38 FLAGELLATE PROTOZOA 



is clearly a unicellular alga. Like animals, Polytoma uvella needs 

 an organic source of carbon, but like plants it can exist on 

 inorganic nitrogen. Polytoma can encyst, and in the encysted 

 state is carried about in dust, etc., to germinate in favourable 

 circumstances elsewhere. 



REPRODUCTION 



Reproduction is usually brought about by a process known 

 as repeated fission, in which binary fission is repeated so as to 

 form four daughters before the young separate, but sometimes 

 there are only two offspring. Fission takes place within the cuticle, 

 this being carried about during the process by the action of the 

 fiagella, which remain attached to one of the daughters. The 

 nucleus divides by a kind of mitosis. The first division is nearly 

 transverse, the second at right angles to it. The fiagella are then 

 withdrawn, each daughter forms two small fiagella, and the 

 cuticle of the parent is dissolved. At intervals of a few days 

 syngamy takes place. Two ordinary individuals come together 

 and fuse, their nuclei joining and their cytoplasm flowing into 

 one mass, which then encysts. After a resting period the zygote 

 divides by repeated fission into eight, each of the daughters 

 grows two fiagella, and the cyst is dissolved. In regard to this 

 process we must notice : (i) that syngamy can occur at any time in 

 the life of the individual, and does not take place between special 

 germ cells which cannot develop without it : in most animals, 

 on the other hand, syngamy is obviously impossible in the adult 

 and can only take place between the germ cells before they develop 

 the rest of the body ; (2) that the gametes are alike, and not, as in 

 most animals, of two kinds, a passive kind, which bears the bulk 

 of the cytoplasm, and an active kind, by which is carried out the 

 locomotion which the process involves. Both gametes in Polytoma 

 are fairly well supplied with cytoplasm and both are motile. 

 Only when one is older than the other is there sometimes a 

 difference in size. 



In Chlamydomonas (Fig. 8) syngamy takes place, not, as might 

 seem possible, between ordinary individuals, but between special 

 small forms which arise by repeated fission of the ordinary forms. 

 These special gametes, however, are like the ordinary individuals 

 in all but size. In some species of Chlamydomonas they are 

 themselves of two sizes, which conjugate large with small, so 



