ABSTRACT
The quantitative representation of the various developmental precursors of the erythrocyte as identified in Romanowsky type dye stained dry film smears of tibiotarsal bone marrow has been assessed in several avian species. The erythroid precursors comprised 92% of all hemopoietic cells in the bone marrow in the three-week-old chicks. In ducks less than 21 days of age active marrow could be found within all regions of the examined bones. After 30 days the marrow was hemopoietically most active in the humerus. Adult Larus ridibundus the black-headed gull maintains a tibiotarsal hemopoietic bone marrow whose erythropoietic cellular composition is comparable to that of the adult Iranian duck. The hemopoietic humeral bone marrow of Gopherus agassizii the desert gopher tortoise maintains at maximum, depending on the techniques of obtaining the cells, only 3–6% erythropoietic precursors. The bone marrow was adopted by the subsequent avians and mammalians as the sole, principal locus for erythropoiesis.
