FROM TWELVE TO THIRTY-SIX SOMITES 143 
folds. The former are of very constant occurrence and _ persist 
a long time (Fig. 93). 
Elsewhere the effect of the twisting of the embryo is rapidly 
compensated so that the secondary folds of the right half of the 
amnion do not persist long. 
The subsequent history of the amnion and chorion is given 
in another place. It should be noted here that the chorion, at 
the stage of seventy-two hours, is continuous peripherally with 
the splanchnopleure at the margin of the vascular area, and that 
it becomes separate from it only as the body-cavity extends 
more and more peripherally. The sero-amniotic connection 
remains throughout the entire embryonic period and modifies in 
an important fashion the subsequent history of the membranes. 
The yolk-sac is the name given to the extra-embryonic 
splanchnopleure, because in the course of expansion of the blasto- 
derm and extension of the extra-embryonic body-cavity over the 
surface of the yolk, it finally becomes a separate sac enclosing 
the yolk. It remains connected by the yolk-stalk with the intes- 
tine until finally, some time after hatching, it is absorbed com- 
pletely. The yolk is absorbed by the entodermal lining and is 
carried to the embryo in solution by means of the vitelline veins. 
Origin of the Allantois. The allantois arises as a diverticulum 
of the hind-gut soon after the formation of the latter by the tail- 
fold. It is not indicated before the formation of the tail-fold as 
stated by some authors, but the tube identified by them as the 
primordium of the allantois at this early stage is really the in- 
testinal diverticulum leading to the anal plate (Fig. 70). At the 
stage of twenty-eight somites the allantois is indicated by the 
depth of the hind-gut, the ventral portion of which in front of 
the anal plate soon becomes constricted from the upper portion, 
and forms the primordium of the allantois. In longitudinal sec- 
tions of an embryo of about thirty-five somites it can be seen to 
include nearly the entire floor of the hind-gut between the anal 
plate and the posterior intestinal portal (Fig. 80). It is lined 
with entoderm and has a thick mesodermal floor in which numer- 
ous small blood-vessels are already present. A transverse section 
(Fig. 81) shows that the thick mesodermal wall is broadly fused 
with the somatopleure in the region of the neck. In other 
words, the allantois is developed within the ventral mesentery. 
It will also be seen by comparing these figures that the amnion 
