During intrauterine life the function of the nephric system apparently is minimal. The task of maintaining the homeostasis of the fetus is fulfilled by the placenta, as evidenced by the absence at birth of abnormalities in fluid and electrolyte balance in newborns with bilateral renal agenesis. At birth, the kidneys have a combined weight of about 25 gin, compared to 40 gm at 3 months of age and 300 gm in the adult. They often demonstrate persistence of a fetal, lobular structure and because of the laxity of the abdominal wall, they are readily palpable. On cut section the superficial renal cortex appears thin, whereas the juxtamedullary and medullary areas are relatively well developed. This corresponds to the more advanced state of development reached by the deeper (more central) parts of the kidney and reflects the centrifugal pattern with characterizes the growth of this organ. Despite its small size and the thin cortex, and unlike many other mammals, the kidney of the full term newborn contains its full complement of nephrons (800,000 to 1,000,000 per kidney). According to Potter and Thierstein,