Another anomalous example of the function of this tissue is found in the hematopoiesis of birds. While there seems to be some evidence that in the spleen and liver red blood cells may be formed, there is perhaps at most times sufficient bone marrow to keep the blood cells at a proper number. There is no inverse relation between the amount of bone marrow and that of splenic bulk; that is in diving birds with their hollow bones the latter is no greater than in gallinaceous birds which have abundant marrow in all the skeleton. It has been suggested that the spleen supplies an activator for pancreatic ferments. This receives a sort of support from the greater size of the organ in carnivores and smaller relative size in ungulates, since in the former concentrations of digestive enzymes are more often needed. Because of the greater excretion of iron in splenectomized animals or those being starved, Fischer has thought that the spleen has some power to metabolize this element; this receives some support from the statements concerning the size of this organ in the carnivorous orders. From experimental studies it appears that some relation exists between the lymphocytes and resistance to implanted tumors. If one apply this idea to the amount of lymphatic tissue and the incidence of spontaneous new growths, it is found that no definite relationship is discoverable either in terms of size of spleen or richness in lymphatic nodes. Carnivores, rodents and marsupials show a high tumor incidence and have a good lymphatic supply. Aves, on the other hand, with a poor nodal apparatus and a variable splenic bulk, show many fewer tumors than do the mammals. The mononuclears of avian blood rise normally as high as 60 per cent., of which 40–45 per cent. are of the small size.