G40 



ANATOMY OP VERTEBRATES. 



In the cold-blooded reptiles, hatched by external heat, indepen- 

 dently of incubation, the course of developement may be inter- 

 rupted for longer periods, without hurt to the embryo, than in the 

 warm-blooded Ovipara. Agassiz states that in Testudinata the 

 common period of hatching may be f postponed for months.' 



In Snakes and Lizards a sharp tooth is developed in the pre- 

 maxillary of the embryo, towards the close of incubation, where- 

 with they cut through the tough egg-shell. 1 The operation of this 

 transitory and purposive weapon has been observed by Weinland : 2 

 it totally disappears in the adult of most Ophidia. For breaking 

 through the more brittle shell in Chelonia the embryo is provided 

 with a sort of horn or hard excrescence above the end of the upper 

 jaw : this afterwards disappears. In the Crocodilia the snout of 

 the nearly hatched young is sufficiently hard to break the egg- 

 shell ; but there is no distinct tubercle, nor any precociously 

 developed premaxillary tooth. 3 



1 cccxxxvr. 



2 cccxxxvr. 3 crc. p. 288. 



452 



Initial steps of Vertol irate Developcmqpt. Gorm-3'olk of 

 Rana temporaria. " cccxxxvr. 



