Structure of the shell and tertiary membranes of eggs of softshell turtles (Trionyx spiniferus)

Eggs of the turtle Trionyx spiniferus are rigid, calcareous spheres averaging 2.5 cm in diameter. The eggshell is morphologically very similar to avian eggshells. The outer crystalline layer is composed of roughly columnar aggregates, or shell units, of calcium carbonate in the aragonite form. Each shell unit tapers to a somewhat conical tip at its base. Interior to the crystalline layer are two tertiary egg membranes: the outer shell membrane and the inner shell membrane. The outer shell membrane is firmly attached to the inner surface of the shell, and the two membranes are in contact except at the air cell, where the inner shell membrane separates from the outer shell membrane. Both membranes are multi-layered, with the inner shell membrane exhibiting a more fibrous structure than the outer shell membrane. Numerous pores are found in the eggshell, and these generally occur at the intersection of four or more shell units.