94 THE FROG. 



considerably in size. The eyes, which as yet have been small, 

 become larger and more prominent. The fore-limbs appear, the 

 left one being pushed through the spout-like aperture of the 

 gill-chamber, and the right one forcing its way through the 

 opercular fold, in which it leaves a ragged hole. The abdomen 

 shrinks ; the stomach and liver enlarge, but the intestine 

 becomes considerably shorter than before, and of smaller dia- 

 meter ; the animal, previously a vegetable feeder, now becomes 

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

 gradually absorbed ; and important modifications, accompanying 

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

 of the pharynx. The kidneys undergo considerable changes ; 

 the bladder is formed ; and sexual differentiation is definitely 

 established. The tail, which is still of great length (Fig. 44, n), 

 now begins 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 FHOG'S EGG. 



1 . Formation of the Egg. 



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. length, shortly after the open- 

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

 of the peritoneum appear along the dorsal surface of the body 

 cavity, close to the root of the mesentery. These genital ridges 

 are found in all tadpoles alike, no difference of sex being esta- 

 blished until a considerably later period. 



Each genital ridge is at first due merely to a modification in 

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



