170 PASTEUR: THE HISTORY OF A MIND 



by puncture inoculate them with fresh corpuscles taken 

 either from a diseased worm, or from its excretions, 

 the worms thus treated are sure to be attacked with a 

 disease which, in its external characters, recalls com- 

 pletely pebrine, and correlatively, the corpuscles thus 

 introduced into their organism, develop there until they 

 have invaded it throughout. The corpuscle is, therefore, 

 the cause of the disease, and pebrine is due, and due 

 solely, to the abnormal development of these little 

 organisms. All uncertainty has disappeared, and Pas- 

 teur adopts anew the doctrine of the corpuscle as cause, 

 and the parasitic theory. 



Foitunately, the progress of the disease is not as rapid 

 as it is certain. It is nearly 30 days after infection be- 

 fore the animal is sufficiently invaded by the parasite 

 to be truly sick, and to be able no longer, for example, 

 to spin its cocoon. As its life in the larval state is only 

 about 35 days long, every worm which comes from a 

 sound egg, that is to say which does not contain at 

 the moment of its birth corpuscles in process of devel- 

 opment, will almost surely produce its cocoon. In order 

 that it should be otherwise the larva must become dis- 

 eased in the first days of its existence, at a time when 

 the malady is still, so to speak, latent in its neighbors, 

 even in the most infected ones, and when there are a 

 thousand chances that it will not come into contact 

 with any mature corpuscles which it could swallow or 

 with which it could be infected through wounds. 

 Therefore, if an egg is sound, that is to say, free from 

 corpuscles, the offspring cannot die from pebrine. Here 

 evidently we have a fact of capital importance, and 

 it is not the only one of this order. 



There results, in reality, from this long period of 

 incubation of the disease, another consequence: that is 

 that the silkworm, passing from 15 to 20 days in its 



