The total length of the embryo at this stage averages 1.06 mm.

The nerve cord extends for the entire length except for a short undifferentiated region at the caudal end (fig. 4E). The anterior half of the nerve cord possesses a cavity which is widest in the region of the prosencephalon (fig. 4A, E). A pair of lateral outgrowths, optic buds, are present. These are solid at first, but quickly develop small cavities (fig. 4A, E, F).

The ectoderm of the head is folded ventrad, forming a head fold with an underlying subcephalic pocket (fig. 4A, F). The head region is enclosed in a thin, double-layered sac of periblastic ectoderm.

The auditory placode is present at this stage (fig. 4B, E, F). Mesenchyme is aggregated on either side of the neural keel in the midbody region where the first somites will form (fig. 4C, E, F).

The chorda-endoderm appears to split longitudinally, with the ventral cord of cells being the solid primordium of the gut and the dorsal cord forming the notochord (fig. 4C, F). The shallow Kupffer’s vesicle marks the posterior limit of the chorda-endoderm (fig. 4D, F). The germ ring, by this stage, has advanced considerably, leaving about one-fifth of the yolk surface uncovered by periblast. This cap of uncovered yolk may be considered homologous to the yolk plug of amphibian eggs.