The further studies of histopathologica features of the renal tumor (nephroblastoma) occuring in chickens exposed to the BAI strain A (myeloblastosis) avian tumor virus are described. The neoplasm appears to be a growth of mixed tissue components consisting of wide range of variation of epithelial and mesnchymal structures in disorders of differentiation from nephrogenic tissue morc than those reported iu 1962. The findings suport the concept that the growth originates in nephogenic elements residual in the postembryonic kidney and suggest a possible mechanism for maintenance of continuous growth as shown in Text figure I.