1. Prehepatic embryonic erythropoiesis is influenced when normal pregnant rats are injected with potent antianemic substances.
2. Both liver extract and PGA (folic acid) cause a reduction in mean cell diameters, but the former is slightly more effective.
3. The rate of mitosis is increased and the relative proportions of individual phases altered.
4. The distribution of light areas in the cytoplasm—negative images of organoids as seen in dry smears—is altered to a greater extent by PGA than by liver extract.
5. The in vitro and in vivo action of drugs on erythropoietic cells are discussed.