In the rabbit kidney, the connecting tubule is a clearly delimited tubular portion interposed between the convoluted part of the distal tubule and the cortical collecting duct. Superficial nephrons have individual connecting tubules; the connecting tubules of deeper nephrons fuse together to form a so-called arcade. The collecting duct is subdivided, with respect to localization, into cortical, outer medullary and inner medullary collecting duct. Connecting tubule and collecting duct are lined by a total of three different cell types: the connecting tubule by the connecting tubule cell and the intercalated cell; the collecting duct by the principal cell and the intercalated cell present only in the cortical and outer medullary segment. The intercalated cell can have different manifestations ("gray", "black", and "light"). The principal cell undergoes gradual alterations from a very complicatedly organized prismatic cell (approximately 7.5 micron cellular height) in the cortical collecting duct to a very simply organized, high prismatic cell (approximately 75 micron cellular height) in the inner medullary collecting duct.