586 RESPIRATION OF THE EMBRYO :-ALLANTOIS. 
whilst circulating in the vascular area, is sufficient. In Fishes, 
no further provision is made for this process; since, by the 
time it would be required, the egg is hatched; the young 
animal comes forth into the medium it is permanently to 
inhabit, its own gills come into play, and the air contained in 
the water can act directly upon the blood circulating in the 
vascular area. But in the higher oviparous animals, whose 
development proceeds further 
before they leave the egg, a 
special provision is made for 
this purpose. On the third day 
of incubation, in the Fowl, a 
bag termed the Allantois (fig. 
322, i) begins to sprout (as it 
were) from the lower end of the 
body ; and gradually enlarges 
(fig. 323), passing round the 
embryo, and beneath its en¬ 
veloping membranes, so as 
ig. jirf.-i/MBjuu uj;’ rowii, wiui me , ■ , , . , 
Allantois, a, over which ramify the almost Completely to inclose 
embryo; and as one side of 
it lies in close proximity with the membrane of the shell, 
it is very advantageously situated for receiving the in¬ 
fluence of the air. It thus serves as the temporary respi¬ 
ratory apparatus of the Chick, up to the time when it is pre¬ 
paring to quit the egg. 1 There is reason to believe that the 
bird then receives air into its lungs, from the air-space formerly 
mentioned (§ 755), which increases in size as the contents of 
the egg diminish in bulk by the evaporation of their watery 
part. By the increased vigour which it thus acquires, it is 
enabled to perform the movements requisite for extricating 
1 If the respiration of the embryo be prevented by rendering the 
shell impermeable to air, its development is completely checked. No 
means of accomplishing this is so effectual, as smearing the shell with 
oil or grease of any kind. Hence the effect of the well-known practice 
of buttering the surface of the egg, in preventing the chick from being 
reared; and the same operation, if performed when the egg is quite 
fresh, will preserve it for some time fit for eating, its decomposition 
bei^g prevented by the complete exclusion of the air. 
