t of innumerable spores,
which have been thrown off in all directions by a minute fungus called
_Empusa muscoe_, the spore-forming filaments of which stand out like a
pile of velvet from the body of the fly. These spore-forming filaments
are connected with others which fill the interior of the fly's body like
so much fine wool, having eaten away and destroyed the creature's
viscera. This is the full-grown condition of the _Empusa_. If traced back
to its earliest stages, in flies which are still active, and to all
appearance healthy, it is found to exist in the form of minute corpuscles
which float in the blood of the fly. These multiply and lengthen into
filaments, at the expense of the fly's substance; and when they have at
last killed the patient, they grow out of its body and give off spores.
Healthy flies shut up with diseased ones catch this mortal disease, and
perish like the others. A most competent observer, M. Cohn, who studied
the development of the _Empusa_ very carefully, was utterly unable to
discover in what manner the smallest germs of the _Empusa_ got into the
fly. The spores could not be made to give rise to such germs by
cultivation; nor were such germs discoverable in the air, or in the food
of the fly. It looked exceedingly like a case of Abiogenesis, or, at any
rate, of Xenogenesis; and it is only quite recently that the real course
of events has been made out. It has been ascertained, that when one of
the spores falls upon the body of a fly, it begins to germinate, and
sends out a process which bores its way through the fly's skin; this,
having reached the interior cavities of its body, gives off the minute
floating corpuscles which are the earliest stage of the _Empusa_. The
disease is "contagious," because a healthy fly coming in contact with a
diseased one, from which the spore-bearing filaments protrude, is pretty
sure to carry off a spore or two. It is "infectious" because the spores
become scattered about all sorts of matter in the neighbourhood of the
slain flies.

The silkworm has long been known to be subject to a very fatal and
infectious disease called the _Muscardine_. Audouin transmitted it by
inoculation. This disease is entirely due to the development of a fungus,
_Botrytis Bassiana_, in the body of the caterpillar; and its
contagiousness and infectiousness are accounted for in the same way as
those of the fly-disease. But, of late years, a still more serious
epizootic has appeared a