The morphology of the haemopoietic cells of the yolk sac in mice with particular reference to nucleolar changes.

Nucleolar changes of haemopoietic cells during the development of the mouse yolk sac blood islands were qualitatively examined by light and electron microscopy. Haemopoiesis in the yolk sac begins at seven days of gestation and, after the formation of vitelline vessels, erythropoiesis appears to be active at ten days. At 7-8 days of gestation, angioblasts have large and reticulated nucleoli in connection with the nuclear envelope either through the invagination of the nuclear membrane, i.e. canals and tubular or lamellar structures, or directly. The nucleoli of the fixed primitive erythroblasts at 8-9 days are of compound type and invaginations of the nuclear membrane are rarely seen. In free erythroblasts, fibrillary areas become obscure. In matured erythroblasts, remnants of the nucleoli are present in the heterochromatin masses. Nucleolar changes in each stage are so characteristic that nucleolar structure can be useful in the recognition of the differentiation stages of yolk sac haemopoetic cells.