THE ORIGIN OF LIFE 157 



reproduction are confined to the protists, but the third kind 



(sporulation) also occurs among the metists. 



Seyial cytogenic reproduction is based upon gametes or 

 mating germ cells. Since complementary gametes are spe- 

 cialized for union with each other to form a single synthetic 

 cell, the zygote, the number of their nuclear threads or chro- 

 mosomes is reduced to one half (the haploid number) at the 

 time of maturation, so that the somatic or tissue cells of the 

 parent organism have double the number (the diploid num- 

 her) of chromosomes present in the reduced or mature gametes. 

 Hence, when the gametes unite to form a zygote, summation 

 is prevented and the diploid number of chromosomes char- 

 acteristic of the given species of plant or animal is simply 

 restored by the process of syngamy or union. The process 

 by which the number of chromosomes is reduced in gametes 

 is called meiosis, and, among the metists, it is distinct from 

 syngamy, which, in their case, is a separate process called 

 fertilization. Among the protists, we have, besides fertiliza- 

 tion, another type of syngamy called conjugation, which 

 combines meiosis with fertilization. 



In sexual reproduction, we have three kinds of gametes, 

 namely: isogametes, anisogametes, and heterogametes. In 

 the type of sexual reproduction known as isogamy, the com- 

 plementary gametes are exactly alike both in size and shape. 

 There is no division of labor between them. Each of the 

 fusing gametes is equally fitted for the double function which 

 they must perform, namely, the kinetic function, which en- 

 ables them to reach each other and unite by means of move- 

 ment, and the trophic function which consists in laying up 

 a store of food for the sustenance of the developing embryo. 

 In anisogamy, the complementary gametes are alike in shape, 

 but unlike in size, and here we have the beginning of that 

 division of labor, upon which the difference of gender or sex 

 is based. The larger or female gamete is called a macro- 

 gamete. It is specialized for the trophic rather than the 

 kinetic function, being rendered more inert by having a large 



