7io 



A MANUAL OF VETERINARY PHYSIOLOGY 



it grows out from the body, being connected to the embryo by 

 means of a stalk, and forms a vascular sac through which blood 

 from the chick's body circulates. The allantois in the chick is 

 a breathing organ ; the air enters through the pores of the shell, 

 and the blood takes up oxygen from the air surrounding the 

 allantoic sac ; an air space also exists at the end of the shell. 

 An allantois exists in the mammal (Figs. 247, 249) ; unlike that 

 of the bird, it does not obtain oxygen from the air, though it is 



Fig. 249. — Diagram of the Foetal Envelopes of a Five Months Horse 

 Embryo (Bonnet). 



a breathing organ in the sense that it furnishes oxygen to the 

 foetus. 



Immediately enveloping the mammalian embryo is the amnion. 

 This arises from the same portion of the embryo which gives 

 origin to the body wall, through ridges which grow up over it 

 (Figs. 246, 249), and eventually form a sac. This sac contains a 

 fluid in which the foetus lives. The fluid, or liquor amnii, is 

 alkaline in reaction, and yellowish in colour during the early days 

 of gestation, but reddish towards the end of it, probably due to 



