According to Dawson,1the malignant melanomas which have an origin in a benign pigmented nevus arise from two sources: (1) from a proliferation of the original nevus cells embedded in the corium and (2) from the cells of the surface epithelium and its interpapillary epithelial processes by a series of transformations analogous to those traced in the genesis of a simple nevus. Those of intra-ocular origin are said to arise in the pigmented cells of the choroid, ciliary processes, iris or cornea at the line of the palpebral fissure. They raise the most difficult aspects of the problem of histogenesis, for while the cutaneous melanomas may in many instances be traced directly to an epidermal origin, it is only the conjunctival pigmented nevi which can be thus traced, and the tumors arising from any part of the uveal tract are held by the great majority of observers to be mesoblastic