Silkworm Disease. 76 



disastrous effect into his own rearing-house, in which he had 

 hitherto only most healthy worms. 



But Pasteur demonstrated not only that the disease could be 

 transmitted from worm to worm, by one or all of these modes, 

 but also that the disease is hereditary, and that under such cir- 

 cumstances it is most fatal, destroying the worms whilst yet 

 small. An experienced eye can see at a glance whether Pebrine 

 is abnormally prevalent in a rearing-house. It is not requisite 

 to see the black spots on the skin of the diseased worms, but it 

 is at once observed that, although the worms have hatched 

 almost simultaneously, and have been reared under precisely 

 similar conditions, there is a marked difference in the size of 

 them. Some, the healthy worms, have progressed favourably, 

 changing their skins regularly, and have attained nearly their 

 full growth. Others, the diseased worms, are more or less re- 

 tarded ; some are very small, and seem to have made scarcely 

 any progress, and there is every intermediate size between these 

 and the healthy worms, according as the subject is more or less 

 infested by the parasites. This want of uniformity of size is, as 

 I have already remarked, very noticeable, and at once suggests 

 the presence of Pebrine. By experiment Pasteur demonstrated, 

 as indeed would seem but natural, that the earlier in life a worm 

 became infected the less likelihood was there of it living to pro- 

 duce silk. It was whilst investigating the meaning of the whole- 

 sale destruction of young larvae in houses which had hitherto 

 been free of Pebrine that he was led to examine the moths and 

 the ova laid by them. On the former he discovered the black 

 spots and the parasites, and these latter he also found in the 

 ova, pointing to the disease being hereditary. 



Supposing worms have been healthy, and yet almost at the 

 last period of their existence as worms they become infected in 

 any one of the ways already noted. It is too late in life for the 

 disease to kill them, or even to prevent them spinning their 

 cocoons ; but nevertheless they bear in them the seeds of the 

 disease. Having spun their cocoons they in ordinary course turn 

 to chrysalids. The newly-disclosed chrysalis contains at the 

 very first nothing but a substance of creamy consistence, having 

 no divisions into head, thorax, and abdomen. As these divisions 

 and the internal economy of the perfect insect are gradually de- 

 veloped, certain of the parasites contained by the diseased worm 

 are enclosed by the newly forming membranes, and in this 

 gradual process some parasites become imprisoned by the mem- 

 branes of the future ova. Thus, when the ova are laid, they 

 contain from the very first the germs of Pebrine, and presage 

 certain and untimely death of the larv« issuing from them. 

 Seeds from different parents would not all be infected, and seeds 

 even from infected parents would not all of them enclose the 



