ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 265 



which in conjugation forms the stationary and migratory micronuclei, 

 and of necessity the non-formation of a syncaryon. 



In the race reared without conjugation for seven years the endomixis 

 has occurred frequently, on an average once each month. As conjugation 

 has been successfully consummated by individuals removed from this 

 culture, it is plain that both endomixis and conjugation are normal 

 phenomena in the life of this same race. 



It seems clear that this culture offers strong physiological evidence 

 against the interpretation of either of the first two so-called reducing 

 micronuclear divisions as actually being a chromosome-reducing division. 

 Since the nuclear changes in endomixis and conjugation are funda- 

 mentally the same, except for the absence of the third micronuclear 

 division in the former, it is justifiable to regard this third micronuclear 

 division which occurs in conjugation as the one directly preparatory, 

 from the standpoint of chromosome reduction and sexual phenomena, 

 for the imminent accession of foreign chromatin in the form of the 

 migratory micronucleus of the other conjugant. 



Endomixis involves the disintegration and absorption of the old 

 macronuclear and micronuclear material, and a re-organization. It thus 

 affords the opportunity for molecular re-arrangement, and therefore may 

 afford the opportunity for the origin of variations within a pure line. 

 Jennings found in pure lines of Paramecium without conjugation 

 evidence of variations about the mean. These might be due to re- 

 arrangements effected in endomixis. Woodruff and Erdmann have not 

 found variations in the structure or in the division-rate in their culture, 

 but they point out that endomixis affords a field for the origin of such 

 variations. It is conceivable that " heritable " variations may result 

 from some rare re-combinations in endomixis. 



Endomixis initiates a new rhythm in the life-history, i.e. a period of 

 increased metabolic activity and therefore a reproductive activity ; and 

 since its fundamental morphological features are almost identical with 

 those preliminary to the formation of the stationary and migratory 

 micronuclei in conjugation, the fact lends strong support to the view 

 that conjugation has a dynamic as well as a hereditary role. Jennings 

 has emphasized the importance of conjugation in connexion with 

 heredity and variation ; Calkins has emphasized the dynamic aspect : 

 both aspects are probably important. 



The authors describe in detail the cytological changes in the re- 

 organization process — the descending phase, the climax, and the 

 ascending phase. We have referred only to the general result that the 

 life of the Paramecium race can proceed indefinitely with endomixis 

 under favourable environmental conditions — conjugation being un- 

 necessary. " Senile degeneration and " physiological death " are not the 

 inevitable result of continued reproduction without conjugation. The 

 cell has an internal regulatory phenomenon, endomixis, which is self- 

 sufficient for the indefinite life of the race. 



Cycles and Rhythms in Paramecium.* — Gary N. Calkins refers to 

 the work of Woodruff and Erdmann, and discusses the question of 



* Amer. Nat., xlix. (1915) pp. 65-76. 



