one of the types of blood-forming cells in vertebrate animals and man. In accordance with the theory of the origin of various blood elements from cells of a single type, hemocytoblast cells form erythrocytes, leucocytes, and megakaryocytes. Hemocytoblast cytoplasm is basophilic because of the high concentration of ribonucleic acid in it. Sometimes it contains azurophilic nuclei or threads. Hemocytoblasts originate in mesenchyme cells. In early stages of vertebrate development, the embryo contains hemocytoblasts in the vitelline vessels (initial or primary type); at later stages and in mature organisms the cells are concentrated in the blood-producing organs (secondary hemocytoblasts). In man, they are in the spinal column and the lymph organs for blood production. Hemocytoblasts are capable of dividing through mitosis.