CHEMOTAXIS 249 



a bacterial infection, and approach and engulf the bacteria 

 in the same manner as they do other foreign particles, and 

 so rid the body of the invaders (Plate I., a and 6). 



The migration of the leucocytes towards the scene of 

 action is explained by Metchnikoff on the hypothesis that 

 they are attracted by the chemical substances elaborated 

 by the bacteria which exert upon them what he termed 

 " positive chemotaxis." In this event the bacteria are 

 removed by the leucocytes, and general infection and 

 death do not occur. In other cases, unfortunately, the 

 bacterial chemical products repel, or perhaps it is more 

 correct to say do not attract, the leucocytes, and " nega- 

 tive chemotaxis " occurs, so that the bacteria are free to 

 grow and multiply, and general infection ensues. All living 

 cells are influenced by positive and negative chemotaxis. 

 If a fine capillary tube containing peptone solution be 

 introduced into a suspension of bacilli, e.g. B. fluorescens 

 liquefaciens, under a cover-glass, and watched microscopi- 

 cally, the bacilli will be attracted to the tube and soon 

 invade its lumen. If, however, a weak acid be substituted 

 for the peptone water, the bacilli will be repelled. Capil- 

 lary tubes filled with the same solutions and sunk in the 

 tissues will exert a similar action upon leucocytes ; the 

 peptone tube will be invaded by leucocytes, the acid one 

 will contain none. The process by which the bacteria 

 are ingested by the leucocytes can be similarly watched. 

 The leucocytes which act in this manner are termed 

 phagocytes, and they are of two classes the macrophages, 

 the large mononuclear leucocytes, and the smaller micro - 

 phages, the polymorphonuclear leucocytes. Certain of 

 the tissue cells and endothelial cells also possess phago- 

 cytic properties. The importance of phagocytosis is also 

 shown by the fact that, while in ordinary susceptible 

 rabbits infection with anthrax is followed by a feeble 

 phagocytosis and the animals succumb, in rabbits vac- 



