XIII] 



FEMALE GENERATIVE ORGANS 



185 



this outgrowth blends with the chorion to fcrm the foetal placenta. 

 The ]3ortion of the allantois remaining within the foetus eventually 

 becomes the bladder, and the point from which the allantois 

 passes out remains after birth as the navel. The chorionic villi 

 contain blood vessels connected with the circulatory system of 

 the developing foetus so that the foetal and maternal circulation 

 are brought into ver}^ intimate relation, being only separated by 

 a very thin wall of protoplasm, but there is no direct communica- 

 tion between them. Nutritive material — protein, fat and carbo- 

 h3'drate as well as oxygen — transfuse through this thin layer, 

 from the mother to the young, while excretory products and carbon 



Fig. 9-5. Seven weeks' horse embiyo (after Ewart from Smith, 

 Messrs BailHere, Tindall and Cox). all. allantoic cavity, 

 am. amnion, c.v. villous process between allantois and yolk 

 sac, y.s. yolk sac, v' and tt' vascular villi, a — c absorbing area 

 of yolk sac, t.g. area of special attachment. 



dioxide pass in the opposite direction from the foetal to the 

 maternal circulation. In this way the placenta acts as a foetal 

 alimentary canal, lung and kidney, but there is never am'- actual 

 contact between the maternal and foetal blood. In later em- 

 bryonic life the foetus retains its connection with the chorion 

 and so with the maternal placenta, by the umbilical cord which 

 is formed partly from the allantois, and contains the umbilical 

 arteries and vein. 



The form and mode of attachment of the placenta are different 

 in the various orders of mammals. The chorionic villi may be so 

 interlocked with the maternal tissue that in parturition some of 

 the uterine mucosa is torn away from the remainder leaving a raAV 

 surface. This is what happens in man and in the dog as ^\ell as 



