Fig. 1. Time-lapse analysis of the initiation of erythroblast circulation. Images were taken of a 6-somite mouse Tg(-globin-GFP) embryo every 6 minutes for a total of 12.1 hours (see Movie 1 in the supplementary material). The yolk sac (YS), heart (H) and somites (S) are indicated. (A) Very few erythroblasts (arrow) are visible in the yolk sac and embryo at the start of the movie (t=0). (B) The same field of view 1 hour later, indicating an increase in erythroblasts. Erythroblasts marked with red arrows are not circulating. (C) Image taken 8.1 hours after the start of the movie, showing more cells moving into the embryo and beginning to fill the heart (H). (D) Fluorescent cells become more evident in the heart towards the end of the time period by 11.7 hours in culture. Clumps of erythroblasts are seen as some cells stop circulating, whereas others continue to move freely.