Total length, 2.02 mm. Tail length, 0.62 mm. Mesencephalon width, 0.42 mm.

At this stage the blastopore is completely closed. Usually a thin streak is visible extend-ing for about 0.4 mm. on the surface of the yolk membrane underlying the tail, marking the site of closure of the blastopore.

Aside from a general difference in size, the most prominent feature distinguishing this stage from stage 12 is the presence of the first indications of visible pigment (fig. 13A). This appears in the outer retinal layer, delineating the eyes of living specimens with a thin dark margin.

In the pharyngeal region, four pairs of gill pouches are present, with the first (spiracular or pseudobranchial) somewhat constricted (fig. 13B). The development of the postanal gut (Kupffer’s vesicle) into the urinary bladder (or vesicle) is indicated by an enlargement and invagination of the former (fig. 13B).

Considerable changes have taken place in the circulatory system. There are usually four pairs of aortic arches present, i.e., the third fourth, fifth, and sixth (fig. 14A). Occasionally the fourth or the sixth pairs are incompletely formed. The ventral aorta poste-rior to the fourth arches is double, and it is noteworthy that the ventricular end of the ventral aorta is anterior to the third aortic arch. The blood flow in the ventral aorta, then, is caudad.

The circle of Willis partially surrounds the infundibulum, and the collateral cross connection of the basilar and the anterior cardinals is greatly reduced or absent. The basilar runs caudad and splits into a pair of vertebral arteries at about the level of the pectoral fin buds.

The pericardial serosa is partially vascularized by branches of the common cardinal veins, and is drained by the sinus venosus.