Gas permeability of American alligator eggs and its anatomical basis

The barrier to gas flux across the eggs of American alligators (Alligator mississippiensis) consists of a calcareous shell and an underlying shell membrane of two layers, a limiting membrane facing the embryo and a fibrous membrane facing the shell. The limiting membrane is penetrated by an immense population (averaging 341,188 cm-2) of tiny pores (averaging 0.51 micron in diameter) and a small population (averaging 190 cm-2) of large pores (averaging 34.6 microns in diameter). An estimated 6% of these pores are open at the onset of incubation, and 22%-24% are open near hatch. The shell membrane is 2.6-10 times less permeable to O2 than the shell. Its permeability nearly quadruples during incubation, is higher at the equator than elsewhere, increases more rapidly when eggs are incubated at 33 degrees C as opposed to 30 degrees C, and appears to depend primarily on its water content. In contrast, the shell's permeability to O2, as well as its water vapor conductance and the number of open pores in it, does not change significantly during incubation.