210 CHELONIA. 



in the chick, is not long in making its appearance. In all 

 Reptilia the vascular channels which arise in the vascular area, 

 and the vessels carrying the blood to and from the vascular area, 

 are very similar to those in the chick. In the Snake the sinus 

 terminalis never attains so conspicuous a development and in 

 Chelonia the stage with a pair of vitelline arteries is preceded by 

 a stage in which the vascular area is supplied, as it permanently 

 is in many Mammals, by numerous transverse arterial trunks, 

 coming off from the dorsal aorta (Agassiz, No. 164). The 

 vascular area gradually envelops the whole yolk, although it 

 does so considerably more slowly than the general blastoderm. 



Ophidia. There is, as might have been anticipated, a very 

 close correspondence in general development between the 

 Lacertilia and Ophidia. The embryos of all the Amniota are, 

 during part of their development, more or less spirally coiled 

 about their long axis. This is well marked in the chick of the 

 third day; it is still more pronounced in the Lizard (fig. 130) ; 

 but it reaches its maximum in the Snake. The whole Snake 

 embryo has at the time when most coiled (Dutrochet, Rathke) 

 somewhat the form of a Trochus. The base of the spiral is 

 formed by the head, while the majority of the coils are supplied 

 by the tail. There are in all at this stage seven coils, and the 

 spiral is right-handed. 



Another point, which deserves notice in the Snake, is the 

 absence in the embryo of all external trace of the limbs. It 

 might have been anticipated, on the analogy of the branchial 

 arches, that rudiments of the limbs would be preserved in the 

 embryo even when limbs were absent in the adult. Such, 

 however, is not the case. It is however very possible that 

 rudiments of the branchial arches and clefts have been preserved 

 because these structures were functional in the larva (Amphibia) 

 after they ceased to have any importance in the adult ; and that 

 the limbs have disappeared even in the embryo because in the 

 course of their gradual atrophy there was no advantage to the 

 organism in their being specially preserved at any period of life 1 . 



Chelonia 2 . In their early development the Chelonia re- 



1 It is very probable that in those Ophidia in which traces of limbs are still 

 preserved, that more conspicuous traces would be found in the embryos than in the 

 adults. 



" Vide Agassiz (No. 164), Kupffer and Benecke (No. 154), and Parker (No. 165). 



