An account is given of the development of the pronephros from 76 hours after fertilization. It was not possible to observe the origin of the duct and the tubules. At 76 hours six short pronephric canals are laid down opposite somites IV to IX, each connected with the dorso-lateral wall of the corresponding nephrotome. They run laterally into the pronephric duct, which opens with its fellow of the opposite side into a groove at the hinder end of the body. A tubule arises from the nephrotome opposite somite X; it is always short, and its nephrostomial canal passes into the inner end of the sixth tubule. Nephrotome IV very soon disappears, the nephrocoele widening out into the splanchnocoele whilst the first tubule eventually comes to open into the dorsal side of the second tubule. This condition is retained until the degeneration of the pronephros. Nephrotomes V and VI, and usually a variable number of those behind this level also, open out into the splanchnocoele, as does nephrotome IV. All these posterior ones, however, again become completely closed in by the formation of a new ventral wall, which grows across from the anterior and lateral sides of the nephrotome. In this way a series of secondary pronephric chambers arise. Glomeruli now develop, and almost at the same time the walls between the chambers break down so that a single elongated chamber remains, in which lies a long glomus. The embryo hatches about 9 days after fertilization, and soon afterwards the pronephros reaches its full development. It then consists of one external nephrostome and five or six internal nephrostomes opening on the one side into the duct and on the other into a long pronephric chamber containing the glomus. The chamber extends from somite VI to between somites XIV and XV, and the nephrotomes down to this level probably take part in its formation. Behind somite X only small rudimentary tubules develop, and the mesonephros begins at somite XVI. The development of the mesonephros was not studied, but the nephrotomes of this region doubtless give rise to the Malpighian capsules. By 20 days after fertilization degeneration has already set in. The tubules move closer together, the most anterior nephrostome being the first to disappear. During this time ciliated peritoneal funnels may develop, which connect the nephrostomial canals with the body cavity; these appear to be simply a phenomenon of degeneration. The pronephros of Acipenser is more primitive and better developed than in other Ganoids and in Polypterus, and the fusion of the chambers is more extensive than in any other vertebrate. The nomenclature of that part of the mesoderm which gives rise to the pronephros is discussed.