94 THE FROG. 



considerably in size. The eYs a^ whicli as yet have bee n small, 

 beco me larger and more prom inent. Th e fore-limbs appear, the 

 lef T^e beinfy pushed thrnngh the spout-like aperture of the 

 gilFchambe r, and the right one forcing its way throug h the 

 opercular fold, in which it leaves a ragged hole. TheaESomen 

 shrmiis ; the stomach and Uver enlarge, but the intestine 

 becomes considerab ly shorter than befor e, and of smaller dia- 

 meter ; the ani mal, previously a vegetable feeder, now b ecomes 

 carnivorous . The gill-clefts close up ; the gills themselv es are 

 gradually absorbed ; and important modifications, accompanying 

 the change in the mode of breathing, occur, in the blood-vessels 

 of the pharynx. Th e kidneys undergo considerable chang es ; 

 the bladder is formed ; and sexual differentiation is definite ly 

 established. The tail, which is still of great length (Kg. 44, ii), 

 now be gins to shorten, and is soon completely absorbed ; the 

 hind legs lengthen considerably, and the animal leaves the water 

 as a frog. 



By preventing tadpoles from breathing air directly, as by 

 placing a wire net an inch or so below the surface of the water 

 in which they are living, the occurrence of the metamorphosis 

 can be indefinitely deferred. Under these conditions tadpoles 

 increase greatly in size, but do not become transformed into 

 frogs. 



In the remainder of this chapter the several stages in the 

 development of the tadpole, and the formation of the various 

 "organs and systems, will be described in detail. 



THE FROG'S EGG. 

 1. Formation of the Eggf. 



The early stages in the formation of the eggs cannot be seen 

 in the adult frog, but must be studied in tadpoles. 



In tadpoles of about 10 mm. leng- j-.h, shortly after the open- 

 ing of the mouth, a_ pair of longitudinal ridge-like thickenings 

 of the peritoneum ap pea r alonp: the dorsal surface of the bo dv 

 cavity, c lose to the root of the mp,sentery. T hese genital ridgfe s 

 are found in all tadpoles alike, no difference of sex being est'a- 

 blished until a considerably later period. 



Each g enital r idge is at first due m erely to a modificatio n in 

 shape of the peritoneal epithelial cells, which, elsewhere flattened, 



