286 



THE EVOLUTIONARY CAREER 



with three such layers, the endoderm, ectoderm and mesoderm 

 {Triplohlastica). In embryogeny we see the distinction : the 

 gastrula is diploblastic, but upon the formation of the mesoderm 

 it becomes triploblastic. In the fully developed animals the 

 Coelenterates represent the diploblastic phase and the other 

 primary groups the triploblastic one. 



(4) Triploblastic animals are divided into those in which there 

 is a coelom, or body cavity, in the mesoderm and those in which 

 this cavity cannot be recognized. And the distinction can be 

 observed in some embryogenies. For a time the mesoderm may 

 be solid and then follows a phase in which it attains a ccelomic 

 cavity. 



We are now left with the primary groups of the animal kingdom 

 — the phyla, or other categories. No investigations made so far 

 enable us to relate together in any other way the ccelomate phyla 

 — at least no attempts to do so have been generally accepted by 

 zoologists. 



94^. The Parallelism of Embryological Phases with 

 Classificatory Groupings. Clearly the embryonic phases occur 

 in the following order (as, for instance, in Echinus and Arnphioxus). 



And the order is also that of the degrees of generality of structure 

 (as numbered) in the classification. But this is inevitable, for we 

 have based the classification on homologies that are founded on 

 embryonic phases. 



95. ON GENERALIZED TECTONIC CHARACTERS 

 All organisms whatever, are, at the beginning, or all through 

 their life-histories, unicellular. All multicellular animals pass 

 through a phase of development which is typically, or atypically, 

 gastrular. All animals that are " higher " (or more elaborated, 

 tectonically) than the coelenterates pass through a further develop- 

 mental phase in which a third germ-layer is added to the two 



