| Male and female fish are distinguished by the rounder belly of the silvery female (gravid with eggs) and the slimmer, pinker male (top panel). Each egg is surrounded by a chorion (shown only for the fertilized egg; second panel). Cleavage of the blastodisc creates a mound of cells sitting on top of the yolk (third panel). These proceed to envelop the yolk, beginning at the sphere stage (fourth panel). Gastrulation begins at 50% epiboly. Involution movements around the margin produce the thickened germ ring, while involution and convergence movements produce a more prominent thickening, the shield, on the future dorsal side of the embryo (fifth panel). Epiboly and gastrulation progress until the entire yolk ball is covered with cells, most of them on the dorsal side (sixth panel). Between 10 and 24 h after fertilization, segmentation generates pairs of somites on either side of the midline, useful as a staging tool. By 18 hpf, 18 somite pairs have formed, the tail begins to extend and the rudiments of the brain and sensory organs (eye and ear) are readily apparent (seventh panel). The embryo hatches from its chorion between the second and third days. By day 5, it has an inflated swimbladder and actively pursues live prey. |
