334 GENERAL ZOOLOGY 



kidneys, to which they are attached. The ovaries of the female 

 frog when mature fill up a large portion of the body-cavity. 

 Attached to the anterior end of the spermaries and ovaries 

 are the fat-bodies (Fig. 165, 34), 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. 167, 1, 2, 3) 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 

 required 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 

 (Fig. 167, 4). Certain areas of the body are covered with 

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

 out 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 (Fig. 167, 5). 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 (Fig. 167, 6) the first or 

 primary gills are replaced by secondary gills, which soon be- 

 come covered with a fold of skin, the operculum. The growth 

 of the operculum 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 some species of frogs the tadpole develops 

 into the adult form in the course of a single season. 



