THE EVOLUTION OF SEX 



13 



plast appearing as a thick green mass on each side of the 

 central nucleus. As no other color appears, it is evident 

 that Ulothrix is one of the Chlorophyceae. 



A— 



Fig. 2. Ulothrix, a Conferva form. A, base of filament, showing lowest holdfast 

 cell and five vegetative cells, each with its single conspicuous cylindrical chloro- 

 plast (seen in section) inclosing a nucleus; B, four cells containing numerous 

 small zoospores, the others emptied; C, fragment of a filament showing one cell 

 (a) containing four zoospores, another zoospore (6) displaying four cilia at its 

 pointed end and just having escaped from its cell, another cell (c) from which 

 most of the small biciliate gametes have escaped, gametes pairing (<f), and the 

 resulting zygotes (e) ; D, beginning of new filament from zoospore ; E, feeble 

 filaments formed by the small zoospores ; F, zygote growing after rest; G, 

 zoospores produced by zygote.— Caxd well, except F and 6, which are after 

 Dodel-Port. 



The cells are all alike, excepting that the lowest one of 

 the filament is mostly colorless, and is elongated and more 

 or less modified to act as a holdfast, anchoring the filament 

 to some firm support. With this exception the cells are all 

 nutritive ; but any one of them has the power of organizing 

 for reproduction. This indicates that at first nutritive and 

 20 



