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ROBERT W. HEGNER 



isolated and families were reared from numbers 7 and 8. Fam- 

 ily av. 7 consisted of twenty-eight specimens; among these were 

 seven that possessed 3 nuclei. There were sixteen specimens in 

 family av. 8, all of which were binucleate. In table 28 are given 

 the distribution of diameters and the mean diameters of the 

 binucleate specimens in family av. 7, the tri-nucleate specimens 

 in family av. 7, and the specimens in family av. 8. 



Another family, labeled avd. 1, was reared from a single speci- 

 men that possessed a shell like that shown in figure 43, B. There 

 were thirty specimens in this family. The original progenitor 

 was 31 units in diameter and its descendants ranged from 23 to 

 29 units, with a mean diameter of 25.47. The shape of the shell 



TABLE 28 



Arcella vulgaris. Table showing the distribution of diameter of the shell in bi- 

 nucleate and trinucleate specimens belonging to family av. 7, and in specimens 

 belonging to families av. 8 and avd. 1. The unit of measurement is J^.S fx 



was constant within this family as well as within families av. 7 

 and av. 8 described above, hence we may conclude that shape 

 as well as size is a character that is inherited within families of 

 Arcella vulgaris. 



The results of these studies on Arcella vulgaris may be stated 

 as follows : 



1 . Families reared in the laboratory from wild specimens reveal 

 the existence of pure lines in this species as regards diameter. 



2. Variations in diameter within families are similar to those 

 observed in A. dentata and A. discoides. 



3. In one family a series of specimens appeared that possessed 

 3 nuclei. These were all larger than the specimens with 2 nuclei, 

 and indicate that here, as in the other species described, there is 



