Age, 7.7 days.

Standard length, 3.29 mm.

Length of caudal fin, 0.23 mm. Mesencephalon width, 0.84 mm.

Small, stellate epineural and cutaneous melanophores are thinly scattered over the entire head and along the middorsal region of the body and tail. The melanophores over the mesencephalon are somewhat larger. Fine, dot-like melanophores appear in the lateral peritoneal lining (fig. 23A).

Both a dorsal and an anal fin bud are present, and the caudal fin possesses at least 10 rays (fig. 22B). The operculum, arising from the mandibular and hyoid gill arches, covers the four remaining gill arches (fig. 23B). Posterior to the last branchial arch (sixth visceral arch), the pharynx possesses a pair of shallow lateral evaginations. The latter are the vestigial seventh visceral pouches.

At the entrance of the ductus choledochus into the intestine, a ventral saccular outgrowth representing the gall bladder is present (fig. 23B). The diffuse pancreas also originates at this point and extends caudad, scattered in small lobules in the mesentery and the enteric serosa. The compact cephalic lobe of the pancreas possesses no crypts or ducts, and its cells are strongly basophilic (fig. 23B). On the basis of its origin, position, and morphology, it is possible that it repre-sents the homologue of islets of Langerhans tissue of the pancreas. The spleen, a highly vascular lymphoid structure, is present at this level to the right of the pancreas (fig. 23B).

The swim bladder has enlarged laterally (to the right side) and posteriorly (figs. 22B, 23B). This new portion, however, possesses a squamous epithelium, in contrast to the basal and left regions which possess a vacuolated columnar epithelium as described for stage 16.

The anterior branch of the ventral aorta has completely degenerated (fig. 14E). The posterior branch curves dorsally to supply the sixth, fifth, fourth, and third aortic arches in that order. The hyoidean (second) and efferent spiracular (first) arteries are smaller. The ventral aorta continues craniad as an unpaired external carotid artery (fig. 14E).