Two transplantable hamster melanomas, the M-melanoma 1 and the A-melanoma 3 of Fortner, which exhibit differing growth and metastasizing properties, have been investigated with the aid of the electron microscope.

The M-melanoma 1 contains melanocytes, which are characterized by the presence of melanin granules, dendritic processes, and cytoplasmic filaments. The melanin granules may be derived from the Golgi apparatus. Melanogenesis starts in small vacuoles which frequently possess fibrillar structures with periodicity and which become increasingly filled with a dense material.

In the A-melanoma 3 can be distinguished two different types of tumor cell—Type 1 and Type 2. Type 1 cells are strikingly rich in free ribonucleoprotein granules and poor in cell organelles, particularly in those characteristic of melanogenesis. They do not contain any pigment or any cytoplasmic filaments. The Type 2 cells (so-called amelanotic melanocytes) possess numerous vacuoles which may contain a dense substance (promelanin?) and probably originate in the Golgi apparatus. Like melanocytes from M-melanoma 1 and from skin, these cells also contain fine cytoplasmic filaments. Marked folds are a conspicuous feature of the surface of these cells.