766 INHERITANCE 



reduction of the chromosomes, so far as factors for size are concerned, 

 must occur at the first or the second of the maturation divisions, not at 

 the third. The fact that they are of the same final size shows that the two 

 descendant clones have the same factors for size. This can occur only if 

 the third maturation division, producing the two pronuclei, is non- 

 reductional. In that case the two pronuclei of an individual are neces- 

 sarily alike, and only when this is so will the nuclear combinations be 

 the same in the two ex-conjugants. If reduction occurred at the third 

 maturation division, the two pronuclei would often be diverse, leading 

 to different final sizes. 



In summary, it may be said that the final size of the descendants of the 

 ex-conjugants is determined by the nuclear constitution, as is shown by 

 the fact that the final sizes are the same for the clones derived from the 

 two conjugants of a pair. But for a long time, for many vegetative 

 generations, the nature of the cytoplasm affects the size of the descend- 

 ants; descendants with cytoplasm of different constitution remain for 

 long periods diverse. The longest period observed by De Garis during 

 which the diversity of cytoplasm persisted was thirty-six days, which 

 would mean about the same number of vegetative generations. 



A similar differential effect of the cytoplasm is seen in the inheritance 

 of other characteristics. Sonneborn and Lynch (1934), before the work 

 of De Garis on size, observed such a cytoplasmic effect in crosses of clones 

 that differed much in the rate of multiplication. If before conjugation 

 one of the clones multiplies rapidly, the other slowly, after conjugation 

 the clone that receives the cytoplasm from the rapid race continues for 

 a time to multiply rapidly, while the other, receiving its cytoplasm from 

 the slow race, continues to multiply slowly. This effect of the diverse 

 cytoplasms continues for about ten vegetative generations. But during 

 that time the difference in fission rate for the two ex-con jugant clones 

 gradually disappears, till at the end of the period the rate of fission is 

 the same in the two. For example, the two clones N 21 and B were 

 crossed by Sonneborn and Lynch. The mean daily fission rate in N 21 

 was 0.67, while in B the fission rate was 1.97, three times as great as in 

 N 21. During the first five days after conjugation, the daily rate for the 

 clones that had received cytoplasm from the slow race N 21 was 1.10; 

 for those that had received cytoplasm from the fast race, B, the daily 

 rate was 2.00. In the second five-day period the rates were respectively 



