THE EVOLUTION OF MAN. 



264 



elongated into a thin stalk, which at first opens outward in 

 a narrow canal. (Of. Fig. 137, /, vol. i. p. 382.) This is called 

 the appendage of the labyrinth recessus labyrinthi, Fig. 246, lr}. 



FIG. 246. Development of the ear -labyrinth of a Chick, in five con- 

 secntive stages (A-E) (cross -sections through the rudimentary skull) : fl, 

 ear-groove ; lv, ear-vesicle ; IT, labyrinth appendage ; c, rudiment of the 

 cochlea ; csp, hind semi-circular canal ; cse, outer semi-circular canal ; 

 jr, jugular vein. (After Reissner.) 



FIGS. 247, 248. Head of an embryonic Chick, on the third day of incuba- 

 tion : 247 in front, 248 from the right; n, rudimentary nose (olfactory 

 groove) ; I, rudimentary eye (ocular groove) ; g, rudimentary ear (auditory 

 groove) ; v, fore-brain ; gl, eye-slit ; o, process of the upper jaw ; u, process 

 of the lower jaw of the first gill-arch. (After Koelliker.) 



FIG. 249. Primitive brain of human embryo of four weeks, in vertical 

 section, and the left half observed from within : v, z, m, h, n, the five grooves of 

 the skull cavity in which the five brain bladders are situated (fore, twixt, 

 mid, hind, and after brains) ; o, primary, pear-shaped auditory vesicle 

 (showing through); a, eye (showing through); no, optic nerve; p, canal of 

 the hypophysis ; t, central skull-pieces. (From Koelliker.) 



