EEPEODUCTION 423 



fact suggests that the sexual cells have been derived from 

 asexual ones, and are a later development, therefore, in the 

 history of the race. 



The more complete differentiation of the gametes into 

 male and female can be observed among several of the 

 families of the Algae. In some species of Ectocarpus and 

 Cutleria the gametes are much like those of Ulothrix, but 

 some are smaller than the others. The larger ones come 

 to rest soonest, and lose their cilia ; one of the smaller 

 more motile ones then fuses with each of the larger. We 

 can in this case speak of the larger as female and the 

 smaller as male. The differentiation is still very rudi- 



FIG. 172. OOGONIUM OF Fiicus, CON- FIG. 173. AN OOSPHERE OF Fucus 



TAINING EIGHT OOSPHERES. (After SURROUNDED BY ANTHEROZOIDS. 



Thuret.) (After Thuret.) 



mentary, as in the event of no fusion taking place the 

 female cell can still develop into a new plant. 



The most complete differentiation of the gametes can 

 be traced in the higher members of the Algae. The females 

 become larger and cease to develop cilia, the males remain 

 small and motile. The former are then called oospheres 

 and the latter antherozoids or spermatozoids. A good 

 example of this stage of differentiation is afforded by Fucus 

 (figs. 172 and 173). 



The structures or organs in which the sexual cells of 

 these plants are formed are known as gametangia. When 

 the gametes are distinctly male and female the gametangia 



