Fig. 2

Extended depth of field projections of whole mounted larvae stained with osmium tetroxide (orientation indicated by compass, apical/frontal is up). a 1-dpf; a pair of frontal epidermal invaginations is present. The vegetally located blastopore has an open connection to the narrow midgut primordium (white arrowhead) which is recognizable by the darkly stained, yolk-laden cells. b 2-dpf; an apical tuft of elongated cilia has formed, the mouth opening has been shifted to the ventral side. The cells of the midgut still contain large amounts of yolk. Note the epidermal mucus gland cells from which mucus was extruded during fixation (black arrowhead). c 3-dpf; larva has an ellipsoid shape. The epidermal mucus gland cells have increased in number while the yolk in the cells of the midgut is considerably diminished. A faintly pigmented eyespot is visible on the ventro-lateral side of the larva. d 4-dpf; the larva has further elongated. The apical tuft and the mucus gland cells are more pronounced while the pigmented eye has enlarged. In the midgut epithelium no dark stained yolk reserves are visible. e 5-dpf; the larva has become longer and more slender. The pigmented eye is clearly visible close to the ventral midline frontal of the mouth opening. f 7-dpf; most advanced larval stage observed. The pigmented eye has moved further towards the midline of the larva. The lumen of the midgut is clearly empty. ap: apical, at: apical tuft, bp: blastopore, cmg: cells of midgut epithelium, do: dorsal, fi: frontal epidermal invaginations, gc: mucus gland cells, l: left, mo: mouth opening, pe: pigmented eye, r: right, ve: ventral, vg: vegetal