IMMUNITY AND SUSCEPTIBILITY. 71

particles by certain of the body-cells which are called
phagocytes. This activity of the cells toward inert
particles had been observed by Virchow as early as 1840,
and toward living* bacteria by Koch in 1878, but was not
carefully studied until 1884. Metsclmikoff divides the
phagocytes into fixed phagocytes, comprising- the fixed
connective-tissue cells, eiidotlielitun, etc., and the free
phagocytes, which are the leucocytes. The terms u phag-
ocyte n and u leucocyte'7 are not to be regarded as synon-
ymous in this connection ; all leucocytes are not phag-
ocytic, the lymphocyte having never been observed to
take up bacteria.

It is obvious that only those cells can be phagocytic
which are without a resisting cell-wall and possess
ameboid movement. When an ameba, in a liquid con-
taining numerous diatoms and bacteria, is watched
through the microscope, an interesting phenomenon is
observed. The ameba will approach one of the vege-
table cells, even though it may be at a distance, will
apprehend and surround it, and within the animal cell
the vegetable cell will be digested and assimilated. The
ameba has no eyes, no nose, no volition, and, so far as
we can determine, no nervous apparatus which gives
it tactile sense, yet it will approach the particle fitted
for its use and swallow it. The attraction which draws
the cells together has been called by Peffer cheniotaxis,
chemiotaxis, or chenwlropism.

Chemotaxis is the exhibition of an attractive force
between cells and their nutriment, ameboid cells and
food-particles, and sometimes between ameboid cells and
inert particles. This attractive force, when operating so
as to draw the ameba to the particle it will devour, is
further named positive chcmotaxis in order to distinguish
it from a repulsive force sometimes exerted causing the
ameboid cells to fly from an enemy, as it were, and which
is called negative chcwotaxis*

The force that operates and guides the ameba in its
movements is exactly the same as that which governs the