512 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 of bacterial 

 products in the medium which poison the organism. Hance (1917) 

 held that they are caused by the concentration of katabolic products 

 derived from the organism and accumulated 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 

 nasvtum, Paramecium caudatnm and Urokphis mohiJis 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 cihate Cnjptochilum 7iigricans. 

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

 of CaCl2 and Fe2Cl6, produced epidemics of conjugations, while 

 weak solutions of the last two salts inhibited conjugations. Still 

 more extensi\e experiments along the same line were made by 

 Zweibaum (1912) on Paramecium candatmn. Dilute salts AICI3 

 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- 

 mecinm. 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 mediinn used. 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 

 onset of conjugation leaves one with the same perplexity that 

 troubles ('hatton, Woodruff, and others and calls forth the same 



