REVERSION IN ORIENTATION TO LIGHT 511 



owing largely to the disappearance of the organisms in my cul- 

 tures before sufficient tests were made. Ten distinct tests were 

 made on two different days. In all of these tests the colonies 

 were thoroughly washed in pure distilled water so as to remove 

 all traces of the culture solution before the acid was added. 

 The colonies were, under the conditions of the experiments, 

 definitely negative in the pure water. In seven there was no 

 indication of reversion in orientation. In one of the remaining 

 tests some of the colonies probably became positive and in the 

 other two a large majority of them clearly became positive. 

 In these two tests the colonies had been in pure water overnight. 

 They were tested in fresh pure water, but I am not certain as to 

 whether or not they were washed in transferring them from the 

 water in which they had been during the night to the fresh water. 

 If pure water containing colonies is left in a watch-glass for some 

 hours, it ordinarily becomes slightly alkaline. It is therefore 

 possible that the solution in which these colonies were tested was 

 slightly alkaline, and it may be that this is the reason why rever- 

 sion was obtained in these tests and not in the others. It is 

 consequently fairly certain that the reversion from negative to 

 positive orientation in Spondylomorum is not due to the action on 

 the organisms of acid or free hydrogen ions, as is maintained by 

 some investigators. 



If this is true, reversion must be associated either with certain 

 chemical compounds produced by the action of the acids, as, 

 for example, salts, or with the concentration of the hydroxyl ions. 

 There is no evidence at hand in favor of the former supposition, 

 as will be demonstrated presently, but there is a certain amount of 

 evidence in favor of the idea that reversion is associated with the 

 concentration of hydroxyl ions. This evidence we shall now 

 present. Under certain conditions, changes in the alkalinity of 

 the culture solution produced marked effects on the sense of 

 orientation; under others it did not. We shall discuss the latter 

 first. 



