284 THE EVOLUTION OF MAN. 



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 -layers. At a later period the 

 remnant of the nutritive yelk forms only a small globular 

 sac, the yelk-sac, or navel-sac {saccus vittelinus, or vesicula 

 umbilicalis, Fig. 70 nb). 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 

 vitellinus), 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 {umbilicus) 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 

 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 of 

 the germ -history of the higher Vertebrates, is, I am con- 

 vinced, entirely false. For the Gastrsea Theory, the full 

 significance of which now becomes evident, teaches us that 



