PHYSIOLOGY. 



33 



The period of gestation in the 



Cow. , 

 Ewe. . 



Sow . . 

 Bitch . 

 Cat . . 

 Rabbit 

 Human 



is 9 months, average 283 days. 



5 

 4 

 2 

 2 

 I 

 9 



If we suppose, then, that impregnation has taken 

 place, and the period of gestation has begun, in 

 the interval certain structures responsible for the 

 nourishment of the fcetus have been formed. (Fig. 13.) 

 These are known as the " foetal envelopes " or 

 "placental membranes." They consist of a membrane 

 •called the "chorion," another the "amnion," the 

 "allantois," the "umbilical vesicle," and the "umbilical 

 cord." 



• The Chorion. 



This is the most externally situated of the 

 placental membranes. By its external surface it is in 

 contact with the inner aspect of the walls of the uterus. 

 In shape it is a regular counterpart of the uterus, which 

 it lines. (Fig. 14.) It is a closed sac, and is principally 

 concerned in the supply of nutrition to the foetus, 

 the process going on from the external surface of this 

 membrane. 



The Amnion. 

 This is the most internally situated of the fcetal 

 -envelopes, and, like the chorion, is a closed sac. It 

 surrounds the foetus, and from its internal aspect a 



