92 ROBEET W. HEGNER 



matin mass are correlated, it follows that spine number and chro- 

 matin masfe are likewise correlated. It is possible, therefore, that 

 although diverse lines were isolated by the selection of speci- 

 mens according to their spine number, what was really being 

 selected was variations in the chromatin mass within the nuclei. 

 The permanency of these diverse branch lines consequently de- 

 pends upon the permanency of the differences in chromatin mass. 

 It does not seem probable that an unequal quantitative division 

 of the chromatin could bring about the permanent diversities 

 that were obtained, since the formation of new chromatin is a 

 regular process during the growth of the cell, and any inequality 

 in chromatin mass following cell division would soon be overcome 

 by the growth of the chromatin. It has been suggested by Jen- 

 nings in Difflugia that ''the substances determining the heredi- 

 tary characters may be distributed with less accuracy than in 

 higher organisms, so that the two products of fission may often 

 receive parts that are not equivalent" (Jennings, '16, p. 524). 

 This may likewise be true of Arcella. The sudden large herita- 

 ble changes (mutations) would, according to this suggestion, be 

 due to large qualitative inequalities and the smaller heritable 

 variations to smaller qualitative inequalities during nuclear 

 division. 



We may recognize in these organisms both fluctuating and 

 heritable variations, but it is of course impossible to distinguish 

 immediately from the shell characteristics which of these two 

 variations is being selected, and hence the rate of isolation of 

 heritably diverse branch lines is, according to the laws of chance, 

 less rapid than it would be if the two types of variations could be 

 determined at once. 



With one exception, all of the characters of Arcella that were 

 studied were found to vary together, the correlation being very 

 high. In Difflugia, however, Jennings worked with several 

 characters that varied independently. Thus ''there is some indi- 

 cation in the pedigrees that hereditarily higher numbers of spines 

 need not necessarily go with hereditarily larger size, though they 

 usually do" (Jennings, '16, p. 519), and "Hereditarily diverse 

 combinations of size and length of spines occur in the different 



