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The cocci are rather small, measuring about o. 7 // in
diameter. When examined in a delicately-stained con-
dition the organisms may be seen to consist of hemi-
spheres separated from each other by a narrow interval.
The contiguous surfaces are flat, thus differing from
the gonococcus, whose contiguous surfaces are concave.
The grouping is not very characteristic. In both liquid
and solid culture-media the organisms either occur in
solid masses or are evenly distributed. It is only in the
organs or tissues of a diseased animal that it is possible to
say that a true Staphylococcus grouping is present.

The organism stains brilliantly with aqueous solu-
tions of the anilin dyes. In tissues it can be beautifully
stained by Gram's method.

The staphylococci grow well either in the presence or
absence of oxygen at a temperature above 18° C., the
most rapi.d development being at about 37° C. Upon the
surface of gelatin plates small whitish points can be
observed in forty-eight hours (Fig. 51). These rapidly

FIG. 51.—Staphylococcus pyogenes aureus: colony two days old, seen upon an
agar-agar plate;   x 40 (Heim).

extend to the surface and cause extensive liquefaction.
Hand in hand with the liquefaction is the formation of
an orange color, which is best observed at the centre of
the colony. Under the microscope the colonies appear