The length of the embryonal area is about 0.82 mm. The entire embryo appears to consist of the neural keel, which is slightly broader at the cephalic end (fig. 3E, F). Serial cross sections show the presence of a second cord of cells extending cephalad from the region of the dorsal lip of the blastopore about two-thirds the length of the embryo (fig. 3B, C, F). The cord of cells, situated ventral to the neural keel and contiguous with it, is provisionally called the chordaendoderm (see description of stage 8).

A small cavity is formed on the under surface of the caudal region of the germinal shield and is identified as Kufler’s vesicle (fig. 3D, F). A fold of periblastic ectoderm covers the anterior tip of the cephalon (fig. 3A, E, F). This two-layered fold is the primordium of the extra-embryonic pericardial membranes. In subsequent stages, the fold is invaded by mesenchyme, and forms two double-layered somatopleural membranes, i.e. pericardial amnion and pericardial serosa (described by Tavolga and Rugh, 1947).