REPTILIA. 



173 



end of the primitive streak, has been described on pp. 170 — 171 

 allantois continues for some 



yd 



The 



Tnci 



time to point directly back- 

 wards ; but gradually as- 

 sumes a more ventral direc- 

 tion ; and, as it increases 

 in size, extends into the 

 space between the serous 

 membrane and amnion, 

 eventually to form a large, 

 highly vascular, flattened 

 sack immediately below the 

 serous membrane. 



The Yolk-Sack. The 

 blastoderm spreads in the 

 Lizard with very great ra- 

 pidity over the yolk to form 

 the yolk sack. The early 

 appearance of the area pel- 

 lucida, or as it has been 

 called by KupfFer and Be- 

 necke the embryonic shield, 

 has already been noted. 

 Outside this a vascular 

 area, which has the same 

 function as in the chick, is 

 not long in making its ap- 

 pearance. 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 oflf 

 from the dorsal aorta (Agassiz, No. 164). The vascular area gradually 

 envelopes 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 



Fig. 130. Advanced embryo of Lacerta mu- 

 ralis as an opaque object \ 



The embryo was 7 mm. in length in the curled 

 up state. 



fh. fore-brain; m6. mid- brain ; c6. cerebellum; 

 au. auditory vesicle (closed) ; ol. olfactory pit ; 

 md. mandible; hy. hyoid arch; 6r. branchial arches ; 

 fl. fore-limb ; hi. hind-limb. 



^ This figure was drawn for me by Professor Haddon. 



