GAS-PERMEABILITY OF AMERICAN ALLIGATOR EGGS AND ITS ANATOMICAL BASIS

The barrier to gas Bur across the eggs of American alligators (Alligat or mississippiensis) consists of a calcareous shell and an underlying shell membrane of two layers, a limiting membrane facing the embryo an d a fibrous membrane facing the shell. The limiting membrane is penetr ated by an immense population (averaging 341,188 cm(-2)) of tiny pores (averaging 0.51 mu m in diameter) and a small population (averaging 1 90 cm(-2)) of large pores (averaging 34.6 mu m in diameter). An estima ted 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 permeabl e to O-2 than the shell, Its permeability nearly quadruples during inc ubation, is higher at the equator than elsewhere, increases more rapid ly 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, th e shell's permeability to O-2, as well as its water vapor conductance and the number of open pores in it, does not change significantly duri ng incubation.