Total length, 1.67 mm. Width of mesencephalon, 0.25 mm.

The tail bud, approximately 0.37 mm. in length, covers most of the area of the blasto-pore (fig. 8A). The latter is small and elliptical or slit shaped. Occasional embryos have been found with much larger blastopores at this stage. The head lies free within a pericephalic (amnionic) cavity only as far back as the beginning of the rhombencephalon (figs. 8B, 9A, B, C). The extra-embryonic membrane is thicker and possesses some mesenchyme at its margins (figs. 8B, 9A-F). This is the first step in the transformation of the periblastic-ectodermal extra-embryonic membranes into the definitive inner pericardial amnion and outer pericardial serosa, each a somatopleural membrane.

The mesencephalon is widened, and the rhombencephalon less so. The infundibular region is deep and distinct (fig. 8B). The rhombencephalon, in the region of the cranial placodes, possesses a thin roof, i.e., the characteristic tela choroidea of the myelencephalon (fig. 9D). The lens placode is fully invaginated, solid, but not yet separated from the superficial ectoderm (figs. 8A, 9B). The olfactory organs appear as a pair of small solid masses of invaginated ectodermal tissue(figs. 8A, 9B). The lateral-line anlagen extend from the level of the first gill pouch to the pectoral fin bud (fig. 9D, E, F).

The endoderm shows little change from the condition in stage 10. The intestinal portion possesses a cavity extending back to the fifteenth somite (figs. 8B, 9H-K). The three gill pouch primordia are more sharply defined (figs. 8A, B, 9D, E).

Nineteen to 20 somites are present at this stage (fig. 8). A pair of pectoral fin buds make their appearance just anterior and lateral to the first somite (figs. 8A, B, 9G). The mesomere is distinguishable from the first to the thirteenth somites. At the level of the second to fourth somites, the mesomere is organized into a nephric duct (fig. 91). The circulatory system consists of only a rather diffuse heart anlage and a single vessel, the dorsal aorta (figs. 8B, 9A-D, H, I).