THE GREEN FROG 339 



female frog when mature fill a large portion of the body 

 cavity. Attached to the anterior end of the spermaries and 

 ovaries are the fat bodies, — lobed organs which attain 

 their fullest development in the spring. These organs are 

 believed to be of use as storehouses of reserve material, ren- 

 dering possible the formation of large numbers of sperma- 

 tozoa or eggs without complete exhaustion of the animal. 

 After the spawning season the fat bodies decrease in size. 



Development. The eggs of frogs (Fig. 175) are laid early 

 in the spring in shallow water in large, jelly-like masses. 

 They are fertilized by the male as they leave the body of the 

 female. Within a week or ten days they hatch, the time re- 

 quired depending largely upon the temperature of the water. 

 At first the young is blind, and is without gills or a mouth ; 

 it fastens itself to weeds and other objects in the water 

 by means of a crescent-shaped, adhesive apparatus at the 

 anterior end. Certain areas of the body are covered with 

 cilia, by the vibration of which the animal is able, even 

 without using its tail, to go forward in the water. Eyes, 

 external gills, and a mouth provided with horny jaws soon 

 appear, and the young, now the familiar tadpole, begins 

 to feed on plant food. The alimentary canal is long and 

 coiled, as it usually is in animals which feed upon plant 

 material. The heart has two chambers. 



As the tadpole increases in size the first, or primary, gills 

 are replaced by secondary gills, which soon become covered 

 with a fold of skin, the operculum. The growth of the oper- 

 culum continues till the gill openings are covered, leaving a 

 small hole usually on the left side. In this fish-like condition 

 the tadpole continues through the summer, and on the 

 approach of cold weather buries itself in the mud, where it 

 hibernates. In the leopard frog and some other species the 

 tadpole develops into the adult form in the course of a 

 single season. 



