132 LOUIS PASTEUR 



swarming with them. Pasteur found that the dis- 

 ease was contagious, that healthy worms could be 

 infected by feeding upon leaves on which diseased 

 worms had been crawling. They could be given 

 the disease by a mere scratch from an infected 

 worm or by the prick of a needle smeared with 

 infectious material. Eggs containing the supposed 

 parasite were found to give rise to diseased larvae, 

 and on this fact, a method of egg selection was 

 practised in order to obtain healthy progeny. Ex- 

 amination of eggs, however, was soon replaced by 

 an examination of the couples that produced the 

 eggs. The procedure recommended by Pasteur in 

 1865 was as follows: 



After the mating, the female, set apart will lay her 

 eggs; then one will open her, as well as the male, in order 

 to search therein for the corpuscles. If they are absent 

 from both male and female, he will number their laying 

 which shall be preserved as eggs absolutely pure, and 

 bred the following year with particular care. There will 

 be eggs diseased in various degrees according to the 

 greater or less abundance of corpuscles in the male and 

 female individuals which have furnished them. 



Pasteur himself became a raiser of silk worms 

 in order to be able to furnish pure "seed," as the 



