Changes in the erythrocytes of the developing opossum, Didelphia virginiana.

Changes in the erythrocyte population during postnatal development of the opossum have been studied. At birth, the red cells of the peripheral blood number less than 600,000/cmm and the characteristic erythrocyte is a large nucleated cell ('megaloblast'). These are rapidly replaced by smaller, nonnucleated cells. The results indicate that the yolk-sac period of blood formation is short and that the hepatic phase of hemopoiesis begins prior to birth. The data also suggest that megaloblasts in the opossum may be able to transform into megalocytes.