Fig. 8. Frequency of blood-forming CD34⁺ CD45^– endothelial cells in embryonic and fetal hematopoietic tissues. CD34⁺ CD45^– cells sorted from the yolk sac, AGM, embryonic liver and fetal bone marrow were grown on MS-5 stromal cells in limiting dilution conditions. Hematopoietic cell colonies developing on or underneath MS-5 stromal cells were scored at week 5 under a phase-contrast inverted microscope. The number of cells seeded per well was plotted against the logarithm of the percentage of negative wells. As calculated from the line of best fit, the cell number at log 37% negative wells is the frequency of hemogenic cells in the starting population. D, days; W, weeks.