206 



THE EVOLUTION OF MAN. 



32, D) ; on the other hand, the flat floor of the cavity is 

 composed of the larger, whiter-coloured cells of the intes- 

 tinal layer, or entoderm (Fig. 82 z). 



^giP^i^j^.-D 





Fig. 32-35. — Fonr longitudinal sections of the segmented egg of a Toad, 

 in four successive stages of evolution. In all, the letters indicate the same 

 parts : F, cleavage-cavity ; D, the roof of this cavity ; R, dorsal half of 

 the germ ; B, intestinal half ; P, the yelk. plug (white circular space at the 

 lower pole) ; z, yelk-cells of the entoderm (the gland-germ of Remak) ; 

 2V, primitive intestinal cavity (^protogaster, or Rusconi's nutritive cavity). 

 The primitive mouth is filled up by the yelk-plug (P) ; s, boundary between 

 the primitive intestinal cavity (N) and the cleavage-cavity (F) ; k, k', section 

 through the swollen circular lip or edge of the primitive mouth (the so- 

 called anus of Rusconi). The dotted line between A: and k' indicates the 

 former connection between the yelk-plug (P) and the central mass of yelk- 

 cells (z). In Fig. 35 the egg has turned round 90°, so that the dorsal half 

 of the germ (R) is seen above ; the intestinal half (B) is now turned down- 

 ward. (After Strieker.) 



