REPRODUCTION. 55 
Fie. 59.—A human egg (much enlarged) from the ovary of a female. The whole egg is a 
simple eas cell, The greater part of this cell is formed by the egg-yelk, by the gran- 
ular cell-substance (protoplasm), consisting of innumerable yelk-granules with a little 
inter-granular substance. In the upper part of the yelk lies the bright, globular, germ- 
vesicle, corresponding 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- 
colored 5 RE? (zona pellucida, or chorion). This is traversed by very numerous 
hair-like lines, radiating toward 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. (Haeckel.) 
ticular of structure from other cells. Its differences are hidden 
ones of molecular structure and functional behavior. In ac- 
cordance with the diverse circumstances under which ova 
mature and develop, certain variations in structure, mostly of 
the nature of additions, present themselves. 
Thus, ova may be naked, or provided with one or more 
coverings. In vertebrates there are usually two membranes 
around the protoplasm of the ovum: a delicate covering (Vi- 
telline membrane), beneath which there is another, which 
is sieve-like from numerous perforations (zona radiata, or 2. 
pellucida). The egg membrane may be impregnated with lime 
salts (shell). Between the membranes and the yelk there is a 
fluid albuminous substance secreted by the glands of the ovi- 
duct, or by other special glands, which provide proteid nutri- 
ment in different physical condition from that of the yolk. 
The general naked-eye appearances of the ovum may be 
learned from the examination of a hen’s egg, which is one of 
