134 GENERAL GROWTH. 



being no pouch behind the last branchial arch. The first of 

 these will form the hyomandibular cleft, the second the hyo- 

 branchial, and the third, fourth and fifth the three branchial 

 clefts. 



Although the pouches of the throat meet the external skin, 

 an external opening is not formed in them till after the larva is 

 hatched. Before this takes place there grow, in the majority of 

 forms, from the outer side of the first and second branchial arches 

 small processes, each forming the rudiment of an external gill ; 

 a similar rudiment is formed, either before or after hatching, 

 on the third arch ; but the fourth arch is without it (figs. 80 

 and 82). 



These external gills, which differ fundamentally from the 

 external gills of Elasmobranchii in being covered by epiblast, 

 soon elongate and form branched ciliated processes floating 

 freely in the medium around the embryo (fig. 80). 



Before hatching the excretory system begins to develop. The segmental 

 duct is formed as a fold of the somatic wall at the dorsal side of the body 

 cavity (fig. 79, u). Its anterior end alone remains open to the body cavity, 

 and gives rise to a pronephros with two or three peritoneal openings, 

 opposite to which a glomerulus is formed. 



The mesonephros (permanent kidney of Amphibia) is formed as a series 

 of segmental tubes much later than the pronephros, during late larval life. 

 Its anterior end is situated some distance behind the pronephros, and 

 during its formation the pronephros atrophies. 



The period of hatching varies in different larvae, but in most 

 cases, at the time of its occurrence, the mouth has not yet 

 become perforated. The larva, familiarly known as a tadpole, is 

 at first enclosed in the detritus of the gelatinous egg envelopes. 

 The tail, by the development of a dorsal and ventral fin, very 

 soon becomes a powerful swimming organ. Growth, during the 

 period before the larva begins to feed, is no doubt carried on at 

 the expense of the yolk, which is at this time enclosed within the 

 mesenteron. 



The mouth and anal perforations are not long in making 

 their appearance, and the tadpole is then able to feed. The gill 

 slits also become perforated, but the hyomandibular diverti- 

 culum in most species never actually opens to the exterior, and 

 in all cases becomes very soon closed. 



