608 LECTURE XXIT. 



pointed out that a sexual difference probably exists even 

 when the gametes are externally similar; that one of them is 

 male and the other female. This is indicated by the fact 

 that a planogamete of Ulothrix or Acetabularia will only 

 conjugate with another derived from a different gamet- 

 angium; and, in the case of Dasycladus, conjugation only 

 takes place between planogametes derived from different 

 individuals. 



We will now trace the gradual external differentiation 

 of the gametes. In Ectocarpus siliculosus and in Scytosiphon, 

 Algae belonging to the group of the Phaeosporeae, there 

 are two kinds of reproductive organs, the unilocular and 

 the multilocular, which both give rise to motile and ex- 

 ternally similar reproductive cells. Those produced in the 

 unilocular organ germinate independently; they are, in fact, 

 zoospores, and the organ producing them, a sporangium. 

 Those produced in the multilocular organs may germinate 



Fig. 66 (after Berthold). Process of conjugation in Ectocarpus. I af the 

 female planogamete coming to rest. II the female planogamete at rest surrounded 

 by male planogametes. /// a e the fusion of the female with a male piano- 

 gamete. 



