1882.] 



Epidemic known as the " Salmon Disease." 



383 



living plants for their support — the Saprolegnice are essentially sapro- 

 phytes ; that is to say, they ordinarily derive their nourishment from 

 dead animal and vegetable matters, and are only occasionally parasites 

 upon living organisms. In this respect they resemble the Bacteria, if 

 the results of recent researches, which tend to show that pathogenic 

 bacteria are mere modifications of saprogenic forms, are to be ac- 

 cepted. 



As I have said, I do not think that the evidence laid before the 

 Commission of 1879 can leave any doubt as to the causation of the 

 salmon disease on the minds of those who are acquainted with the 

 history of the analogous diseases in other animals and in plants. 

 Nevertheless, this evidence, valuable as it is, suggests more questions 

 than it answers, and in November, 1881, hearing that the disease had 

 broken out in the Conway, I addressed myself to the attempt to 

 answer some of these. 



It was already known that when the papyraceous slough-like sub- 

 stance which coats the skin of a diseased salmon is subjected to 

 microscopic examination, it is found to be a mycelium, or fungus-turf, 

 composed of a felt- work of fine tubular filaments or hyphce, many of 

 which are terminated by elongated oval enlargements, or zoosporangia. 

 Within these the protoplasm breaks up into numerous spheroidal par- 

 ticles, each less than -20V0 °f an inch in diameter. These, the zoospores, 

 are set free through an opening formed at the apex of the zoosporangium, 

 and become actively or passively dispersed through the surrounding 

 water. Herein lies the source of the contagiousness or infectiousness 

 of the disease. For any one of these zoospores, reaching a part of the 

 healthy skin of the same or of another salmon, germinates and soon 

 gives rise to a mycelium similar to that from which it started. 



But I could find no satisfactory information as to the manner in 

 which the fungus enters the skin, how far it penetrates, the exact 

 nature of the mischief which it does, or what ultimately becomes of 

 it ; nor was the identity of the pathogenic Saprolegnia of the salmon 

 with that of any known form of saprogenic Saprolegnia demonstrated. 

 It appeared to me, however, to be useless to attempt to deal with the 

 disease until some of these important elements of the question were 

 determined. 



To this end, in the first place, I made a careful examination of the 

 minute structure of both the healthy and diseased skin, properly 

 hardened and cut into thin sections ; and, in the second place, I tried 

 some experiments on the transplantation of the Saprolegnia of the 

 living salmon to dead animal bodies. Perhaps it will conduce to 

 intelligibility if I narrate the results of the latter observations first. 



The body of a recently killed common house-fly was gently rubbed 

 two or three times over the surface of a patch of the diseased skin of a 

 salmon and was then placed in a vessel of water, on the surface of which 



