86 ROBERT W. HEGNER 



Why were new shells produced by apparently normal speci- 

 mens in the various lines of Arcella dentata and A. polypora 

 and then cast off empty? Why was the change from the uni- 

 nucleate to the binucleate condition in A. dentata always accom- 

 panied by empty shell formation? ^Vhy were not changes from 

 the binucleate condition to the uninucleate condition in A. 

 dentata likewise accompanied by the throwing off of empty 

 shells? Why were no empty shells formed in A. polypora and 

 A. discoides when changes in nuclear number occurred? Why 

 did the uninucleates in the lines of A. dentata and A. discoides 

 all finally become binucleate? What determines the genera- 

 tion when nuclear changes occur? Why did uninucleate and 

 binucleate specimens of A. polypora obtained from pieces all 

 produce progeny with a greater number of nuclei, and thus 

 return to the average condition of the line? Why did not the 

 trinucleate race of A. vulgaris persist? 



The formation of empty shells by apparently normal speci- 

 mens of A. dentata may have resulted from the failure of 

 the nuclei to divide. This may in turn have been due to the 

 lack of sufficient cytoplasm to initiate division of the nuclei, 

 although the factors that are responsible for cell division were 

 in operation. 



Similarly in A. polypora cell division may have begun at a 

 time when the total amount of chromatin within the organism 

 was in excess, but was not completed normally because of the 

 presence of an insufficient amount of cytoplasm. 



The production of an empty shell when nuclear doubling 

 occurs in A. dentata may be accounted for in the following 

 way. Having reached the stage when division normally takes 

 place, the nucleus of the organism divides and a new shell is 

 formed. Both nuclei and all of the cytoplasm remain in the 

 parent shell, however, instead of one nucleus and half of the 

 cytoplasm separating from the rest and occupying the new 

 shell, as usual. What causes this process is not clear. In A. 

 discoides and A. polypora under similar conditions no empty 

 shells are formed. Racial differences must be assumed to 

 account for this condition. Evidently cell division in these 



