Enrichment of progenitors in extraembryonic (yolk sac) versus intraembryonic regions. Numbers of primitive MEP, definitive MEP, or unipotential Meg-CFC hematopoietic progenitors (as determined by immunohistochemistry) and erythroblasts in all intraembryonic tissues versus yolk sac. Fold enrichment was calculated as the ratio of intraembryonic progenitors to erythroblasts divided by the ratio of yolk sac progenitors to erythroblasts.³⁵  Tissues from individual experiments were grouped and averaged by stage (± SEM).