246 INTRODUCTION TO CYTOLOGY 



continued vigor of the race in such animals, and that without a periodic 

 recurrence of the process the race would become increasingly senescent 

 and finally die out. The long-continued experiments of Woodruff, 

 Jennings, and others showed that this is not what happens. Given a 

 properly regulated environment, these organisms will give rise to thou- 

 sands of successive generations with no conjugation and no decline in 

 vigor. The natural conclusion seemed to be that protoplasm does not 

 necessarily become senescent through continued vegetative activity. 



The problem assumed a new aspect with the discovery of endomixis by 

 Woodruff and Erdmann (1914). In conjugation, as shown above, the 

 micronucleus in each individual initiates a series of three mitoses, one 

 of the resulting nuclei passing as a gamete nucleus to the other individual, 

 where a fusion takes place. Since some of the products of the division of 

 the fusion nucleus become macronuclei, conjugation involves a replace- 

 ment of the old macronucleus by material from the micronucleus, as 

 well as the exchange and fusion of micronuclei. In endomixis, on the 

 other hand, there is such a replacement but no exchange or fusion. For 

 example, in Paramcecium aurelia, which has two micronuclei, it is found 

 that after every 40 or 50 generations (by fission) the macronucleus dis- 

 integrates in the cytoplasm, while each micronucleus undergoes two 

 successive mitoses (rather than three). All but one or two of the 

 resulting nuclei degenerate, those which remain later dividing to produce 

 the new macronuclei and micronuclei. 



The question of whether the race can continue to live without either 

 conjugation or endomixis has awaited answer for several years. In 

 Paramcecium aurelia and P. caudatum the periodic recurrence of this 

 nuclear reorganization is coincident with a rhythmic acceleration in 

 metabolic activity, which at least in part explains the ability of these 

 animals to maintain their vigor without conjugation. Furthermore, if 

 endomixis ceases, the cultures die. But in other species endomixis takes 

 place in the encysted stage or is lacking altogether, yet the metabolic 

 rhythms occur and the race continues. This indicates some factor other 

 than endomixis as the cause of the periodic acceleration of vital activity. 

 The results of several researches" point to the general conclusion that 

 neither conjugation nor endomixis is necessary for the continued existence 

 of the race, although under environmental conditions inducing senile 

 change both processes have rejuvenating effects. 



Endomixis thus resembles syngamy in its stimulative effect on metab- 

 olism and development, but its possible relation to variation and heredity 

 is not so clear. There is no evidence that in endomixis there is any 

 meiotic disjunction of chromosomes in the formation of the degenerating 

 nuclei; in fact, there are certain indications that in the process of conjuga- 



3' Woodruff and Spencer (1924), Spencer (1924), E. L. Moore (1924). See 

 especially Woodruff (1925), Calkins (1926), and Jennings (1920). 



