160 



PRINCIPLES OF ANIMAL BIOLOGY 



and are nonmotile; these are called ova (singular, ovum), or eggs. Other 

 gametes are minute, often a very small fraction of the size of the ova of 

 the same species. These are poorly supplied with nutritive material, 

 have a very small cytosome, and usually are motile; they are kno^\^l as 

 spermatozoa (singular, spermatozoon). The individuals in which eggs 

 develop are females, and those in which spermatozoa develop are males. 

 Sexual Reproduction in Metazoa. — In metazoa the germ cells (ova 

 and spermatozoa) are the only cells which retain the power of uniting 

 to initiate the development of a new metazoan individual. All other 

 cells have completely lost this power. As the time for sexual repro- 

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



A B 



Fig. 128. — Sperm cell and ovum. A, spermatozoon of rabbit; B, fertilized ovum of 

 Nereis with two polar bodies, ph. (B from Wilson, " The Cell.") 



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

 described in detail in Chap. 16, but its essentials may be stated here. 

 In the ovum it consists in the main of two cell divisions 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 nonfunctional. In the 

 sperm cell the process does not differ essentially from that in the ovum, 

 except that it 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 illus- 

 trated in Fig. 128. 



When mature spermatozoa and eggs of the same or closely related 

 species are brought together, the actively motile spermatozoa meet and 



