68 OUTLINES OF CHORDATE DEVELOPMENT 



ceases to feed; a casting, or ecdysis, of the outer layer of the 

 skin takes place; the horny jaws are thrown off; the large 

 frilled lips shrink up; the mouth loses its rounded suctorial 

 form and becomes much wider; the tongue, previously small, 

 increases 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 diameter; the animal now becomes carnivorous. 

 The gill-clefts close up; the gills themselves are gradually ab- 

 sorbed; 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 estab- 

 lished. The tail, which is still of great length, now begins to 

 shorten, and is soon completely absorbed; the hind legs lengthen 

 considerably, and the animal leaves the water as a frog." 

 (Marshall, " Vertebrate Embryology," pp. 93-4). 



It frequently happens that a cool season or a scarcity of 

 food delays this period of metamorphosis until late in the 

 summer, and not infrequently the winter is passed in the tad- 

 pole stage, metamorphosis of the then overgrown tadpoles 

 occurring the following warm season. Indeed in some species 

 (e.g., bull frog, R. catesbiana] this is normal. The length 

 of the period w r hich elapses after metamorphosis, before sexual 

 maturity, varies widely. Some species seem to mature the 

 second spring after metamorphosis, i.e., when two years old, 

 others not until two or three years later. 



II. THE GERM CELLS AND THEIR PRODUCTION 



A. THE EGG AND SPERM AT THE TIME OF SPAWNING 



The fully formed ovum is a spherical cell varying, in different 

 species of frogs, from 1.5 to 3.0 mm. in diameter about 1.75 



