32 Life and Death, Heredity and Evolution 



by parts of the reserve nucleus. Every forty to fifty gen- 

 erations the macronucleus breaks up and is absorbed and 

 disappears, just as happens when conjugation is to occur. 

 Then each reserve nucleus divides (in Paramecium aurelia) 

 so as to produce eight (see Figure 9). Six (or seven) 

 of these disappear by absorption, like the active nucleus. 

 The remaining one later produces by division the new active 

 nucleus (macronucleus), and the reserve nucleus or nuclei. 

 That is, the active nucleus is replaced every forty or fifty 

 generations by material from the reserve store. This whole 

 process Woodruff and Erdmann call endomixis. 



This process has been found to occur, not only in the 

 single race in which Woodruff and Erdmann first found it, 

 but in many other races, and in. another species of Para- 

 mecium. And in another infusorian, Stylonychia, the Rus- 

 sian investigator, Fermor (1913), states that a similar proc- 

 ess occurs at the time of encystment. After multiplying 

 for five or six weeks the animals lose all their appendages 

 and other organs, gather into a sphere and form a sort 

 of thin shell about them. Then the two active nuclei disap- 

 pear and are replaced from the reserve nuclei, which have 

 united to form one. The very brief account of this process 

 by Fermor was published earlier than the work of Woodruff 

 and Erdmann. It has been stated by Calkins that a similar 

 process occurs in the encystment of Didinium (1915 a), and 

 in Uroleptus (1919). It seems probable that it will be found 

 generally in the infusoria. 



The discovery of this process of replacement of the active 

 nucleus by the reserve nucleus evidently puts a new face 

 on the matter. To the question whether the living sub- 

 stance uses itself up in functioning, so as to require replace- 

 ment, it seems to answer "yes !" even more definitely than any 

 discovery that conjugation was necessary would have done. 



