring, we may conveniently distinguish the unguiculate type as having a true chorionic placenta and the ungulate type iis having an allantoic placeuta. Evolution of the Placenta. — As regards the evolution of the placenta, our conceptions are still very obscure. The oj)inion was ong, and perhaps still is, generally prevalent that the placenta is primarily an organ of the allantois. This notion was one of those theories which sometimes become current without ever having been supporteil by adequate proof, and are repeated until tradition has rendered them venerable and age gives them a dignit}' their worth d<K»s not entitle them to. The principal support of this theor}" was derived from the ftict that the albmtois is connectcnl with tln^ j)lacontal circulation. Balfour in 18S1 (•* Works," I., 743) sought to uKxlify this view by attributing importance to the relations of the* yolk-sac, which he l)elieved to be the means of maintaining the circulation.

  • ErroupouKly. us I believe.

In his "Comparative Embryology," II., 249, Balfour reprints most of the article cited. Minot, 98, 433, laid stress upon the role of the chorion and upon the fact that the placenta is necessarily always a product of the chorion, and further upon the fact that the allantois in man is permanently (and in the rabbit primarily) merely a stalk of connection between the embryo and the chorion. The investigations mentioned in this chapter which have been recently published seem to me to greatly strengthen my view. It is by the chorion that the ovum is attached, except in certain rodents in which the development has obviously been modified. It is from the chorion that the foetal villi grow out. On the other hand, it is evident that the yolk-sac is primitively a product of the splanchnopleure and distinct from the somatopleuric chorion; the failure of the mesoderm and coelom to spread completely over the yolk (entoderm of the blastodermic vesicle) in certain mammals does not alter the fundamental relations. It is true that in certain marsupials the chorion is very imperfectly separated from the yolk-sac, but it does not appear that this represents an ancestral stage of the mammalia ; on the contrary, it is probably a purely marsupial modification. I am therefore unable to recognize any reason for connecting the evolution of the placenta with the yolk-sac or vitelline circulation. The role of the allantois is secondary ; it serves as a medium of blood supply, either, as we have seen, as a carrier of vascular trunks to supply the circulation of the chorion (unguiculates) or bringing its own circulation into play by growing together with a non- vascular chorion.

The question remains whether the unguiculate or the ungulate type of placenta is to be regarded as the more primitive. At first thought the resemblance of the foetal envelopes of ungulates to those of Sauropsida leads us to conclude that the allantoic placenta must be the more primitive; the resemblance referred to consists in the early complete separation of the chorion (serosa) from the other parts and in the development of the allantois as a large free vesicle. But the ungulates are highly modified mammals not related closely to the lower placentalia, while the unguiculates do merge into a generalized mammalian type. When we consider further that the lower unguiculates show the typical chorionic placenta in its full perfection, the conclusion is unavoidable that this is the nearer type to the ancestral. In fact, the placenta appears in animals with the chorionic type of the organ before the allantois becomes free, and the great size of the allantoic blood-vessels is connected primitively, not with the allantois, but with the already important chorionic circulation; the placenta is hero interpolated in the ontogeny before the specialization of the allantois, which functions as the vascular pathway between the chorion and embryo, both primitively and permanently. The enlargement of the allantois in ungulate mammals is a supervening change, effected perhaps by an atavistic recurrence to reptilian ontogeny.

Ryder, 87.6, has advanced the theory that the zonary placenta is older than the discoidal, but Minot, 98, 434, has shown that this view is untenable.

The degenerative changes in the uterus occur, so far at present kno^^^l, only in connection with the chorionic placenta ; in the ungulates the uterine mucosa is modified in structure in connection with the development of the placenta, but the modifications are not known to be degenerative; hence in the allantoic placenta the maternal blood flows in maternal blood-vessels, and it is always separated by maternal connective tissue and epithelium from the chorion.

Theory of the Placenta. — According to the views explained in the preceding pages, I hold the placenta to be an organ of the chorion; that primitively the chorion had its own circulation, and formed the discoidal placenta by developing villi which grew down into the degenerating uterine mucosa; by the degeneration of the maternal tissues the maternal blood is brought closer to the villi, and the degeneration may go so far that all the tissue of the uterus between the villi disappears ; a layer of the mucosa is preserved between the ends of the villi and the muscularis uteri to form the so-called decidua; the placenta receives its foetal blood by the means of large vessels running in the mesoderm of the allantois. From this discoidal chorionic placenta the zonary placenta of camivora, the diffuse placenta of the lower primates, and the metadiscoidal placenta of man have been evolved.

A second type of placenta, perhaps evolved from the first, is found in ungulates, and is characterized by a vascular allantoic vesicle uniting with a non- vascular chorion to form the foetal placenta, and by the absence of degeneration in the maternal tissue. This type is the allantoic placenta, which offers many interesting modifications.

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Cite this page: Hill, M.A. (2024, February 26) Embryology 1897 Human Embryology 17. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/1897_Human_Embryology_17

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