40 GENERAL ZOOLOGY 



in the water by a single spermatozoon entering each egg. The 

 jelly surrounding each egg swells to several times its original 

 diameter and serves as a protective covering for the developing 

 embryo. When the embryo reaches the length of six or seven 

 millimeters it hatches by wiggling out of the gelatinous capsule. 

 This takes place about two weeks after fertilization, depending 

 upon the temperature of the water, a low temperature prolonging 

 the time. The embryo, now a larva, has a blunt head and a short 

 median fin, but no limbs or even a mouth. At either side of the 

 head are rudiments of external gills. A mouth soon forms from 

 an indentation (invagination) in the outside layer of cells covering 

 the ventral surface of the head, communicating with the anterior 

 end of the pharynx. The larva then begins to feed on water 

 plants and grows rapidly. The external gills become enclosed 

 in a fold of the skin, called the operculum, which forms a respira- 

 tory chamber, opening on the left side by a spiracle. With the 

 formation of the operculum, the external gills are resorbed and 

 are replaced by internal gills, which develop on the edges of gill 

 slits located in the wall of the pharynx on either side. The 

 forelegs develop inside of the operculum and at first are not 

 visible externally. The hind limbs appear later at the base of the 

 tail. 



At about the end of the third month of larval development 

 metamorphosis takes place. Metamorphosis involves profound 

 changes, both external and internal. Briefly, these consist in the 

 completion of the development of the lungs with accompanying 

 changes in the circulatory system ; an enlargement of the stomach 

 and liver and a shortening of the intestine; the resorption of the 

 operculum and the liberation of the forelimbs; disappearance 

 of the gills and closure of the gill slits; and finally the resorption 

 of the tail. During the larval period the mouth is provided with 

 horny jaws which are cast off with the molting of the skin 

 accompanying metamorphosis. During the larval period the 

 diet is largely plant material. After metamorphosis, the frog 

 becomes more carnivorous in its feeding habits. Correlated with 

 this there occurs a shortening of the alimentary tract, in keeping 

 with well-known fact that flesh eaters have relatively shorter 

 digestive tracts than plant eaters. 



Hibernation. — Following the breeding season and throughout 

 the summer the frog is an active feeder, storing up a reserve 



