94 BIOLOGY 



each begins to develop into a new filament like that from 

 which it originated. The growth into the new filament is by 

 division; the cells after dividing remain attached together in 

 the form of a long chain. 



The second method of reproduction is by conjugation and 

 reminds us of that in Pandorina. In this case, the contents 

 of some of the cells break up into a large number of parts 

 instead of a small number, and these, by the bursting of the 

 cell wall, are finally liberated into the water ; Fig. 41 c. 

 They are then found to possess two flagella, instead of four 

 like the zoospores, and by means of these they swim around. 

 These small spores are, however, unable to grow into new 

 threads.* After the spores have been swimming about foi 

 some time they come in contact, as shown in Figure 41 d, and 

 fuse together, the fusion being identical with that already 

 described in Pandorina; see page 74. There are thus formed 

 conjugation spores known as the zygospores (Gr. zygon = yoke). 

 These zygospores, after a time, produce by division several 

 more spores which, upon becoming free, soon begin to divide 

 and grow into new filaments like those with which we started, 

 This kind of reproduction is very similar to that of Pandorina 

 and clearly suggests the sexual reproduction which occurs in 

 higher organisms. 



In the organisms thus described, we have examples which 

 cannot properly be called unicellular, nor on the other hand 

 can they be called multicellular; each one of these cells carries 

 on by itself all the functions of the organism, whereas in multi- 

 cellular organisms, as we shall presently see, the different cells 

 have different functions to perform, and the cells that make 

 up the individual are not all alike as they are in the forms 

 already described. We must look upon the Pandorina and 

 Ulothrix as intermediate between the unicellular and the 

 multicellular forms. In this way they illustrate the general 



* Sometimes, however, they do grow into very short threads which are 

 much smaller than the original. 



