178 ORIGIN OF SEX 



food, etc., zoospores were formed that were incapable of further 

 growth. But, by the union of two of these zoospores, a cell was 

 formed possessed of renewed vigor and capabilities of growth. 

 So we can think of the sexual cells or gametes as zoospores 

 that are lacking in the materials essential to growth and of the 

 sexual process as a union of the two bodies for the purpose of 

 bringing together the missing material and supplying the neces- 

 sary energy for growth. 



A closely allied genus, Chlamydomonas (Fig. loi) has a life 

 history very similar to that of Sphaerella but the formation of 

 the gametes reveals a variation that gives us an understanding 

 of how these bodies came to differ and finally became distin- 

 guishable as male and female gametes. In some of the species 

 of Chlamydomonas, to be sure, the gametes are similar and can 

 not be distinguished as male or female (Fig. loi, B). This 

 stage of sexuality is called isogamous, meaning similar gametes. 

 In other species, there is a decided variation in the size of the 

 gametes, certain cells or plants producing large and less active 

 cells that are termed female gametes, while other plants produce 

 numerous minute male gametes that swim actively about (Fig. 

 loi, C, D). This stage of sexuality, where the gametes differ 

 in size is called heterogamous, meaning unequal gametes. This 

 variation in the size of the gametes which we call the differentia- 

 lion of sex is due to the amount and nature of the material stored 

 in them, the male gametes being formed in a cell in larger num- 

 bers than in the case of the females. These heterogamous 

 gametes do not differ in nature from the isogamous gametes. 

 They are also lacking in some substance that renders them in- 

 capable of growth unless fusion of two gametes is effected. 



(&) Colonial Unicellular Algae. — The various stages in the 

 evolution of sex are also well illustrated in several genera of 

 unicellular green algae that live together in colonies. These 

 colonial forms are also of interest in that they help us to under- 

 stand how unicellular plants became associated — at first quite 

 independent of one another — and this led to the dependence of 

 one plant upon another and finally to a distribution of work 

 among the plants of the colony, so that certain plants or cells 



