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Development of the Fetal Membranes and Placenta

The Allantois(Figs. 25to28).The allantois arises as a tubular diverticulum of the posterior part ofthe yolk-sac; when the hind-gut is developed the allantois is carried backward with it and then opensinto the cloaca or terminal part of the hind-gut: it grows out into the body-stalk, a mass of mesodermwhich lies below and around the tail end of the embryo. The diverticulum is lined by entoderm andcovered by mesoderm, and in the latter are carried the allantoic or umbilical vessels.

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In reptiles, birds, and many mammals the allantois becomes expanded into a vesicle which projectsinto the extra-embryonic celom. If its further development be traced in the bird, it is seen to project tothe right side of the embryo, and, gradually expanding, it spreads over its dorsal surface as a flattenedsac between the amnion and the serosa, and extending in all directions, ultimately surrounds the yolk.Its outer wall becomes applied to and fuses with the serosa, which lies immediately inside the shellmembrane. Blood is carried to the allantoic sac by the two allantoic or umbilical arteries, which arecontinuous with the primitive aort, and after circulating through the allantoic capillaries, is returned tothe primitive heart by the two umbilical veins. In this way the allantoic circulation, which is of the utmostimportance in connection with the respiration and nutrition of the chick, is established. Oxygen is takenfrom, and carbonic acid is given up to the atmosphere through the egg-shell, while nutritive materialsare at the same time absorbed by the blood from the yolk.

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FIG. 26Diagram showing later stage of allantoic development with commencing constrictionof the yolk-sac. (See enlarged image)

FIG. 27Diagram showing the expansion of amnion and delimitation of the umbilicus. (See

enlarged image)

In man and other primates the nature of the allantois is entirely different from that just described. Hereit exists merely as a narrow, tubular diverticulum of the hind-gut, and never assumes the form of a

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vesicle outside the embryo. With the formation of the amnion the embryo is, in most animals, entirelyseparated from the chorion, and is only again united to it when the allantoic mesoderm spreads overand becomes applied to its inner surface. The human embryo, on the other hand, as was pointed outby His, is never wholly separated from the chorion, its tail end being from the first connected with thechorion by means of a thick band of mesoderm, named the body-stalk (Bauchstiel); into this stalk thetube of the allantois extends(Fig. 21).

The Amnion.The amnion is a membranous sac which surrounds and protects the embryo. It isdeveloped in reptiles, birds, and mammals, which are hence called Amniota; but not in amphibia andfishes, which are consequently termed Anamnia.

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In the human embryo the earliest stages of the formation of the amnion have not been observed; inthe youngest embryo which has been studied the amnion was already present as a closed sac (Figs.24and32), and, as indicated on page 46, appears in the inner cell-mass as a cavity. This cavity isroofed in by a single stratum of flattened, ectodermal cells, the amniotic ectoderm, and its floorconsists of the prismatic ectoderm of the embryonic diskthe continuity between the roof and floorbeing established at the margin of the embryonic disk. Outside the amniotic ectoderm is a thin layer ofmesoderm, which is continuous with that of the somatopleure and is connected by the body-stalk withthe mesodermal lining of the chorion.

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FIG. 28Diagram illustrating a later stage in the development of the umbilical cord. (Seeenlarged image)

When first formed the amnion is in contact with the body of the embryo, but about the fourth or fifthweek fluid (liquor amnii) begins to accumulate within it. This fluid increases in quantity and causes theamnion to expand and ultimately to adhere to the inner surface of the chorion, so that the extra-embryonic part of the celom is obliterated. The liquor amnii increases in quantity up to the sixth orseventh month of pregnancy, after which it diminishes somewhat; at the end of pregnancy it amountsto about 1 liter. It allows of the free movements of the fetus during the later stages of pregnancy, andalso protects it by diminishing the risk of injury from without. It contains less than 2 per cent. of solids,consisting of urea and other extractives, inorganic salts, a small amount of protein, and frequently a

trace of sugar. That some of the liquor amnii is swallowed by the fetus is proved by the fact thatepidermal debris and hairs have been found among the contents of the fetal alimentary canal.

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In reptiles, birds, and many mammals the amnion is developed in the following manner: At the pointof constriction where the primitive digestive tube of the embryo joins the yolk-sac a reflection or foldingupward of the somatopleure takes place. This, theamniotic fold(Fig. 29),first makes its appearanceat the cephalic extremity, and subsequently at the caudal end and sides of the embryo, and graduallyrising more and more, its different parts meet and fuse over the dorsal aspect of the embryo, andenclose a cavity, the amniotic cavity. After the fusion of the edges of the amniotic fold, the two layersof the fold become completely separated, the inner forming the amnion, the outer the false

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amnion orserosa. The space between the amnion and the serosa constitutes the extra-embryoniccelom, and for a time communicates with the embryonic celom.

FIG. 29Diagram of a transverse section, showing the mode of formation of the amnion in the

chick. The amniotic folds have nearly united in the middle line. (From Quains Anatomy.)

Ectoderm, blue; mesoderm, red; entoderm and notochord, black. (See enlarged image)

FIG. 30Fetus of about eight weeks, enclosed in the amnion. Magnified a little over two

diameters. (Drawn from stereoscopic photographs lent by Prof. A. Thomson, Oxford.) (See

enlarged image)

The Umbilical Cord and Body-stalk.The umbilical cord(Fig. 28)attaches the fetus to the placenta;its length at full time, as a rule, is about equal to the length of the fetus, i.e., about 50 cm., but it may begreatly diminished or increased. The rudiment of the umbilical cord is represented by the tissue whichconnects the rapidly growing embryo with the extra-embryonic area of the ovum. Included in this tissueare the body-stalk and the vitelline ductthe former containing the allantoic diverticulum and theumbilical vessels, the latter forming the communication between the digestive tube and the yolk-sac.The body-stalk is the posterior segment of the embryonic area, and is attached to the chorion. Itconsists of a plate of mesoderm covered by thickened ectoderm on which a trace of the neural groovecan be seen, indicating its continuity with the embryo. Running through its mesoderm are the two

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umbilical arteries and the two umbilical veins, together with the canal of the allantoisthe last beinglined by entoderm(Fig. 31).Its dorsal surface is covered by the amnion, while its ventral surface isbounded by the extra-embryonic celom, and is in contact with the vitelline duct and yolk-sac. With therapid elongation of the embryo and the formation of the tail fold, the body stalk comes to lie on theventral surface of the embryo (Figs. 27and28), where its mesoderm blends with that of the yolk-sacand the vitelline duct. The lateral leaves of somatopleure then grow round on each side, and, meeting

on the ventral aspect of the allantois, enclose the vitelline duct and vessels, together with a part of theextra-embryonic celom; the latter is ultimately obliterated. The cord is covered by a layer of ectodermwhich is continuous with that of the amnion, and its various constitutents are enveloped by embryonicgelatinous tissue,jelly of Wharton. The vitelline vessels and duct, together with the right umbilicalvein, undergo atrophy and disappear; and thus the cord, at birth, contains a pair of umbilical arteriesand one (the left) umbilical vein.

FIG. 31Model of human embryo 1.3 mm. long. (After Eternod.) (See enlarged image)

Implantation or Imbedding of the Ovum.As described (page 44), fertilization of the ovum occurs in

the lateral or ampullary end of the uterine tube and is immediately followed by segmentation. Onreaching the cavity of the uterus the segmented ovum adheres like a parasite to the uterine mucousmembrane, destroys the epithelium over the area of contact, and excavates for itself a cavity in themucous membrane in which it becomes imbedded. In the ovum described by Bryce and Teacher7thepoint of entrance was visible as a small gap closed by a mass of fibrin and leucocytes; in the ovumdescribed by Peters,8the opening was covered by a mushroom-shaped mass of fibrin and blood-clot(Fig. 32),the narrow stalk of which plugged the aperture in the mucous membrane. Soon,however, all trace of the opening is lost and the ovum is then completely surrounded by the uterinemucous membrane.

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The structure actively concerned in the process of excavation is the trophoblast of the ovum, whichpossesses the power of dissolving and absorbing the uterine tissues. The trophoblast proliferatesrapidly and forms a network of branching processes which cover the entire ovum and invade anddestroy the maternal tissues and open into the maternal bloodvessels, with the result that the spaces

in the trophoblastic network are filled with maternal blood; these spaces communicate freely with oneanother and become greatly distended and form the intervillous space.

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