XI PLACENTA 583 



aperture in the shell is enlarged, and the young bird is hatched and 

 begins a free life. 



In the higher Mammalia the allantois talces on a further 

 important function. The mode of development of the 

 amnion and allantois in the Rabbit is similar to that 

 described above in the case of the Bird. But the later 

 history of the allantois is widely different, owing to the 

 modifications which it undergoes in order to take part in 

 the formation of the placenta, the structure by means of 

 which the fcetus receives its nourishment from the walls of 

 the uterus, with which the blastodermic vesicle (p. 565) 

 becomes adherent. The foetal part of the placenta is formed 

 from the outer layer of the amnion (serous membrane, 

 Fig. 156, sh) in a limited disc-shaped area where the distal 

 portion of the allantois coalesces with it. The membrane 

 thus formed {chorion) develops vascular processes — the 

 clwrionic villi ( ft) — which are received into depressions — 

 the uterine crypts — in the mucous membrane of the dorsal 

 wall of the uterus which constitutes the maternal portion of 

 the placenta. The completed placenta with its villi is sup- 

 plied with blood by the allantoic vessels, and the blood 

 supply of the uterus is at the same time greatly increased ; 

 the fcetal and maternal capillaries and sinuses are thus 

 brought into intimate relation with one another in the 

 placenta, and diffusion can take place between them, 

 nutrient matter and oxygen diffusing from the blood of 

 the mother into that of the fcetus, while excretory sub- 

 stances pass from the blood of the foetus into that of the 

 mother. 



The discoidal placenta of the Rabbit is of the type termed 

 deciduate, the villi of the placenta being so intimately con- 

 nected with the uterine mucous membrane that apart of the 

 latter comes away with it at birth in the decidua, or after 



