The last decade has witnessed a revival of interest in the problem of the origin of the nevus cell in the pigmented nevus. This problem has always had an attraction, on account of the not infrequent instances of malignant degeneration of such nevi. There is a vast amount of literature on the subject. As is often the case in medicine, recent technical advances have rendered much of it obsolete. The great majority of the studies on the histogenesis of the nevus cell have been made on static material. Adequate studies of the earliest stages of rapidly evolving pigmented moles are rare. I had the opportunity to observe and study the sudden appearance, rapid evolution and maturation of an extensive eruption of pigmented nevi in a boy of 12 years. With the histologic material obtained from that source it was possible to evaluate the more recent theories of the origin of