REPRODUCTION 205 



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 

 afi'ecting the entire protoplasm, and 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 of the succession 

 of di^■ision processes which characterize reproduction upon growth 

 and metabolism is clearly evidenced by simple starvation experi- 

 ments, 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 indi^•idual 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 Stylomjchia, 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 Colpidiiim 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- 

 \iduals of Paramecium caudaium, P. aurelia and Pleurotricha 

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

 di^'ision rate in five days in the 40-drop test, the lowest in a 2-drop 

 test. Also in Uroleptus mohilis, in a sixty-day test in which 1 

 indi^•idual, 2, 3 and 4 individuals were isolated daily in a single drop 

 of medium the highest division rate was shown by the solitary indi- 

 vidual in a drop as shown in the following table: 



10 individuals, 1 to a drop, each di^^ded 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 di\'ided in the sixty days . . 54.2 " 



Environmental conditions which alter the permeability of the 

 cell, thereby enhancing or retarding metabolic activities do, however, 

 have a corresponding efiect upon the division rate. Age of indi- 

 viduals, or the protoplasmic organization at different periods of the 

 life cycle likewise has a determining eft'ect on the rate of division, the 

 differences, as shown in the following table, being due to the differ- 

 ences in the reactions of the protoplasm to the same medium under 



