Total length, 1.81 mm. Tail length, posterior to anus, 0.48 mm.

Width of mesencephalon, 0.38 mm.

When embryos at this stage are freed from their membranes, they exhibit slow twitching movements of the body and tail. The extra-embryonic membranes are complete, forming a two-layered sac and enclosing the head of the embryo, including, posteriorly, the auditory vesicles. These membranes,namely, the pericardial amnion and pericardial serosa, are indicated in figure 10B. The blastopore is usually closed at this stage, but a faint raphe, marking the closure of the lips, may be distinguished. This raphe is located postanally and underlies the tail bud. The mesencephalon is considerably widened and possesses thickened sides and floor (fig. 10A).

The metencephalon is small and relatively poorly distinguishable from the following myelencephalon, the latter possessing its characteristic tela choroidea and visible neuromeres (fig. 10A). The optic stalks are thin (fig. llA, B). The infundibular region of the diencephalon is large and deep (fig. llA, B).

Five pairs of cranial nerves can be ident-fied and traced in serial cross sections reconstructed in figure 10B. The trigeminal is the largest and most anterior, running ventrally and posteriorly to fuse with the facial (VII). The facial and auditory nerves arise at thesame point, just anterior to the auditory vesicle. The auditory (VIII) branches off and fuses with the otocyst (VII) continues ventrally tissue. The facial and posteriorly to fuse with the trigeminal (V). The glosso-pharyngeal (IX) and vagus (X) arise separately and can be traced for a short distance postero-ventrad.

The pharyngeal portion of the gut is flat, with three pairs of flanges extending Iaterad and dorsad (fig. 11A). These are the primordia of the first three pairs of gill pouches. The first and third possess small grooves, and the second a second pouch small, shallow will become the pouch. The first true gill cleft. Posterior to the third pouch is the intestinal portal leading into the straight in-testine and ending in a groove-shaped anus. Posterior to the anus is the shallow postanal gut (Kupffer’s vesicle) (figs. 10B, 1lA).

Twenty-one to 22 compact somites may be discerned, the anus being at the level of theeighth (fig. 10A, B). The mesomeric region is relatively undifferentiated at the level of the pectoral fin bud and the first somite. Posteriorly, a pair of nephric ducts extend to the level of the seventh somite where they taper off into a solid cord of cells (fig. 10B). A pair of cords of primordial germ cells are aggregated into gonadial ridges in the region of the second to fifth pairs of somites. The position of these is indicated in figure 11A.

The circulatory system at this stage is a complete circuit, but little visible blood is present. The heart beat is irregular. The blood circuit can be traced in this stage bothin serial cross sections and in India ink in-jected specimens (fig. 12A-D). The blood enters the elongate sinu-atrial region of the heart in front of the tip of the head. A pulsating ventricular region bulges slightly towards the right side, and empties into a short conus and ventral aorta (fig. 12A, B). The third aortic arches (first to develop) are well formed, and a single fourth arch is usu-ally present on the left side (fig. 12A, B). Leading forward from the third arches are a pair of internal carotid arteries supplying the extensive optic plexi. Running caudad, the two radices aortae fuse into a single dorsalaorta just posterior to the level of the auditory vesicles. At that point, a pair of small subclavian arteries are given off. The dorsal aorta extends into the tail region as the caudal artery, giving off numerous interseg-mentals along the way (fig. 12A, B).

A pair of large anterior cardinal veins lead from the optic plexi to the ducts of Cuvier (fig. 12C, D). The caudal vein is drained by the vitello-caudal vessel emptying into the yolk sac circulation at a level just posterior to the anus. The caudal vein is also drained by the postcardinals. The left postcardinal is a vestigial structure, whereas the right is well developed (fig. 12C, D). A circle of small arterial vessels is present beneath the mesencephalon, posterior to the infundibulum (fig. 12C). These blood vessels arise from the internal carotid plexus in the eye, give off a pair of cerebellar arteries, and fuse into a single median basilar artery. The basilar artery is short and, at this stage, is connected by a collateral cross connection to theanterior cardinals (fig. 12C). This circle of blood vessels, as will be shown later, comes to surround the infundibulum and consequently may be considered comparable to the circle of Willis, present in birds and mammals.

The ducts of Cuvier lead into the extensive vascular yolk sac (fig. 12C, D).