186 CHLAMYDOMONAS, SEXUAL REPRODUCTION 



divisions in the parent-cells, are small and quick-moving micro- 

 gametes (Fig. 97, g). The zygote is produced by one of the 

 latter approaching a passive macrogamete and fusing with it 

 (Fig. 97, i-k). 



This kind of sexual union in which the two gametes are unlike 

 is described as anisogamous, in contrast to the isogamous process, 

 with fusion of similar gametes, found in the majority of species 

 of Chlamydomonas. Since in higher forms of plants, where the 

 differentiation between the gametes is more extreme, the female 

 are motionless and the male alone motile, the state of affairs 

 obtaining in C. monadina can be regarded as a simple phase of 

 differentiation of sex. 



C. monadina is also peculiar in having gametes provided 

 with a cell-membrane (Fig. 97, g and h), a feature also seen 

 in a few other species of this genus and of its immediate allies, 

 although in all other members of the Vegetable Kingdom the 

 sexual cells are naked. 



Prolonged desiccation and extremes of temperature leave 

 the thick-walled zygospores unharmed, even when the pool 

 dries up. As the caked mud flakes and becomes powdery, 

 both it and the zygospores are whirled away in windy weather 

 as dust. If the zygospores are blown into water, their colour 

 sooner or later changes to green, owing to the absorption of the 

 yellowish oil, and the contents divide successively into a small 

 number of parts which are liberated as new organisms by the 

 bursting of the thick membrane. It is in this way that Chlamy- 

 domonas and similar forms reach, and develop in, almost any 

 suitable piece of water. 



The genus Carter ia, individuals of which are not infrequent 

 in fresh-water, resembles Chlamydomonas in all essential re- 

 spects, except for the possession of four cilia (Fig. 98, F). 

 Another motile unicellular form, commonly found in small 

 pools of water, is variously known by the generic names Hcema- 

 tococcus, Sphaerella, and sometimes Protococcus ; the last name 

 is, however, altogether antiquated. The individuals (Fig. 98, 

 A and C), though moving with the aid of two cilia and of much 

 the same shape and size as those of Chlamydomonas, differ 

 somewhat in structure. The protoplast is separated from the 

 firm bounding membrane by a wide transparent layer of mucilage 



