THE EVOLUTION OF MAN. 



Fro. 62-69. Diagrammatic transverse sections through the most im- 

 portant germ-forms of the ideal Primitive Vertebrate (Fig. 52-61). 89 



FIG. 62. A. Transverse section through the Gastrula; two-layered germ 



FIG. 63. B. Three-layered germ. 



FIG. 64. C. Four-layered germ (four secondary germ-layers). 



