124 ^^^ GERM-CELLS 



guishable from the surrounding somatic cells. As development pro- 

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

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

 formations 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 i^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 very small quantity of cytoplasm. 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 apparatus, 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 destructive or katabolic processes by which the energy neces- 

 sary for these movements is set free.^ When finally matured, there- 

 fore, 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 afterward that 

 the spermatozoon was proved to be of the same nature. 



A. The Ovum 



The animal egg (Figs. 58-59) 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 viicropyle, through 

 which the spermatozoon enters (Fig. 63). It contains an enormous 

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

 spicuous nucleolus known to the earlier observers as the germinal 

 spot. In many eggs the latter is single, but in other forms many 



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. 



