THE PROTOZOAN LIFE CYCLE. 23 I 



out until the race in each case became extinct. Two of the eight 

 individuals representing different pairs continued to multiply in 

 cultures for more than a year, the method employed being the 

 same as that used in previous experiments. 1 The other six died 

 before reaching the fourth generation. One of the two success- 

 ful exconjugants reorganized as Paramccinin aurclia, the others 

 as P. caudatum. The can datum form was much more vigorous 

 than the aurclia form and kept up in its division rate with a ninth 

 individual that was chosen for control from the individuals that 

 had not conjugated in the original culture. At the end of two 

 months the aurclia form had divided 5 5 times, the caudatum 

 form 76 and the control 77 times. In the period between the 

 last of April and the end of June the aurclia form gradually lost 

 its specific aurclia characters and became more and more like 

 caudatum. The two micronuclei were reduced to one, one being 

 absorbed, and by the seventieth generation the original aurclia 

 form could not be distinguished morphologically from the origi- 

 nal caudatum form, while its division energy became much 

 greater than that of the other types in culture, an energy which 

 lasted through more than 300 generations. 



Without entering into a discussion of the interesting biological 

 features of this apparent change of species which I have published 

 elsewhere, 2 I would merely call attention to the fact that here at 

 any time, during the first 45 generations at least, the thousands 

 of cells that might have been formed would have been classified 

 as P. aurclia, while study of the life history shows that it is only 

 a temporary form assumed by P. caudatum and is to be interpreted 

 as a mere variant or sport. These results merely emphasize the 

 importance of studying the entire life cycle, as a basis for proto- 

 zoan species. 



It is particularly important that these specific distinctions 

 should be clearly recognized in that field of protozoan research 

 which is most important at the present time, the pathogenic. 

 Here more than in any other field of biological study is the need 



Calkins, " Studies on the Life History of Protozoa," I., Arch. f. Entwk., XV., 

 No. i, 1902. 



2 Paramecium aurelia and Paramecium caudatitin, in "Biological Studies" by 

 the pupils of William Thompson Sedgwick, Chicago, 1906. 



