2 ROBERT W, HEGNER 



3. Observations and experiments on Arcella polypora 35 



a. Origin of the specimens studied 35 



h. Relations between nuclear number and diameter in wild specimens. . 35 



c. Variations in diameter within families 36 



d. Relations between nuclear number and diameter within small families 



and between families 37 



e. Relations between nuclear number and diameter within a large fam- 



ily. Family ap. 5 38 



1. Correlation in diameter between parents and immediate off- 



spring 38 



2. Results of selection for lines with large and small diameters. ... 38 



3. Relations between nuclear number and diameter during the 



selection period 41 



4. Results of selection for large and small diameters within groups 



with the same number of nuclei 42 



5. Changes in nuclear number and their results 44 



6. Results of a further increase in nuclear number 45 



7. Uninucleate and binucleate specimens obtained by cutting 



experiments 46 



/. Differences. between different families with regard to nuclear num- 

 ber and diameter 50 



1. Variations in nuclear number and diameter in wild specimens and 



within small families 50 



2. Differences between family ap. 5 and families ap. 38, ap. 39, ap. 



69, and ap. 34 51 



a. Families ap. 38, ap. 39 and ap. 69 51 



h. Family ap. 34, Comparison of nuclear number and diameter 



with family ap. 5 52 



g. Relation between diameter of shell and diameter of mouth in wild 



populations and within families 54 



/i. Summary of results of observations and experiments on Arcella 



polypora 59 



4. Observations and experiments on Arcella discoides 61 



a. Pure line 61 



b. Variations in diameter and in nuclear number 62 



c. The reproduction of uninucleate pieces 64 



d. Summary of results of observations and experiments on Arcella 



discoides 65 



5. Observations and experiments on Arcella vulgaris 65 



6. The relations between chromatin mass and cell size 67 



a. Chromatin mass and cell size within two families of Arcella dentata 67 



h. Chromatin mass and cell size within two families of Arcella polypora 71 



c. The possible relation between the chromatin mass and the death of 



certain specimens of Arcella dentata 74 



d. Summary of the studies of chromatin mass and cell size 76 



7. Discussion 76 



