628 



VETERINARY PHYSIOLOGY 



out, a vesicle, filled with fluid, and at first communicating with 

 the posterior gut, is developed (fig. 240, all.). This is the 

 allantois. In man it never attains any size, but in most of the 



Am. 



Fig. 243.— Schematic section through the pregnant uterus of the Mare to 

 show the large allantoic sac, All., filled with fluid surrounding the 

 amniotic sac ; Am., the fluid in which the foetus floats. 



lower animals it spreads all around and encloses the amnion, 

 and is distended with a large quantity of fluid. This fluid has 

 all the characters of urine, and when fluorescin is injected into 



B 



C A 

 Fig. 244. — Schematic section of one cornu of the uterus of a ruminant at an 

 early stage of gestation to show the elongated umbilical vesicle, A, 

 and allantois, B, and the embryo in the amniotic sac, C. 



the maternal circulation, it appears in the foetus before it shows 

 in the allantoic fluid. It is almost certainly produced from the 

 first by the foetal kidney. 



III. The Nourishment of the Foetus. 



The Placenta is formed on the foetal side by these processes; 

 on the maternal side by the increased growth and increased 

 vascularity of the maternal mucosa. 



