ONTOGENESIS 



233 



Therefore, in animals of complicated structure many 

 different things are going on at the same time that must 

 be separately and individually described. Again, as 

 the embryo of a vertebrate is at no stage of its embryonal 

 development self-supporting, all of the developmental 

 processes look toward future needs, making no pro- 

 vision for immediate needs, which are provided for by 

 the yolk or derived through the placenta (oxygen, nu- 

 triment, etc.) from the parent. 



Thus, in phylogenetic development, it has been 

 shown that the first need of the organism is food, and 

 the first specializations have to do with the vegetative 

 functions, so that the vegetative organs are the first to 

 evolve, the first cell differentiation being the separation 

 of the outer cells into protective coverings and the inner 

 cells into nutrition providers. Following this means 



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

 (Galloway after Brehm.) 



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. 



