The significance of the observation by Staedeler and Frerichs (1858) of "colossale Quantitaten" of urea in the blood and tissues of some skates and a shark remained obscure for four decades. In 1897 Bottazzi reported that the freezing point depressions of elasmobranch blood and of sea water were approximately equal, that is, they had approximately the same total solute particle, or osmolal, concentrations. But whereas sodium and chloride make up the bulk of solute particles in sea water, Rodier (1899) observed that urea accounted for about one-third of the solute particles of elsamobranch blood. Rodier also noted that the freezing point depression of the fish blood was generally slightly greater than that of the animal's sea-water environment, an observation which has been re-