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VETERINARY PHYSIOLOGY 



or more parts of the surface of the ovum, and the Placenta is 

 thus formed. This acts as the foetal lung, giving the embryo 

 the necessary oxygen and getting rid of the waste carbon dioxide. 

 It is the foetal alimentary canal supplying the necessary material 

 for growth and development ; and it is the foetal kidney through 

 which the waste nitrogenous constituents are thrown off. 



In the mesoblast, through which the allantoic arteries pass out, 



FIG. 175. Longitudinal Section through the human uterus and ovum at the fifth 

 week of pregnancy. D.S., decidua serotina, which will become the 

 placenta; D.R., decidua reflexa ; D.V., the uterine mucous membrane 

 called the decidua vera. 



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

 posterior gut, is developed (figs. 172, all., 173 and 174). This is 

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

 the lower animals it spreads all round and encloses the amnion. 



3. Festal Circulation 



The performance of these functions by the placenta is 

 associated with a course of circulation of the blood somewhat 

 different to that in the post-natal state (fig. 177). 



The blood coming from the placenta to the foetus is collected 

 into a single umbilical vein, u.v., which passes to the liver, /. 

 This divides into the duetus venosus, d.v., passing straight through 

 the organ, and into a series of capillaries among the cells. From 



