OVIPARA. 121 



Oviparous production consists, essentially, in this — the young ani 

 mal is not attached to the parietes of the oviduct, but remains sepa- 

 rated from it by its most external envelope. Its aliment is prepared 

 before hand, and enclosed in a sac attached to its intestinal canal; 

 this is what is called the vitellus, or yolkof eggr, of which the young 

 animal is a sort of appendage, at first imperceptible, which is n/wjr- 

 ished and augmented by absorbing the fluid of tliat yolk. Such of 

 the Ovipara as breathe with lungs have the egg furnished with a 

 highly vascular membrane, which appears to serve for the purposes 

 of respiration; it is connected with the bladder, and is analogous to 

 the allantoid of the Mammalia. It is neither found in Fishes nor in 

 the Batrachians, which latter, when young, respire, like Fishes, by 

 branchiae. 



CLASS IL 



AVES. 



Birds are Oviparous Vertebrata, with double systems of circula- 

 tion and respiration, organized for flight. 



Their lungs, undivided and attached to the ribs, are enveloped by 

 a membrane pierced with large holes, which allow the air to pass 

 into several cavities of the chest, lower part of the abdomen, arm- 

 pits, and even of the interior of the bones, so that not only is the 

 surface of the pulmonary vessels bathed in the ambient fluid, but 

 that also of an infinitude of vessels in other parts of the body. Thus, 

 in certain respects, Birds respire by the branches of the aorta, as 

 well as by those of the pulmonary artery, and the energy of their ir- 

 ritability is in proportion to their quantum of respiration.(l) The 

 whole body is so constructed as to profit by this energy. 



Their anterior extremities, destined to sustain them in flight, could 

 neither serve them for standing, nor for prehension; they are bipeds 

 then, and pick up objects from tlie earth with tiieir mouth; their 



(1) Two common Swallows consume as much pure air as a Guinea-Pig. 



Q 



