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THE EVOLUTION OF MAN. 



Fifi. 02-69. — Diagrammatic trausvevso sections throngU the most iu 

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



Fig. 62. — A. Transverse section through the Gastrula; two-layered ger 



Fig. 63. — B. Three-layered germ. 



Fig. 64'. — C. Four-layered germ (four secondary germ-layers). 



