BLACK TTJMOR OF THE CATFISH 
11 
MICROSCOPIC STUDY 
Under the lower powers of the microscope the sectioned material of the timior 
appears as a mass of small black bodies, usually more or less rounded, so thickly 
distributed among the connective tissue fibers of the skin that they almost com- 
pletely obscure everything else. These bodies appear to be cystlike masses of the 
cocci, and scattered individual cocci may also be observed everjTvhere in great 
numbers. 
In smears made from fresh material the same cystlike bodies are present, along 
with scattered cocci, and under the oil immersion the cocci within the cyst may 
often be seen in active Brownian movement. It was at first supposed that this 
movement might be due to some activity on the part of the organism, but in smears 
made from material macerated in strong potassium hydroxide the same movement 
was evident. In fresh smears the cysts may be seen to break under slight pressure 
of the cover glass, liberating large numbers of the cocci. The cocci are densely 
black with an endogenous pigment, and they are so abundant that it is difficult 
to examine sectioned material with satisfaction, even when cut to only two or three 
microns. 
From my observations it appears that the tumor begins its development in 
the outer layer of the dermis and later involves all of this layer, expanding it to 
many times its original thickness. The dermal blood vessels are enlarged, but I 
have seen no evidence of bleeding, even in old tumors. As the disease progresses 
small fingerlike projections of the tumor invade the epidermal layer and make their 
way to the surface (figs. 6 and 8). Eventually the epidermis sloughs off over the 
older portion of the tumor, leaving a ragged surface formed by the dermal connect- 
ive tissue fibers and the substance of the tumor. A slight pressure of the tumor 
at this stage causes the extrusion of the inky-black fiuid containing the cocci, 
as indicated above. 
STUDIES AND EXPERIMENTS 
As some form of fungus had been suggested as the cause of the disease, my 
first studies were directed toward this point. Ordinary fish fungus {Saprolegnia 
sp.) is not black, but some species of Aspergillum and some other fungi do produce 
black pigment. Some fishes were found with fungus on the tumors, but a little 
study revealed the fact that this fungus was not black and that it sometimes occurred 
elsewhere on the same fish where the skin had been abraded and where there was 
no evidence of the black tumor. Moreover, when growing in the tumor, the fungus 
was not found in the deeper portion. Careful search was made for any fungus in 
sectioned and fresh material and by macerating with strong potassium hydroxide, 
which breaks dov/n the animal tissue but leaves fungi intact. Fungus as a cause 
of the tumors was thus thrown out of consideration. 
The question of an animal organism as the causative factor was also con- 
sidered, especially as the Myxosporidia often produce tumorhke growths, but no 
evidence of any such could be observed. 
The hypertrophy of the pigment layer of the skin, due to stimulation by an 
organism or otherwise, at first seemed a possibility. The extension of the black 
tumors into unpigmented areas of the mouth, gill chamber, pharynx, and even 
