at the end of three or four hours they attained from ten to twenty
times their original length. At the end of a few additional hours
they had formed filaments in many cases a hundred times the length of
the original rods. The same filament, in fact, was frequently
observed to stretch through several fields of the microscope.
Sometimes they lay in straight lines parallel to each other, in other
cases they were bent, twisted, and coiled into the most graceful
figures; while sometimes they formed knots of such bewildering
complexity that it was impossible for the eye to trace the individual
filaments through the confusion.

Had the observation ended here an interesting scientific fact would
have been added to our previous store, but the addition would have
been of little practical value. Koch, however, continued to watch the
filaments, and after a time noticed little dots appearing within them.
These dots became more and more distinct, until finally the whole
length of the organism was studded with minute ovoid bodies, which lay
within the outer integument like peas within their shell. By-and-by
the integument fell to pieces, the place of the organisms being taken
by a long row of seeds or spores. These observations, which were
confirmed in all respects by the celebrated naturalist, Cohn of
Breslau, are of the highest importance. They clear up the existing
perplexity regarding the latent and visible _contagia_ of splenic fever;
for in the most conclusive manner, Koch proved the spores, as
distinguished from the rods, to constitute the _contagium_ of the fever
in its most deadly and persistent form.

How did he reach this important result? Mark the answer. There was
but one way open to him to test the activity of the _contagium_, and
that was the inoculation with it of living animals. He operated upon
guinea-pigs and rabbits, but the vast majority of his experiments were
made upon mice. Inoculating them with the fresh blood of an animal
suffering from splenic fever, they invariably died of the same disease
within twenty or thirty hours after inoculation. He then sought to
determine how the _contagium_ maintained its vitality. Drying the
infectious blood containing the rod-like organisms, in which, however,
the spores were not developed, he found the _contagium_ to be that which
Dr. Sanderson calls 'fugitive.' It maintained its power of infection
for five weeks at the furthest. He then dried blood containing t