The mature ova, just prior to or during fertilization, average 1.6 mm. in diameter. They are of a clear yellow color with peripherally arranged fat globules of various sizes. When the egg is damaged, the globules are found to remain embedded in a more gelatinous cortical region. The globules are composed of a colorless oil of apparently lower viscosity.

Unfertilized, degenerating eggs are of frequent occurrence, particularly in virgin females, and can be identified easily by the aggregation of oil globules at one pole and the presence of irregular strands of coagulated cytoplasm on and below the egg surface, usually near the polar cap of oil globules. A similar condition characterizes degenerate, early gastrula stages, but these can be distinguished by the presence of coagulated cytoplasm on the egg surface only.