256 THE PHILOSOPHY OF BIOLOGY 



such a formation of the middle layer, or mesoblast, and 

 of the mesoblastic or coelomic cavities also actually 

 occurs. Let us assume therefore that all multicellular 

 animals have descended from a primitive Gastrea-form 

 essentially similar in morphology to the gastrula larva ; 

 and let us assume that all ccelomate animals have 

 descended from a form in which a third layer of cells, 

 or mesoblast, became intercalated between the other 

 two. These two assumptions are the bases of the 

 classic phylogenies of the last century ; all Ccelenterate 

 animals have descended from a Gastrea-form, and all 

 animals higher than the Ccelenterates have been evolved 

 from a three-layered form. Implied in this hypothesis 

 is also a third one, that the Gastrea-stage of evolution 

 possesses such a degree of stability that it has persisted, 

 though in an obscure condition it may be, in the 

 development of nearly all multicellular animals. The 

 triple germinal layers, endoderm, ectoderm, and meso- 

 derm, which first became distinct from each other in 

 the primitive ccelomate animal, also acquired a high 

 degree of stability, and they have been transmitted by 

 heredity to all animals higher than Ccelenterates. The 

 Gastrea and the three germinal layers are therefore to 

 be sought for in the developmental stages of all the 

 higher animals, and they have usually been found. 

 Let it be admitted that they may make a transient 

 appearance — that they may be obscured in many ways, 

 still they ought to be there. 



The Gastrea-Theorie ceased to be useful, as a means 

 of description, or a working hypothesis of investigation, 

 after the rise of experimental embryology. It could 

 not be proved that the process of development by 

 gastrulation and the cleavage of a mesodermal layer 

 are so very conservative that they have persisted 

 throughout the greater part of the evolution of the 



