8 



permits me to refer. He finds that the serum of a young rat has just as 

 much power of attracting the leucocytes of the mouse as serum coming 

 from an adult rat. Nevertheless, the latter has the power of preventing 

 the development of anthrax in mice. Serum from a very young rat can 

 exhibit no such action. While acknowledging that the evidence is now 

 strongly in favour of Metchuikoff's contention that a negative may 

 eventually be changed to a positive chemiotactio state, it is hard to see 

 in this a complete elucidation of sundrv difficulties. Take, for instance, 

 a case that Metchnikoff has made his own— the case of recurrent fever. 

 Here, during the access of the fever, spirilla swarm in the blood, while 

 none are to be discovered within the leucocytes or other phagocytes ; 

 the crisis occurs, and in a very few hours, not a single free spirillum is 

 discernible, but a fair number are to be seen within the cells of the 

 spleen, there undergoing degeneration. It is difficult to comprehend 

 that a negative has been so abruptly changed into a positive chemiotaxis. 

 It is difficult not to opine that there is some factor added to and beyond 

 chemiotaxis and phagocytosis, determining the crisis in acute zymotic 

 disease. I have already laid stress on the fact that Metchnikoff freely 

 admits the possible action of some other factor or factors not as yet 

 included in his philosophy, and here it seems to me that other factors 

 must be invoked. Otherwise, the more one studies the phagocyte theory 

 and compares it with those which have been brought forward within the 

 last year or two to replace or augment it, the more, as I shall proceed to 

 show, one sees that it fulfils the conditions of a good theory and embraces 

 the widest circle of phenomena. 



II. 



The literature which has owed its origin to Metchnikoff's studies upon, 

 phagocytosis and immunity has already attained to such proportions that 

 it is impossible within the limits of a comparatively short article to do 

 more than touch upon some of the main points that have been raised, 

 and to attempt to show wherein is the strength and wherein the weakness 

 of the counter theories. 



Undoubtedly phagocy^-es are not always readily determinable. 

 Leucocytes in general, as all know who have studied films of pure blood 

 under the microscope, break up with extreme ease. Thus, unless special 

 precautions are taken, and special methods employed, it is quite possible 

 to see only free micro-organisms in the field, when, previous to trans- 

 mission to the slide, all had been confined within phagocytes. Thus, 

 whereas a few years ago there were many prepared to deny that 

 phagocytosis occurred, now, when the methods to be employed are more 





