184 PHAGOCYTOSIS 



become granular and then disappear. They also absorb or engulf 

 motile bacteria which may for a while continue to display their 

 motility. Digestion is performed by means of an acid fluid, as can 

 be demonstrated by feeding them on litmus or on substances colored 

 with neutral red. In some instances at least, digestion proceeds not 

 only in feebly acid, but also in neutral and feebly alkaline solution. 

 For this reason the digestive ferment is regarded as a trypsin rather 

 than a pepsin. 



Another phagocyte is the ameba, which by means of its pseudo- 

 podia surrounds and digests not only dead particles, but also micro- 

 scopic plants, animals and bacteria. Intracellular digestion in these 

 unicellular animals has been carefully studied with most interesting 

 results. They take into their bodies both dead and living bacteria, 

 algae and infusoria. It is easy to watch the behavior of living cells 

 after engulfment. Some amebas live exclusively on living cells and 

 some restrict their food to bacteria. Indeed, some are so particular 

 that they prefer one bacterial species, and when supplied with mixed 

 cultures they devour their favorite, leaving others unharmed. Mouton 

 studied a species of amebas which had been fed through many gen- 

 erations wholly on colon bacilli. 



It is especially worthy of note that when colon cultures are fed 

 to those amebas, the bacilli may be agglutinated or bunched outside 

 the amebas and entire clumps are engulfed at once. The similarity 

 between this process and the agglutination of typhoid bacilli with the. 

 serum of one ill with this disease is striking and significant. One is 

 justified in supposing that in each case the phagocytes (in typhoid 

 fever, the body cells) pour out a secretion which prepares the bacilli 

 for absorption. Of course, in the agglutination of typhoid bacilli 

 with serum, the phagocyte is not present, but remains in the body. 

 However, agglutination is not essential and single bacilli may be 

 absorbed and digested. In absorption, the bacteria are taken into a 

 vacuole and the process of intracellular digestion begins. When the 

 bacilli have been previously stained with neutral red, they are seen 

 to become cherry red in the vacuole, showing that the digestive fluid 

 is acid. The ferment is known as the ameba-enzyme. It may be 

 obtained by dissolving or extracting masses of amebas. It acts in 

 feebly acid, neutral, and feebly alkaline solutions. It readily digests 



