The condition of the bone marrow corresponds with fair accuracy to that which one is accustomed to see in the human being. Certainly this holds good for the mammals, while among the birds, the few observations upon which we feel like relying indicate a nodular erythropoiesis of rather striking character. In the areas of reddening as seen grossly there will be found under the microscope an orderly arrangement of large red cells with loose chromatic nuclei about a very much larger cell of the same type, apparently the primary erythroblast. Outside of this group, red cells such as appear in the circulating fluid, are rather irregularly distributed in a marginal zone. I have seen small areas like this in apparently normal marrow, but the central grouping was not so large as in the anemic cases; it thus appears that we probably have the anatomy of erythropoiesis. The deposition of pigment in the birds is in much coarser granules than among the mammals, in the former case large masses sometimes obscuring several liver cells or apparently blocking a lymphatic sinus; the Kupffer cells do not seem to be heavily laden. The extramedullary formation of blood cells has been a matter of considerable interest and study in the human being, and as far as it concerns the circulating mononuclears, the general opinion seems to be that such a histogenesis exists. A decision in the negative is perhaps reached by the majority in the case of erythropoiesis, and as far as my observations go, this holds for all mammals. It seems worth while, however, to record an occasional finding in some birds, especially anemic ones, which may be of importance in their erythropoiesis. The adult red cell is a clearly formed ellipse with a distinct, deeply stained, sharply outlined nucleus of a shape corresponding to that of the whole cell. Young red cells have a more nearly circular outline but almost truly circular nucleus, the short diameter being at least proportionately greater than is the corresponding short diameter of the whole cell; this is also the nucleus whose internal structure resembles that of the human plasma cell. Groups of such cells have been seen in the interstices of the liver, sometimes as many as twelve, in a rather orderly formation. An excess seems at times visible in the spleen but not in orderly arrangement. Observations are under way toward determining the relation of this finding to the amount of marrow, the condition of the blood and the habits of the bird. Primary Anemia. As already specified primary anemia is apparently causeless, aside from the assumption that it is a disease of the marrow itself. Since there are only four cases, representing three orders and they cannot be grouped as could the secondary variety, the individual instances will be discussed separately.