[Differentiation of the epithelium of the pronephros and the primary kidney in frogs].

The kidneys of tadpoles of different developmental stages were examined in preparations processed histologically and histochemically. It was found that differentiation of the provisory excretory organ tubules in frogs was "shortened" or "accelerated" after P. P. Ivanov's terminology, and developed differently as compared with differentiation of tubules of the definitive organ of excretion -- the primary kidney. When differentiating the epithelium of the proximal portion of the primary kidney nephron passes the stage of the high prismatic false-stratified epithelium. The pronephros tubules do not pass this stage and the epithelium becomes a strict monolayer from the very beginning. No mitoses are observed in the pronephros tubule epithelium even at the earliest differentiation stages. Later on, the beginning of tubule functioning, and with the reduction, and later disappearance of yolk granules in the epithelium solitary mitoses make their appearance. The mitotic activity of the primary kidney tubule epithelium is very high (70%) at the early stage of differentiation. Then its mitotic activity decreases (30%), and after the beginning of the tubule functioning mitoses in its epithelium become solitary.