The diameter of this later blastula or germ ring is 0.8 mm. (fig. 1C). The thickness at the center is 30 microns. This region is composed of three to four layers of cells averaging 10 to 15 microns in size. The peripheral germ ring region is 0.06 mm. thick, 0.1 mm. broad, and consists of 10 to 15 layers of very small, rapidly dividing cells (fig. 1D). The thinner central region has few mitotic figures and forms a syncytium. It may therefore be termed the periblastic ectoderm, since there is some doubt as to whether its origin is the same as that of the periblast described by Wilson (1889). The segmentation cavity is still very thin, with the under surface of the blastula often in contact with the underlying yolk (fig. 1D). A narrow and incomplete zone of junction is present.