PHAGOCYTOSIS 191 



seen projecting their pseudopodia, surrounding and engulfing bacteria. 

 The bacterium is taken into a vacuole filled with clear fluid. If the 

 bacterium be motile its movements may continue in the vacuole, but 

 soon active movement ceases. Vibrios crumble into granules and 

 finally disappear. Bacilli become pale and transparent, then are seen 

 only in shadow and finally this is lost. Cocci swell up, become trans- 

 parent and then are lost to sight. However, some bacteria, such as the 

 tubercle and leprosy bacilli, remain in the microphage quite indefi- 

 nitely without recognizable change. The same is true of certain 

 spores. In these cases, there is no digestion, but the body is protected 

 from the multiplication of the organism. Phagocytes take up litmus 

 granules which show no change in color after inclusion, but bacilli 

 stained with neutral red become cherry red in the vacuole, showing 

 that the fluid is feebly acid. While this is the rule, in some phago- 

 cytes, as in the giant cells containing tubercle bacilli, the reaction is 

 plainly alkaline. There can be no question that phagocytes engulf and 

 finally destroy bacteria. A phagocyte, which has recently engulfed a 

 living bacillus, when brought in a hanging drop under the microscope, 

 soon dies itself and the liberated bacterium may begin to multiply, 

 but when the inclusion has continued for a longer time, the liberated 

 bacterium does not grow. There has been some discussion over the 

 question of the identity of the digestive ferments of the mononuclear 

 and the polynuclear phagocytes. A priori we should expect them to 

 be different, and the weight of evidence supports this view. Exudates 

 especially rich in mononuclear leukocytes are active as hemolytic 

 agents but have only feeble germicidal action, while those rich in 

 polynuclear leukocytes have no hemolytic action, but are powerful 

 bactericides. Is this difference in both the complement (alexin or 

 cytase) and the fixator (sensitizer or amboceptor) or in the latter 

 only? This question has given rise to some marked differences of 

 opinion but a positive conclusion is not justified by any undisputed 

 evidence and we will leave it for future investigators to answer. 



In concluding this part of our subject, it must be admitted that 

 Metschnikoff and those who have labored in the same field have 

 demonstrated that phagocytosis is an important factor in natural 

 immunity. 



