UROLEPTUS MOBILIS ENGELM. 467 



CONCLUSIONS 



These experimental results enable us to view the phenomena 

 of fertilization from still another angle. It has been demon- 

 strated both for Paramecium by Woodruff and Erdmann and 

 for Uroleptus that renewal of vitality or rejuvenescence may be 

 accomplished without the union of two individuals in conjuga- 

 tion. The phenomena of 'endomixis' precede this renewal in 

 Paramecium, and the phenomena accompanying encystment 

 precede it in Uroleptus. In the present experiments we prevent 

 union of nuclei after the two individuals have united in conjuga- 

 tion and obtain the same results as though conjugation had 

 continued to the end. What is it that starts the mechanism 

 resulting in maturation divisions and reorganization leading to 

 renewal of vitality and a new life-cycle? For Uroleptus at 

 least there is no doubt that rejuvenescence follows as a result 

 of normal conjugation, nor is there any less doubt that it follows 

 reorganization during encystment. Rejuvenescence, therefore, 

 is a phenomenon or an aggregate of phenomena which may be 

 treated independently of conjugation. 



Whether there is a parallel between these results with cut 

 conjugating Uroleptus and certain types of parthenogenesis in 

 Metazoa I am not ready to state. Such a problem demands a 

 knowledge of the cytological happenings of the cut individual 

 prior to formation of the new macronucleus, and up to the 

 present I have dealt only with the living fragments. These 

 living fragments show certain definite nuclear structures and 

 happenings, and so far as they go, they indicate that the ex- 

 perimental animals during reorganization do not differ from 

 normal ex-conjugants. 



In any attempt to interpret these phenomena, emphasis must 

 be laid on the fact that cutting the cell and severing protoplas- 

 mic connections with the second individual in conjugation does 

 not stop the succession of cellular processes after they are once 

 started. If we cut off the anterior end of a cell which has just 

 separated from another after conjugation, the operation does 

 not prevent the development of a new macronucleus nor absorp- 

 tion of the old macronucleus, nor does it interfere with rejuvenes- 



