THE ALG^ 163 



filaments, swarm-cells of a somewhat different kind are 

 formed. These are characterised by having two cilia instead 

 of four, but in other respects are like the former kind, 

 though smaller. They are produced to the number of 

 8, 16, 32, or even more, in a mother-cell, by successive 

 bipartition, just as in the former case. Except for the 

 number of cilia there is no visible difference between 

 the two kinds. It is these small swarm-cells which are 

 the sexual cells of Ulothrix. When they escape from 

 their mother-cell they swim off through the water just 

 like the larger kind of spore. If, however, they meet, as 

 often happens, with another swarm of their own kind, 

 just set free from another cell of the same or of a 

 different thread, an extraordinary process takes place. 

 When two swarm-cells from different mother-cells happen 

 to touch one another they at first become entangled by 

 their cilia, and the pair go on spinning through the 

 water together. They gradually become more closely 

 united ; their bodies come into contact laterally, and 

 soon begin to fuse. The fusion starts at the pointed 

 colourless ends, and after these parts are quite joined 

 up the opposite ends remain for a time separate. Soon 

 however, in fact within a very few minutes, fusion is 

 complete ; the two cells have become one, and now 

 constitute a single four-ciliated spore (see Fig. 70). Its 

 origin from two cells can long be recognised by the two 

 eye-spots and the two chlorophyll-bodies. 



The movements do not last long after fusion is 

 complete. The spore resulting from this union with- 

 draws its four cilia and comes to rest, attaching itself 

 like an ordinary zoospore by the colourless end. In most 

 cases the two cells which unite are of about the same 

 size ; sometimes it happens that a smaller cell unites with 



