THE EARLY DEVELOPMENT OF THE FROG 67 



tive tract gives the body a well-rounded form, sharply marked 

 off from the narrow tail, upon which develop large dorsal and 

 ventral fin-like folds of skin; this is the only locomotor organ 

 in the tadpole. A period of rapid growth follows upon this 

 voracious feeding; the rate of growth depending upon tem- 

 perature and food supply. 



Immediately after hatching external gills develop rapidly on 

 the sides just back of the head, and for a time these are the 

 only respiratory organs, but about the time the mouth opens 

 four pairs of gill slits successively perforate the pharyngeal 

 wall, and their borders become folded forming the true internal 

 gills: thereupon the external gills gradually diminish and after 

 a few days disappear completely. At this time the branchial 

 region becomes covered over externally by a protecting oper- 

 cular fold of integument, the opercular cavity thus formed 

 finally remaining open on the surface only by a single excur- 

 rent pore or " spiracle" on the left side. 



During the next few weeks, while the tadpole continues to 

 feed almost incessantly, there are few external changes except 

 the general increase in size. About four or five weeks, ordi- 

 narily, after hatching (much sooner at room temperature) the 

 limb buds appear, first the anterior pair within the opercular 

 cavity and therefore not visible externally, and soon after the 

 posterior pair either side of the cloaca (Fig. 22, H). By the 

 end of the second month these have enlarged and become 

 jointed. 



For some time previous to this the tadpoles have been coming 

 to the surface occasionally to expel small bubbles of air from 

 the slowly developing lungs, and to gulp down a fresh supply, 

 and as this aerial respiration increases the internal gills retro- 

 gress and the gill slits diminish. 



If developmental conditions have been favorable and food 

 abundant, about the end of the third month the period of 

 metamorphosis commences during which, in the space of a few 

 days, the tadpole loses many of its characteristic structures 

 adapted to aquatic life and rapidly, almost suddenly, assumes 

 the characteristics of the amphibious frog. "The tadpole 



