The compact blastula is very flat, not more than 30 to 50 microns thick in the center, and averages 0.4 mm. in diameter (fig. 1A). The center is composed of two layers of irregular polyhedral cells 15 to 20 microns in diameter. The cells are smaller and more numerous peripherally, averaging 5 to 10 microns, and are arranged in three or four layers. Around the rim of the blastula there is a narrow zone of junction. A very thin, flat, segmentation cavity can be discerned between the blastula and the yolk surface (fig. 1 B).