All the proteids contain phosphorus in addition to the five chemical elements, carbon, oxygen, hydrogen, nitrogen, and sulphur, which are common to the other albuminoids. Another interesting feature in their history is that the action of the gastric juice divides them into their two constituents:—the nuclein, which is deposited and resists the destructive action of the digestive liquid, and the albumin or histone, which on the contrary experiences this action with the usual consequences. Thus the gastric juice furnishes a process which is very simple and very convenient in the analysis of the proteids. _Localization of the Nucleo-Proteids._—What we said before as to the important physiological rôle of the cellular nucleus may arouse the expectation that in it will be found the living matter which is chemically the most differentiated, the albuminoids of highest rank—_i.e._, the nucleo-proteids and their constituents. Not that they would not be found in the protoplasm of the rest of the cell, but there is certainly a risk that they would be less concentrated there and more blended with accessory products; they are there connected with much more secondary vital functions. This conclusion inspired the early researches of Professor Miescher, of Basle, in 1874, and, twenty years later, those of Professor Kossel, one of the most eminent physiological chemists in Germany. In fact, these compounds have been found in all tissues which are rich in cellular elements with well-developed nuclei. The white globules of the blood furnished to Lilienfeld the first nucleo-histone ever isolated. The red globules themselves, when they possess a nucleus, which is the case in birds and reptiles as well as in the embryo of mammals, contain a nucleo-proteid which was easily isolated by Plosz and Kossel. Hammarsten, the Swedish chemist, who has acquired a great reputation from his researches in other domains of biological chemistry, prepared the nucleo-proteids of the pancreas in 1893. They have been obtained from the liver, from the thyroid gland (Ostwald), from brewers’ yeast (Kossel), from mushrooms, and from barley (Petit). They have been detected in starchy bodies and in bacteria (Galeotti). § 2. CONSTITUTION OF NUCLEINS. _Constitution of Nucleins._—Our path is already marked out if we wish to penetrate farther into the constitution of these proteids, which are the immediate principles highest in complexity among those which form the living protoplasm. We must analyze the two components, the albumins and the histones on the one hand, and the nucleins on the other. As for the nucleins, this has already been done, or very nearly so.