PHAGOCYTOSIS 295 



the study of the phenomenon more promising than any of the others 

 so far offered. 



It is true, on the other hand, that such a theory in no way ac- 

 counts for the apparently selective positive chemotaxis which is 

 exerted by different substances. Thus the preponderance of poly- 

 nuclear leukocytes in foci and serous cavities containing organisms 

 like staphylococci, meningococci, streptococci, and others is in con- 

 trast to the lymphocytic accumulation in the pleural, subarachnoid', 

 and peritoneal spaces infected with tubercle bacilli. Some writers 

 have spoken, therefore, of active and passive leukocytosis according 

 to whether or not the cells attracted seemed to possess ameboid 

 motility. That surface tension phenomena alone do not account for 

 this is clear. But it must be remembered that even tubercle bacilli, 

 though eventually attracting few polynuclears and many lympho- 

 cytes, will cause an active polynuclear accumulation in the perito- 

 neum and pleura when first injected, and are actively phagocyted. 

 Later when the lesion is established and the bacilli are lodged in the 

 tissues the polynuclears give way to the lymphocytes, which even 

 then never accumulate in such proportion as do the microphages in 

 acute suppurative lesions. It may well be that the chemotaxis origi- 

 nally attracting the polymorphonuclear leukocytes is the same in 

 every case, but that a continued irritant, especially one well sur- 

 rounded by tissue elements as are the organisms within the tubercles, 

 may cease to exact any chemotactic influence, the accumulation of in- 

 active lymphocytes possibly being due to a progressive death of these 

 cells carried into the neighborhood of the lesion by the normal circu- 

 lation of the lymph. 



