Haemopoietic centres in the developing angelfish,Pterophyllum scalare (cuvier and valenciennes).

The first haemopoietic centres in the embryo ofPterophyllum scalare are found in the blood islands of the yolk sac. These results are in contrast to the classical theory of blood formation in teleosts, which maintains that the first blood formation occurs intraembryonically, in the so-called intermediate cell mass of Oellacher. InPterophyllum, the intermediate cell mass forms only the axial blood vessels. Haemopoiesis in the post-embryo is carried out by the pronephros. This organ remains haemopoietic to the adult stage. In the adult, the pronephric tubules are degenerated; the organ is filled with haemopoietic tissue and also contains strands of adrenal tissue. The adult kidney (mesonephros) is also haemopoietic, though to a much lesser degree than the pronephros.The blood islands in the yolk sac form only stem cells (haemocytoblasts) and proerythroblasts. Released into the circulation, they differentiate and mature into round, disc-like erythrocytes (erythrocytes-E). Erythropoiesis in the pronephros produces elliptical erythrocytes (erythrocytes-ImA). Thus for the latter part of the postembryonic phase, until complete absorption of the yolk, there is a mixed erythrocyte population in circulation. During metamorphosis into the laterally-compressed adult, the adult type of erythrocyte (erythrocyte-A) makes its first appearance. Leucocytes and thrombocytes appear much later in development than the red blood cells. They are formed in the pronephros and are seen in circulation only after the yolk has been absorbed.