CHEMOTAXIS 183 



forward until checked by any one of several influences, as pointed out 

 by Wells, as (1) Until the chemotactic substance has been used up or 

 removed, or from any of the causes that terminate inflammation; (2) 

 the leukocytes may reach a point where the chemical stimulant is so 

 generally diffused that surface tension is decreased equally in all di- 

 rections and motility stops; (3) the leukocytes may reach a place where 

 toxins or other chemical substances coagulate their cytoplasm or fer- 

 ments cause their solution; (4) they may be blocked by a dense wall of 

 leukocytes and other cells while being held fixed by the chemical attrac- 

 tion that diffuses through this wall. These factors would explain the 

 formation of the wall of leukocytes about an area of infection. When, 

 for example, the abscess has ruptured or has been incised, with removal 

 of the chemotactic substances, there may be less chemotactic substances 

 in the center of the inflamed area than there is further out; hence the 

 leukocytes will move away from the center toward the periphery, follow- 

 ing the chemotactic substances back into the blood-vessel and lymph- 

 stream. This would explain the dispersion of living leukocytes at the 

 close of an inflammatory process. 



General leukocytosis can be explained equally well by assuming that 

 the chemotactic substances from the area of inflammation, reaching the 

 blood-stream, pass through the bone-marrow, lowering surface tension 

 and attracting leukocytes into the blood-stream as long as it contains 

 more chemotactic substances than the marrow. 



The exact chemical nature of chemotactic substances is unknown. 

 In bacterial infection the toxins, and especially the protein of dead micro- 

 organisms, are regarded as mainly responsible for the occurrence of posi- 

 tive chemotaxis. Chemotaxis and phagocytosis of chemically inert 

 particles, such as coal-dust, stone-dust, and pigments, are more difficult 

 to explain on this physical basis of alteration in surface tension. It is 

 probable that the death of tissue-cells, brought about by these materials, 

 may produce the chemical stimulant responsible for a mild but definite 

 chemotactic influence. Although the movement of amebae and similar 

 higher animals cannot be fully explained on this physical basis, the 

 surface tension theory best explains leukocytic movement. Although 

 the ameba may possess some special property that endows it with the 

 power of selecting and engulfing a food particle, it would appear to be 

 entirely unreasonable to assume that a simple, undifferentiated, and 

 naked leukocyte possesses similar powers. The physical theory, there- 

 fore, appears to be the most reasonable offered in explanation of the 

 ameboid movements of these simple cells. 



