288 



THE EVOLUTION OF MAN. 



FIGS. 258-260. Lyre-shaped germ-shield of a Chick, in three consecutive 

 stages of development; seen from the dorsal side; enlarged about twenty 

 times. Fig. 258, with six pairs of primitive vertebrae. The brain is a sim- 

 ple bladder (M>). The spinal furrow from x remains wide open ; behind, at 

 z, it is much enlarged, mp, Marrow-plates ; sp, side-plates ; y, limit be- 

 tween the pharynx cavity (sh) and the head-intestine (vd). Fig. 259, with 

 ten pairs of primitive vertebrae. The brain has separated into three 

 bladders : v, fore-brain ; m, mid-brain ; h, hind-brain ; c, heart ; dv, yelk- 

 veins. The spinal furrow is still wide open (z). mp, Marrow-plates. 

 Fig. 260, with sixteen pairs of primitive vertebrae. The brain has separated 



