STUDIES ON THE SERUM OF VACCINATED ANIMALS. 15 



varieties of bacteria. With greatest delicacy they have reacted to 

 a new chemical substance and each one takes up numerous Proteus 

 organisms, recognizable by their rod shape, but still refuses the 

 streptococci which remain scattered throughout the preparation 

 outside the cells. The protoplasm of the phagocytes acts rapidly 

 on the bodies of the bacteria that have been engulfed. These 

 Proteus bacilli soon manifest changes in their reaction to dyes: a 

 preparation from the exudate taken a short time after the second 

 injection or kept for a few hours in a moist chamber shows, with a 

 contrast stain of eosin and methylene blue, phagocytes containing 

 reddish rods (Proteus) ; outside the cells are seen numerous strep- 

 tococci stained blue. The organisms that have been taken up, then, 

 take an acid stain instead of the usual methylene blue. 



We may conclude, then, that the leucocytes of the peritoneal 

 exudate have been neither killed nor paralyzed by a toxin from the 

 streptococcus. Their faculty of engulfing has remained intact, 

 but they refuse to enter in contact with the streptococcus owing 

 to the fact that they receive a negative chemical stimulation from 

 this organism. 



It may be clearly seen from this experiment that phagocytes are 

 capable of choosing with great delicacy between organisms offered 

 them on account of their reaction to chemical substances. It is 

 then very probable that when an animal is inoculated with a given 

 bacterium the phagocytes first take hold of those individual or- 

 ganisms that are most attracting, that is to say, the least dangerous. 

 From this standpoint it is easy to understand that the increase in 

 virulence by passage through the animal body is due at least in part to 

 selection on the part of the phagocytes. The evolution of the in- 

 fection which has just been described gives evidence of this fact. 

 The injection of the relatively scanty culture of streptococcus 

 caused a gathering of leucocytes (it is known that the injection of 

 simple bouillon without the presence of bacterial products, also 

 causes this phenomenon). Leucocytes in the beginning take up a 

 certain number of bacteria, but they fail, although present in rela- 

 tively larger numbers than the bacteria, to absorb certain individuals 

 particularly endowed with repelling properties. These more viru- 

 lent organisms may multiply, and furnish new individuals of like 

 virulence, and similarly insusceptible to phagocytic action. 



