292 THE EVOLUTION OF MAN. 



is formed of both primary germ-layers, we will adopt Van 

 Beneden's name, and call it the intestinal germ-disc (Gas- 

 trodiscus). 



The small, circular, dull whitish spot which lies at a 

 particular point on the outer surface of the bright-coloured, 

 transparent, and spherical " intestinal germ- vesicle," and 

 which is the " intestinal germ-disc " (Gastrodiscus), has long- 

 been known to naturalists, and was compared with the 

 " germ-disc " of Reptiles and Birds. Sometimes, therefore, it 

 was called the "germ-disc" (discus hlastodey^Tnicus), some- 

 times the '" embryonic spot " (iache eviihryonnaire), but 

 more usually the germ-area {area gerininativa). The 

 further evolution of the germ proceeds especially from this 

 germ-area. On the other hand, the greater part of the 

 intestinal-germ- vesicle of Mammals is not directly employed 

 in the formation of the future body, but in the formation of 

 the transitory '' navel-vesicle." The embryo-body pinches 

 itself off from the latter more and more, in proportion as 

 it grows and develops at the expense of the latter ; the 

 two become no longer connected except by the yelk-duct 

 (the stalk of the yelk-sac) ; and the latter forms the indirect 

 communication between the cavity of the navel-vesicle and 

 the intestinal cavity in the course of development (Fig. 70). 



The germ-area, or the intestinal germ-disc of Mammals, 

 originally consists, like the germ-disc of Birds, merely of 

 the two primary germ-layers, each of which is formed of a 

 single cell-stratum. Soon, however, a third cell-stratum, the 

 rudiment of the middle fibrous layer {"'(nesoderma), appears 

 in the middle of the circular disc, between the two earlier 

 strata. According to most observers, the mesoderm arises 

 Irom the inner primary germ-layer ; according to others, on 



