88 ROBERT W. HEGNER 



Variations observed in the number of nuclei in A, dentata, A. 

 discoides, and A. vulgaris were slight; in A. vulgaris specimens 

 were obtained which possessed 1, 2, and 3 nuclei, respectively, 

 whereas in A, dentata and A. discoides all of the specimens were 

 either uninucleate or binucleate. For these three species the 

 binucleate condition is evidently normal and all of the uninu- 

 cleates and trinucleates eventually reverted to this state. In A. 

 polypora, however, considerable variation in nuclear number 

 was noted within each pure line. Thus the number in family ap. 

 5 ranged from 3 to 7 and in family ap. 34 from 5 to 10. Nuclear 

 changes were of frequent occurrence in these families and rever- 

 sion to the modal condition was constantly taking place. 



5. Pure lines 



The data already published (Hegner, '19) prove that a great 

 many pure lines exist in Arcella dentata with respect to spine 

 number and diameter of the shell. They also demonstrate a 

 close correlation between these two characters. The observa- 

 tions recorded in the present contribution indicate that pure 

 lines likewise exist in the other species of Arcella that have been 

 studied. The characteristics that have been noted particularly 

 are: 1) in Arcella vulgaris, the diameter and shape of the shell; 

 2) in A. discoides, the diameter of the shell, diameter of the mouth 

 of the shell, and, 3) in A. polypora, the number of nuclei, size of 

 the chromatin bodies within the nuclei, diameter of the shell, 

 and diameter of the mouth of the shell. A correlation was also 

 demonstrated between the diameter of the shell and the diameter 

 of the mouth of the shell in this species. 



6. The isolation of heritably diverse lines in Arcella polypora 

 within a family during vegetative reproduction 



It has been shown by Jennings ('16) in Difflugia, by Middleton 

 ('15) in Stylonychia, by Root ('18) in Centropyxis, and by Hegner 

 ('19) in Arcella dentata, that by selection, heritably diverse 

 lines may be isolated within a family during vegetative repro- 

 duction. No extended attempt was made to discover whether 



