228 



THE EVOLUTION OF MAN. 



superficial cleavage (segmentatio superjicialis, Plate III. 

 Fig. 25-80). This form is entirely unrepresented among 

 Vertebrates. It, however, plays the most important part 



Fig. 46-49. — Gastrulation 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); b, clcavage-cells. 



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

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



Fig. 48. — (C) Germ-vesicle in the pi'ocess of inversion (Blastula in- 

 vaginata) ; e, exoderm ; i, entoderm ; n, nutritive yelk ; re, thickened edge ; 

 s, cleavage-cells. 



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

 cavity. 



