66 JenkinSON, Germinal Layers of Vertebrates. 



In the first place, it is postulated that the primary layers 

 of the Gastrula are completely homologous, that is, homo- 

 phyletic or homogenous, throughout the Metazoan series ; 

 and in the second, that they bear certain constant and 

 invariable relations to the organization of the adult on 

 the one hand, and to that of the reputed ancestral form 

 on the other. Haeckel recognises this perfectly, ' Der 

 ' vvesentliche Inhalt dieser Gastraea-Theorie,' he says 

 ' beruht auf der Annahme einer wahren Homologie (ohne 



* welchen die Gastraea-Theorie nicht haltbar ist) der primi- 

 ' tiven Darmanlage und der beiden primaren Keimblatter 



* bei alien Thieren mit Ausnahme der Protozoen. " 



Further, since, as is held, it is phylogeny which is 

 repeated in and ultimately determines the form which 

 .ontogeny takes, any doubt which exists as to the homo- 

 logy of an organ in the fully developed form may be 

 completely set at rest by a reference to the germinal layer 

 in which it arises, the homologies of the germ-layers in 

 their turn being determined by reference to their mutual 

 anatomical relations or, when possible, their mode of origin 

 in the segmenting ^^^. It is origin then, and not destiny, if 

 we push these theories to their logical conclusion, which is 

 the final criterion of the homologies, whether of the organs 

 of the adult or of the layers of the germ. 



The first question accordingly which we have to 

 answer is, are the so-called primary layers always homo- 

 logous? Let us consider this in the light of the facts 

 which vertebrate development has taught us. 



At the close of segmentation the microleicithal verte- 

 brate ovum consists of an animal hemisphere of small ceUs 

 and a vegetative hemisphere of large cells, identified 

 by Haeckel with the primary ectoderm ahd endoderm 

 respectively. In the megalecithal eggs segmentation is 



^^ Jenaische Zeitschr., vol. 8, p. lo, 13, 1874. • . 



