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The endoderm can be separated from the ectomesoderm of the area vasculosa of the early chick embryo. The development of the separated germ layers in organ cultures shows that the endoderm is necessary for the normal formation of blood islands by mesodermal cells. In the absence of endoderm the mesodermal cells undergo erythropoiesis in dispersed groups of cells but do not form endothelium.
Fred H. Wilt (Fri,) studied this question.