342 



THE EVOLUTION OF MAN.i 



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

 stages of development, seen from the dorsal sm-face (about 20 times the! 

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

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

 end it is very wide open at z ; rap, medullary plates ; sj>, side-plates ; y, 

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

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

 v, fore-brain ; ii), mid-brain ; ?i, hind-brain ; c, heart; civ, yelk-veins. Thej 

 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 : -!•, fore-brain ; z, iwixt-brain ; m, mid-brain ; h, 

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

 yelk-veins; ))!ji, medullary plates ; wu', primitive vertebra. 



