15 



But this does not appear to be the case. Mr. List, 

 the Chief Constable of Berwick, has been good enough, 

 at my request, to repeat some experiments which he had 

 formerly made upon the effect of confining diseased salmon 

 and sea trout in coops in the tideway of the estuary of the 

 Tweed at Berwick ; and, in June 1882, he sent me two sea- 

 trout which had got completely well under these circum- 

 stances, though signs of the situation of the patches of 

 disease remained. Careful examination of the skin in these 

 regions by means of sections prepared for the microscope, 

 revealed no trace of the fungus ; so that it would seem 

 that the parasite is completely rooted out by the sea 

 water. 



In a river which remains year after year the seat of 

 epidemic salmon disease, therefore, Saprolegnia must be 

 permanently resident there, in some shape or other ; and 

 the fish which ascend are infected by this stationary store 

 of the fungus. 



In the experiments which have been described, the 

 fungus has been brought into direct contact with the fly 

 or the fish to be infected ; but this is not at all necessary. 

 In a full-grown specimen of the fungus, the great majority 

 of the hyphae end in sporangia, and one of these may 

 contain a hundred or more minute zoospores. These, 

 when ripe, are ejected from the sporangium, and each is 

 propelled by a pair of cilia through the water. By these 

 means, and by the currents in the water, these zoospores 

 may be drifted a long way, and if any one of them 

 reaches a fish, it may germinate, penetrate its skin, and 

 give rise to the disease. 



The quantity of zoospores which may be produced from 

 a mass of Saprolegnia no larger than that which covers 

 an ordinary fly, is prodigious, and a few diseased salmon 



