embryonic hematopoiesis trait Any measurable or observable characteristic related to the function of or processes in the development of primarily large, nucleated erythroblasts as well as some megakaryocytes and primitive macrophages occurring in blood islands in the yolk sac, generally occurs from E7-E11 in the mouse and through 10 weeks in humans. Trait.ontology VT:0003396 primitive hematopoiesis embryonic development trait blood cell development trait