484 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
only some slides of the brain, heart, liver, and spleen, hurriedly prepared in the field 
from two perch which had been dead an unknown length of time when taken. 
Apart from the gorging of the heart and congestion of the liver already mentioned 
and a noticeable amount of cellular degeneration in the liver and especially in the 
spleen, these slides gave no definite hint of the nature of the disease. This degenera- 
tion, very much more abundant in the herring, consisted of a conversion of the con- 
tents of the cells into a yellow, dark brown, or black mass of minute spherical gran- 
ules which had the appearance of micrococci ; but as they did not stain with aniline 
they were very probably pigment granules instead; a supposition rendered more plaus- 
ible by the equal or greater pigmentation of the viscera which I have since noticed 
in many seemingly healthy fishes. These altered cells were more abundant near the 
larger blood vessels, and where considerable numbers of them had undergone degen- 
ei ation their walls were often broken down, and the cells were thus replaced by a col- 
lection of their dark yellow or black contents. 
From a general study of my fish collections and of the conditions prevailing in 
Lake Mendota a number of additional facts of some significance may be selected. 
(1) The herring, or so-called whitefish, of the lake were perishing in extraordi- 
nary numbers during the entire period of this outbreak, with symptoms precisely like 
those of the perch, and taking into account the relative numbers of these species in 
the lake, perhaps in as large or even larger proportion. These herring, like the perch, 
are, as is well known, bottom feeders, and in midsummer remain in the deeper waters. 
Furthermore, they die every summer, according to the uniform testimony of those with 
whom I talked, in precisely the same way as in 1884, but in very much smaller num- 
bers. The condition of the bodies of the fresh herring examined, two of which were 
taken before death, was precisely that of the diseased perch, except that there was a 
greater amount of cellular degeneration of the viscera, particularly of spleen and kid- 
ney. Substantially all the substance of the former organ except the gorged blood 
vessels was replaced by masses of the spheiical granules already described, or by cells 
filled with them, and the kidneys of the specimens examined were so loaded with them 
as to be black to the naked eye. 
There follows from the above a considerable probability that the perch were 
affected by the same cause as the herring, or else that the disease was a contagious 
one and taken from the herring directly. It is further likely that this cause is present 
every year, as is shown by the regular death of a small number of the latter fish, but 
that its action was greatly intensified in 1884. 
(2) The majority of the perch dying were full grown, and absolutely no young were 
seen either by myself or by any one with whom I talked. The captain of the passenger 
steamer, who spends most of his time on this lake, had seen none dead less than 5 
or 6 inches in length and of an estimated age of three or four years. The smallest 
specimens which I saw were at least half grown; but according to Professor Birge, of 
the University of Wisconsin, a few specimens were seen not over 3 or 4 inches long. 
(3) There was a marked contrast in food between the dead and diseased fishes and 
the healthy ones taken by the use of the seine in the shallow waters along shore. 
The former had eaten, almost without exception, little or nothing but a large red Chi- 
rouomus larva living, as shown by the notes on collections given in the preceding 
part of this paper, in the mud of the deeper water, while the healthy fishes taken in 
the seine had not fed at all upon these large red larvae, but only upon smaller white 
