342 



THE EVOLUTION OF MAN. 



uia 



mp 



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

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

 natural size). Fipf. 103, with six pairs of primitive vertebra?. Brain is a 

 simple bladder {hh). The spinal furrow is open bcliind x ; at the posterior 

 end it is very wide open at z ; mp, medullary plates ; sp, side-plates ; y, 

 limit between throat-cavity {sh) and head-intestine {yd). — Fig. 104, with 

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

 V, fore-brain ; m, mid-brain ; h, hind-brain ; r, heart ; dv, yelk-veins. Tho 

 medullary furrow continues wide open at tho posterior end (z) ; mp, medul- 

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

 parted into five bladders : r, fore-brain ; z, twixt-brain ; m, mid-brain ; h, 

 hind. brain ; n, after-brain; a, eye-bladders ; cj, ear-vesicles ; '', heart; dr, 

 yelk-veins; mp, medullary plates; uic, primitive vertebra;. 



