70 U. S. BUREAU OF FISHERIES. 



give rise to an opacity or white spot at that place. It is therefore entirely 

 possible and even probable that the disease may have more than one cause, 

 and that its progress is variable. Certainly the ordinary occurrence is not 

 epidemic in character, differing thus from Bataillon's case and some encoun- 

 tered by Hofer. 



Oeneral morphological features. — The yolk of trout fry that have just started 

 the absorption of the sac presents a characteristic appearance under normal 

 conditions. Beneath the enveloping layer of cells are located the embryonic 

 blood vessels and what might be called an absorption area. The latter* har- 

 bors periblast cells more or less irregularly distributed and surrounds the 

 yolk, which is structureless and homogeneous under ordinary magnification, 

 and takes a dense stain. 



In yolk affected by white spot the diseased region showed uneven extent of 

 the absorption area. Various stages of disintegration characterized such pic- 

 tures, tracts of transformed yolk and yolk spheres of various sizes penetrating 

 into the still unaffected material. Blood vessels follow the advancing dis- 

 integration, so that they are often found in the center of the yolk, whereas 

 under normal conditions at such a stage of development they are entirely at 

 the surface. Location of the white spot in the yolk sac is variable, but in a 

 general way the following may give some idea of the distribution. The 

 diseased specimens were Loch Leven fry from Saratoga, Wyo. (early stage of 

 sac absorption), 42 being examined. 



Diseased area close to liver 9 



Touching or surrounding oil globule 8 



Close to both liver and oil globule 12 



Near heart 3 



Po.sterior tip of yolk 10 



42 



In no case was the spot located entirely in the interior of the yolk ; that is. 

 it always came in contact with the yolk enveloping cell layer at some point. 



The question of bacterial ayencij. — In some cases bacteria are certainly in- 

 volved. Examples were furnislied by rainbow-trout eggs from Cape Vincent, 

 N. Y. (stage shortly before hatching), which had been shipped there from 

 Wytheville, Va., some weeks previously. Microscopic examination showed at 

 least two kinds of bacteria present in the disintegrating yolk, and these were 

 found side by side in the individual cases or also in seemingly pure cultures. 

 The affected yolk had broken up as usual, and the bacteria were clustered 

 most thickly around the yolk spheres. Similar conditions were observed in 

 the few cases of the disease present in brook-trout eggs from a hatchery at 

 Taunton, Mass. 



But this serves to show only that bacteria may be involved in the progress 

 of the disease — ^not that they are to be considered as causative agents. The 

 random distribution of diseased eggs among healthy eggs is a fair proof in itself 

 that the infection is not contagious or, better, that normal and healthy eggs 

 ai-e not attacked by the bacteria. There must, then, be a predisposition in some 

 eggs that renders them liable to infection. It is of some note to observe that 

 eggs with high mortality rates had generally (not always) undergone ship- 

 ment from a more or less distant station. Naturally they had, therefore, xmder- 

 gone a certain amount of rough handling, and this made it very possible that 

 the primary cause of the disease was to be found in some injury thus sus- 

 tained by means of which the bacteria gained access to the yolk. To gain 

 some data on this point, 200 brook trout in the fry stage (yolk sac at early 

 stage of absorption;* fry, 18 mm.) were isolated and the yolk sac of each 

 pricked to a slight depth with a sharp needle. In a few cases the fishes were 

 injured in the operation, and in still others the wound caused partial loss of the 

 yolk. In what may be called successful, operations the wound caused no loss 

 of yolk and appeared only as a very small white area at the surface of the lat- 

 ter, the fishes showing no immediate ill-effects on liberation. One hundred 

 uninjured fry were kept under similar conditions for control purposes. After 

 10 days a little more than 2.5 per cent of the opei-ated fishes had entirely recov- 

 ered and showed no signs of the injury, and somewhat less than 50 per cent 

 showed a growth of the artificially produced white opaque area. A third of 

 the latter died in the course of these 10 days. Development of fungus elimi- 

 nated a greater part of the remaining 25 per cent. Of the control 2 per cent 

 died during this period, neither they nor the survivors showing a definite 



