5 - Eggshell structure and formation in eggs of oviparous reptiles

Summary

Introduction

The vast majority of contemporary reptiles are oviparous and deposit eggs with some sort of enveloping shell (Packard & Packard, 1988). The eggshell separates the developing embryo from its environment, modulates the movement of water and gases into and out of the egg, and serves as a source of calcium during embryogenesis (Packard & Packard, 1984, 1988, 1989a,b). Structure of eggshells is quite diverse, ranging from the small, flexible, parchmentshelled eggs of many squamates to the large, rigid-shelled eggs of crocodilians. None the less, certain common themes unite structure of eggs of all oviparous reptiles.

Eggshell structure

Scanning electron microscopy has been used to examine structure of eggshells from a large number of species (Table 5.1). Many studies of eggshells have been quite superficial, but a number of generalisations about structure of reptilian eggshells can still be made. Shells of reptilian eggs are composed of an outer, inorganic layer underlain by an organic (shell) membrane comprised of multiple layers of fibres (Fig. 5.1). Fibres within a layer of the membrane may form a random felt-work or may be highly parallel, and may be closely apposed to one another or widely spaced. The number of fibrous layers in the shell membrane varies among species, and the different layers comprising the shell membrane vary morphologically. This variability, coupled with the occasional occurrence of avian-like air cells in reptilian eggs has led to the interpretation that some reptilian eggs exhibit both an inner and an outer shell membrane (Packard & Packard, 1979).