In all of the vertebrates, beginning from the most primitive ones, an universal principle of structural, functional and chemical organization of the nephron is noted. The main trend of progressive evolution of kidney function (both in phylogenesis of vertebrates and ontogenesis of birds and mammals) is the increase of the intensity of filtration-reabsorption mechanism of urine formation. The development of secretory apparatus is considered as an adaptation to the conditions in which this mechanism is not sufficient to provide the necessary level of excretion. It is suggested that secretory mechanism is formed in close relation to cellular system of reabsorption of sodium and chloride. The presence in lamprey kidney of a loop which is similar to Henle's loop in mammals and birds indicates that the development of the system of osmotic concentration conditioned by the formation in the kidney of the medulla and from a sharp increase in renal arterial blood supply.