When Pasteur started for Alais (June 16, 1865), entrusted with this scientific mission by the Minister of Agriculture, his mind saw but that one point of interrogation, “What caused these fatal spots?” On his arrival he sympathetically questioned the Alaisians. He received confused and contradictory answers, indications of chimerical remedies; some cultivators poured sulphur or charcoal powder on the worms, some mustard meal or castor sugar; ashes and soot were used, quinine powders, etc. Some cultivators preferred liquids, and syringed the mulberry leaves with wine, rum or absinthe. Fumigations of chlorine, of coal tar, were approved by some and violently objected to by others. Pasteur, more desirous of seeking the origin of the evil than of making a census of these remedies, unceasingly questioned the nursery owners, who invariably answered that it was something like the plague or cholera. Some worms languished on the frames in their earliest days, others in the second stage only, some passed through the third and fourth moultings, climbed the twig and spun their cocoon. The chrysalis became a moth, but that diseased moth had deformed antennæ and withered legs, the wings seemed singed. Eggs (technically called seed) from those moths were inevitably unsuccessful the following year. Thus, in the same nursery, in the course of the two months that a larva takes to become a moth, the pébrine disease was alternately sudden or insidious: it burst out or disappeared, it hid itself within the chrysalis and reappeared in the moth or the eggs of a moth which had seemed sound. The discouraged Alaisians thought that nothing could overcome pébrine. Pasteur did not admit such resignation. But he began by one aspect only of the problem. He resolved to submit those corpuscles of the silkworm which had been observed since 1849 to microscopical study. He settled down in a small _magnanerie_ near Alais; two series of worms were being cultivated. The first set was full grown; it came from some Japanese seed guaranteed as sound, and had produced very fine cocoons. The cultivator intended to keep the seed of the moths to compensate himself for the failure of the second set, also of Japanese origin, but not officially guaranteed. The worms of this second series were sickly and did not feed properly. And yet these worms, seen through the microscope, only exceptionally presented corpuscles; whilst Pasteur was surprised to find some in almost every moth or chrysalis from the prosperous nursery. Was it then elsewhere than in the worms that the secret of the pébrine was to be found?