288 BIOLOGY OF THE PROTOZOA 



Of these external conditions other factors than the supply of 

 food may, and apparently do, play a part. Enriques (1903, 1905, 

 1909, etc.) has long maintained that the phenomena of degeneration 

 and senescence are caused at bottom, not by internal conditions 

 but by external causes, apparently by the accumulation in the 

 medium of bacterial products which poison the organism. Hance 

 (1917) held that they are caused by the concentration of katabolic 

 products derived from the organism and accumulate in the medium. 

 Enriques also makes the statement that upon filtering the liquid 

 in which conjugating forms are present and adding non-conjugating 

 individuals to it, the latter will conjugate; on the other hand a 

 similar liquid with non-conjugating individuals if filtered and 

 used as medium for conjugating individuals, will act as a deterrent 

 to conjugation. Repeated attempts on our part with Didinium 

 nasutum, Paramecium eaudatum and Uroleptus mobilis have failed 

 utterly to confirm these results. There is more evidence for his 

 conclusion that salts in the medium are necessary for conjugation, 

 a conclusion based upon his experiments with NaCl, NaBr, and 

 Nal in certain concentrations, on the ciliate Cryptochilum nigricans. 

 These particular salts together with strong solutions (1 to 10,000) 

 of CaClo and FeoCls, produced epidemics of conjugations, while 

 weak solutions of the last two salts inhibited conjugations. Still 

 more extensive experiments along the same line were made by 

 Zweibaum (1912) on Paramecium eaudatum. Dilute salts, A1C1.3 

 in particular, added to the medium after a long period of rich feeding, 

 followed by a period of hunger of five to six weeks (sic) produced 

 almost complete epidemics. No salts at all, or very strong salts 

 added to the medium caused no conjugations. These results are 

 certainly suggestive but the experiments should be repeated with 

 carefully controlled material and with some other type than Para- 

 mecium. With this organism Hopkins (1921) failed to confirm 

 these results. Some rather incomplete and unconvincing experi- 

 ments by Baitsell (1912) may also be cited in this connection. Two 

 lines of Stylonychia from the same ancestral cell, were cultivated 

 on different media; one line on hay infusion, the other on beef 

 extract. Individuals of the former line refused to conjugate while 

 those of the latter line conjugated. From this Baitsell concluded 

 that the determining condition was the medium used. Chatton 

 (E. and M., 1931) concludes that certain types of food will induce 

 conjugation in Paramecium while other types will not. Calkins 

 and Gregory (1914) found that in the same medium some lines 

 would conjugate regularly while other lines from the same ancestral 

 cell would not conjugate at all or conjugate only after nine months 

 of continued culture (see also Hopkins, 1921). 



A full consideration of the evidence that has accrued in support 

 of the thesis that external conditions are alone responsible for the 



