NUCLEOPLASMIC RELATIONS IN ARCELLA 



41 



3. Relations between nuclear number and diameter during the 

 selection period. The difference between the efficacy of selec- 

 tion in the two species can easily be accounted for on the assump- 

 tion that there is a quantitative relation between the nuclei 

 and the cytoplasm associated with them. In A. dentata all but 

 a negligible number of the specimens used in the selection ex- 

 periments possessed two nuclei. In A. polypora, on the other 

 hand, all of the specimens in the low line that were measured 



TABLE 12 



Arcella polypora. Family ap. 5. Correlation table for diameter oj the shell in 

 parents and progeny during the first period of fifty-four days when no selection 

 was practiced. The unit of measurement is 4-3 fj^. Coefficient of correlation, 



0.442 ± 0.041 



were provided with 4 nuclei, whereas all those in the high line 

 that were measured contained 5 or 6 nuclei. The conclusion 

 that these facts suggest is that the members of the low line are 

 smaller than those of the high line because of their lesser num- 

 ber of nuclei. They also indicate that the frequent large dif- 

 ferences in diameter between parent and offspring may be due 

 to a change in nuclear number, that is, when the offspring was 

 much larger than the parent an increase in nuclear number 

 probably occurred, and when the offspring was much smaller, 

 a decrease in nuclear number was responsible. No very great 



