One of the most recent and the most comprehensive memoirs upon the terrible epidemic had been presented to the Academy of Sciences by M. de Quatrefages. One paragraph of this paper had forcibly struck Pasteur. M. de Quatrefages related that some Italian naturalists, especially Filippi and Cornalia, had discovered in the worms and moths of the silkworm minute corpuscles visible only with the microscope. The naturalist Lebert affirmed that they might always be detected in diseased silkworms. Dr. Osimo, of Padua, had even perceived corpuscles in some of the silkworms' eggs, and Dr. Vittadini had proposed to examine the eggs with a microscope in order to secure having sound ones. M. de Quatrefages only mentioned this matter of the corpuscles as a passing remark, being doubtful of its importance, and perhaps of its accuracy. This doubt might have removed from Pasteur's mind the thought of examining the significance of these little corpuscles, but, amid the general confusion of opinions, Pasteur was attracted to the study of these little bodies all the more readily because it related to an organic element which was visible only with the microscope. This instrument had already rendered such services to Pasteur in his delicate experiments on ferments, that he was fascinated by the thought of resuming it again as a means of research. I. On June 6, 1865, Pasteur started for Alais. The emotion he felt on the actual spot where the plague raged in all its force, in the presence of a problem requiring solution, caused him at once to forget the sacrifices he had made in quitting his laboratory at the École Normale. He determined not to return to Paris until he had exhausted all the subjects requiring study, and had triumphed over the plague.