^1^ ^OOGENESIS ^1^ 



separate completely, (x) adhere irregularly, or (3) 

 adhere in regular geometrical fashion. 



There is no reason for believing that these three 

 paths of development w^ere not follov^ed simultane- 

 ously — that is, that animal life did not from the very 

 first develop in three divergent w^ays (fig. A, p. ^38). 

 There is no logic in the assumption that the earliest 

 animals were necessarily of the single-celled or pro- 

 tozoan type. All of the single celled animals that 

 we know are quite as highly specialized as are any 

 other animals, though they are specialized in a wholly 

 different way. 



While it is most reasonable to suppose that all 

 three alternatives were realized from the very start, 

 if any one of the three alternatives were to precede 

 the others it would presumably be the development 

 of a more or less formless sponge-like mass from which 

 on the one hand single-celled creatures were derived 

 through a complete separation of the cells after divi- 

 sion, and on the other hand the structurally more 

 complex animals were derived through the arrange- 

 ment of the cells as they divided in regular geo- 

 metrical fashion. The almost complete individuality 

 of certain cells in sponges and especially the behavior 

 of dissociated sponge cells would seem to support this 

 view. But the most probable supposition is the 

 simultaneous appearance of all three lines of devel- 

 opment from the primitive single cell. 



We have seen that regular adhesion of cells after 

 division in geometrical fashion results in the forma- 

 tion of a two layered cup called a gastrula (figs. 116, 



[193] 



