DEVELOPMENT OF DIGESTIVE CAVITY AND ALLANTOIS. 585 



say that the digestive cavity in Vertebrata is formed by the 

 pinching-off (as it were) of a small portion of the general sac 

 of the yolk. In the Mammalia, the remainder of the yolk-bag- 

 is completely separated from this by the closure of its narrow 

 orifice, and is afterwards thrown off; so that only a very 

 small portion of the germinal membrane is received into the 

 permanent structure. But in 

 Birds and other oviparous 

 animals, the whole of the yolk- 

 bag is ultimately drawn into 

 the abdomen of the embryo ; 

 the former gradually shrinking 

 as its contents are exhausted ; 

 and the latter enlarging, so as 

 to receive it as a little pouch 

 or appendage. In Fishes, the 

 hatching of the egg very com- 

 monly takes place before this 

 process has been completed ; 

 so that the little Fish swims 

 about with the yolk-bag hang- 

 ing from its body. 



760. The embryo, like the 

 adult, has need of Respiration ; 

 partly that its own heat may be 

 kept up; and partly that the 

 carbonic acid liberated in the various processes of nutrition, 

 may be set free. Owing to the peculiar structure of the 

 membrane covering the albumen and forming the basis of the 



Fig. 321. A, MORE ADVANCED EM- 

 BRYO OF FOWL, connected only by 

 the vitelline duct b, with the yolk- 

 bag a a, over which are distributed 

 the blood-vessels cc; B, early form 

 of the anterior extremity a, and of 

 the posterior extremity b. 



Fig 322. DIAGRAM OF THE FORMATION OF THE ALLANTOIS, . 

 (The other references as in fig. 320.) 



shell ( 755), the outer air is enabled to gain access to the 

 interior of the egg; and at first its action upon the blood, 



