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A great difference exists in the adult bone marrow of central bones as compared with outlying bones of the mammalia and avia, the distal bones being at a great disadvantage from the standpoint of blood cell production. Several experimental procedures are reported by which this disadvantage is overcome and in consequence fatty marrow of outlying bones is replaced by red marrow occurring chiefly at the epiphyseal regions, unless a low oxygen stimulus is also provided when marrow of the diaphysis becomes involved. A common factor in all of the experiments was an elevation of temperature beyond that prevailing in these distal regions, and it is felt that the evidence warrants the opinion that the cause of improvement is thermal. In some experiments, blood cell formation was increasing while the heat was adversely affecting the testis. The experiments permit construction of a general theory of fat distribution in bone marrow. In certain grafts of precartilage to other rats, normal differentiation into bone, cartilage, and marrow occurred, while in others cartilage and very small amounts of primitive marrow developed with slight, or no bone formation. Cartilage was always successfully engrafted. The capacity to form sinusoids in bone marrow is determined by the nature of the tissue rather than by the ingrowing endothelium.

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