192 ON THE SAPROLEGNIE^. 



out in the rivers of North Wales, and in the Tay and North Esk, 

 in Scotland. 



The first symptom of this disease is the appearance of small 

 greyish or ashy discolourations of the skin, usually upon those parts 

 of the body which are devoid of scales, such as the top and sides 

 of the head, and the bases of the fins. When a patch of diseased 

 skin has once appeared, it rapidly increases in size, and runs into 

 any other patches which may have appeared in its neighbourhood. 

 The marginal zone, constantly extending into the healthy sur- 

 rounding skin, retains its previous characters, while the ashy cen- 

 tral part changes. It assumes the consistency of wet paper, and 

 can be detached in flakes, like a slough, from the skin which it 

 covers. If the subjacent surface is now examined, it will be found 

 that the epidermis, or scarf-skin, has disappeared, and that the 

 surface of the derma, or true skin, is exposed. The affection, how- 

 ever, is not confined to the epidermis. As the patch acquires 

 larger dimensions, the derma, or true skin, in its centre, becomes 

 subject to a process of ulceration ; and thus a deep bleeding sore 

 is formed, which eats down to the bones of the head, and sends 

 off burrowing passages, or sinuses, from its margins. In severe 

 cases the disease may extend far into the interior of the mouth, 

 the edges of the fins become ragged, the gills are said to be 

 attacked, and cases of the blinding of the fish, by extension of the 

 disease over the eyes, are reported. 



On advancing within the margin of the diseased area, hyphse 

 of the Sap7olegnia are seen to penetrate horizontally between the 

 cells of the epidermis, thrusting them asunder as the roots of an 

 ordinary plant thrust themselves into the soil, thus giving rise to 

 the radiating ridges, which here make their appearance. Proceed- 

 ing further towards the centre of the diseased patch, the hyphae 

 become more numerous, and take a vertical as well as a horizontal 

 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 hyphas are 

 directed inwards, and, traversing the epidermis, pierce the super- 

 ficial layer of the derma. Yet, nearer the centre, the epidermis is 

 completely broken up into fragments, and detached cells are seen 

 lying in the thick mycelium of the fungus, which now begins to 



