36 VETERINARY OBSTETRICS. 



The XTmbilical Vesicle. 



This is a small, pear-shaped structure, which is- 

 only seen in the very early stages of foetal life. Its 

 function is not very well understood, but it is believed, 

 to supply the foetus with nourishment until the 

 membranes have assumed that function. By its narrow- 

 extremity, the umbilical vesicle is in contact with the 

 small intestine of the fcetus ; and by its other or broad 

 extremity, it is in contact with or rests upon the inner 

 aspect of the chorion. As soon as the placental 

 circulation is established, this structure commences to 

 undergo atrophy, and all that remains of it at birth is a 

 brownish colored fibrous cord. 



The PLACENTAL CIRCULATION, or, as it is sometimes 

 called, the "vascularization," varies in the fcetus of 

 different animals. 



In the Cow, as already stated, there are a large, 

 number of vascular processes in the pregnant animal, 

 called cotyledons. These are in contact with the- 

 external surface of the chorion, and the blood of the 

 parent is thus brought into close contact with the 

 blood of the foetus at these situations. There is no 

 direct continuity of the circulation, the blood of the 

 parent being spread out in very small vessels on the 

 surface of the maternal cotyledons, while that of the- 

 foetus is distributed in a similar manner, on the 

 opposing surface of the chorion. Through the walls- 

 of these opposing vessels the exchange takes place. 



In the Mare, instead of the blood-vessels being- 

 found at certain parts, they are distributed. all over ther. 



