ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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bo found in the cells, and the results were of the most serious kind, in 
spite of the intense phagocytosis induced by the microbes of these 
diseases. This fact proved that the phagocytes and all the other means 
of defence were, under certain conditions, and at certain times, powerless 
to effect any good results ; they had done their best to take up the 
microbe, but these had adapted themselves to the interior of the cells, 
and had conquered. It was not sufficient that the microbes should be 
eaten up, it was essential that they should also be digested by the 
phagocytes. Even in those cases where the struggle was going against 
the human organism, these cells still were the aggressors. It had been 
frequently observed in tuberculosis and leprosy that the bacilli had been 
killed in the interior of certain of these cells. The theory asserted that 
a struggle occurred between the microbes and the cells, but it did not 
imply that the bacilli always won the day. Phagocytosis only occurred 
in immune animals ; in animals susceptible to the disease it was either 
not to be observed, or it was incomplete. 
He then proceeded to discuss the question whether immunity was 
the consequence of this power of the cells to digest the virulent microbes. 
As had been said, the cells of a refractory animal took up the microbes, 
which, it would appear, under favourable circumstances remained inert 
in the interior of the cells. 
Numerous facts had been alleged to show that the microbes at the 
time they were taken up by the phagocytes were not degenerated, but 
were, on the contrary, in acoudition of full activity. Thus, to take only 
one example, it had been found that in frogs the bacilli which had been 
taken up by the leucocytes remained alive within the protoplasm of the 
cell ; this was apparent from their movements. In lymph taken from 
the body of a pigeon numerous bacilli were to be seen imprisoned in the 
leucocytes, and these bacilli could be watched growing, actually under 
the eye of the observer, within the interior of dead phagocytes ; they 
could be seen to elongate, to push out the protoplasm, distort the 
form of the cell, and finally to make their escape. Another demon- 
stration of the importance of the action of phagocytes was afforded by the 
fact that even in immune animals the microbes were found to increase 
when kept out of the reach of the leucocytes ; thus, if a rabbit were 
inoculated in the anterior chamber of the eye, where there were no cells, 
the bacteria grew freely, and their development was only checked when 
the leucocytes had after a time migrated in large numbers, and began to 
take the microbes into their interior. It thus appeared that phago- 
cytosis was a very general phenomenon, and one which was very effica- 
cious in checking the advance of the organisms ; when it failed, the 
individual succumbed to the virulence of the bacteria. The question 
remained, What was the mysterious force which attracted the cells 
towards the microbes? Why were the leucocytes, which in immune 
animals destroyed the microbes, incapable of seizing upon them in 
non-immune animals ? 
In 1883, Metschnikoff propounded his theory of phagocytosis. This 
theory rested on two assumptions ; first, that the cells were attracted to 
the microbes in virtue of a special sensibility manifested towards all 
foreign bodies introduced into the tissues ; the second was that this 
power of seizing upon the virulent microbes in immune animals 
