Development of the metanephros can be subdivided into four periods.
The first period begins at S16 and lasts until week 15. The ampulla of the ureteric bud divides into a cranial and a caudal branch, forming the shape of an anchor; the future longitudinal shape of the kidney is thus determined. Both wings of the anchor form an ampulla. The next division also occurs in a dichotomic way, but this time asymmetrically (long and short branches). Subsequent divisions are asynchronous. They occur more frequently at the poles than between the poles, which leads to the characteristic shape of the kidneys. From S16 to approximately S19, which is phase 1 of this period of metanephros development, 3-6 divisions take place. These divisions give rise to the renal pelvis, the major calices and the minor calices. In the region of the future calices, divisions occur very quickly; the adjacent ducts dilate, their walls break up and a single space is formed (dilation and fusion). At phase 2 (up to the 10th division, approximately weeks 13 and 14), the papillary ducts (10-25 collecting ducts per renal papilla) appear. At week 15, during phase 3, an additional 5-7 divisions lead to the ramification of the system of collecting ducts.