492 LORANDE LOSS WOODRUFF AND RH. ERDMANN 



that a similar condition follows conjugation, and therefore that 

 one aspect of conjugation (though it is unnecessary for the life 

 of the race under proper environmental conditions) is dynamic. 

 Certain observations which we have made on mass cultures 

 of Paramaecium aurelia indicate that endomixis and conjuga- 

 tion may occur simultaneously in different animals of the same 

 culture, thus strongly suggesting that the same general conditions 

 lead to both phenomena — one animal meeting the conditions 

 one way and another by the other, and that both phenomena 

 fill essentially the same place in the economy of life of Para- 

 maecium aurelia. Therefore it is evident that the vegetative 

 aspect of the life history of Paramaecium is periodically inter- 

 rupted by periods in which a dynamic reorganization occurs 

 either by endomixis or by conjugation. 



IX. ENDOMIXIS AND ITS RELATION TO PARTHENOGENESIS 



One might at first glance consider endomixis as a case of par- 

 thenogenesis, owing to certain similarities of the two phenomena. 

 In parthenogenesis the new individual arises from one nucleus 

 which is not a syncaryon, i.e., the fusion of completely inde- 

 pendent nuclei. In endomixis the new nuclear apparatus is of 

 micronuclear origin without the formation of a syncaryon. Both 

 parthenogenesis and endomixis therefore are apocaryomictic phe- 

 nomena which must be contrasted with caryomictic phenomena 

 such as conjugation, copulation and, to a certain extent, autogamy. 

 The introduction of chromatin belonging either to another indi- 

 vidual or to another nucleus of the same individual gives an 

 opportunity for an extensive rearrangement of the nuclear sub- 

 stance which cannot be effected to such a high degree in either 

 parthenogenesis or endomixis. 



Although both parthenogenesis and endomixis are followed 

 by Entwicklungserregung they afford some features which are 

 not identical. In parthenogenesis there is a chromatin reduc- 

 tion which occurs and is compensated for either in the egg itself 

 or in some later period of the life cycle of the race. 



