DEVELOPMENT OF DIGESTIVE CAVITY AND ALLANTOIS. 585 
say that the digestive cavity in Yertebrata 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 Bespiration ; 
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 c c; b, early form 
of the anterior extremity a, and of 
the posterior extremity b. 
Fig 322.— Diagram of the Formation of the Allantois, i. 
(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, 
