ENVIRONMENT ON ARCELLA 



433 



theses is added in the figures (figs. 1, 2, and 3). In another place 

 (Hegner, '20) the writer has shown that this change from the 

 binucleate to the uninucleate condition is accompanied by a 

 decrease in size, as indicated also by these experiments. The 



14-35 (n) 



14-33 (n) 



Fig. 1 Arcella dentata. Part of the pedigree of family 14. The numbers 

 indicate the number of spines and the diameter in units of 4.3 m- For example, 

 the original progenitor (16-34) possessed sixteen spines and was 34 units in diame- 

 ter. The letters and numbers in parentheses should be interpreted as follows : 

 (s) = specimen produced while parent was in sodium silicate solution; {n) = 

 specimen produced while parent was in normal medium; (c) = specimen with 

 crinkled shell; {I) = specimen with only one nucleus; {?) = indeterminate 

 number of spines. 



outlines in figures 4 and 5 show the decrease in the length of the 

 spines. In many cases the spines did not extend beyond the 

 edge of the shell, being represented only by ridges on the dorsal 

 surface of the shell. The experiments extended over the period 

 from March 18 to May 15, 1918. The largest family, no. 14 



