218 



THE EVOLUTION OF MAN. 



FIGS. 221-223. Lyre-shaped (or sole-shaped) germ-shield of a Chick, in 

 three consecutive stages of evolution, seen from the dorsal surface : about 

 twenty times enlarged. Fig. 221, with six pairs of primitive vertebras. 

 The brain a simple bladder (Tib). The medullary furrow is wide open from 

 the point x, very wide at z. mp, Marrow (or medullary) plates ; sp, side- 

 plates ; y, boundary between the throat cavity (sh) and the head-intestine 

 (vd). Fig. 222, with ten pairs of primitive vertebrae. The brain consists of 

 three bladders : v, fore-brain ; m, mid-brain ; h, hind-brain, c, Heart ; dr, 

 yelk-veins. The medullary furrow is wide open behind (z). mp, Marrow- 

 plates. Fig. 223, with sixteen pairs of primitive vertebrae. The brain 

 consists of five bladders: v, fore-brain; z, twixt-brain; m, mid-brain ; h, 

 hind-brain ; n, after-brain, a, Eye-vesicles ; g, ear- vesicles ; c, heart ; dr r 

 yelk- veins ; -nip, marrow-plate, uw, primitive vertebrae. 



