In a discussion of the chemical heterogeneity of protagon, the author previously alluded to the probability that phrenosin and cerebron were identical.1 Thierfelder recently published in rejoinder some opinions to the contrary.2 Reexamination of the facts in the case have convinced the writer that Thudichum's phrenosin, Gamgee's pseudocerebrin, Parcus's cerebrin, Kossel and Freytag's cerebrin, Thierfelder's cerebron and Koch's cerebrin were practically the same. The slight discrepancies among the figures for percentage elementary composition were probably due to slight proportions of inevitable impurities in each preparation.Of the products referred to, phrenosin and cerebron have been subjected to the most thorough study. The descriptions of each are in close harmony. Each has been found to yield, on hydrolysis with dilute sulfuric acid, apparently the same proportions of a sugar (galactose), a nitrogenous base (sphingosin) and a peculiar organic acid. The following data were obtained on direct analysis ...