THREE STAGES IN VERTEBRATE PHYLOGENY. 



285 



fche real state of the case is originally just the opposite. 

 The Gastrula, in the body- wall of which the two primary 

 germ-layers appear from the first as closed tubes, is the 

 original germ- form of all Vertebrates, as of all Invertebrate 

 animals; and the fiat germ-disc, with its flatly extended 

 germ-layers, is a later, secondary germ-form, which arose 

 in consequence of the kenogenetic formation of the large 

 nutritive yelk, and the consequent extension of the germ- 

 layers over the surface of the latter. 91 The curving of these 

 germ-layers, which actually occurs, and their coalescence 

 into tubes is, therefore, not original and primary, but a much 

 later, tertiary incident of evolution. Accordingly, the three 

 following stages of germ-formation must be distinguished in 

 the Phylogeny of Vertebrates : 



A. First Stage : 



B. Second Stage : 



C. Third Stage : 



Primary 



Secondary 



Tertiary 



(Palingenetic) 



(Kenogenetic) 



(Kenogenetic) 



Process of Germ- 



Process of Germ. 



Process of Germ- 



formation. 



formation. 



formation. 



From the first, the 



The germ-layers 



The germ-layers form 



germ-layers form closed 



spread themselves out 



a flat germ-disc, the 



tubes. 



in the form of a leaf, 



edges of which incline 



No nutritive yelk. 



in consequence of the 



toward each other, and 





formation of a large 



coalesce into ti closed 





yelk- sac from the cen- 



tube. 





tre of the intestinal 







tube. 





If this view is correct, and, as the logical conclusion 

 from the Gastrsea Theory, I am obliged to believe it is so, 

 then the explanation of the process as at present accepted 

 must be exactly reversed. The yelk-sac must no longer 

 be treated as though it were originally distinct from the 



