HEREDITY IN THE PROTOZOA 305 



appear dubious until we know whether these di- 

 visions occur in the pairs which are spht in the 

 same way as in the conjugating individuals. It 

 has also been shown by Woodruff and Erdmann 

 that at intervals a process occurs in Paramecium 

 that they call endomixis, in which the individual 

 undergoes changes that are like the first steps taken 

 before conjugation. The old macronucleus breaks 

 down and is absorbed. The micronucleus divides 

 twice. Two of the four resulting nuclei disappear, 

 one becomes the new micronucleus, and the other 

 a new macronucleus. If reduction appears at this 

 time, we should expect that sudden differences in 

 the rate of division might appear. Erdmann finds 

 positive evidence that supports this expectation. If 

 the third micronuclear division, that does not appear 

 in endomixis but does appear prior to conjugation, 

 is the reduction division, then it may be difficult 

 to reconcile such an interpretation with the com- 

 parison of the split-pairs that Jennings has made. 



There is some evidence, in forms other than 

 Paramecium, showing that reduction in the number 

 of chromosomes takes place prior to interchange. 

 In the infusorian Didinium nasutimi there are 16 

 chromosomes. In the first of the three maturation 

 divisions of the micronuclei each daught(^r gets 

 10. In the second division the 10 separate into two 

 groups of 8 each. In the third division each of 

 the 8 divides giving 8 to each daughter. The full 

 number would then be restored by conjugation. 



In Uroleptis, Carchesium, Operculina, and Anoplo- 



