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 vertebrae. 

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

 the points, very wide at z. nip, 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 ; dv, 

 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 ; \ 

 hind-brain ; n, after-brain, a, Eye-vesicles ; g i ear- vesicles ; c, heart ; dv, 

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



