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 



