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 

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

 simple bladder (hb). The spinal furrow is open behind x ; at the posterior 

 end it is very wide open at z ; mp, medullary plates; sp, si de -plate s ; y, 

 limit between throat-cavity (s?i) and head -intestine (vd). — Fig. 104, with 

 10 pairs of primitive vertebras. 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 (z) ; mp, medul- 

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

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

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

 yelk-veins ; mp, medullary plates j uw, primitive vertebrae. 



