452 



THE EVOLUTION OF MAN. 



FIGS. 158-160. Transverse section through an early larval form of 

 Imphioxus. (Diagrammatic, after Eolph.) (Cf. Fig. 152, p. 424.) Tn Fig. 

 158 there is a free passage from without into the intestinal cavity (D), 

 through the gill-openings (). In Fig. 159 the lateral longitudinal folds 

 of the body-wall, the gill-roof, are forming, growing downwards. In Fig. 

 160 these side-folds have grown towards each other and their edges have 



