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86 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

articulated threads, one kind occurred in i^pirillum form, eight
were motile, fourteen immotile.
Formation of spores was observed in three cases only, the
others appearing to develop by fissation alone ; eight liquefied
gelatine. Fourteen were cultivated on slices of potato, five of
them growing rapidly, one in particular soon spreading over the
whole surface of the slice and liquefying it completely to a depth
of one to two millimeters. The others did not thrive well on
potato. Of fifteen kinds which were cultivated on the white of
egg, four grew well, evolving considerable quantities of sulphu-
retted hydrogen (HgS) and later ammonia (XH3). The white
of egg was converted into a semi-transparent pasty mass, which

FiQ. 4 3. Fig. 44.

gradually disappeared. Seven grew slowly, and four not at all.
On sections of normal undecalcified dentine certain bacteria
seemed able to exist for some days, presumably till the organic
substance exposed on the surface of the section was consumed.
No growth occurred on enamel.
These bacteria showed, as far as they were examined, differ-
ences in relation to the action of atmospheric air. Ten of them
were strictly obligatory aerobic, growing only with free access
of air; four were not strictly obligatory aerobic, growing better,
but not exclusively, when air was admitted ; eight were faculta-
tive aerobic, that is, they seemed to grow equally well w^ith or
without oxygen.
If from the deposits about the necks of the teeth we make line-