Here we see, as would be expected, that on the high proteid diet, there was a large excretion of total nitrogen and of urea; while on the low proteid diet, nitrogen and urea were correspondingly diminished. The point to attract our attention, however, is the marked difference in the percentage of urea-nitrogen in the two cases; a difference which amounts to about 26 per cent. A similar difference is to be noted in the percentage of uric acid-nitrogen. Lastly, it is to be observed that in spite of the great difference in the extent of metabolism on the two days--an excretion of 16.8 grams of nitrogen, as contrasted with 3.6 grams--the _amount_ of creatinin-nitrogen is essentially the same. Folin finds that these peculiarities in the percentage distribution of excreted nitrogen hold good in all cases where there is this wide divergence in the amount of proteid katabolized, and, further, that there is a gradual and regular transition from the one extreme to the other. He sees in these results evidence that there are in the body two forms of proteid katabolism, essentially independent and quite different. One kind is extremely variable in quantity, while the other tends to remain constant. The variable form has its own particular kind of waste products, of which urea is the chief. The constant katabolism, on the other hand, is largely represented by creatinin and to a lesser degree by uric acid. The more the total katabolism is reduced, the more prominent become creatinin and uric acid, products of the constant katabolism; while urea, as chief representative of the variable katabolism, becomes less conspicuous. Folin suggests the term _endogenous_ or _tissue_ metabolism for the constant variety, while the variable form he would name _exogenous_ or _intermediate_ metabolism.