The presence of a parasite having been brought to public notice in the blood of animals suffering from splenic fever, at the very time when Pasteur had shaken the belief in spontaneous generation, people grew accustomed to the idea that the stricken animals might have contracted the germs of the malady from the outer world, without any spontaneous birth, strictly so called, of the disease. This opinion was strengthened by a knowledge of the spores of the splenic bacillus. Pasteur, aided by Messieurs Chamberland and Roux, commenced experiments with a view to solving this difficult etiological question. The first experiments took place in the fields of a farm at the village of St. Germain, near Chartres. Several groups of sheep were fed on lucern grass which had been sprinkled with artificially-reared splenic fever bacteria, or with their germs or spores. Although all the sheep of the same group absorbed an immense number of the spores of the parasite, many survived, even after being visibly affected. Those that died showed all the symptoms of what is called spontaneous splenic fever. The period of incubation lasted as long as eight or ten days, although, in its latter stages, the disease exhibited those startling features which have caused a belief that the incubating period is a very short one--short, that is to say, for those conditions of contagion where the parasite is not deposited in its pure state under the animal's skin. But if prickly plants (notably the pointed ends of dried thistle leaves, or beards of barley blades cut into little bits about a centimeter in length) were added to this infected food, the mortality increased to a striking extent. On examination after death, the lesions of these animals were found to be similar to those observed in sheep which were attacked by splenic fever in sheds, or which died of the disease in the open fields. From that time forward, the idea which had been predominant in the minds of Pasteur and his fellow-workers during all their inquiries, was materially strengthened. They were convinced that the animals which died of blood disease in the department of Eure et Loire had been infected by germs or spores of the splenic microbe contained in their food; but the question remained, Whence came these germs?