xii PLACENTA 601 



the remainder of the yolk-sac has been drawn into the ccelome, and the 

 ventral body-walls have closed round it. On the shell being broken 

 the allantois gradually shrivels up, respiratory movements begin, the 

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

 begins a free life. 



In the higher Mammalia the allantois takes 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 foetus receives its nourishment from the walls of 

 the uterus, with which the blastodermic vesicle (p. 582) 

 becomes adherent. The fatal part of the placenta arises in 

 a limited disc-shaped area of the outer layer of the 

 amnion (serous membrane, Fig. 167, sh\ where the distal 

 portion of the allantois coalesces with it. The membrane 

 thus formed (chorion) develops vascular processes the 

 chorionic villi (pl\ which are received into depressions 

 the uterine crypts, in the thickened mucous membrane of 

 the dorsal wall of the uterus which constitutes the maternal 

 portion of the placenta. The 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 foetal 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 foetus, 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 



