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. 91 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 Gastraea 

 Theory, 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 



