THE DIFFERENTIATION OF SEX 327 



directive stimuli to cause whole organisms or their organs 

 to move or bend in response to stimuli. The sperm finds 

 the opening, enters, and fuses with the egg. As soon as 

 this fusion has occurred a change comes about in the egg 

 resulting in the formation of a membrane over its surface 

 preventing the entrance of other sperms. This is a 

 simple case of chemical regulation. 



The Differentiation of Sex. — In Ulothrix we can 

 speak of sex although there is no distinction between the 

 sexes. Not only may the two fusing gametes come from 

 the same individual but they are both motile and indis- 

 tinguishable in every respect. In (Edogonium there is 

 not only sex but two sexes, at least in respect to the 

 gametes themselves. One is always small and highly 

 motile, while the other is large, stored with food, non- 

 motile, and remains in the cell where it was formed. In 

 some cases there is a distinction of sex between the fila- 

 ments such that one filament forms sperms only and the 

 other eggs only. In still other species of (Edogonium 

 the male filament is very small compared to the female. 

 In these species there are also two kinds of swarm spores 

 differing in size. The small ones produce male filaments 

 and the large ones females. 



Other illustrations of the same sort of differentiation 

 of sex can readily be found among the Algae and Fungi.' 

 Moreover, it is easy to extend the range of observation 

 and find illustrations intermediate between these, so as 

 to show that sex has arisen many times among plants 

 through a change in the habits of swarm spores and that 

 two sexes have been developed by a gradual loss of motion 

 and increase in size and stored food by one gamete to de- 

 velop an egg. Practically all the higher plants have 

 sexual reproduction fundamentally like this but with 

 added complications in the way the gametes are formed 

 or in the sequence of development during the entire life 

 cycle. 



Alternation of Generations is found in some Algae 



