ENLARGED PERICARDIUM IN LEBISTES EMBRYO 



189 



of the various embryological stages were made from living spécimens with the aid 

 of a Wild Stereomicroscope M 5. * 



The spécimens were fixed in alcoholic Bouin solution. Due to the brittleness 

 of the yolk following fixation, différent clearing agents were used: a) methyl- 

 benzoate, b) terpineol-alcohol (after Wetzel, 1931), c) isopropoylalcohol- 

 paraffin mixture with subséquent swelling of the paraffin blocks in distilled water 

 (after Wessing and Claes, 1958). Alternatively, the blastodisk or embryo was 

 separated from the yolk after fixation, thereby rendering subséquent swelling of 

 the paraffin blocks unnecessary. The embedding wax used was Merck paraffin 

 (mP 56-58° C) with addition of some bees'-wax. Sections were eut at 5\l and 

 7(jl. Stains used were acid hemalum (Mayer), hematoxylin (Weigert), hematoxylin 

 (Heidenhain), hematoxylin (Prenant), counterstained with benzopurpurin, eosin, 

 orange G and picroindigocarmin. 



3. RESULTS 



For anyone not completely familiar with teleostean development, the 

 following introductory remarks a) — d) are included: 



a) The surface ectoderm in the teleosts is made up of two différent layers: 1) the 

 epidermic stratum (couche supérieure of Henneguy), which is the upper sheet. 

 It consists of a layer of flattened cells, which does not contribute to the forma- 

 tion of the neural System. It persists into adult life as the outermost layer 

 of the epidermis; 2) the strata of "nervous layer" cells (couche profonde of 

 Henneguy), in the following abbreviated as "nervous ectoderm". Thèse 

 strata give rise to the central nervous System, the sensé organs and the epidermis. 



b) The neural System, as other Systems in teleosts, is laid down as a solid mass 

 of cells and only later hollows out. 



c) The periblast (yolk sac entoderm) refers to a layer found superficially on the 

 yolk. It overgrows the yolk with the advancing blastoderm (germ) ring 

 during epiboly. Its nuclei are scattered in a cytoplasmic syncytium and, at 

 the end of cleavage, appear greatly increased in size. They develop pro- 

 tubérances and loose the capacity to divide by normal mitoses. 



d) The urinary bladder in the teleostean embryo lies dorsally to the rectum and 

 opens just behind the anus. In Lebistes reticulatus the bladder increases 

 greatly in size during development. It grows as a bilobed structure into the 

 yolk sac and at the end of the embryonic peried almost fills the space originally 



* Obtained with a grant from the Royal Dublin Society. 



