220 BIOLOGY: GENERAL AND MEDICAL 



surface to outgrow and surround the inner concave 

 surface, the fusion or concrescence of many of the folds, 

 the growth of groups of cells at certain points to form 

 organs and so bring about the final evolution of the 

 foetus. 



The external morphological development and the 

 resemblances it embraces can be more quickly understood 

 by an examination of the accompanying illustrations 

 taken from Romanes than from a lengthy description. 



Internal development progresses simultaneously with 

 external development. Perhaps so much of this has 

 been left to the imagination of the reader that it may be 

 well to consider certain features more particularly. 

 Let us begin by remarking that it progresses simply or 

 complexly according to the future simplicity or complex- 

 ity of the species to which the embryo belongs. Let it 

 also be understood that in spite of complexity it pro- 

 gresses rapidly, and that the complexity incidental to 

 the phylogenetic position of the animal whose embryo 

 is examined is not the result of the consecutive forma- 

 tion of the internal organs, but of their simultaneous 

 formation. 



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 



