332 



THE EVOLUTION OF MAN. 



retained, the germ-layers, even in the earliest stage, form 

 closed tubes, which may be immediately referred to the 

 tubular shape of an elongated Gastrula. (Cf Figs. 62-69.) 



When, therefore, it was generally thought that the 

 main object of the germ-history of Vertebrates was to 

 derive the later organization of these from a primitive, 

 flat, discoid form, the two-layered germ-disc (or the three- 

 layered germ-shield), a grave error was committed.^^ For 

 this flat, circular germ-disc, and the flat, sole-shaped germ- 

 shield which arose from the former, are phylogenetic form- 

 ations, which arose only secondarily, in consequence of the 

 accumulation of a large mass of nutritive yelk in the 

 primitive intestine of the primary Gastrula ; and so when, 

 at a later period, the dorsal side of the flat germ-shield 

 arches, and its edges bend towards each other and coalesce 

 into tubes on the ventral side, the process is neither primary 

 nor secondary, but tertiary. 



A right conception of the formation of the intestine is 

 evidently the real point on which a thorough knowledge of 

 these important germinal processes depends. The greatest 

 difficulties are solved when a clear and correct conception 

 of the formation of the intestinal canal has been acquired. 

 For the primitive intestine is, according to the Gastryea 

 Theorj^, the earliest and the most important organ of the 

 animal body. In order to gain this clear idea of the forma- 

 tion of the intestinal tube and the parts attached to it, it is 

 especially necessary to note accurately the important modi- 

 fication undergone by the intestinal-glandular layer of the 

 mammalian germ. This, as has been said, is at first a 

 simple layer of cells (an epithelium), which lines the inner 

 surface of the globular intestinal germ- vesicle. It is a 



