432 ROBERT W. HEGNER 



excess of sodium silicate might facilitate shell production and 

 bring about the formation of variations, such as longer spines. 

 The method employed was to make up daily or every other day 

 culture media as usual and then add one drop of sodium silicate 

 to 100 cc. of the medium. Solutions of greater strength were 

 tried, but the organisms did not thrive in them. 



The experiments were begun with thirty specimens taken from 

 the lines that were being used at the time for selection 

 experiments. From these, five families were reared as follows: 



Family 14 with 135 specimens 

 Family 15 with 20 specimens 

 Family 20 with 11 specimens 

 Family 25 with 9 specimens 

 Family 26 with 19 specimens 



No difficulty was experienced in obtaining large families, the 

 number of specimens recorded being limited only by the amount 

 of time available to care for them. 



a. Fission rate 



The rate of fission of specimens grown in the sodium silicate 

 medium decreased immediately from an average of one division 

 in 2.50 days to one division in about four days. Evidently the 

 presence of sodium silicate affected adversely the food material 

 upon which the Arcellas were feeding or else hindered the feeding 

 process or the rate of digestion and assimilation. 



h. Size and spine number 



Instead of an increase in spine number and length and in the 

 size of the shell as was expected, the immediate result of the 

 changed medium was a decrease in all these characters. The 

 pedigrees shown in figures 1, 2, and 3 illustrate the differences in 

 diameter and spine number very clearly. Many of the specimens 

 were badly crinkled, which, no doubt, accounts in part fOr their 

 smaller diameter; in others the binucleate condition was lost and 

 uninucleates appeared. Wherever this occurred a 1 in paren- 



