BULLETIN OF THE UNITED STATES FISH COMMISSION. 441 



next iioiiit of consideration is the relation of the fungus to tlie affection 

 of the skin. Is the growth of the fungus the cause of that affection, 

 or does the fungus merely find a favorable nidus in the products of the 

 affection ■? 



The Saprolegniw, as we have seen, habitually grow on dead animal and 

 vegetable substances; and it is therefore a fair supposition that some 

 morbid affection may cause the local death of the skin of the fish; and 

 that the fungus simply implants itself in the dead tissue, as if it were 

 the dead body of a fly. 



On the other hand, our knowledge of the destructive epidemics caused 

 by Empusa in flies, Botrytis in silkworms, and Entomophthora in other 

 caterpillars, and of the multifarious fungi which produce bunt, smut, 

 and mildew in plants, affords at least equal ground for the supposition 

 that the ulceration and destruction of the skin are caused by the inva- 

 sion of healthy fish by the Sainolegnia. The decision of this question 

 is obviously of the greatest importance. 



Direct experimentation by infection of healthy salmon, in the manner 

 m which dead flies were infected from the diseased salmon, being out of 

 the question, at present, on account of its practical difficulties, the only 

 profitable way of investigation lay in the study of the minute structure 

 of the healthy and of the diseased skin, so as to ascertain the exact 

 relation of the fungus to the morbid appearances. 



The skin of the salmon, like that of vertebrated animals in general, 

 consists of a superficial, cellular, non-vascular, scarf skin, or epidermis^ 

 covering a deep fibrous and vascular true skin, or derma. The former is 

 divisible into a superficial, a middle, and a deep layer of cells, the last 

 being in immediate contact with the derma. The deep cells are verti- 

 cally elongated, the middle ones more or less broadly spindle-shaped or 

 rounded, while the thin superficial layer cousists of flattened cells. The 

 deep cells are constantly multiplying by fission, and their jn^ogeny be- 

 come middle cells, the outermost of which, for the most part, becoming 

 flattened, give rise to the superficial layer, which is continually shed and 

 replaced. Some of the cells of the middle layer, however, enlarge, take 

 on a more or less spheroidal form, and become filled with a mucous 

 fluid. As they rise to the surface, they open and pour out this fluid, 

 which lubricates the surface of the fish. In any vertical section of a 

 properly i)repared portion of salmon skin more or fewer of the openings 

 of these cells are to be seen. The derma is composed of matted bun- 

 dles of connective-tissue, traversed by blood-vessels and nerves, and 

 containing numerous lymphatic spaces. The superficial layer of the 

 derma contains a number of dark pigment cells, of which there is a 

 close-set zone immediately beneath the ei)idermis. 



In a thin vertical section of the skin of the head of a salmon, which 

 has passed from the sound skin through the centre of a diseased patch, 

 the various structural elements which have been described, disposed 

 with great regularity, are alone visible in the healthy part of the sec- 



