i68 



ORIGIN AND NATURE OF SEX 



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 



Fig. ioi. 



Fig. I02. 



Fig. ioi. Features in the life history of Chlamydomonas: A, character 

 of the mobile plant. B, conjugation of isogamous gametes. C, a plant 

 dividing to form numerous small male gametes. D, a plant forming tsvo large 

 female gametes. E, male and female gametes about to conjugate. 



Fig. 102. Features in the life history of Pandorina: A, a colony of plants. 

 B, each plant of the colony dividing to form a new colony. C, the plants of 

 a colony escaping as gametes. D, the conjugation of two gametes of un- 

 equal size. E, later stage in the conjugation. F, gametospore or resting 

 spore. G, large zoospore formed from the gametospore. FI, a colony formed 

 by the division of the zoospore, G. 



cells that are termed female gametes, while other plants produce 

 numerous minute male gametes that swim actively about (Fig. 

 IOI, C, D). This stage of sexuality, where the gametes dififer 



