The nitrogen-containing crystalloids of blood have interested physiologists and clinicians for almost a century and in some respects the questions at issue are but little clearer to-day than in the time of Bright. Methods of research have changed and the data secured are more accurate, but it is remarkable in reviewing the literature that many of the conclusions of the later investigations are a simple rewording of the earlier. The facts have, however, been amplified and our knowledge made more complete without as yet any unaminity of opinion as to the significance of these facts. The amount of data at our disposal now is not inconsiderable and it may not be a useless task in the beginning to survey this field. Urea was first detected in the urine (matière savonneuse), by Rouelle, Jr., in 1773, and in 1821 Prevost and Dumas observed an accumulation of urea in the blood of