218 



THE EVOLUTION OF MAN. 



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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 vertebrae. 

 The brain a simple bladder (Jib). 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 vertebra?. The brain consists of 

 three bladders : v, fore-brain ; m, mid-brain ; Ji, hind-brain, c, Heart ; dv, 

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

 plates. Fig. 223, with sixteen pairs of primitive vertebras. 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 ; dv, 

 yelk- veins ; mp, marrow-plate, uiv, primitive vertebras. 



