Ultrastructural criteria that prove the similarities between amphibian and human tumors.

Toads fed with 0.5 mg 7,12-dimethylbenz(a)anthracene twice a week for 12 weeks displayed liver tumors in 27 out of 100 cases. Electron micrographs of liver tumors showed some criteria of malignancy, such as the presence of nuclear pockets, pseudoinclusions, dilatation of the cisternae of rough endoplasmic reticulum and loss of cell junctions. These features are closely similar to those of true malignancy described in humans and other mammals.