EMBRYOLOGY OF TADPOLE AND CHICK 



517 



and development. The adoption of a standard makes an understanding 

 possible of any special modifications one may find. 



Then, so much work has been done in this field that a thorough 

 understanding of the embryology of the frog is essential to anyone who 

 wishes to do advanced work in the zoological sciences. 



Just as there are various groups of birds with different hatching 

 periods, so different species of frogs also vary as to the length of time 

 the embryo requires before being able to emerge from the egg. But, 



Fig. 315. 

 Examples of tailed and tailless Amphibia. 



unlike the birds, the frog passes through a process of metamorphosis, 

 which simply means that even after the embryo's emergence from the 

 egg, it does not have the adult form, but must pass through still further 

 changes before becoming a full-fledged frog. In the frog the form that 

 is assumed at the time of hatching and which later changes on arrival 

 at adult life, is called the tadpole stage. 



Temperature has much to do with both the rapidity with which a 

 frog's egg develops, and with which the tadpole develops into an adult 

 frog. This, however, is not unlike the hen's egg, for, it will be remem- 

 bered that after the hen's egg was fertilized and the embryo had already 

 begun to develop (before the egg has been laid), such an egg could be 

 placed in a moderately cool place for many days, which would result in 

 all development ceasing. If the egg is then placed under a hen, or in 

 an incubator, the embryo again begins to develop. 



We shall arrange our study of the frog under two headings : First, 

 the true embryonic period. This extends from the time the egg is fer- 

 tilized through blastulation and gastrulation. During this time the germ 

 layers as well as the larval and tadpole organs form. This period ex- 

 tends up to the time the tadpole emerges from the egg. 



Second, the larval period, from the time of hatching to the time the 

 legs are formed, the tail is thrown off, and the animal has become a full- 



