Hemopoietic cells in the liver and spleen of embryonal and early neonatal mice were karyometrically examined by light microscopy. Hepatic hemopoiesis started at 11 days gestation, and proerythroblasts and less differentiated cells with nuclei larger than 7 micron in diameter, appeared in the primitive hepatic cords. After 12 days, the liver contained a number of mature erythroblasts. The immature large hemopoietic cells disappeared from the liver at 16 days gestation. Splenic hemopoiesis began at 16 days gestation. Hemopoietic cells with nuclei measuring larger in diameter than 8 micron appeared in the splenic cords, and the immature cells could be seen even at 2 days after birth. Development of embryonic hemopoiesis from the liver to the spleen is discussed in relation to yolk sac and bone marrow hemopoiesis.