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Embryogenesis of the hemopoietic tissue was histologically studied in Rana temporaria at successive developmental stages, beginning from neurula and up to the end of metamorphosis. Primary foci of hemopoiesis (yolk sac, pronephros, hepatic diverticulum) were also studied radioautographically. Histological description of the hemopoietic tissue in R. temporaria is presented according to common stages of development in the species investigated. The data concerning the early and, in fact, simultaneous appearance of primary blood cells (PBC) in embryonic foci, as well as the results of the radioautographic investigation on the dynamics of PBC proliferation demonstrate that there possibly exist some local sources of hemopoiesis in embryonic hemopoietic organs.

Temporal hemopoietic organs in the quail embryo are the yolk sac, heart, mesonephros, metanephros, liver, pancreas, small intestine and its mesentrey. In some organs either only erythroid cells (the heart, small intestine and its mesentery), or only granulocytes (the pancreas) are formed. In other temporal organs cell differentiation of both erythrocytic and granulocytic series takes place; as a rule, the erythrocytic series predominate. Maximum of erythro- and granulocytopoiesis in all the temporal hemopoietic organs occur on the 9th--11th incubation days. By the end of the second week of incubation the hemopoietic activity either essentially decreases or stops. The definitive hemopoietic organs are the bone marrow, spleen and organs of the lymphoid system--the thymus and the bursa Fabricii. Differentiation of erythrocytes is especially active in the spleen; granulocytes are predominantly formed in the bone marrow. In the thymus and bursa Fabricii the phase of temporal granulocytohemopoietic activity precedes lymphocytopoiesis. A complete change of erythrocytic population takes 5--6 days of incubation. The definite erythrocytes, even before they appear in blood islets of the yolk sac, are identified in the cardiac hemopoietic focus and in the kidneys of the embryo extremities on the 3d--4th day of incubation. A comparative investigation on development of the hemopoietic tissue in embryogenesis of the quail, hen and starling has been performed.