ENLARGED PERICARDIUM IN LEBISTES EMBRYO 



197 



now occupies most of the yolk sac. Pectoral fins beat rhythmically. Movements 

 of lower jaw become évident. Pelvic fin primordia appear, and rays develop in 

 dorsal fin. The anterior tip of the head pushes through the pericardial sac by 

 separating the pericardial folds. Thèse subsequently narrow to a small band, 

 the so-called neckstrap. This neckstrap is still supplied with blood from the 

 anterior branches of the ductus Cuvieri. The vitelline blood circulation is 

 greatly reduced (Fig. 5a). 



b) Semicircular canals of the labyrinth develop. 



The vascularization of the neckstrap remains restricted to the external sheet 

 of the pericardium and is still continuous with the vitelline circulation. The 

 vessels of the neckstrap do not traverse the dorsal midline (Fig. 6e, 7e). 



Stage 8 (25-30th embryonic day) 



a) Embryo measures up to 8.5 mm. Opercular movements have started and 

 resuit in a dorsal splitting of the neckstrap. The remnants of the neckstrap slide 

 down and perivitelline fluid can now leave the gill cavity freely (Fig. 5b). Urinary 

 bladder is emptied, which results in the yolk remnants being taken up into the 

 body. Subsequently birth takes place. The newly born fish feeds immediately 

 and swims to the surface to fill the swimbladder. 



b) After ovulation, the highly folded wall of the urinary bladder is conspic- 

 uous. Body cavity is filled with the remnants of the yolk; ductus Cuvieri seen 

 amidst the yolk. 



Remnants of the neckstrap are seen overlying the labyrinth. Its vessels 

 break down within a few days after birth. 



4. DISCUSSION 



1) ECTODERMAL HEADFOLD IN TELEOSTS 



There is, as far as can be ascertained, only one publication that gives histo- 

 logical évidence of the formation of the ectodermal headfold in a teleost: 

 Henneguy (1888), in his treatise on the development of the trout, reports how 

 at the stage when the lens begins to develop (22 somites), the head région starts 

 to lift from the yolk sac (Fig. 2c-d). 



Wilson (1891), in his excellent description of the embryology of the sea bass 

 {Serranus atrarius) mentions that in this species the headfold begins to be 

 developed late in embryonic life. Earlier, the head is buried in the yolk and the 

 eye cup is in direct contact with the periblast of the yolk sac. As an adaptation 

 to the late appearance of the ectodermal headfold, the lens is formed from cells 



