In many aspects hematopoiesis in newborn rodents, especially in rats, resembles hema- topoiesis in the human fetus in the 6th-7th month of gestation. In man the transition from the stage of liver to bone marrow erythropoiesis takes place at this time (Bessis, 1973). In rodents, however, the liver is almost the only place where hematopoiesis occurs until birth. Thereafter it is replaced to a growing extent by the bone marrow, which so far consists mainly of immature mesenchymal cells (Maximow, 1910; Cuda, 1970). Thus hematopoietic precursor cells appear in the sternum only around 30 h after...