The establishment of the nature of the chemical relationship of the purin bodies, the diureides which form the chief end-product of proteid metabolism in birds and reptiles, with urea, the endproduct in mammals, is not the least important of the valuable contribution of E. Fischer to physiological chemistry. The constant presence of diureides in the mammalian urine, and of urea in the urine of birds, shows that,no hard-and-fast line exists between the metabolic processes in the two groups, and the fact that in the mammalian foetus an important end-product of metabolism is a diureide—allantoïn—is a further proof of the close affinity of the tissue changes throughout the vertebrate series.