r subjected the development of the corpuscles to a searching
investigation, and followed out with admirable skill and completeness
the various modes by which the plague was propagated. From moths
perfectly free from corpuscles he obtained healthy worms, and
selecting 10, 20, 30, 50, as the case might be, he introduced into the
worms the corpusculous matter. It was first permitted to accompany the
food. Let its take a single example out of many. Rubbing up a small
corpusculous worm in water, he smeared the mixture over the
mulberry-leaves. Assuring himself that the leaves had been eaten, he
watched the consequences from day to day. Side by side with the
infected worms he reared their fellows, keeping them as much as
possible out of the way of infection. These constituted his 'lot
temoin,'--his standard of comparison. On April 16, 1868, he thus
infected thirty worms. Up to the 23rd they remained quite well. On the
25th they seemed well, but on that day corpuscles were found in the
intestines of two of them. On the 27th, or eleven days after the
infected repast, two fresh worms were examined, and not only was the
intestinal canal found in each case invaded, but the silk organ itself
was charged with corpuscles. On the 28th the twenty-six remaining
worms were covered by the black spots of _pebrine_. On the 30th the
difference of size between the infected and non-infected worms was
very striking, the sick worms being not more than two-thirds of the
bulk of the healthy ones. On May 2 a worm which had just finished its
fourth moulting was examined. Its whole body was so filled with the
parasite as to excite astonishment that it could live.

The disease advanced, the worms died and were examined, and on May 11
only six out of the thirty remained. They were the strongest of the
lot, but on being searched they also were found charged with
corpuscles. Not one of the thirty worms had escaped; a single meal had
poisoned them all. The standard lot, on the contrary, spun their fine
cocoons, two only of their moths being proved to contain any trace of
the parasite, which had doubtless been introduced during the rearing
of the worms.

As his acquaintance with the subject increased, Pasteur's desire for
precision augmented, and he finally counted the growing number of
corpuscles seen in the field of his microscope from day to day. After
a contagious repast the number of worms containing the parasite
gradually augmented until finally it