The total length at this stage is 1.45 mm., and the width at the mesencephalon is 0.13 mm. A bluntly rounded tail bud, 0.20 mm. long, overhangs the still open blastopore (fig. 6A, B). The extra-embryonic membrane encloses the head as far back as the auditory vesicle (figs. 6A, B, 7A-F). The subcephalic pocket extends to about the middle of the mesencephalon.

The three primary divisions of the brain are easily seen, with the mesencephalic portion possessing the thickest walls (fig. 6A). The telencephalic portion of the prosencephalon is small and acuminate (figs. 6A, 7A). The diencephalic region is relatively long and deep. The deepest point represents the site of the infundibulum (fig. 6B).

The optic vesicles are folded into optic cups with a thickened sensory layer (figs. 6A, 7A). The optic stalks are hollow at this stage. The lens placodes are partially invaginated (figs. 6A, 7B). Small olfactory placodes are present (figs. 6A, 7A). The auditory placode has invaginated as a solid mass and, at this stage, has formed a small cavity (figs. 6A, B, 7F). No endolymphatic ductor vestige of it is ever present. Two aggregations of neural crest cells are visible in the cross sections and are shown in figure 6A and B. The anterior of these is the first cranial placode (fig. 7E), and the other is the second cranial placode (fig. 7F), primordium of the seventh and eighth cranial nerves. Three or four of the first spinal nerve primordia are also present in the form of neural crest (fig. 7J).

The cranial portion of the endoderm is flattened and extended dorso-laterad to form the first evidences of the first three gill pouches, which are thickened grooves at this stage (figs. 6B, 7D, E, G). Just anterior to the level of the first somite, there is an intestinal portal opening into a small tubular cavity which extends posteriorly from that point to about the level of the sixth somite (figs. 6B, 71, J). Caudad, the endodermal cord is solid (figs. 6B, 7K, L), and Kupffer’s vesicle is greatly reduced.

Twelve or 13 pairs of somites are present (fig. 6). From the second somite to the ninth or tenth, a mesomeric region (nephrogenic cord) can be distinguished (fig. 7J). The heart anlage is still a diffuse mass of mesenchyme (figs. 6B, 7B, C, D). The dorsal aorta may be found as a thin-walled, flattened tube lying between the notochord and the endoderm in the region of the first four or five somites (fig. 7 J). Blood-cell anlagen are found inside and around the dorsal aorta.

A pair of thickened grooves of ectoderm are present along the lateral limiting sulci, extending from the level of the first cranial placode up to about the first somite (fig. 7E). These structures are believed to be the primordia of the rather reduced lateral line system typical of the Poeciliidae.