The first haematopoietic sites in Brachydanio rerio are formed by the "intermediate cell mass", situated between somite and lateral plate. The sequential sites are in the endocardium, pro-and mesonephros. The results are compared with those of Pterophyllum scalare, which has its first blood anlage on the yolk sac.The classical theory of blood formation in teleosts maintains that the first haematopoietic sites are situated intraembryonally, in the intermediate cell mass of OELLACHER (6,7,10,(13)(14)(15)(18)(19)(20). This placed the teleosts in direct contrast to the chondrichthyes and the amniota. Subsequently, teleosts were found which have additionally or exclusively, extraembryonic haematopoietic sites, i.e. blood islands situated on the yolk sac_. ( 1-4, 16, 17).The sequential sites of haematopoiesis -operating from the embryonic to the adult phase -were established and discussed for Pterophyllum scalare, a "blood island teleost" (1 ). It seemed, therefore, particularly interesting to determine the sequential sites in an "intermediate cell mass teleost", in order to compare and contrast ontogenesis of blood forming sites in both types. Brachydanio rerio was chosen because there were indications that its first blood anlage is in the intermediate cell mass (4, 11).
MATERIAL AND METHODSFertilized eggs (diameter 1.15 mm) of Brachydanio rerio (HAMILTON-BUCHANAN) were reared in the laboratory. At 27°C the embryonic phase lasts 3 days and the postembryonic phase, until complete absorption of the yolk, 4 days (1 1). The embryos were staged according to the description of HISAOKA and BATTLE (8).Fixatives used were Helly's and Stockard's fluids. Sections were cut serially at 5, 6 and 7 M, stained with 1) haematoxylin (Weigert) counterstained with picroacid fuchsin (van Gieson) and 2) May-Grunewald, counterstained with Giemsa. Benzidine tests were performed according to the method described by DOHERTY, SUH and ALEXANDER (5).
RESULTS
Embryonic phaseAccording to the stageing of HISAOKA and BATTLE (8) gastrulation covers stages 1-17. At the closure of the blastopore, the embryo possesses two pairs of somites.12 Somites (early stage 19): Auditory placode and eye primordia are present. In the anterior and mid-trunk regions (somites 1 -10) the lateral plate is detached from the somite, * This paper is dedicated to Prof. F. Verzar for the celebration of his 90th birthday.
