228 



THE EVOLUTION OF MAN. 



superficial cleavage (segmentatlo sitperjicialis, Plate III. 

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

 Vertebrates. It, however, plays the most important part 



Fig. 16-19. — Gastrulation of a Hen's epfg:. All four figures reprepent 

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

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

 part (perpendicularly shaded) is represented. 



Fig. 46. — (A) Mulberry-germ (Morula); h, clcavage-celTs. 



Fig. 47. — (-C) Germ-vesicle (Blastula) ; s, cleavage-cavity; h, blasto- 

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



Fig. 48. — (C) Germ. vesicle in the process of inversion (Bla.Hula in. 

 vagina ta) ; e, exoderm ; i, entoderm; n, nutritive yelk; k', thickened edge ; 

 s, cleavage-cells. 



Fig. 49. — (D) Gastrula (DiscogastruJa) of Chick : d, primitive intestinal 

 cavity. 



