256 SUSCEPTIBILITY AND IMMUNITY. 
hardly a single white corpuscle in the interior of which bacilli can- 
not be seen. Many corpuscles contain isolated bacilli only; others 
have thick masses in their interior, the nucleus being still recog- 
nizable ; while in others the nucleus can be no longer distinguished ; 
and, finally, the corpuscle may become a cluster of bacilli, breaking 
up at the margin—the origin of which one could not have explained 
had there been no opportunity of seeing all the intermediate steps 
between the intact white corpuscle and these masses” (Fig. 78). It 
will be noted that in the above quotation Koch affirms that the 
bacilli penetrate the leucocytes and multiply in their interior. Now, 
the theory of phagocytosis assumes that the bacilli are picked up by 
the leucocytes and destroyed in their interior, and that immunity de- 
pends largely upon the power of these “‘ phagocytes” to capture and 
destroy living pathogenic bacilli. 
The writer suggested this as an hypothesis as long ago as 1881, 
in a paper read before the American Association for the Advance- 
ment of Science, in the following language: 
“It has occurred to me that possibly the white corpuscles may 
have the office of picking up and digesting bacterial organisms which 
Fia. 78.—Bacillus of mouse septicemia in leucocytes from blood of mouse (Koch), 
by any means find their way into the blood. The propensity exhib- 
ited by the leucocytes for picking up inorganic granules is well 
known, and that they may be able not only to pick up but to assimi- 
late, and so dispose of, the bacteria which come in their way, does 
not seern to me very improbable, in view of the fact that amoeba, 
which resemble them so closely, feed upon bacteria and similar or- 
ganisms.” ? 
At a later date (1884) Metschnikoff offered experimental evi- 
dence in favor of this view, and the explanation suggested in the 
above quotation is commonly spoken of as the Metschnikoff theory. 
1“ A Contribution to the Study of Bacterial Organisms commonly found upon 
Exposed Mucous Surfaces and in the Alimentary Canal of Healthy Individuals.” II- 
lustrated by photomicrographs. Proceedings of the American Association for Ad- 
vancement of Science, 1881, Salem, 1882, xxx., 838-94. Also in Studies from the 
Biological Laboratory, Johns Hopkins University, Baltimore, vol. ii., No. 2, 1882. 
