The total length of the early tail bud stage is 1.23 mm. The head has expanded considerably, primarily as a result of the growth of the optic vesicles (fig. 5A, G). The width of the mesencephalon region is 0.08 mm. The extra-embryonic periblastic membrane encloses the head to the level of the auditory placodes (fig. 5H). The blastopore is elliptical in shape, about 0.5 mm. in its longest axis. A small tail bud, 0.05 mm. in length, is present, overhanging the anterior lip of the blastopore (fig. 5G, H).

The nerve cord is tubular throughout, except in the relatively undifferentiated tail bud region. The neurocoele is widest in the head region, where primary divisions into prosencephalon, mesencephalon, and rhombencephalon can be distinguished (fig. 5G).The optic vesicles are large, hollow, and connected to the prosencephalon by solid optic stalks (fig. 5A, G, H). In cross section, lens placodes are shown (fig. 5A). The first cranial placode of neural crest (anlage of the fifth cranial nerve) is present (fig. 5B, G, H).

The gut endoderm is a cylindrical cord of cells extending from Kupffer’s vesicle (post-anal gut) anteriorly to the level of the first cranial placode where the pharyngeal region becomes flattened (fig. 5B, C, D, H). The notochord can be traced from the blastopore lip region to the level of the auditory placode (fig. 5D, E, F, H).

Five to seven pairs of somites may be present in this stage (fig. 5G, H). Occasionally somites appear as early as stage 8. An aggregation of mesenchyme may frequently be found ventral to the nerve cord, anterior to the first cranial placode. This is interpreted as the anlage of the endocardium.