DEYELOPEMEXT OF EEPTILIA. 



639 



losa,' with the omphalo-mesenteric vessels ; b is part of the allantois 

 with the allantoic or ' umbilical ' vessels, i. The outline of the cara- 

 pace is just marked on the back of the embryo, and the proportion 

 of the vertebral column not so modified appears to be greater, as 

 is its resemblance to the type-form of Eeptile, than in the adult. 



The condition of the carapace and the outward form of a Fresh- 

 Water Tortoise {Emys) is shown in figure 451. The amnios, a, is 

 turned back to show the posi- 

 tion of the limbs and head in 4jl 

 the egg : b is the part of the 

 allantois ; c the remnant of the 

 yolk. The central opening of 

 the plastron, which is perma- 

 nent in the marine Chelonia, 

 is seen at this period in all the 

 order, but is quickly filled up 

 in the land and fresh-water 

 species. The chief speciality 

 in the developement of the 

 scaled Eeptiles, compared with 

 each other, relates to that of 

 the carapace and plastron of 

 the Chelonia : and this has 

 been explained at pp. 557-9, 

 and illustrated in figs. 369-72. 



When the Turtle is hatched, 

 the bones of the head show 

 different degrees of ossification. 

 The 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 an 

 outer layer of bone, the rest being gristle ; the basioccipital and 

 basisphenoid begin to be ossified from the centre ; the alisphenoids 

 and exoccipitals are still cartilaginous. 



The limbs begin to show the digital divisions soon after the 

 carapace is outlined, and the cartilages of the metacarpals and 

 metatarsals are first discernible ; the phalanges are composed of 

 compacted polygonal cells at near the term of incubation, which 

 then become ' cartilage cells,' widely divided by blastema. The 

 long bones of the limbs show a thin outer crust of bone inclosing 

 cartilage, which is progressively ossified, solidifying the shaft, 

 without subsequent excavation of any medullary cavity. 



VOL. I. TT . 



Embryo of an Emys. 



