THREE STAGES IN VERTEBRATE PHYLOGENY. 



285 



the 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 ail 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, tertiarv incident of evolution. Accordingly the three 

 following stages of germ-formation must be distinguished in 

 the Phylogeny of Vertebrates : 



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

 from the Gastraea 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 



