44 



ROBERT W. HEGNER 



types. Most of the changes consisted in an increase or a de- 

 crease of one nucleus, but in a few cases a doubUng of the nuclear 

 number from 3 to 6 occurred. Thus the specimen labeled 

 ap, 5. b. a. 1. 1. 1. 1. 1. was 27 units in diameter and possessed 

 3 nuclei. On May 27th it gave rise to an offspring that was 

 29 units in diameter and possessed 6 nuclei. The parent, how- 

 ever, retained the original number, 3. Six later offspring of this 

 parent all contained 3 nuclei and had a mean diameter of 24.33 

 units, whereas the three descendants of the 6-nucleated off- 

 spring all possessed 6 nuclei and had a mean diameter of 29.50 



Slanatar of eholl 



Fig. 33 Arcella polypora. Family ap. 5. Curves for the variations in diam- 

 eter in groups of specimens arranged according to nuclear number; plotted from 

 table 15. The ordinates are numbers of specimens; the abscissae are diameters, 

 and the numbers at the top of the curves indicate the number of nuclei in the 

 corresponding groups of specimens. 



units. The difference of 5.17 units in the mean diameter of 

 these two groups is obviously due to the difference in nuclear 

 number. 



In another case a specimen (ap. 5 b. a. 1.1.4) 22 units in 

 diameter and with 3 nuclei gave rise to four progeny. Three 

 of these had 3 nuclei and a mean diameter of 24.33; the fourth 

 was 27 units in diameter and possessed 6 nuclei. The three 

 descendants of the latter had 6 nuclei and a mean diameter 

 of 28 units. The differences were not as great when they 



