Total length, 2.31 mm.

Tail length, 0.84 mm. Mesencephalon width, 0.56 mm.

The pericardial serosa is well vascularized and is continuous with the extensive yolk sac membrane (fig. 16C). Pigment is limited to the retinal layer but is more evident and darker than in stage 13 (fig. 16B).

The telencephalon is short, extending somewhat ventrad between two small olfactory bulbs (fig. 16A). A pair of small, flattened telencephalic vesicles are present. In the same transverse section (fig. 16A), the anterior region of the diencephalon is visible, with a thin dorsal roof (anterior tela choroidea). The mesencephalic (optic) lobes are large and thick walled, encroaching anteriorly over the diencephalon (figs. 15A, B, 16C, D). The optic stalks are vestigial or absent, originating from a broadly flattened optic recess in the floor of the diencephalon. Extending dorsad between the optic lobes is a narrow conical projection of diencephalic origin, i.e., the epiphysis (fig. 16C). The infundibulum projects posteriorly from a somewhat larger hypothalamic region (figs. 16C, 17A), and just posterior to the level of the infundibulum is a solid midventral mass of cells growing from the superficial ectoderm, i.e., the hypophysis (fig. 17B). The metencephalon is short and poorly distinguishable from the myelencephalon (fig. 17A, B), the latter being recognizable by the position of the cranial nerves and the presence of a posterior tela choroidea. The neurocoele is narrow in the spinal cord region and constricted to form a dorsal and a ventral canal (fig. 18A).

The retina exhibits three layers (fig. 16B). The outermost is the thin pigment layer. The sensory layer is divided into an outer nuclear layer and a paler staining, inner, nerve fiber layer.

The otocyst is ellipsoidal in shape (fig. 15), with the lateral walls thin and the median and ventral walls considerably thickened and fused with the fibers of the eighth nerve (fig. 17C).

The pharyngeal region possesses four pairs of gill pouches (figs. l5B, 17C). The first is greatly constricted, and the third and fourth pairs are shallow, dorsolateral evaginations of the broad, flat pharynx. The intestinal portal has moved forward to a position ventral to the third pair of pouches. Primordia of the fifth pouches are visible in sections as flat, solid outgrowths of the pharynx (fig. 17D).

Posterior to the fifth pouch primordia, the short esophagus may be identified (fig. 18A). It possesses an irregular lumen lined by a
single layer of cuboidal cells with basophilic cytoplasm. Immediately in front of the liver anlage (figs. 15B, 18C) is a widened region of
the gut, about 30 microns in length and possessing a squamous type of epithelium arranged in three or four layers (fig. 18B). This widened region is presumed to be a vestigial stomach, absent in all subsequent stages.

The liver primordium, projecting to the left side of the intestine with a plexus of hepatic blood vessels, is a solid mass of undifferentiated cells (figs. 15B, 18C). The swim bladder primordium is not yet visible, but its site of future development is marked by a plexus of blood vessels on the right side of the intestine at the level of the vestigial stomach (fig. 18B, C).

The intestine is a straight tube, identifiable from the level of the liver primordium by its columnar, vacuolated epithelium and small circular lumen (figs. 15B, 18C, D). The intestine ends in a slit-shaped anus (fig. 15B).

Twenty-four to 25 myotomes are present at this stage (fig. 15), and striated muscle fibers can easily be identified in the sections. The mandibular arch is large, and the hyoid, third, fourth, and fifth arches are progressively smaller. The pectoral fin buds are broad at the base and have a thin distal margin (figs. l5A, B, 18A, B, C).

The notochord is dorsoventrally flattened, originating at a level posterior to the otocysts (figs. 17D, 18A-D), extending into the tail, where it becomes more rounded. The posterior end is curved dorsad (fig. 15B).

The ventral aorta runs caudad, giving off five pairs of aortic arches (fig. 14B). The second arch (afferent spiracular artery or hyoidean artery) is thin and slender. The third arch is the largest, and the fourth, fifth, and sixth are progressively smaller, with the sixth arch appearing as a branch of the fourth (fig. 14B). A hepatic plexus is formed around the liver primordium, drained by a short hepatic vein, which, in turn, empties into the left common cardinal vein.

The urinary vesicle (formed from the postanal gut) has invaginated more deeply than in stage 13 and possesses a slit-shaped open ing (fig. 15B). The nephric ducts become evident at the level of the esophagus, possess a lumen, and extend caudad to about the sixth or seventh somite, becoming solid at about the fourth. No tubules are present at this stage.