REPRODUCTION 



185 



develops into a single or paired prominence, the "holder," often 

 improperly called "sucker," acting as an adhesive apparatus 

 by means of which the helpless larva fixes itself at first to 

 the outer surface of the mucilaginous envelope of the egg, and 

 later to weeds or submerged objects. Eyes are absent. A 

 small bud-like tubercle on each side of the posterior border of 

 the head is the rudiment of the external gills, and vertical folds 

 in front of and behind the bud represent the visceral clefts, the 



Fig. 13.- — A, Development of Hylodes martinicensis. 1, embryo seven or 

 eight days old ; 2, twelve days old ; 3, young just hatched. B, Stages in meta- 

 morphosis, of Common Frog. 1, tadpoles soon after hatching; 2, tadpole with 

 external gills, from above ; 7, young frog. 



intervals between which will later become converted into the 

 four branchial arches. 



As the larva grows, the tail lengthens and shows a mus- 

 cular portion with chevron-shaped divisions (myotomes), 

 bordered above and below by a vertical membrane. The ex- 

 ternal gills appear as digitate or branched appendages; the 

 olfactory pits shift more forwards, 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 pig- 



