DEVELOPMENT OF THE LEOPARD FROG 419 



by mitosis. Many of the steps in differentiation have been deter- 

 mined, and some of the underlying causes are known, but the 

 problem of why some cells develop in a certain manner at a given 

 period, and thus form a particular part in such definite correlation 

 with other parts of the embryo, is still the central and for the most 

 part unsolved problem of Embryology. 



Metamorphosis and Juvenile Stages. — The changes by which 

 the tadpole larva is converted into a young frog are termed the 

 metamorphosis, as in other cases where a larva becomes metamor- 

 phosed into an adult (c/. Fig. 11 H to K, p. 23). Externally, 

 the globular body assumes the color and shape of the frog; the 

 skin is cast, and with it the horny hps; the small round mouth 

 changes into the large wide mouth of the frog, with concomitant 

 changes in the shape of the head; the eyes become prominent; 

 the tympanic membranes appear; the fore legs break through 

 the operculum; the hind legs grow rapidly to their adult propor- 

 tions; while the tail dwindles to a mere stump in the final stages 

 when the individual begins to hop about as a miniature frog. 

 The internal changes are even more profound. The gills and gill 

 shts disappear, and other parts of the gill apparatus either degen- 

 erate or become incorporated in modified form into other organs; 

 the tongue enlarges; the circulatory system becomes adapted to 

 pulmonary respiration; the lungs are completely developed; the 

 digestive tract, which has a very long and coiled intestine adapted 

 for the food of the tadpole, becomes changed by the enlargement 

 of stomach and hver and by actual shortening of the intestine; 

 and the sex glands become further differentiated. The young frog 

 which is thus produced from the tadpole within a few days, is 

 called a juvenile or youthful stage, according to the terminology 

 used for developmental stages throughout the Animal Kingdom. 

 If the eggs of the leopard frog have been laid in March, the indi- 

 vidual is hkely to reach such a stage by the early part of July. It 

 then grows rapidly during the summer and again the next year, 

 usually becoming sexually mature at two years of age. The age 

 that may be attained by frogs is not definitely known, but it 

 seems unhkely that they live many years. Bull frogs nine years of 

 age are reported by the New York Zoological Park. Toads prob- 

 ably live for a much longer period. 



