3/6 Problem of Rejuvenescence in Protozoa [June-September 



the absence of the introduction of foreign nuclear (and cytoplasmic) 

 material into the cell. 



For reasons advanced elsewhere, we hold that endomixis is not 

 parthenogenesis, but it is not necessary at this time to enter into a 

 more or less academic discussion in regard to the exact Classifica- 

 tion of endomixis among Entwicklungserregimg phenomena. 



In the light of the discovery of the details of endomixis, by the 

 daily study of pedigreed cells of this race, a survey of the cells, 

 which had been preserved at intervals during the previous seven 

 years of its life, revealed a number of the crucial stages of endo- 

 mixis, thus showing that the process has been in progress ever since 

 the race has been bred, and is not, as Hertwig^ suggests, a develop- 

 ment during long subjection to culture. That it is not even a pecu- 

 liarity of this race is evident from the fact that we have found 

 endomixis in four other distinct races selected at random — three 

 from America and one from Germany. Further, Hertwig, in 1889, 

 incidentally noted in a mass culture, in which conjugation had 

 not been observed for a long time, certain animals whose nuclear 

 structure apparently indicated isolated stages of the process which 

 have been elucidated in our cultures. Therefore, it seems well 

 established that endomixis is of general and probably universal 

 occurrence in Paramcecinm aurelia. It also occurs, with essentially 

 similar features, in all the races of Paramcocium caudatum which 

 we have studied.^*^ 



Now, in regard to the significance of endomixis from the stand- 

 point of our subject — rejuvenescence in protozoa: It is clear that 

 the cycle emphasized by Maupas, Calkins and others, is merely a 

 phantom which has continually receded as each successive investi- 

 gator has approached the problem with improved culture methods, 

 until it has vanished with this eight-year-old culture. What remains 

 then is the rhythm and in the light of endomixis — the underlying 

 cytological phenomenon of which the rhythm is an outward physio- 

 logical expression — the whole problem takes on a new aspect. The 

 cell automatically reorganizes itself periodically by a process which, 

 in its main features, simulates conjugation, but without a contribu- 

 tion of nuclear material from another cell. 



»Hartwig: Biol. Cent., 1914. 



10 Erdmann and Woodruff : Journ. Exp. Zool., 1916. 



