n 



nor need I give a resume of the separate researches of later yeure. It will 

 he sufficient to state his theory as it at present stands, amplifying such 

 portions of it as are seen to need fuller treatment. And the theory may 

 be stated as follows : — 



(1) That two forms of leucocytes, present in the blood and lympii, 

 the largo uninuclear " macrophages " and the sn^aller " microphages " 

 which tend to the raultinuclear, are, under certain conditions, capable of 

 incepting such bacterid as have gained entry into the system. 



(2) That the splenic corpuscles, endothelial colls, and sundry other 

 cells of mesodermal origin, possess the same property, though it is 

 called into action to a less extent. 



(3) That these phagocytes incept and destroy living and active 

 microbes. 



(4) That the more acute the disease, the less the phagocytosis that 

 can be determined ; the more chronic, the more extensive the phagocy- 

 tosis ; or otherwise : the more virulent and rapidly proliferating the 

 microbes, the less the potentiality of the phagocytes to incept and 

 destroy them, and vice versa. 



(5) That this potentiality of the phagocytes depends upon the 

 " chemiotaxis " exerted by the jjroducts of bacterial growth. In the 

 case of the more vii'ulent microbes the leucocytes are repelled from the 

 focus of infection ; there is a negative chemiotaxis, and thus, in the 

 absence of phagocytosis, the proliferation of the microbes takes place 

 without hindrance ; whereas, the less virulent microbes and their pro- 

 ducts attract the leucocytes ; they exert a 2}ositive chemiotaxis, so that 

 there is an emigration of leucocytes through the capillary walls to the 

 focus of infection, and the leucocytes taking up the microbes arrest the 

 infective process. 



(G) That the leucocytes and other mesodermal cells may become 

 accustomed, and eventually attracted, to substances by which at first 

 they are repelled, and thus a negative may be transformed into a positive 

 chemiotaxis. Hence, as in acute zymotic diseases, the microbes may at 

 first rapidly proliferate, and phagocytosis manifest itself only after a 

 certain period, leading then to resolution of the disease. 



(7) That the cells having once acquired positive chemiotactic properties 

 in relation to the products of any specific microbe retain and transmit 

 these properties through a series of coll generations, the length of which 

 varies according to the microbe, the extent of the primary reaction, and 

 the idiosyncrasies of the individual. 



(8) That, consequently, immunity and the cure of disease are mainly 

 brought about by the activity of special cells, the i^agocytes, — being 

 primarily dependent upon the attraction existing between these cells and 

 the products of bacterial metabolism. 



(9) Yet while this is the main factor it is quite possible, nay, probable, 



