SAPROLEGMA IN RELATION TO THE SALSION DISEASE. 327 



surface. Others of the vertical hypljae^ on the contrary, are 

 directed inwards ; and, as root-hjphoe, not only traverse the 

 deep layer of the epidermis, but pierce the superficial layer 

 of the derma, and penetrate into its substance for a short 

 distance. Yet nearer the centre the epidermis is completely 

 broken up into fragments and detached cells, which lie in the 

 meshes of the thick mycelium formed by the horizontal and 

 vertical stem-hyphae of the fungus. The vertical stem-hyphse 

 attain their full length, often branching, and begin to de- 

 velop zoosporangia. Towards the .derma the root-hyphse 

 are so numerous and close set that they are often separated 

 by interspaces which hardly exceed their own diameter where 

 they penetrate the superficial layer of the derma. Moreover, 

 they branch out in the latter to a depth of a tenth of an 

 inch, often penetrating the bundles of connective tissue. 

 Their ultimate ramifications usually end in curiously swollen 

 extremities. Still more towards the centre of an ulcerated 

 patch, the place of the epidermis is taken by the felted 

 mycelium of the Saprolegnia, the superficial layer of the 

 derma has disappeared, small vessels have often been laid 

 open, and blood has been effused. 



All these appearances become perfectly intelligible if we 

 suppose that, when Saprolegnia spores reach the surface of 

 the body of a live salmon, they behave in the same manner 

 as we know they do when they reach the surface of the body 

 of a dead fly. If it should light upon one of the apertures 

 of the mucous cells an easy road into the soft interior of the 

 epidermis is open to the hypha of the germinating spore. 

 But, apart from this, the flat superficial cells are certainly as 

 easy to pierce as is the tough cuticle of a fly. No doubt, as 

 in the fly, the hypha grows directly inwards, and piercino- 

 the superficial layer of the derma, comes into direct relation 

 with the abundant nutriment it finds there. The fungus 

 then ramifies, on the one side, in the derma, on the other in 

 the epidermis, sending off" in the latter vertical branches 

 ■which soon develop sporangia, and horizontal branches, 

 which are driven, like subsoil ploughs, into the middle layer 

 of cells. The zoosporangia emit multitudes of zoospores, 

 many of which are deposited on the epidermis in the neigh- 

 bourhood of the first; and, penetrating it in the same way, 

 add to the Saprolegnia plantation. Tiius the disease con- 

 stantly spreads centvifugally ; and, as the oldest and most 

 luxuriant growth of Saprolegnia is in the centre, so is the 

 mechanical destruction of the epidermis first effected there. 

 But it is in this region also that the greatest number of root- 

 hyphse penetrate the derma. They cannot fail to interfere 



