Development of the Collecting System
The collecting ducts of the metanephros arise from the ureteric bud, which is an outgrowth of the mesonephric duct, near its entrance to the cloaca. The elongating ureteric bud initially penetrates the metanephric mesoderm and then subsequently dilates, forming the primitive renal pelvis.
Development of collecting system, available at: http://web.uni-plovdiv.bg/stu1104541018/docs/res/skandalakis%27%20surgical%20anatomy%20-%202004/Chapter%2023_%20Kidneys%20and%20Ureters.htm
The ureter is formed from the stalk of the ureteric bud and the collecting tubules are formed as a result of repeated branching of the bud. The collecting tubules which undergo repeated branching, forms the successive generations of tubules. The first four generations of the tubules enlarge and fuse together to form the major calyces. The successive four generation of tubules form the minor calyces. The remaining generations of tubules form the collecting tubules of the permanent kidney and are approximately 1 to 3 million in number.