Age, 5.5 days.

Standard length, 2.52 mm. Length of caudal fin bud, 0.09mm.

Mesencephalon width, 0.68 mm.

This stage is characterized by the appearance of melanophores other than retinal. The retinal melanophores are obscured by pigment in the choroid layer, the latter contain ing numerous iridiodytes in addition to melanophores (fig. 20A). About 10 to 20 small, stellate epineural melanophores appear in the dorsal region above the metencephalon and myelencephalon (fig. 20A).

The optic lobes of the mesencephalon are greatly widened and thickened, forcing the eyes into an almost frontal position, and these lobes overlie the metencephalon and diencephalon (fig. 19A, B). The choroid fissure of the retina is distinct and possesses numerous fibers of the optic nerve (fig. 20B). The hypothalamic region of the diencephalon has enlarged so that it partially overlies the hypophysis and has pushed the infundibulum into a slightly more posterior position. Three bluntly rounded lobes can be distinguished as constituting the otocyst (fig. 19A, B): a ventral, saccular portion; an anterior, medial portion possessing a single large otolith; and a posterior portion with a single small otolith.

Lateral extensions of the mandibular and hyoid arches forming opercula are present here. The opercula are short and blunt, concealing only the first of four pairs of gill arches. Twenty-five somites are present, this number being only one fewer than the full complement (fig. 19). A caudal fin bud is present (fig. 19B).

The esophagus is short and distinct. The gastric region, described for stage 14, is absent. From the anteriormost intestinal region three primordia may be traced (fig. 20B). To the right is the short, tubular primordium of the swim bladder, surrounded by a plexus of blood vessels. Just posterior to the above and also extending to the right is a solid endodermal outgrowth in which pancreatic cells and ductules can be distinguished. At the same level, to the left of the intestine, is the liver (figs. 19B, 20B). The intestine is somewhat S-shaped in ventral-view and ends in the anus at about the level of the sixth somite (figs. 19B, 20B).

The most important development in the circulatory system is in the region of the ventral aorta. At this stage, the ventral aorta is split into two vessels (fig. 14C). The anterior of these drains a pair of small, efferent, spiracular arteries (first aortic arches), and supplies a pair of small efferent spiraculars (second aortic arches), and the third and fourth pairs of aortic arches. The posterior ventral aorta is connected to the common base of the fifth and sixth aortic arches and also supplies the fourth arch in part (fig. 14C).

At a level just anterior to that of the pectoral fin buds, a few mesonephric tubules have developed in the mesomere region dorsal to the mesonephric duct. The meso-nephric ducts possess short extensions 50 to 100 microns anterior to the above region. These extensions may be vestigial homologues of the pronephric ducts. Posteriorly, the mesonephric duct can be traced up to the base of the urinary vesicle. The latter structure has invaginated considerably and lies in a position dorsal to the anal portion of the intestine, its distal (or cephalic) end being slightly bilobed (figs. 19B, 20B).