ONTOGENESIS 



221 



must be provided for transporting the nutriment so 

 that it may easily reach the cells not contiguous to the 

 source of supply. The last system to appear is the 

 correlating and coordinating central nervous system. 



In the ontogenetic development of the higher animals 

 the conditions are different, for the nourishment of the 

 embryonal tissues is provided for by the egg yolk or by 

 the placenta, so that the digestive organs need not be 

 early perfected. The size of the embryo and the arrange- 



Fio. 97. The metamorphosis of the frog. The numbers indicate the sequence. 

 (Galloway after Brehm.) 



ment of its parts are also such that the circulatory 

 organs need not be active before a considerable general 

 complexity of structure is attained. There is, however, 

 among vertebrates a general dominance of the nervous 

 system, so that instead of the organic systems developing 

 one after the other, as in phylogenetic progress, ontoge- 

 netic development is so modified that the dominant sys- 

 tem first makes its appearance, and in vertebrate em- 

 bryos the central nervous system which takes precedence 

 of all others in importance is one of the first to appear. 



