82 Elementary Zoology. 



" ureters "). At the anterior end of the testes are a pair of 

 lobed bodies, the fat bodies. The ovary of the female is much 

 more extensive than the testis, but it has the same fat body 

 attached to its anterior end. The eggs are shed freely into the 

 ccelom, and are caught up by the open mouths of the oviduct ; 

 the oviducts are much-coiled tubes which open into the cloaca. 



The Life-History of the Frog. 



The male frog at the breeding season develops a thick 

 glandular pad upon the index fingers. This assists the male 

 in clasping the female firmly, which is done during the period 

 of oviposition, the milt being shed upon the ova as they are 

 extruded. The eggs are enveloped in a thick transparent 

 coat derived from the walls of the oviduct, which contains 

 mucin, and swells up when brought into contact with water. 

 The actual eggs themselves are smallish round bodies, black at 

 one pole and white at the other. 



The Tadpole is hatched out at a very early period of its 

 development. It is the rule of the frog tribe for tadpoles to be 

 produced ; but the rule is not without exceptions. . In a few 

 cases there is no tadpole stage at all, the young frogs making 

 their way out of the egg. When the tadpole is hatched it has 

 no mouth, and is therefore still dependent upon the yet un- 

 absorbed yolk for its nutrition. It has a horse-shoe-shaped 

 sucker below the future position of the mouth, by means of 

 which it moors itself to leaves or stones. It has pairs of 

 external gills, which are outgrowths of the skin, and possibly 

 represent the simple cutaneous gills of some invertebrate 

 ancestor. In various marine worms, for example, there are 

 gills of this character. Later, the mouth becomes apparent, 

 and its interior is furnished with a series of horny teeth, the 

 precise arrangement of which has been shown to be charac- 

 teristic for the tadpoles of different kinds of frogs. The 

 alimentary canal grows long, and is coiled in a peculiar fashion, 

 somewhat like the spring of a watch ; this form of the alimen- 

 tary tract is associated, in the common frog, with a purely 



