PHYSIOLOGY. 33 
The period of gestation in the 
Cow. .... is 9 months, average 283 days. 
EWeevcae & “ % isa 
SOW. e.e% 4 . e izo | 
Bitch eka « 2 - e 56-65 “ 
Caiteenaue 2 - ss 55 St 
Rabbit... 1 : a 
Human... 9 7 a 280 «= 
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 foetus 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 fcetus, 
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 
