﻿THE AIR OF TOWNS. 35 



silkworm was characterized by the appearance of black spots. It 

 showed itself, moreover, iu the stunted and unequal growth of the 

 caterpillars, in the languor of their movements, fastidiousness in regard 

 to food, and premature death. The black spots which appeared 

 through the transparent skin of the silkworm had been examined and 

 proved to be living cor]3uscles. These gradually took possession of 

 the intestinal canal and spread, finally filling the silk cavities so that 

 the worm when its appointed time came went automatically through 

 the process of spinning, but without j)roducing any silk. This was 

 already known when Pasteur came ui^on the scene. By careful and 

 constant use of the microscope he followed the life of these fatal 

 corpuscles. 



The life of the silkworm is like that of any ordinary caterpillar. 

 When hatched from the egg the worm, which is not much larger than 

 a pin's head, begins to feed and grow, casting his skin from time to 

 time when his coat gets too tight until, having attained a length of 

 almost 2 inches, he suddenly stops feeding and, having found a suit- 

 able spot, he begins to spin his silk web around him. 



Within the cocoon he remains dormant for a time in the chrysalis 

 state, and then iu the form of the moth makes his way out of his silk 

 prison. The puzzle which had baffled previous investigations was 

 this: The eggs and the worm might appear sound and healthy and yet 

 produce in the one case diseased worms and in the other, although 

 spinning their silk cocoons, produce diseased moths or eggs. Pasteur 

 proved that "the corpuscles maybe incipient in the egg and escape 

 detection, germinal in the worm and baffle the microscope." As the 

 worm grows the corpuscles grow 5 in the chrysalis they are more dis- 

 tinct, and in the moth they invariably appear. A diseased moth then 

 lays infected eggs which, owing to the minuteness of the corpuscles, 

 appear healthy. Moreover, a diseased worm may infect a healthy one. 

 Feeding together, corpuscles are transferred from the diseased to the 

 healthy worm, and the infected worm, without immediately showing 

 signs of disease, may spin its cocoon and eventually lay its eggs; but 

 the eggs are all tainted. Instead, then, as silk growers were in the 

 habit of doing, of selecting the eggs for the next year's growth from the 

 moths which had survived the most successful cocoons, the microscope 

 was brought to bear on the moths when the presence of these diseased 

 corpuscles was invariably made evident. This is the practice now 

 adopted by all silk growers, and numbers of women skilled with the 

 microscope examine each moth as it emerges from tlie cocoon. 



Here we have, then, the first distinct connection between living germs 

 and the cause of disease, of infection, and of hereditary taint. The 

 constant strain of microscope work, which restored to France lier silk 

 industry, produced partial paralysis from which Pasteur never quite 

 recovered. 



It would be easy to multiply examples to which this great discovery 

 has given rise. Tuberculosis, diphtheria, wool-sorter's disease, leprosy, 



