Nephrogenic rest: A fragment of embryonic tissue in the kidney retained after the period of embryonic development. Nephrogenic rests are precursors (forerunners) of Wilms tumor. Two distinct categories of nephrogenic rest are recognized on the basis of their topographical relation to the renal lobe. Intralobar nephrogenic rests occur within the renal lobe and develop as the result of events early in renal development. Pelilobar nephrogenic rests are confined to the periphery of the renal lobe and are related to late kidney development.