THE DEVELOPMENT OF THE FROG'S EGG 335 



tadpole, begins to develop legs. The hind legs first 

 appear, afterward the fore legs sprout out, and finally 

 the tail begins to wither away, until the form of the four- 

 legged tailless frog (anuran) is fully developed. In the 

 leopard frog, as in the toad, this change of form (metamor- 

 phosis) is completed during the first summer, but in case 

 of the bullfrog and green frog the tadpole passes through 

 the winter in the immature state, and does not complete 

 its metamorphosis until the second summer. Conse- 

 quently, 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 Chap- 

 ter XVII., the gills and the fin on the tail of the tadpole 

 are lost in the metamorphosis. Necturus, on the con- 

 trary, retains gills and tail-fin, so that its acquisition of 

 legs is almost the sole indication 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 develop- 

 ment, 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 budding of leaves on a tree, the regeneration of the 

 parts of a Hydra, or the healing of a cut in the skin. In 

 the case of most of the higher plants and animals, the ripe 

 egg will not develop until it is " fertilized," that is, until a 

 germ from another individual has fused with the ripe egg. 



