42 J. MCA. KATER AND R. D. BURROUGHS. 



(2) For the work on hydrogen-ion concentration the ordinary 

 hay infusion \vas used and the P h w.i^ .iltered by either sodium 

 hydroxide or acetic acid. The hydrogen-ion concentration was 

 determined by the colorimetric method. 



(3) The problem of metabolic by-products was approached in 

 two different ways: first, by filtering the cultures through a 

 porcelain filter and replacing the medium with fresh hay infusion, 

 and, second, by removing the fluid from a thriving culture, in 

 the same way, and replacing with medium taken from an old 

 culture in which there were none active, all being encysted. In 

 the one case the by-products were kept at a minimum, in the 

 other they were increased beyond normal. 



(4) To obtain a low food content in the cultures the hay 

 infusion was replaced with distilled water and various per- 

 centages of distilled water and fresh hay infusion. 



It has become rather customary, in recent years, to carry on 

 work of this kind on organisms that have all been derived irom a 

 single individual. By this method one avoids comparing different 

 strains, but, on the other hand, the results are applicable only to 

 that particular strain, which probably can never be duplicated. 

 The organisms used in the present work were all derived from a 

 single culture, not from a single individual. It was thought, 

 at the beginning, that by running a great number of cultures 

 conclusions could be arrived at that would be applicable to the 

 species, and that minor variations could be smoothed out by 

 amount of data. The remarkable correspondence in the different 

 cultures of any one series has entirely justified this procedure. 

 This agreement has been so great that it has been possible to give 

 only one result for all cultures of a series, with only two ex- 

 ceptions. 



The condition of the cultures, in respect to active forms, is 

 designated by the following terms: very poor, poor, fair, good, 

 and excellent; and, with regard to cysts by: very few, few, some, 

 many, and very many. These indefinite terms will doubtless 

 arouse criticism. To this we can only reply that the differences 

 in both active and encysted forms in the different series was so 

 striking in all cases that they could easily be placed in their 

 proper class, and, secondly, that it is tin relationship which is of 

 importance and not the actual numbers. 



