POLYTOMA 



123 



Reproduction is usually brought about by a process known 

 as repeated fission, in which binary fission is re- 

 peated so as to form four daughters before the 

 separate, but sometimes there are only two off- 

 Fission takes place within the cuticle, this being 

 about during the process by the action of the 

 which remain attached to one of the daughters, 

 nucleus divides by a kind of mitosis. The 

 division is transverse and the second longitudinal. 



Reproduction. 



young 

 spring, 

 carried 

 fiagella 

 The 

 first 



When it is completed the fiagella are withdrawn, each 

 daughter forms two small fiagella, and the cuticle of the 

 parent is dissolved. At intervals of a few days conjugation 

 takes place. Two or- 

 dinary Polytomce come 

 together and fuse, their 

 nuclei joining and their 

 cytoplasm melting into 

 one mass, which then 

 encysts. After a rest- 

 ing 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 (1) that 

 since Polytoma, like Amoeba, shows no division into germ- 

 substance and body-substance with special nuclei, con- 

 jugation can occur at any time in the life of the individual, 

 and does not need to take place directly after the separa- 

 tion of the germs : in the frog, on the other hand, con- 

 jugation is obviously impossible in the adult and can only 

 take place between the little germs, before they develop 

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

 as in the frog, 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. The 



s.g. 



Fig. 75. — Polytoma uvella : three 

 stages in ordinary fission. 



c.v., Contractile vacuole ; cu. t cuticle ; 

 nu., nucleus ; s.g., starch grains. 



