228 



THE EVOLUTION OF MAN. 



superficial cleavage (segmentatio superficialis, Plate III. 

 Fig. 25-30). This form is entirely unrepresented among 

 Vertebrates. It, however, plays the most important part 



FIG. 46-49. Gastmlation of a Hen's egg. All four figures represent 

 perpendicular, half-diagrammatic sections through the middle of the thin, 

 circular tread, or germ-disc. Of the nutritive yelk (n) only the contiguous 

 . part (perpendicularly shaded) is represented. 



FIG. 46. (A) Mulberry-germ (Morula); fc, cleavage-cells. 



PIG. 47. (B) Germ-vesicle (Blastula) ; s, cleavage-cavity; b, blasto- 

 derm-cells ; w, thickened or swollen edge of the germ-disc. 



FIG. 48. (C) Germ-vesicle in the process of inversion (Blastula in- 

 vaginata) ; e, exoderm ; i, eutoderm ; n, nutritive yelk ; w, thickened edge ; 

 s, cleavage-cells. 



FIG. 49. (D) Gastrula (Discogastrula) of Chick : d, primitive intestinal 

 cavity. 



