THE DISEASES OF SILK WORMS 131 



what we are dealing with, and we shall be in a 

 better position to grapple with it. In the search 

 for the cause for the disease previous investigators 

 had made some observations which gave Pasteur 

 a point of attack. Guerin-Meneville in 1849 na d 

 seen in blood of silk worms some small, oval cor- 

 puscles which subsequent observers discovered in 

 various parts of the body and even in the eggs. It 

 was found that these bodies increase in number as 

 the disease progresses, but there was much doubt 

 as to what relation they bear to the disease. 



When Pasteur began his investigations he was 

 unaware of most of these previous findings, and he 

 rediscovered a number of things already known. 

 On the evening of his arrival at the scene of action 

 he had observed the oval bodies in the tissues of 

 diseased worms. He soon made himself familiar 

 with the occurrence of these minute bodies in all 

 stages of the life history of their host from the egg 

 to the mature moth. The question which naturally 

 occurred to Pasteur in the light of his previous in- 

 vestigations was, What relation do these small oval 

 bodies bear to the disease? The supposed para- 

 sites do not occur, at least in abundance, in healthy 

 worms; they increase in numbers as the disease 

 progresses, and worms dying of pebrine are often 



