2<)2 THE EVOLUTION OF MAN. 



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

 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 blastodermicus), some- 

 times the "embryonic spot" (tache embryonnaire), but 

 more usually the germ-area (area germinativa). 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 (mesoderma), appears 

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

 strata. According to most observers, the mesoderm arises 

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



