At week 7, from the 6th division of the ureteric bud, an interaction occurs between the ampullae and the metanephrogenic blastema. A nephrogenic vesicle appears in the vicinity of each ampulla. It fuses together with the ampulla through its tubule and differentiates into a nephron. The connection of the nephron to the ampulla causes the ampulla to divide. One of the two new ampullae retains the nephron, while the other induces the formation of a new nephron. The ampullae migrate towards the renal periphery, together with the nephrons from the hilum of the kidney. The kidney differentiates into a renal cortex and a renal medulla.
The second period of metanephros development lasts from week 14 until week 22. It is characterized by the formation of nephron arcades. The ampullae cease dividing. An ampulla already carrying a nephron can induce an additional nephron. The connecting tubule of the original nephron shifts away from the ampulla with which it was initially linked, and terminates in the connecting tubule of the new nephron. This process is repeated several times, so that a nephron arcade with 4 to 7 nephrons can develop around a collecting tubule. The nephrons are linked to each other through the connecting tubules. Only the newest nephron is directly attached to the ampulla.
In the third period, which lasts from week 22 until week 36, the ampullae grow into the periphery of the renal cortex; as they do so, they induce a series of new nephrons.
In the fourth period, which is during the last month of pregnancy, the ampullae disappear. Nephrons are no longer formed. The loops of Henle and the convoluted tubules develop: interstitial growth commences.