Two types of immature erythrocytic series in the human fetal liver. 
Cells of erythrocytic series in the human liver obtained from 109 embryos 28 to 49 days after ovulation and 76 fetuses between 8 to 22 weeks of gestation were investigated by light and electron microscopy.  Antisera against fetal hemoglobin (Hb-F) were used in the immuno-peroxidase method to identify erythroblasts in the embryonic and fetal liver.  Immunoperoxidase staining for Hb-F revealed that most of the hemopoietic cells found in fetal hepatic parenchyma were erythrocytic in nature.  The cells of the erythrocyte series consisted of large immature cells which were usually invaginating into the cytoplasm of hepatocytes, and small mature erythroblasts which tended to gather in the subendothelial spaces of the sinusoids.  The early hepatic erythroid progenitor cells observed in the intercellular spaces of the hepatocytes until 10 weeks of gestation (the early stage of hepatic hemopoiesis) distinctly differed in ultrastructure from the late hepatic erythroid progenitor cells which appeared after 10 weeks of gestation (the late stage of hepatic hemopoiesis).  These findings indicate that the progenitor cells of the erythrocytic series and the hemopoietic stem cells in the early stage of hepatic hemopoiesis are morphologically different from those in the late stage, and that the cells of erythrocytic series in the liver in the early stage differ in the course of maturation from those in the late stage.