CLASS AVES. d 



exterior of these envelopes. Its nourishment is pre- 

 pared beforehand, and inclosed in a sac attached to its 

 intestinal canal : it is this which is called the mtellus, 

 or yolk of the egg, in which the young, at first imper- 

 ceptible, is inserted and nourished, and augments 

 by absorbing the liquor of the yolk. Oviparous ani- 

 mals which respire by lungs, have, moreover, in the 

 egg, a membrane plentifully supplied with vessels, 

 which seem to serve the purposes of respiration ; they 

 are attached to the bladder, and represent the allan- 

 tois of the mammalia. It is not found in the fish, 

 nor in the batracian reptiles, which, when young, 

 respire like the fish, by gills. 



Many of the cold-blooded oviparous animals bring 

 forth their young developed, and without the shell or 

 other membrane which separates them from the mo- 

 ther ; these are called false vivipara. 



THE SECOND CLASS OF VERTEBRATED ANIMALS. 



THE BIRDS 



Are oviparous, vertebrated animals, with double cir- 

 culation and respiration^ organized for flight. Their 

 lungs are not divided, but fixed to the ribs, and are 

 enveloped by a membrane pierced by great holes, 

 which permit the air to pass into many cavities in the 

 chest, lower belly, arm-pits, and even the interior of 

 the bones; so that the exterior fluid not only bathes 



B 2 



