90 THE GERM-CELLS 



In the earlier stages of embryological development the progeni- 

 tors of the germ-cells are exactly alike in the two sexes and are in- 

 distinguishable from the surrounding somatic cells. As development 

 proceeds, they are first differentiated from the somatic cells and then 

 diverge very widely in the two sexes, undergoing remarkable transfor- 

 mations of structure to fit them for their specific functions. The 

 structural difference thus brought about between the germ-cells is, 

 however, only the result of physiological division of labour. The 

 female germ-cell, or ovum, supplies most of the material for the body 

 of the embryo and stores the food by which it is nourished. It is 

 therefore very large, contains a great amount of cytoplasm more or 

 less laden with food-matter {yolk or deutoplasm\ and in many cases 

 becomes surrounded by membranes or other envelopes for the pro- 

 tection of the developing embryo. On the whole, therefore, the early 

 life of the ovum is devoted to the accumulation of cytoplasm and the 

 storage of potential energy, and its nutritive processes are largely 

 constructive or anabolic. On the other hand, the male germ-cell or 

 spermatozoon contributes to the mass of the embryo only a very 

 small amount of substance, comprising as a rule only a single nucleus 

 and a centrosome. It is thus relieved from the drudgery of making 

 and storing food and providing protection for the embryo, and is 

 provided with only sufficient cytoplasm to form a locomotor appara- 

 tus, usually in the form of one or more cilia, by which it seeks the 

 ovum. It is therefore very small, performs active movements, and 

 its metabolism is characterized by the predominance of the destruc- 

 tive or katabolic processes by which the energy necessary for these 

 movements is set free. 1 When finally matured, therefore, the ovum 

 and spermatozoon have no external resemblance ; and while Schwann 

 recognized, though somewhat doubtfully, the fact that the ovum is a 

 cell, it was not until many years afterwards that the spermatozoon 

 was proved to be of the same nature. 



A. THE OVUM 



The animal egg (Figs. 42, 43 A) is a huge spheroidal cell, sometimes 

 naked, but more commonly surrounded by one or more membranes 

 which may be perforated by a minute opening, the micropyle, through 

 which the spermatozoon enters (Fig. 45). It contains an enormous 

 nucleus known as the germinal vesicle, within which is a very conspic- 



1 The metabolic contrast between the germ-cells has been fully discussed in a most sug- 

 gestive manner by Geddes and Thompson in their work on the Evolution of Sex ; and these 

 authors regard this contrast as but a particular manifestation of a metabolic contrast charac- 

 teristic of the sexes in general. 



