610 PLACENTA CHAP. 



tory organ, gaseous exchange readily taking place through 

 the porous shell ; its cavity is an embryonic urinary 

 bladder, excretory products being discharged into it 

 from the kidneys. 



At the end of incubation the embryo breaks the shell by 

 means of a little horny elevation or caruncle at the end of the 

 beak. By this time the remainder of the yolk-sac has been 

 drawn into the coelome, 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 relations of the 

 amnion and allantois in the rabbit are essentially similar 

 to those 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 blastocyst 

 (p. 586) first becomes adherent over the future placental 

 area by the proliferation of the cells of its outer 

 layer (trophoblastic ectoderm) in this region, which forms 

 irregular processes extending into the thickened mucous 

 membrane on the dorsal side of the uterus (Fig. 

 166, e'}. As the embryo develops, it sinks down and 

 causes the vascular " yolk-sac " or umbilical vesicle (ys) 

 to be doubled in and take on a flattened form (Figs. 167, 

 ds, 166 and 168, ys) : this early becomes attached to the 

 mucous membrane of the uterus, and represents a vitel- 

 line or yolk-sac placenta such as occurs in some viviparous 

 dogfishes ; but its nutritive function in the mammal is of 

 only minor importance. 



The foetal part of the placenta is formed from the 

 outer layer of the amnion (serous membrane) in a 



