THE SILKWORM ITS DISEASES AND IMPERFECTIONS. 73 



The corpuscles are excessively minute, shining, oval bodies, so 

 small as to require the highest powers of the microscope for their 

 examination. They fill the silk tubes of the infected insect, 

 which may then go through the movements of spinning but with- 

 out being able to produce any thread ; they enter its digestive 

 canal, and spread throughout its body, and are not confined to 

 any one stage of its existence, but appear equally in the egg, 

 larva, pupa, and perfect insect. Wherever they are found, there 

 the disease becomes developed, and where they do not occur, the 

 disease is equally absent. For these reasons, it is not merely 

 hereditary, but also contagious and infectious. For, as some of 

 the corpuscles are sure to be passed with the excrement of a 

 diseased individual, any leaves that happen to become soiled with 

 this, and are then eaten by a healthy individual, will serve to 

 convey into its body the insidious foe. Or, again, if a healthy 

 individual should be bitten or scratched by a diseased one, the 

 minute corpuscles may in this manner be introduced into its 

 system, and infect it. In these ways, of course, a single individual 

 may infect a whole brood. Pebrined silkworms may die during 

 any of their ages, though here, as with muscardine, the greater 

 number perish in the last. This applies only, however, when the 

 disease is inherited. If, on the other hand, a healthy caterpillar 

 should become infected by contact with diseased ones, or by germs 

 in the dust of the room, it will usually complete its metamorphoses, 

 and may even form an excellent cocoon ; but it will inevitably 

 transmit the seeds of death to the next generation, and ruin its 

 posterity. This accounts for the fact, so unaccountable before 

 Pasteur's experiments, that eggs selected from moths that had 

 issued from the finest cocoons, frequently yielded caterpillars that 

 soon developed the disease and perished without yielding a crop 

 of silk. 



It was in the year 1865 that Pasteur, at the earnest solicitations 

 of his friend, the celebrated chemist Dumas, undertook to 

 investigate the disease. It had then been raging in France for 

 upwards of twelve years ; the silkworms had sickened and died 

 in multitudes, every year increasing the mortality, and the silk 

 industry was in consequence, brought to the verge of ruin. The 

 weight of cocoons produced in 1853 was twenty-six millions of 

 kilogrammes, producing a revenue of one hundred and thirty 

 millions of francs. During the preceding twenty years, the 

 revenue had doubled, and there seemed every reason to believe 

 that future years would see a further augmentation. Then came 

 the pebrine, and by the time Pasteur commenced operations, the 

 annual yield had become reduced to four million kilogrammes of 



