62 APPLIED BIOLOGY 



others; toads' eggs will hatch within three or four days 

 after being laid and those of wood-frogs develop almost as 

 rapidly. 



61. Metamorphosis. The time necessary for a tadpole 

 to become full grown (Fig. 24) and to metamorphose into 

 a toad or a frog varies with species, and with food and temper- 

 ature of water. Toad and wood-frog tadpoles may metamor- 

 phose within one or two months after hatching. Such 

 rapid development is important, for these animals often lay 

 their eggs in temporary pools which become dry in late 

 spring or early summer. Bull-frogs and some green frogs 



may live in the tadpole stage 

 at least a year, for in early 

 spring there are in permanent 

 ponds many large tadpoles 

 FIG. 24. Full-grown tadpole, m, showing no signs of develop- 



mouth; n, nostril; e, eye; g, gill-slit; j ng l egs an d shortening tails, 

 a, anus; Z, leg beginning to develop. 



(From McMurrich.) and these are at least one 



year old. Probably some of 



these will become frogs in the second summer when somewhat 

 over one year old, while others may live still another year in 

 the tadpole stage. 



The nature of the metamorphosis of a tadpole into a frog 

 is popularly misunderstood. The tadpoles are said to " lose 

 their tails," and this is often taken to mean that the tails 

 drop off. This is not true. In collecting tadpoles in ponds 

 one often finds specimens with short stubby tails and well- 

 developed legs, and others showing various intermediate 

 stages between these and tadpoles with long tails and no 

 legs. The explanation of these different conditions is that 

 after the legs develop the tail is absorbed. First, the tissues 

 inside the end of the tail are disintegrated by the white 

 blood-cells, which " eat " the particles of tissue and carry 

 them back into the body. In a short time after this process 

 starts, the tip of the tail appears withered. The white 



