2l8 



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 tbe dorsal surface : about 

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

 The brain a simple bladder QiV). The medallary furrow is wide open from 

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

 plates ; y, boundary between the throat cavity {sK) and the head-intestine 

 {vdu). Fig. 222, with ten pairs of primitive vertebras. The brain consists of 

 three bladders : r, fore-brain; w, mid-brain • /i, hind-brain. c, Heart ; d\\ 

 yelk-veins. The medullary furrow is wide open behind (s). inp, Marrow- 

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

 consists of five bladders: v, fore-brain; 2, twixt-brain ; w. mid-brain; ^, 

 hind-brain ; w, af ter-brain. 0, Eye-vesicles ; g, ear. vesicles ; r, heart 5 (Jv, 

 yelk- veins ; uip, marrow-plate, uit', primitive vertebrae. 



