3/6 THE EVOLUTION OF MAN. 



a 



FIG. 127. 



FIG. 128. 



FIG. 130. 



FIG. 131. 



FIG. 127. Human egg between the twelfth and thirteenth day. After 

 Allen Thomson. 1. Not opened ; natural size. 2. Opened, and enlarged. 

 Within the outer tufted membrane (c/ionon) the small curved germ lies upon 

 the left of the upper side 'of the large intestinal germ-vesicle. 



FIG. 128. Human egg on the fifteenth day. After Allen Thomson. 

 Natural size, and opened. The small germ lies in the upper right-hand part 

 of the right half. 



FIG. 129. Human germ on the fifteenth day, taken from the egg; 

 enlarged : o, yelk-sac ; b, region of the neck (where the medullary fuiTow is 

 already closed) ; c, head part (with open medullary furrow) ; d, hind part 

 (with open medullary furrow) ; e, a shred of the amnion. 



FIG. 130. Human egg between the twentieth and twenty-second day. 

 After Allen Thomson. Natural size ; opened. The outer tufted membrane 

 (chorion) forms a capacious vesicle, to the inner wall of which the small 

 g3rm (above, on the right) is attached by a short navel-cord. 



FIG. 131. Humaii germ between the twentieth and twenty-second day, 

 taken out of the egg; enlarged : a, amnion; b, yelk-sac ; c, lower jaw process 

 of the first gill-arch; d, upper jaw process of the same; e, second gill-arch 

 (behind it are two other small arches). Three gill-openings are very plainly 

 seen ; /, rudiments of the fore-limbs ; g~, ear-vesicle ; h, eye ; i, heart. 



