DEVELOPMENT. 



297 



water, either singly and attached to water plants (e.g. newt), 

 in clumps (e.g. frog), or in strings (e.g. toads) ; but in some cases 

 they are deposited out of the water, in situations in which the 

 larvae can easily reach it. 



Development.* The eggs, which possess a considerable 

 'amount of food yolk, are relatively small, and undergo (except in 

 the Gymnophiona) a total, but unequal segmentation (Fig. 169). A 

 gastrula is formed by a modification of the process of invagina- 

 tion, and the blastopore in some cases (e.g. newt) persists as the 

 anus and in some cases (e.g. frog) closes up, the anus being a 

 later perforation on its site. A neurenteric canal is very generally 

 present. An amnion and allantois are not formed, though a 

 cloacal bladder, an organ homologous with the allantois, arises 

 as a median ventral diverticulum of the cloaca. The embryos 



FIG. 163. Unequal segmentation of the frog's egg in ten successive stages (after Ecker). 



are also without any external yolk-sac constricted off from the 

 body, the yolk being enclosed at an early period by the side- 

 walls of the body. With a few exceptions, which have already 

 been referred to and are mentioned again in the systematic part, 

 the young leave the egg in an immature condition and undergo 

 the later part of their development as free-swimming larvae. f 

 In the Anura these larvae are generally known as tadpoles. 

 The larvae are always aquatic and breathe by gills (see p. 278) 

 and possess other larval organs, e.g. a pronephros, and lateral 

 line sense organs. Their vascular system is on the piscine type 

 which gradually gives place to that characteristic of air-breathing 



* Balfour, Comparative Embryology, vol 2, 1883. Marshall, Vertebrate 

 Embryology, 1893, Brauer, op. cit. 



f The resemblance in certain cranial characters between the anuran 

 larva and the marsipobranchii has already been referred to (p. 96). 



