[Hematopoiesis in the yolk sac of several representatives of the Sauropsida].

Hemopoiesis in the yolk sac of chicken and crocodile embryos was studied at different developmental stages. Primary intravascular erythropoiesis is closely related to the formation of sanguineous islands appearing in the zone of the yolk growth in the visceral mesoderm. With the development of folds enlarging the surface of the yolk resorbtion, around the vessels running within the folds there appear foci of primary granulopoiesis. The process begins in the paravasal mesenchyma which is gradually disguised by hemopoietic cells (in chicken embryos--at the stage of 8 days, in crocodiles--23 days of incubation). The granulopoiesis continues in chicken embryos during 1/3 of the incubation period, in crocodiles--during 2/3 of the incubation time. The leukopoiesis foci are developed more intensively in crocodiles as well at the size of the yolk sac folds. Leukocytic accumulations disappear in the crocodile sac after hatching (38-40 cm). The change of primary erythropoiesis for the secondary one in the chicken is preceded by the appearance of megaloblastic forms of erythrocytes with the compact homogeneous cytoplasm. They are absent from crocodiles.