NUCLEOPLASMIC KELATIONS IN ARCELLA 91 



9. The effects of selection upon the production of heritably diverse 

 lines in shelled rhizopods 



Jennings ('16), Root ('18), and the writer (Hegner, '19) have all 

 found it possible by selecting certain specimens within a line 

 produced by vegetative reproduction to obtain branch lines that 

 differed from one another in heritable characters. In neither Dif- 

 flugia, upon Avhich Jennings worked, nor Centropyxis which Root 

 used for his researches are the nuclei visible in the living animal. 

 These investigators did not know, therefore, whether nuclear 

 changes occurred in their organisms or not. The heritable di- 

 versities reported by Jennings took place principally by gradual 

 variations, but sudden changes (mutations) also appeared. 

 Both sorts of variations may have been due to changes in the 

 nuclear number, the effects of which were in certain cases slowly 

 and in other cases more quickly revealed by the external charac- 

 ters. In the selection work carried on by the writer on Arcella 

 dentata, the nuclear condition of the organisms was under con- 

 stant observation, and, although empty shells appeared, no speci- 

 mens with more than two nuclei were found and only a very few 

 with one nucleus were discovered. The heritable diversities 

 that were obtained were, therefore, not due to changes in nuclear 

 number. There is evidence, however, that they may have been 

 due to changes in the quantity of chromatin contained in their 

 nuclei. Unfortunately, these diverse branch lines were allowed 

 to die out before it was discovered that differences in chromatin 

 mass existed in the different pure lines. As shown in part 6 of 

 this contribution, the ^ze of the members of a line of Arcella den- 

 tata is correlated with the quantity of chromatin in their nuclei, 

 and the chromatin mass in specimens belonging to a line contain- 

 ing small members is less than in a line composed of large speci- 

 mens. It is, therefore, possible that the diverse branch lines 

 resulted from inequalities in the distribution of the chromatin 

 masses to the daughter nuclei during fission. Diameter of shell 

 and spine number are closely correlated in these organisms (Heg- 

 ner, '19), and thus when spine number was used as a basis for 

 selection, size was also involved. Similarly, since size and chro- 



