Fig. 2. Initiation of plasma flow. 10x10³ M_r fluorescent dextran was injected into the heart (Hrt) of early mouse embryos. The embryos were incubated for 10 minutes and images were taken at 5x magnification. The presence of fluorescence in the yolk sac after such a short incubation was interpreted as the result of flowing blood plasma. In most 2-somite embryos (5 out of 6 injected embryos), dextran remains localized to the heart (A and B; arrow in B). In one case, however, fluorescent dextran could be observed throughout the yolk sac (ys) (C and D; arrow in D). Plasma circulation is consistently observed after the 3-somite stage (E, 3 somites; F, 6 somites). A and C are images taken with brightfield illumination overlaid with fluorescence images of fluorescein-dextran within the vessels (fluorescent microangiographs); B,D,E and F are microangiographs.