. On the anatomy of vertebrates [electronic resource] . 9-72. When the Turtle is hatched,the bones of the head showdifferent degrees of premaxillary and preman-dibular are most advanced for the purposes of feeding; the maxil-lary, the back part of the mandible, the prefronto-nasal, frontal,and parietal come next in hardness. The superoccipital shows anouter layer of bone, the rest being gristle; the basioccipital andbasisphenoid begin to be ossified from the centre ; the alisphenoidsand exoccipitals are still cartilaginous. The limbs begin to show the digital divisions soon af
. On the anatomy of vertebrates [electronic resource] . 9-72. When the Turtle is hatched,the bones of the head showdifferent degrees of premaxillary and preman-dibular are most advanced for the purposes of feeding; the maxil-lary, the back part of the mandible, the prefronto-nasal, frontal,and parietal come next in hardness. The superoccipital shows anouter layer of bone, the rest being gristle; the basioccipital andbasisphenoid begin to be ossified from the centre ; the alisphenoidsand exoccipitals are still cartilaginous. The limbs begin to show the digital divisions soon after thecarapace is outlined, and the cartilages of the metacarpals andmetatarsals are first discernible; the phalanges are composed ofcompacted polygonal cells at near the term of incubation, whichthen become 6 cartilage cells, widely divided by blastema. Thelong bones of the limbs show a thin outer crust of bone inclosingcartilage, which is progressively ossified, solidifying the shaft,without subsequent excavation of any medullary cavity. VOL. I. TT. 640 ANATOMY OF VERTEBRATES. In the cold-blooded reptiles, hatched by external heat, indepen-dently of incubation, the course of developemeDt may be inter-rupted for longer periods, without hurt to the embryo, than in thewarm-blooded Ovipara. Agassiz states that in Testudinata thecommon period of hatching may be e postponed for months. In Snakes and Lizards a sharp tooth is developed in tbe pre-maxillary of the embryo, towards the close of incubation, where-with they cut through the tough The operation of thistransitory and purposive weapon has been observed by Weinland :2it totally disappears in the adult of most Ophidia. For breakingthrough the more brittle shell in Chelonia the embryo is providedwith a sort of horn or hard excrescence above the end of the upperjaw: this afterwards disappears. In the Crocodilia the snout ofthe nearly hatched young is sufficiently hard to break the egg-shell ; but there is no distinct
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