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We have attempted to describe virtually all forms of malignant melanoma which affect man except those arising in the eye. The vast majority of malignant melanomas clearly fall into one of three kinds: (1) malignant melanoma of the superficial-spreading type, (2) malignant melanoma of the lentigo-maligna type, and (3) malignant melanoma of the nodular type. The developmental biology of a primary neoplasm is illustrated by discussing and illustrating the evolution of these three dominant forms of melanoma. Primary malignant melanoma of the superficial-spreading type and of the lentigo-maligna type develop through a characteristic biphasic growth pattern: an initial radial-growth phase, followed by a vertical-growth phase. The radial-growth phase of these melanomas is only rarely associated with the development of metastases, while the vertical-growth phase is commonly associated with subsequent metastatic disease. The phenomenon of the vertical-growth phase is apparently, therefore, a qualitative step in the development of a primary malignant melanoma. Malignant melanoma of the nodular type is an example of a primary tumor without a precursor developmental stage such as a radial-growth phase.