We observed mouse visceral yolk sacs (VYS) in the early and middle gestational periods. Animals: The VYS of mouse of various ages were examined; embryonic days 9 (E9), 10 (E10), 11 (E11) and 12 (E12). Tissues were processed for light and transmission electron microscopic observation as reported previously. Results and discussion: Erythrocyte-like globules were observed in the cytoplasm of the endodermal epithelial cells (E12) (Fig. 2a, b). On the other hand, erythrocyte-like globules were not found in the early periods of gestation (E9, E10, E11) (Fig. 1). Therefore, we think that erythrocyte-like globules took place of blood islands.
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