4 



that others, with the extent and nature of whose action we are nt present 

 uimc(iuainted, aid in combating bacterial invasion. Among these may 

 be mentioned the febrile reaction. 



The wise discernment that further obsorTatiuns and the consideration 

 of a greater number of facts may lead to an expansion or modification of 

 the theory has, it would seem, prevented MetchnikoiT from placing it 

 in a succinct finite form, and so fossilising it. However, I believe that 

 the above, while not Metchuikoffs "ipsissima verba," is a full and 

 correct statement of his theory as it stands at present. In his last 

 "Etude 8ur I'immunite, ' which has come into my hands after going to 

 press, M. MetchnikofF introduces a further expansion of the theory ren- 

 dering it yet more complete. To this modification I shall refer in the 

 second portion of my paper. That the theory has shown itself capable 

 of expansion is evident by the stress now laid upon chemiotaxis as com- 

 pared with the standpoint of a little more tlian a year ago. It will be 

 well to state more fully the observations which have led to this expansion. 



Already, in 1883, in his paper upon iutracellular digestion as per- 

 formed by mesoderm cells of the invertebrata, MetchnikofF noted that 

 these did not take up indiscriminately every particle that came across 

 their path, but exercised a choice, incepting some and leaving others 

 untouched. And in the succeeding year, when discussing the relation- 

 ship of phagocytes to the anthrax bacilli, he attributed acquired 

 immunity to the " progressive habit exhibited by phagocytes of assimi- 

 lating substances which at first they avoided."* But until some law 

 based upon numerous determinations could be laid down as to this 

 selective action, these observations and the explanation of them could 

 not be regarded as adequate, and there remained a distinctly weak place 

 in the theory. 



It is to the botanists that we owe the first advance. Engelmann, in 

 1881, determined that certain chemical substances excite the lower motile 

 organisms and showed that oxygen energetically attracts certain bacteria. 

 Later, Stahl made a series of important investigations upon the Plasmo- 

 dium of iEthalium septicum, an organism living in tan pits — in infusion 

 of oak bark. Placed on the surface of a glass moistened with water, 

 this Plasmodium remains motionless until a drop of infusion of oak bark 

 is allowed to fall near to it ; now the plasmodium moves rapidly towards 

 the infusion. Other liquids bring about a different result — thus, a drop 

 of one-half per cent solution of glucose, placed so as to be just in contact 

 with the organism, causes it to move rapidly away from the drop; and so 

 it is with solutions of different salts. What is of especial interest is that 

 Stahl found that if this experiment with, for example, the glucose solu- 

 tion, be repeated frequently, the plasmodium no longer flies away; but, on 



* Slctchnlkoff, Virehoto't Archiv, Bd. XCVII, p. 518, 1884. 



