Our knowledge of the development of the excretory system in both vertebrates and invertebrates is as yet very incomplete, perhaps more so than of any other system. Until quite recently the whole of the urogenital system of the vertebrates was supposed to be derived from the niesoblast. This view received a sudden check when, on the publication of Graf Spee's researches on the guinea pig in 1884, the segmental (pronephrie) duct was shown to have an epiblastic origin. Hensen, indeed, had noted the fact some years previously, but no notice had been taken of his discovery until Graf Spee called attention to it. Hensen has recently taken up the subject again, and Flemming has published a confirmatory account for the rabbit. Thus there appears no further room for doubting the epiblastic origin of the segmental duct in mammals. It does not necessarily follow that the whole excretory system has an epiblastic origin, but further information is required on the subject. Towards the end of 1886 Van Wijhe demonstrated the epiblastic origin of the segmental duct in Elasmobranchs, and during the present year Von Perenyi has announced that the epiblast plays a similar part in Rana and Lacerta. During the past few months I have been enabled to confirm Von Perenyi's researches so far as Rana is concerned, and have also found that, in regard to the formation of the segmental duct, Teleostean fishes, and probably also birds, are in agreement with other types. The epiblastic origin of the segmental duct is probably a feature common to the Vertebrata generally.