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Summary: The ultrastructure of the nephrostomial tubule and the gonadal crest of Acipenser ruthenus were studied by transmission and scanning electron microscopy. Opisthonephric nephrostomial tubules begin to appear at the end of the first week after hatching and are regularly segmentally arranged and completely developed in the period between 10 and 35 days after hatching. Fully differentiated nephrostomial tubules connect the vestibular area of the Malpighian corpuscle with the coelomic cavity as short, open canals. Before reaching the latter, each nephrostomial tubule widens to a funnel-like structure and ends with a round opening, the nephrostome. The outer rim of the nephrostome protrudes slightly into the coelomic cavity forming the nephrostomial lips. The nephrostomial tubule is lined by one layer of cuboidal or low-columnar cells, equipped with an apical set of long kinocilia, which point generally in the direction of the Malpighian corpuscle. The cilia of the distal funnel region, however, point toward the coelomic cavity. The cells of the nephrostomial lips preserve a lower degree of cellular differentiation than the rest, display a blastema-like appearance and proliferate by frequent mitoses. Proliferating cells of the nephrostomial lips spread out on the coelomic surface and replace the flat mesothelium of lateral plate origin here. Furthermore, cells of the nephrostomial lips also show the tendency to grow downward, giving the epithelium of the lip region a multilayered appearance. Proliferating cells of the medial nephrostomial lips associate with accumulations of germ cells and form the primordium of the gonad (gonadal crest). Within the crest, the intercellular space between the germ cells and the surrounding epithelial supporting cells is filled with a basal lamina-like, electron-dense substance and may contain short, interlocking processes of the two cell types. The large germ cells of this early period have lobulated, electron-lucent nuclei, spherical mitochondria with inclusions and abundant narrow profiles of the smooth endoplasmatic reticulum. [Copyright &y& Elsevier]