4.56 ZOOLOGT 



change of form {metamorphosis) is completed during the first 

 summer, but in case of the l^uhfrog and green frog the tadpole 

 passes through the mnter in the immature state, and does not 

 complete its metamorijhosis until the second summer. Conse- 

 cjuentlj', it is not uncommon to find quantities of large tadpoles 

 in ponds at the time the ice breaks up in the spring. 



Since Amblystoma and Necturus do not lose their tails, 

 the metamorphosis which they undergo is less profound than 

 that of the frog. In Amblystoma, as stated in Chapter XVII., 

 the gills and the fin on the tail of the tadpole are lost in the 

 metamorphosis. Necturus, on the contrary, retains gills and 

 tail-fin, so that its acquisition of legs is almost the sole mdi- 

 cation of metamorphosis. 



General Laws of Development. — Development consists 

 of an unfolding of potentialities wrapped up in the germ ; an 

 awakening in orderly succession of processes lying dormant 

 there. But the causes which control development, the causes 

 which determine when this process and that shall awaken, are 

 still too obscure for us to attempt to picture them in detail. 

 This much is certain, that the causes of development from the 

 egg are the same as those of l)udding of leaves on a tree, the 

 regeneration of the parts of a Hydra, or the healing of a cut in 

 our skin. In the case of most of the higher plants and animals, 

 the ripe egg wall not develop until it is " fertilized," that is, 

 until a germ from another indi\'idual has fused with the ripe 

 egg. But the ripe egg of many of the lower plants and animals 

 requires no fertilization for development, and the meaning of 

 the fertilization ]irocess is quite obscure. 



The developing eggs of all the higher animals pass through 

 much the same sort of early stages. The egg " cleaves " into 

 a number of cleavage spheres, each of which is destined to give 



