342 



THE EVOLUTION OF MAN.' 



mp 



Figs. 103-5. — Lyre-shaped germ-shield of a Chick, in three consecutive ] 



stages of development, seen from the dorsal surface (about 20 times the j 



natural size). Fig. 103, with six pairs of primitive vertebrae. Brain is a i 



simple bladder {Kb) . The spinal furrow is open behind x ; at the posterior ; 



end it is very wide open at z ; mp, medullary plates ; sjo, side-j^lates ; y, i 



limit between throat-cavity {sh) and head-intestine (vd). — Fig. 101, with ] 



10 pairs of primitive vertebrse. The brain has parted into three bladders : | 



V, fore-brain ; m, mid-brain ; h, hind-brain ; c, heart ; dv, yelk- veins. The ^ 



medullary furrow continues wide open at the posterior end (2;) ; mj^, medul- < 

 lary plates. — Fig. 105, with 16 pairs of primitive vertebrae. The brain has 



parted into five bladders : v, fore-brain ; z, iwixt-brain ; m, mid-brain ; 7?-, « 



hind. brain ; n, after-brain ; a, eye -bladders ; g, ear -vesicles ; c, heart j dv, J 

 yelk-veins ; mp, medullary plates ; ^ixv, primitive vertebrge. 



