3.42 



THE EVOLUTION OF MAX. 



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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). Fi. 103, with six pairs of primitive vertebra. Brain is a 

 simple bladder (7i&). The spinal furrow is open behind x ; at the posterior 

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

 limit between throat-cavity (sTi) and head-intestine (vd). Fig. 104, with 

 10 pairs of primitive vertebrae. 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 ; Ji, 

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

 yelk- veins ; mp, medullary plates ; uu; primitive vertebrae. 



