[The development of yolk circulation in normal rat embryos].

To reveal the developing vessels of the yolk placenta the erythrocytes were stained with benzidine. The development of vessels was studied in vivo and in transplantation of embryos in the homologous blood serum. The pattern of branching of blood vessels in the yolk sac of 12-day embryos was established. The development of the blood circulatory system started on the 9,5 day (the anlage of sanguineous islands), the definitive state was acquired by the 12th day. The development of vessels in vitro after removal of the parietal layer of the placenta is dependent on the mophological stage at the moment of transplantation. In explantation of embryos with 2--3 pairs of somites which are being already formed, the yolk vessels develop as those in control. In transplantation of presomite embryos the development of vessels is sharply disturbed, in transplantation of embryos of the egg cylinder stage the vessels are not developed. The inducing role of the yolk placenta perietal layer in the development of blood vessels in the vascular layer is supposed.