440 ROBERT W. HEGNER 



spicuous change brought about by the presence of sodium sili- 

 cate is the almost complete loss of spines. The color of the 

 shell, which becomes brown in a normal medium remains a pale 

 greenish yellow in a sodium silicate solution. Specimens reared 

 in sodium silicate and then returned to a normal medium regain 

 the fission rate, size, spine length, and color or characteristic of 

 the race. 



c. Specimens of Arcella dentata are able to grow and repro- 

 duce in a medium containing from 0.25 to 1 per cent of alcohol. 

 Alcohol, however, is shown, to be injurious, as indicated by the 

 retarded fission rate and irregularities in the shells of the 

 offspring. 



d. Several experiments seem to indicate that temperature 

 influences the length of the spines of Arcella dentata, and that 

 the lower the temperature the smaller the spines become. 



e. Wild specimens of Arcella polypora that possessed a bent, 

 oval shell with an oval mouth opening gave rise under labora- 

 tory conditions to offspring with a fiat circular shell and a 

 circular mouth opening. The bent, oval condition is probably 

 due to an unknown environmental factor. 



/. The environmental factors to which specimens of Arcella 

 have been subjected cause distinct variations from the normal 

 racial conditions, but these modifications persist only so long 

 as the modifying factors are operative. No heritable diversi- 

 ties were observed that were due to the changed conditions. 

 The experiments bring out no data that affect the results 

 obtained by Jennings, Root, and the writer in isolating heritably 

 diverse lines within families of fresh-water rhizopods during 

 vegetative reproduction. 



