EGG -CELLS. 135 



the germinal point (punctum germinativum). But these 

 last two parts, the germinal spot and the germinal point, are 

 only of subordinate importance ; only the first two parts are 

 of primary importance, the protoplasm (Vitellus) and the 

 nucleus (vesicula germinativa). 



In many lower animals, for example, in Sponges and 

 Medusae, the egg-cells retain their entirely simple original 

 nature until fertilization. But in most animals they 

 undergo certain changes before that time ; they sometimes 

 acquire certain additional Protoplasm, which serves for the 

 nourishment of the egg (food-yelk), sometimes outer en- 

 velopes or membranes, which serve for its protection 

 (egg-membranes). An envelope of this sort appears on all 

 mammalian eggs in the course of their further develop- 

 ment. The little sphere is surrounded by a thick covering 

 of a perfectly transparent, glass-like nature, which is dis- 

 tinguished as the zona pelluclda, or chorion (Fig. 11). When 

 this is very closely examined under the microscope, very 

 fine radial lines may be distinguished, traversing the zona ; 

 these are very fine canals. The human egg cannot be 

 distinguished from that of most other Mammals either 

 in its immature or in its more complete condition. Its 

 form, its size, its composition, are approximately the same 

 in all. In its fully developed condition, it has an average 

 diameter of $ of a line, or 0'2 millimetres. If the mam- 

 malian egg is properly isolated and held on a glass plate 

 toward the light,, it appears to the naked eye as a very fine 

 point. The eggs of most of the higher Mammals are of 

 exactly the same size. The diameter of the spherical egg- 

 cell almost always measures from -^ to $fo of a line (1 ^ 

 of a millimetre). It has always the same spherical form, 



