206 



THE EVOLUTION OF MAN. 



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

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

 tinal layer, or eutoderm (Fig. 32 z\ 



,-V 



FIG. 32-35. Four 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) ; s, yelk-cells of the entoderm (the gland-germ of Eemak) ; 

 N, 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) ; Tt, fc', section 

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

 called anus of Rusconi). The dotted line between fc and &' indicates the 

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

 cells (j). 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.) 



