170 Vertebrate Embryology 



rest of the thickness of its walls is formed by the splanch- 

 nopleuric mesoblast. 



" The allantois of the chick is homologous with the 

 bladder of the frog. It differs mainly from this in the 

 fact that, while arising in the same manner, it is not con- 

 fined within the body of the embryo, but, growing rapidly, 

 passes out beyond this as a thin-walled vascular sac (Figs. 

 38, 72, and 80), which spreads out in close contact with 

 the inner surface of the egg-shell, and acts as the respi- 

 ratory organ of the embryo during the greater part of its 

 development. 



" In the chick the allantois commences to form about 

 the middle of the second day. At this time the tail fold 

 is not yet established, so that the allantois (Fig. 51, Al.) 

 appears at first as a pocket-like fold of the splanchno- 

 pleure, lying a short way behind the embryo, and with 

 its cavity opening ventralwards. 



" On the formation of the tail fold, early on the third 

 day, the part of the splanchnopleure from which the 

 allantois arises becomes doubled forwards under the 

 embryo to form the ventral wall of the gut, and the al- 

 lantois now appears as a saccular depression of the 

 ventral wall of the hind-gut (Fig. 38, C). 



" During the third day the allantois increases con- 

 siderably in size, projecting downwards and forwards, 

 as a hollow, thick-walled bud from the ventral surface 

 of the hind-gut, into the body cavity, or space between 

 the somatic and splanchnic layers of the mesoblast. 

 During the fourth day, by its further growth, the allan- 

 tois passes out beyond the embryo, and turns up along 

 its right side, into the space between the two layers of 

 the amnion, which, from the mode of formation of the 



