80 PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 



reaction. Through the action of this acid the compact mass of bac- 

 teria is broken into minute fragments, which ultimately mix with the 

 protoplasm as digested food. Although nothing further is definitely 

 known about it, it is quite probable that the product of this digestive 

 action is the formation of soluble peptones similar to the products of 

 proteid digestion in the higher animals. This is rendered the more 

 probable because of the extraction of a pepsin-like ferment from the 

 myxomycete Fulicjo varians by Krukenberg, and from the huge 

 ameboid rhizopod Pelomyxa palustris by Dixon and Hartog. 



The problem of the nature of the digestive processes in protozoa 

 has an interest in connection with other questions of more vital impor- 

 tance. The nature of the digestive reaction in phagocytes in response 

 to the food matters supplied are involved in the general subject of 

 intracellular digestion. While the initial experiments of Engelmann, 

 Metchnikoff, Le Dantec, Greenwood, and others showed that there 

 is an acid reaction in the gastric vacuoles of certain forms of protozoa, 

 their conclusion that digestion here is entirely due to the action of some 

 ferment-like pepsin acting in an acid medium were apparently pre- 

 mature. The extraction by Krukenberg from fuligo, and by Dixon 

 and Hartog from pelomyxa, of a digestive ferment which dissolves 

 proteid in an acid medium, undoubtedly lends support to their view. 

 But, on the other hand, Mouton ('02) extracted a digestive ferment 

 from ameba which dissolves gelatin and fibrin in an alkaline medium, 

 while Mesnil and Mouton ('03) extracted a similar ferment from para- 

 mecium. These observers, therefore, insist that the digestive fluid is 

 more like trypsin than like pepsin. 



An intermediate position was taken by Metalnikoff ('03), who, 

 on the basis of repeated observations, claimed that the reactions in 

 the paramecium vacuole are first acid and then alkaline. Feeding 

 paramecium with powdered alizarin, which is colored reddish violet 

 in an alkaline medium in which paramecium lives, he found that 

 the vacuoles are at first of this same color. In from five to fifteen 

 minutes the color changes from red to yellow, showing an acid reaction, 

 and this, after from ten to fifteen minutes more, is changed again to 

 the red, showing an alkaline reaction. Not all vacuoles are thus 

 colored, a few giving the alkaline reaction throughout. Metalnikoff 

 concluded, therefore, that proteid digestion in these protozoa follows 

 the same course as in higher animals, a ferment acting in an alkaline 

 medium following one which acts in an acid medium. 



Nierenstein, repeating these experiments, confirmed MetalnikofFs 

 observations, but came to the conclusion that the acid medium plays 

 no part in the actual digestion of the food, serving merely to kill the 

 living organisms taken in. Metalnikoff, however, in a later publication 

 maintains that the bacteria swell in the acid medium and thus undergo 

 the first steps in the process of digestion. These results differ to some 



