2l8 



THE EVOLUTION OF MAN. 



Figs. 221-223.-— Lyre-shapcd (or sole-sbapecl) germ-shield of a Cliick, in 

 three consectitivo stages of evolntioii, seen from the dorsal surface: about 

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

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

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

 plates ; •?/, boundary between the throat cavity (sh) and the head-intestine 

 (I'J). Fig. 222, with ten pairs of primitive vertebi'a:'. The brain consists of 

 three bladders : v, fore-brain; ?«, mid -brain ; /i, hind-brain, c. Heart ; dv, 

 yelk-veins. The medullary furrow is wide open behind (.':). 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 ; 7t, 

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

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



