Age, 6.9 days.

Standard length, 2.80 mm.

Length of caudal fin, 0.12 mm. Mesencephalon width, 0.79 mm.

The small, stellate, epineural melanophores are more numerous and scattered over the brain region, extending along the dorsal midline slightly beyond the pectoral fin buds.

The opercula are larger, covering the first three pairs of gill arches (fig. 21 B).

The full complement of 26 myomeres is present (fig. 2lA, B). Beyond the last myomere and the upturned tip of the notochord is the compressed caudal fin bud with primordia of six fin rays (fig. 21 B). An anal fin primordium is present (fig. 21B).

The intestine is S-shaped in ventral view. At the anterior end of the intestine, near where it gradates from the esophagus, is the liver, connected to the left side of the intestine by means of a bile duct (fig. 21B). At the same level is the pancreas, extending ventrally and to the right of the intestine in a series of nests of cells embedded in the mesentery. The anteriormost end of the pancreas consists of a more compact lobe of relatively undifferentiated tissue. Connected to the right side of the intestine, at this level, is the ductus pneumaticus, characterized by a columnar epithelium and a fine circular lumen. The swim bladder is small, about the width of the intestine, flattened, and in a position dorsal to the intestine. Its epithelium is high columnar and highly vacuolated. The swim bladder extends retroperitoneally and caudad to about the level of the first myomere (fig. 21B). This structure is described in greater detail in stage 18.

The gonadial ridges originate near the base of the swim bladder and are attached to the dorsal peritoneum. They extend caudad to the level of the fourth or fifth myomeres. The mesonephric ducts possess a lumen throughout their length and empty into the base of the urinary vesicle. Mesonephric tubules are numerous at the level of the pectoral fin buds, and a few renal capsules and glomeruli are visible.

The ventral aorta is still double, but the anterior branch is reduced in size. A fine, unpaired external carotid (or lingual) artery branches off the base of the anterior branch of the ventral aorta (fig. 14D).