DEVELOPMENT AND METAMORPHOSIS. 87 



diagram, copied from Thiele's memoir, shows roughly 

 the shape of this organ, and its position with respect 

 to the mouth at the close of the first larval period. 



Fig. 33. 



p G H 



A. Discoglossus pictus. b. Pelobates fuscus. c. Bufo vulgaris. 

 d. Bufo viridis. e. Rana esculenta. F. Rana temporaria. 

 G. Hyla arborea. H. Bombinator pachypus. 



As the embryo grows the tail elongates and shows 

 a muscular portion with chevron-shaped myotomes, 

 bordered above and below by a membranous crest. 

 The gills become digitate or branched ; the olfactory 

 pits shift more forward, and become converted into 

 functional nostrils communicating with the mouth; 

 the eye may be detected at the side of the head, 

 appearing first as a pigmented ring under the trans- 

 parent epidermis; the mouth becomes bordered by 

 fleshy lips ; the anus is perforated ; and the larva 

 is able to feed, having thus far subsisted on the 

 vitellus contained in the abdomen. 



The external gills, two or three in number on each 

 side, the second and third often rudimentary and con- 

 cealed under the first, are largest and persist longest 

 in Rana temporaria. In this species and in the toads 

 they are strongly pigmented, like the body ; in most 

 others they are not, or but feebly, pigmented. A little 

 smaller but likewise branched in Rana esculenta, Bufo 

 vulgaris, and Discoglossus, they remain very short and 



