442 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



tion. But, on advancing within the margin of the diseased area, hyphoe 

 of the Saprolegnia are seen to penetrate horizontally between the cells 

 of the middle layer, thrusting them asunder with so much force that 

 the cells become bent and distorted, and adhere to a hyj^ha as if they 

 were spitted on it. And, in fact, it is because bundles of such hyphaj 

 are thrusting themselves in this manner, as the roots of an ordinary 

 l)lant thrust themselves into the soil, between the epidermic cells, that 

 the radiating ridges which appear on the marginal area of the diseased 

 patch are formed. Close up to the free ends of these hypha3, however, 

 the epidermis is perfectly healthy; and this fact suftices to prove that 

 the growth of the fungus is the cause of the morbid affection of the 

 epidermis, and not its consequence. 



Proceeding further towards the centre of the diseased patch, the 

 hyph?e become more numerous and take a vertical as well as a hori- 

 zontal direction. Of the vertical ones, some traverse the epidermis out- 

 wards, thrusting aside and disturbing its cells, and terminating in short 

 free ends on the surface. Others of the vertical hyphae, on the contrary, 

 are directed inwards ; and, as root-hypha?, 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-hyphte of the fungus. The vertical stem-hyphse attain 

 their full length, often branching, and begin to develope zoosporangia. 

 Towards the derma, the root-hyphre are so numerous and close- set that 

 they are often separated by interspaces which hardly exceed their oavu 

 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 j)enetrating 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 or the epidermis 

 is taken by the felted mycelium of the iSaprolegnia, 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 bodj" 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. l^o doubt, as in the fly, the liypha 

 grows directly inwards, and jjiercing the superficial layer of the derma, 

 comes into direct relation with the abundant luitriment 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 



