206 BIOLOGY OF THE PROTOZOA 



the controls divided in twenty-four hours (see Hartmann's simi- 

 lar experiments with Ameba, p. 239). In Uroleptus, Uronychia 

 and similar forms, however, the many nuclei fuse to form one com- 

 pact and relatively small nucleus prior to division. It would seem 

 that such changes in relative volume of nucleus and cytoplasm are 

 better interpreted as the effects of underlying conditions which lead 

 to division rather than as the direct cause of division. 



None of these theories is of much value in analyzing the antecedent 

 phenomena of division. These must be sought in the reactions of 

 different substances constituting protoplasm. Division of the cell 

 itself is a last step in a progressive series of reproductive changes 

 affecting the entire protoplasm, the constituents of which— micro- 

 somes, mitochondria, plastids, chromomeres, kinetic elements, etc.— 

 have already divided. It is in the division of these fundamental 

 granules in the make up of protoplasm that we must look for the 

 underlying causes of cell division. The dependence upon growth 

 and metabolism of the succession of division processes which char- 

 acterize reproduction is clearly evidenced by simple starvation exper- 

 iments, division ceasing with cessation of metabolic activities. There 

 is a possibility that environmental conditions play a more direct 

 part in reproduction than is indicated by their relations to metab- 

 olism. Thus Robertson (1921) concludes that a catalase (X sub- 

 stance) is secreted by the living cell which directly enhances division. 

 He found that two individuals, or more, of Enchelys farcimen in a 

 drop of culture medium would divide from four to sixteen times more 

 rapidly than a single individual in a similar drop, the result being 

 interpreted as due to contiguity of individuals. This, however, is a 

 direct contradiction of Woodruff's (1911) results with Paramecium 

 and Stylonychia, according to which the division rate is reduced 

 by accumulation of products of metabolism in the medium. Nor 

 is Robertson supported by other observers. Cutler (1924) for 

 example, found for Colpidium colpoda that the division rate depends 

 upon the number of bacteria present as food, and that increase in 

 number of individuals in a drop means a decrease in the individual 

 division rate. Greenleaf (1924) similarly found that solitary indi- 

 viduals of Paramecium caudatum, P. aurelia and Pleurotricha 

 lanceolata isolated in 2, 5, 20 and 40 drops of medium, gave a 

 highest division rate in five days in the 40-drop test, the lowest 

 in a 2-drop test. Also in Uroleptus mobilis, in a sixty-day test in 

 which 1 individual, 2, 3 and 4 individuals were isolated daily in 

 a single drop of medium the highest division rate was shown by 

 the solitary individual in a drop as shown in the following table 

 (see also table on next page) : 



10 individuals, 1 to a drop, each divided in the sixty days . . 74. 1 times 



20 individuals, 2 to a drop, each divided in the sixty days . 59 . 5 



30 individuals, 3 to a drop, each divided in the sixty days . . 54 . 7 



40 individuals, 4 to a drop, each divided in the sixty days . . 54 . 2 



