AMPHIBIA. 



109 



posterior region of the embryo. The hind end of the embryo grows 

 out into a rounded prominence, 

 which rapidly elongates, and 

 becomes a well-marked tail 

 entirely formed by the elon- 

 gation of the post-anal section 

 of the body. The whole body 

 has a very decided dorsal flex- 

 ure, the ventral surface being- 

 convex. Fig. 78 represents an 

 embryo of Bombinator in side 

 view, with the tail commen- 

 cing to project. The longitu- 

 dinal section (fig. 76) is taken 

 through an embryo of about 

 the same age. In the cephalic 

 region important changes have 

 taken place. The cranial flex- 

 ure has become more marked, 

 but is not so conspicuous a 



feature in the Amphibia as in most other types, owing to the small 

 size of the cerebral rudiment. The mid-brain is shewn at fig. 78 a 

 forming the termination of the long axis of the body, and the optic 

 vesicles (a) are seen at its sides. 



The rudiments of the inandibular (d), hyoid (), and first bran- 

 chial (e'} arches project as folds at the side of the head, but the 



FIG. 77. EMBRYOS OF THE COMMON FROG. 

 (After Eeinak.) 



A. Young stage represented enclosed in 

 the egg membrane. The medullary plate is 

 distinctly formed, hut no part of the medullary 

 canal is closed, bl. hlastopore. 



B. Older embryo after the closure of the 

 medullary canal, oc. optic vesicle. Behind 

 the optic vesicle are seen two visceral arches. 



FIG. 78. LATERAL VIEW OF AN ADVANCED EMBRYO OF BOMBINATOR. (After Gotte.) 



a. mid-brain; a', eye; &. hind-brain; d. mandibular arch; d'. Gasserian ganglion; 

 c.hyoid arch; e'. first branchial arch; /".seventh nerve; /'. glossopharyngeal and 

 vagus nerve; </. auditory vesicle ; i. boundary between liver and yolk-sack; k. suctorial 

 disc; 7. pericardia! prominence; m. prominence formed by the pronephros. 



visceral clefts are not yet open. Rudiments of the proctodaeum and 

 stomoclseum have appeared, but neither of them as yet communi- 

 cates with the mesenteron. Below the hyoid arch is seen a peculiar 

 disc (k} which is an embryonic suctorial organ, formed of a plate of 



