OF VERTEBRATE ANIMALS. 63 



ence, from which the vertebrata are divided into oviparous and 

 viviparous or mammalia. 



57. The oviparous vertebrata, are particularly alike in 

 their mode of generation, they have also some common cha- 

 racters of organization in the nervous system, and in the bones 

 which envelop it. 



Oviparous generation consists essentially in the germ being 

 inclosed in its envelopes, with nutritious matter sufficient to 

 nourish it until it be hatched, so that if the egg remain in the 

 interior it does not attach itself to the parietes of the oviduct, 

 but remains separate. The nutriment of the young one is con- 

 tained in a sac which is a part of its intestine, and which is 

 called the yolk. The germ is at first a mere imperceptible ap- 

 pendage, but as fast as it receives its nourishment, it increases 

 in size by the absorption of the yolk which is proportionally 

 diminished, and finally disappears near the period of hatching. 

 The embryos of the ovipari with lungs, birds, and, except the 

 batrachians, reptiles, have moreover a very vascular membrane, 

 which appears to assist in respiration, and which is a prolonga- 

 tion of the bladder: it is called the allantoid; it is not found 

 either in fishes or the batrachian reptiles, whose young are pisci- 

 form. Particular fish and reptiles retain their eggs within 

 them until the time of their hatching; such are called ovovi- 

 vipari. 



The prolongation of the spinal marrow in the cranium, pre- 

 sents in the ovipari, highly developed tubercles, called the quad- 

 rigemina; the cerebrum and cerebellum, on the contrary, are 

 very slightly so, and there is no pons varolii nor corpus callo- 

 sum. The bones of the cranium are either very quickly con- 

 solidated, or remain a long time divided; their senses are not 

 as perfect as those of the vivipari; the lower jaw, which is 

 very complex, is articulated by a concave facette with a pro- 

 jecting part of the temporal bone, that is distinct from its pe- 

 trous portion; the orbits are only separated by a membrane or 

 a bony plate of the sphenoid. When they possess anterior 

 members, the clavicles unite and form a fourchette (as the 

 merry-thought of a chicken) while the elongated coracoid apo- 



physes are articulated with the sternum. The larynx is sim- 

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