IMMUNITY AND SUSCEPTIBILITY. 71 



particles by certain of the body-cells which are called 

 phagocytes. This activity of the cells toward inert 

 particles had been observed by Virchow as early as 1840, 

 and toward living bacteria by Koch in 1878, but was not 

 carefully studied until 1884. Metschnikoff divides the 

 phagocytes into fixed phagocytes, comprising the fixed 

 connective-tissue cells, endothelium, etc., and the free 

 phagocytes, which are the leucocytes. The terms u phag- 

 ocyte" and "leucocyte" are not to be regarded as synon- 

 ymous in this connection ; all leucocytes are not phag- 

 ocytic, the lymphocyte having never been observed to 

 take up bacteria. 



It is obvious that only those cells can be phagocytic 

 which are without a resisting cell-wall and possess 

 ameboid movement. When an ameba, in a liquid con- 

 taining numerous diatoms and bacteria, is watched 

 through the microscope, an interesting phenomenon is 

 observed. The ameba will approach one of the vege- 

 table cells, even though it may be at a distance, will 

 apprehend and surround it, and within the animal cell 

 the vegetable cell will be digested and assimilated. The 

 ameba has no eyes, no nose, no volition, and, so far as 

 we can determine, no nervous apparatus which gives 

 it tactile sense, yet it will approach the particle fitted 

 for its use and swallow it. The attraction which draws 

 the cells together has been called by Peffer chemotaxis, 

 chemiotaxis, or chemotropism. 



Chemotaxis is the exhibition of an attractive force 

 between cells and their nutriment, ameboid cells and 

 food-particles, and sometimes between ameboid cells and 

 inert particles. This attractive force, when operating so 

 as to draw the ameba to the particle it will devour, is 

 further named positive chemotaxis in order to distinguish 

 it from a repulsive force sometimes exerted causing the 

 ameboid cells to fly from an enemy, as it were, and which 

 is called negative chemotaxis. 



The force that operates and guides the ameba in its 

 movements is exactly the same as that which governs the 



