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.

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.

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In the premetamorphic larval green toad, B. viridis viridis, as in other anurans, the skin is made up of a fibrous dermis and an epidermis of stratified epithelium. The effects of bromocriptine, an antiprolactin drug, on the premetamorphic skin of B. viridis viridis was examined. Bromocriptine, dissolved in rearing water at four different concentrations, induced a number of changes in the skin of treated tadpoles. In rough sequence of appearance, these changes include: retraction of the melanocyte dendrites, synchronous burst of the apical vesicles of the superficial epithelial cells, gradual disappearance of the melanosomes from the epithelial cells and widening of the intercellular spaces. In addition, macrophages appeared in the superficial dermis amongst the retracted melanocytes. White crystals were observed on the skin surface and similar crystals were ingested by the macrophages. Prolonged treatment with bromocriptine resulted in hypertrophy and extraction of some epidermal cells. Deep melanocytes of the mesenteries were not affected by bromocriptine-treatment indicating that the drug did not penetrate deep into the tadpole tissue. Whether the macrophages observed in the dermis were recruited from deeper tissues or were converted melanocytes is another issue in need of study.

The flank skin of the premetamorphic stage of the green toad, Bufo viridis viridis, was studied by transmission electron microscopy. The skin was typically larval amphibian, made up of a highly fibrous and vascular dermis to the inside and a highly cellular epidermis to the outside with a distinct basal lamina separating the two layers. The epidermis is three-cells thick. The epidermal cells, mostly keratinocytes, are closely packed together and are attached to each other through desmosomes and interdigitations. The basal keratinocytes are anchored to the basal lamina through hemidesmosomes. In addition to the keratinocytes, some other types of cells known in the amphibian larval skin were found. These cells include mucus-secreting, mitochondria-rich cells, Merkle cells and flask cells. Anew type of cells, the dark cells, are described in this paper. The dark cell rests on the basal lamina. It is a ramified cell with a number of cytoplasmic processes intervening in between the keratinocytes. The cytoplasm is strikingly dark and rich with polysomes and granular endoplasmic reticulum.