FERTILITY 209 



these individuals one of the nuclei remains small and 

 becomes the new micronucleus, the other enlarges to 

 form the new macronucleus. Thus from each ex- 

 con jugant four new paramcecia are produced, which 

 now proceed to divide in the ordinary way, i.e. the 

 micronucleus and the macronucleus elongate and divide 

 at each division of the animal. 



It is customary to regard some phase in this process 

 as involving a reduction division in the sense that a 

 separation of the paired factors takes place. If this 

 occurs prior to interchange of micronuclei (E), then each 

 ex-conjugant corresponds to an egg after fertilization. 

 It is conceivable, however, that segregation might oc- 

 cur in the two divisions that follow conjugation, which 

 would give a different interpretation of the process 

 than the one followed here. 



On the first of these two hypotheses two new strains 

 result after conjugation. Each is a recombination of 

 factors contained in the two parents. If the two par- 

 ents were alike, i.e. homozygous, in many factors, and 

 different, i.e. heterozygous, in a few, the two individuals 

 would be more alike than were the original races from 

 which they came. This is, in fact, what Jennings has 

 shown to be the case, at least he has shown that on 

 the average the ex-con jugants are more like each other 

 than were the original strains. 



Calkins has obtained some new and important facts 

 concerning the likeness and unlikeness of the new 

 strains that result from conjugation. He has used 

 wild, i.e. not weakened, individuals, and has followed 

 the history of the four lines resulting from the first 

 four individuals produced by each ex-conjugant. The 



