62 



ROBERT W. HEGNER 



27. Two groups may be recognized in the table, one with a dis- 

 tribution of from 21 to 26 units, and a mean of 23.32 units, and 

 the other with a distribution of from 28 to 34 units, and a mean 

 of 29.69 unifs. FamiUes with diameters intermediate between 

 these groups could no doubt have been reared and also families 

 with smaller diameters, but the latter were too small to care for 

 easily, and intermediate families were not reared because of lack 

 of time. Variations in diameter in A. discoides are evidently 

 less than in A. polypora and more nearly like those previously 



Figure 40 



Fig. 40 Arcella discoides. Sketch showing the structure of a typical speci- 

 men. X 310. 



Fig. 41 Arcella discoides. Family ad. 3. Part of pedigree showing the sud- 

 den appearance of a large temporary variation. 



found in A. dentata. The smaller range of variation in A. dis- 

 coides and A. dentata is probably due to the constancy in nu- 

 clear number, there being 2 nuclei in every specimen, except in 

 a few unusual cases, whereas, as has been shown above, the large 

 variations of A. polypora are accompanied by changes in nuclear 

 number. 



b. Variations in diameter and in nuclear numher 



Several abrupt but temporary variations were noted in the 

 families of A. discoides. In family ad. 3 a parent 32 units in 

 diameter and with 2 nuclei gave rise to an offspring 30 units in 

 diameter, but with only 1 nucleus. The parent retained 3 nuclei, 

 but the next offspring, which was the same size as the parent, 

 possessed 2 nuclei and the parent also became binucleate. 



