STUDIES ON LIFE-HISTORY OF PROTOZOA. 2O3 



conjugations with endogamous and exogamous unions is very 

 instructive. By " wild ' is meant pairs that are captured while 

 conjugating in the natural pond water. There were only three 

 pairs of these but of the six ex-conjugants five continued to live in 

 the regular culture medium. Had the original number of pairs 

 been larger, the result would be more satisfactory but I believe 

 the proportion of fertile results would be as high as five out of 

 six, or, expressed in percentages, as high as 83 per cent. Of 

 the exogamous conjugations, i.e., those of diverse ancestry, there 

 were 24 pairs selected at different periods, and 48 ex-conjugants 

 isolated, of which only three continued to live in the same cul- 

 ture medium, or about six per cent. Of the endogamous unions, 

 i.e., conjugations in which the gametes were of the same ances- 

 try, 1 6 pairs in all were selected and 32 ex-conjugants isolated, 

 of which two only continued to live, again about six per cent. 

 How are we to account for the great discrepancy between the 

 proportion of fertile wild ex-conjugants and those obtained from 

 the cultures ? / belicre tliat it can be accounted for on the 

 ground of chemical differences in tJic composition of tJie protoplasm 

 of the different individuals. In the natural habitat of these forms 

 no two individuals have the same environment for long periods 

 and it may well be that no two parts of the pond have an identical 

 composition. When two organisms unite, as Paramccciiun unites, 

 and part of the micronucleus of one unites with a similarly-de- 

 rived part of the micronucleus of the other, the result is the 

 formation of a new chemical substance in each organism, for the 

 fusion nucleus in all probability is different in minute composition 

 from either of the old ones. Now, on the other hand, when the 

 organisms had been in the same culture medium for long periods, 

 as was the case with the As and Bs of these experiments, there 

 was no chance for them to be different from one another in 

 chemical composition, and the ex-conjugants, after a few genera- 

 tions, died in the great majority of cases. In all such cases the 

 conjugations were normal, micronuclei were exchanged, the old 

 macronuclei became disintegrated and new ones were properly 

 formed, but something was lacking and the ex-conjugants died. 

 I believe this " something " has to do with the chemical make-up 

 of nucleus and cytoplasm ; and I further believe that the failure 



