there were two cultures of silkworms, two broods (deux
chambrées): the one finished and ascended to the
heather, the other coming out of the fourth molt. The
first had gone along admirably. The worms had climbed
up all at one time, and appeared so vigorous that they
were preparing to make use of all the cocoons for the
egg-laying. The second had dragged along, and presented
a bad appearance; the worms were languishing, ate little,
and did not grow. The sequel proved that this appear-
ance was not deceptive: the harvest of cocoons was
almost a failure.

Now, on examining with the microscope the chrysalids
and the moths of the culture which had succeeded well,
corpuscles were found everywhere in them, while there
were corpuscles only exceptionally in the worms of the
bad brood. And this was not an exceptional fact, for,
by searching in the neighborhood, Pasteur found a mul-
tiplicity of similar cases.

What did this mean? The corpuscles and the disease
of silkworms were, therefore, two distinct things. Could
worms be very healthy and behave properly, like the
worms of the first culture, and nevertheless give cor-
puscular chrysalids? Could they be sick, like the worms
of the second, and not contain corpuscles? To-day we
know that if Pasteur did not find out this it was because
he investigated badly, confounding in his inexperience
two diseases. There is one in which the corpuscle plays
a rôle, another in which it does not. But Pasteur did
not know this, having only discovered it later. And, in
the meantime, the disturbing and imperious question
confronted him: what conclusion is to be drawn from the
preceding observation?

In order to decide, it was prudent to wait and see what
would become of the cocoons of the bad brood. In fact,
in studying them day by day, as they developed, Pasteur

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