306 Vertebrate Embryology 



THE BODY-STALK AND PLACENTA 



In the ungulate mammals (horse, pig, sheep, 

 etc.) the foetal placenta is formed, practically, 

 from the allantois, though the chorion is intim- 

 ately concerned. In the unguiculate mammals 

 (cat, dog, monkey, man, etc.) the conditions 

 are rather different and will here be briefly 

 described. 



As seen in Figures 93-94, the embryo of the 

 unguiculate mammal, when it becomes enclosed 

 in the chorionic vesicle, retains a stalk-like 

 connection between its posterior end and the 

 inner surface of the chorion ; this stalk is mainly 

 of mesoblast and is called the body-stalk. Into 

 this stalk extends, as a narrow diverticulum 

 from the hind-gut, the allantois. In the ungu- 

 lates, where the body-stalk is not persistent, 

 the allantois becomes a large vascular structure 

 as in the chick, and is known as a free allantois. 

 In the unguiculates, on the other hand, it is 

 relatively small, though varying in size ; in man 

 it is merely a long narrow tube extending into 

 the body-stalk where it ends blindly. 



In the ungulates the chorion is not vascular, 

 though it comes into very close union with the 

 vascular allantois. 



In the body-stalk of the unguiculates there 



