174 PRINCIPLES OF ANIMAL BIOLOGY 



In general terms the basis of sexaal reproduction is the union of two 

 cells to form a single cell, the zygote, which by its subsequent divisions 

 produces a new individual if the uniting cells are of metazoan origin, 

 or a new series of individuals if they are of protozoan origin. Not all 

 cells are capable of uniting in this way, and cells which are capable of this 

 act are called gametes, or, in the metazoa, germ cells. Certain gametes are 

 relatively large, contain a considerable amount of nutritive material, and 

 are non-motile. Such gametes are named macrogametes, ova (singular, 

 ovum), or eggs. Other gametes are minute, often a mere fraction of the 

 size of the macrogamete of the same species. These are poorly supplied 

 with nutritive material, have little cytoplasm, and usually are motile; 

 they are known as microgametes, spermatozoa (singular, spermatozoon), 

 sperms, or sperm cells. The terms macro- and microgamete are usually 

 reserved for use in speaking of the gametes of Protozoa. They indicate 

 size relations, or they are very general terms, while ovum or egg and 

 sperm or spermatozoon are the names used for gametes of metazoan 

 animals. The individuals in which eggs develop are known as females, 

 and those in which sperms develop as males. 



Sexual R eproduction in Metazoa. — In metazoa the germ cells, ova and 

 sperms, are the direct descendants of the primordial (first) germ cells. 

 They retain their power of uniting to initiate the development of a new 

 metazoan individual. All other cells have completely lost this power. 

 In a sense the germ cells are primitive cells, resembling the Protozoa 

 in respect to their potentialities. They are stored in the metazoan 

 body where they live at the expense of the somatic cells but without 

 taking part in the general body functions. As the time for sexual repro- 

 duction draws near the germ cells undergo a certain process of develop- 

 ment or of preparation for the sexual act. This preparatory process, 

 known as maturation, is discussed in detail in Chapter X but its essentials 

 may be stated here. Maturation of the ovum consists in the main of 

 two mitoses by which three or four cells are produced. Of these cells one 

 is much larger than the others and its nucleus has one-half the usual 

 number of chromosomes. The small cells are called polar bodies and are 

 non-functional. Maturation of the sperm cell does not differ essentially 

 from maturation of the ovum, except that the process results regularly 

 in the formation of four relatively small cells of about equal size, all of 

 which are usually functional. Like the eggs they have half the usual 

 number of chromosomes. The male germ cells must then be transformed, 

 by a striking change of shape, into spermatozoa. A sperm cell and an 

 ovum with polar bodies are illustrated in Fig. 133. 



When matured sperms and eggs of the same or closely related species 

 are brought together, the actively motile sperms meet and penetrate the 

 eggs. Usually but one sperm can enter an egg. After the entrance of 

 the sperm, either by a change in the surface of the egg or by some other 



