THE ALLANTOIS AND UMBILICAL CORD. 49 
posterior part of the embryonic area, and the area is curved upon itself so that its 
convexity looks towards the entodermal sac, and its concavity towards the amnion. 
After the embryonic area has increased in extent, and when the folding off of 
the embryo has commenced, the anterior end of the area and the posterior end of 
the primitive streak remain relatively stationary as in other mammals, the cephalic 
and caudal folds appear, and the curvature of the greater part of the area is 
reversed, but the most posterior part retains its original position, lying for a time 
parallel with the caudal fold; afterwards, however, it assumes a more horizontal 
position. This posterior section of the embryonic area contains the diverticular 
process of the entodermal sac which is called the allantois; it also contains the 
blood-vessels, allantoic arteries and veins, which pass between the embryo and the 
placenta. It is in relation at first with the amnion, it appears to be entirely 
behind the embryo, and it is called the “body-stalk.” At a later period, 
when the stalk of mesoderm—the allantoic stalk—which connects it with the 
inner surface of the chorion is elongated, this part of the embryonic area 1s 
reversed in position, its anterior end is carried forwards till it forms the posterior 
boundary of the umbilical orifice, and it forms the ventral wall of the body from 
the umbilical to the genital region. 
The Allantois.—The allantois plays an important part in the formation of the 
placenta. It consists of two portions, an entodermic diverticulum from the ventral 
wall of the cloacal part of the hind-gut, and a mesodermal covering. The ento- 
-dermic diverticulum appears in the human subject, before the hind-gut is defined, 
as a hollow blind protrusion from the blastodermic cavity; it extends behind the 
primitive streak into the mesoderm of the body-stalk, but as the folding off of the 
embryo proceeds and the body-stalk is carried forward into the ventral wall of the 
embryo, the position of the diverticulum is altered, and ultimately, when the 
folding off is completed, it springs from the ventral part of the cloaca, runs forward 
to the umbilical orifice and passing through it, projects for a short distance still 
invested with the mesodermal covering primarily obtained from the body-stalk. 
The ventral part of the cloaca is afterwards eonverted into the bladder, while the 
rectum is formed from the dorsal part. 
_ The mesodermal sheath which surrounds the entodermic diverticulum extends 
beyond it to the inner surface of the chorion; the part which extends beyond the 
diverticulum is at first extremely short, indeed it is only recognisable as a layer of 
mesoderm uniting the body-stalk and chorion, but as development proceeds and 
the body-stalk is absorbed into the embryo it is elongated, and it forms the 
allantoic stalk by which the embryo retains its connexion with the chorion, and 
along which pass the allantoic or umbilical arteries to, and the corresponding veins 
from, the chorionic villi. 
After the separation of the cloaca into bladder and rectum, that portion of the 
allantois which lies in the body of the embryo, between the apex of the bladder 
and the umbilical orifice, is gradually converted into a fibrous cord, the urachus. 
The entodermal diverticulum disappears, and after birth, when the placental circu- 
lation ceases, the umbilical arteries are transformed into fine fibrous strands. The 
remainder of the allantois which lies outside the body of the embryo, and which 
takes part in the formation of the umbilical cord and placenta, is separated from 
the embryo at birth. 
The Umbilical Cord.—The umbilical cord is essentially a mesodermal structure 
which connects the embryo with the placenta, serving as a passage for the allantoic 
vessels to and from the feetal portion of the latter organ. It replaces the body- 
stalk and the allantoic stalk, which were earlier provisions for the same purpose, 
and it is formed by the fusion of the allantoic stalk with part of the vitello- 
intestinal duct and the remains of the yolk-sac. 
The vitello-intestinal duct is at first a relatively wide channel which connects 
the primitive gut with the yolk-sac; and it passes through the umbilical orifice. 
In later stages, as the body-stalk is swung round into the ventral wall of the body, 
the allantoic stalk, which projects from the end of the body-stallk, is brought into 
close relation with the distal end of the vitello-intestinal duct and the remains of 
the yolk-sac; the mesodermal constituents of the three structures then fuse 
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