This neoplasm has been observed from the time of its inception in a number of animals. Thus it has been learned that the proliferative stage of the neoplastic process is preceded and accompanied by a striking vascular reaction. Intense hyperemia invariably occurs in that region of the mucosal surface which later becomes the site of neoplastic proliferation. Furthermore, by direct microscopic observation of the living tumors the atypical structure and arrangement of the blood vessels become apparent. A study of the significance of these vascular phenomena in their relation to the inception and growth of the tumor is now in progress.

It has been found possible to transmit the catfish tumor to fish of the same species by implanting fragments of the tumor into the anterior chamber of the eye. Also, by taking advantage of an anatomical peculiarity of the catfish cornea, it has been possible to embed the tumor fragments in normal tissue where it could still be readily observed both in the gross and microscopically. The growth of the transplants in the eye has been followed by periodic examination of the living tumor by means of the slit lamp microscope. In the anterior chamber, the tumor characteristically forms dense membranes which spread over the inner surface of the cornea. In this manner growth continues until the tumor fills the chamber. Between the two layers of the cornea, tumor growth is expansive.

Attempts to transplant the tumor to the anterior chamber of two other species of fish and to frogs, were unsuccessful. Implantation of the catfish epithelioma in alien species of fish excited no exudative response, whereas in a less closely related species of animal, the leopard frog, a pronounced exudative reaction resulted.