HEREDITY IN THE PROTOZOA 305 
appear dubious until we know whether these di- 
visions occur in the pairs which are split 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 nasutum there are 16 
chromosomes. In the first of the three maturation 
divisions of the micronuclei each daughter gets 
16. In the second division the 16 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- 
