IMMUNITY AND SUSCEPTIBILITY. 63 



Roser suggested a relation between immunity and the 

 already familiar phenomenon of phagocytosis. Stern- 

 berg in the United States and Koch in Germany observed 

 the same thing, but little real attention was paid to the 

 subject until 1884, when Metchnikoff appeared, with his 

 careful observations upon the daphnia, as the great cham- 

 pion of the theory which is now known as * ( Metchni- 

 koff 's theory of phagocytosis." 



Phagocytosis is the swallowing or incorporating of 

 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. Metchnikoff divides the 

 phagocytes into fixed phagocytes, comprising the fixed 

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

 phagocytes, which are the leucocytes. The terms "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 



