284 THE EVOLUTION OF MAlf, 



towards each other and coalescing (Fig. 70). The germ 

 growing at the expense of the nutritive yelk, the latter 

 continually becomes smaller; it is completely surrounded 

 by the growth of the germ-layei'S. At a later period the 

 remnant of the nutritive yelk forms only a small glf)bular 

 sac, the yelk-sac, or navel-sac (sacciis vittelinus, or vesicula 

 UTnhilicalis, Fig. 70 nh). This is surrounded by the intes- 

 tinal layer, and connected with the central portion of the 

 intestinal tube by a thin stalk, the yelk-duct (ductus 

 viteUinus), and, in most Vertebrates, is at last completely 

 absorbed by the intestinal tube (Fig. 70 H). The point at 

 which this happens, and at which the intestine finally 

 closes, is the intestinal navel. In Mammals, in which the 

 remnant of the yelk-sac remains outside and gradually 

 dwindles, the yelk-duct pierces the outer ventral wall to the 

 last. The navel cord parts at birth at this point, which per- 

 manently remains as the navel (umhilicus) in the outer skin. 



As in the germ-history of the higher Vertebrates, based 

 chiefly on that of the Chick, the distinction between the! 

 germ (or formative yelk) and the nutritive yelk (or yelk I 

 sac) has up to the present time been regarded as original, 

 the flat, leaf-shaped rudiment of the germ-disc has also 

 necessarily been regarded as the original germ-form, and] 

 the greatest weight has been laid on the fact that these! 

 flat germ-layers curve, and thus become hollow trenches,! 

 and that, by the concrescence of their edges, they become! 

 closed tubes. 



This view, which has governed all past expositions o| 

 the germ-history of the higher Verteljrates, is, I am coi 

 vinced, entirely false. For the Gastraea Theor}^, the fu 

 significance of which now becomes evident, teaches us thi 



