136 



THE EVOLUTION OF MAN. 



always the same characteristic thick covering ; always the 

 same clear, round germinal vesicle with its dark germinal 

 spot. Even under the highest magnifying power of the 





Fig. 11. — A human egg (much enlarged) from the ovary of a female. 

 The whole egg is a simple spherical cell. The greater part of this cell is 

 formed by the egy-yelk, by the granular cell-substance (protoplasm), con- 

 sisting of innumerable yelk-gran ule.s with a little inter-granular substance. 

 In the upper part of the yelk lies the bright, globular, germ -vesicle, corre- 

 sponding with the cell-kernel (nucleus). This contains a darker germ-spot, 

 answering to the nucleolus. The globular yelk mass is surrounded by 

 a thick, light-coloured egg-membrane (zona pellucida or chorion). This is 

 traversed by very numerous hair-like lines, radiating towards the central 

 point of the mass; these are the porous canals, through which, in the course 

 of fertilization, the thread-shaped, active sperm-cells penetrate into the 

 egg-yelk. 



