HEREDITY IN THE PROTOZOA 299 



take place independently of the mechanism of Men- 

 delian heredity that applies only to genes carried by 

 the chromosomes. All that need be said here is that 

 as yet no evidence for such influence has been found. 



Second, it remains to be shown whether in ad- 

 dition to this somatic selection in the Protozoa, if 

 it be such, there may be another form of inheritance 

 of the same kind as that prevalent in the Me^tazoa, 

 based on differences in the chromatin of theproto- 

 zoon individual. Let us now examine the evidence 

 bearing on this question. 



In the process of conjugation in the Protozoa 

 there are phenomena that are often compared with 

 those that occur in higher forms before and at the 

 time of union of the sperm and egg. Whether 

 previous to nuclear interchange there is reduction 

 in the number of the hereditary elements and 

 whether through interchange there are brought about 

 recombinations of elements derived from the two 

 parents are questions that can only be settled, as 

 in the Metazoa, by a study of characteristics of 

 parents (conjugants) and offspring. 



Taking up first the mechanism, that of Para- 

 mecium may serve for example (Figs. 70 and 71). 

 During the time of fusion the macronucleus breaks 

 up and its fragments are later absorbed. The 

 micronucleus divides, and its daughter halves divide 

 again, producing four micronuclei. Three of these 

 are scattered in the protoplasm and disa]o]:)ear; one 

 again divides into two. One of its halves passes 

 into the other conjugant, and there unites with 



