which are being constantly studied by the chemist. They also are to be found in great variety. It has been difficult to separate them one from the other, to characterize them rigorously, or, in other words, to classify them. However, it has been done now, and we distinguish three classes which are differentiated at once from the physiological and from the chemical points of view. The first comprises the complete or typical albuminoids. They are the proteids or nucleo-albuminoids. They are to be found in the most active and most living parts of the protoplasm, and therefore in the spongioplasm of the cell and around the nucleus. The second group is formed of albumins and globulins, compounds already simpler, fragments derived from the destruction of the preceding, into which they enter as constituent elements. In the isolated state they do not belong to the really living protoplasm; they exist in the cellular juice, in the interstitial and circulating liquids in the blood and in the lymph. The third category comprises real but incomplete albuminoids. They are to be found in the portions of the economy which have a specialized or attenuated life, and are destined to serve as a support to the more active elements—i.e., they contribute to the building up of the bony, cartilaginous, conjunctive, elastic tissues. They are called albumoids. It is naturally the first group, that of the proteids—i.e., of the complete and characteristic compounds of the living substance—upon which the attention of the physiologists must be fixed. It is only quite recently that the clear definition of these substances has been given, and proteid compounds detected in the confused mass.
The Nucleo-proteids.—This progress in the char-
