igo ORIGIN OF SEX AND OF THE SOMA 



single cell has four flagella and the zygote therefore 

 eight), but eventually it loses the flagella, becomes 

 perfectly spherical and secretes a thick cell wall 

 (Fig. 22, d), going into the resting state, in which con- 

 dition it can resist a certain amount of desiccation 

 (cf. Mucor, Pythium, etc.). On germination the proto- 

 plasmic contents of the zygote divide to form two or 

 four ordinary (vegetative) individuals. 



(c) Differentiation of Sex. — When the gametes are all 

 equal in size, as they are in several species of Chlamy- 

 domonas, they are called isogametes.^ But in certain 

 species they are of two sizes, the larger derived from 

 fewer, the smaller from a greater number of divisions 

 of the mother cell. Such gametes are called hetero- 

 gametes^ Conjugation then, so far as has been 

 observed, takes place, only between a large and a 

 small gamete. In one species (C. monadina), in which 

 the details of the process have been carefully observed, 

 and in which all the gametes have cell walls, the 

 anterior ends of the two come into close contact, as 

 in the case of isogametes, and the cell walls of the 

 two become fused together (Fig. 23, a). The proto- 

 plasm of the small gamete separates from its wall 

 (sometimes secreting a new wall round its hinder 

 end. Fig. 23, B), and shps through the channel 

 formed by the fusion of the two into the cavity of 

 the large gamete, where it eventually fuses completely 

 with the body of the larger, nucleus with nucleus and 

 cytoplasm with cytoplasm — the chloroplasts with their 

 pyrenoids remaining distinct longest (Fig. 23, c). 

 The mass of protoplasm (zygote) so formed contracts, 

 becomes spherical, and secretes a thick cell wall within 

 the cell cavity of the large gamete (Fig. 23, c). 



' Greek laog, equal, and ya/iim. ' eregos, other (different). 



