Introduction

Research on the water relations of reptilian embryos apparently originated with the descriptions by Dendy (1899), Brimley (1903) and Coker (1910) of swelling over the course of incubation by the flexible-shelled eggs of several chelonians and lepidosaurians (a term used here to refer to Squamata plus Sphenodontia; Benton, 1985). However, most of the current interest in this subject derives from the landmark studies by Cunningham and his associates (Cunningham & Hurwitz, 1936; Cunningham & Huene, 1938; Cunningham, Woodward & Pridgen, 1939) and by Fitch & Fitch (1967). These investigations were the first to demonstrate that reptilian eggs and embryos are affected profoundly by the availability of water in their environment, and they revealed also that the water relations of reptilian eggs differ in important ways from those of avian eggs. In retrospect, however, the experiments suffered from the absence at the time of a generally accepted method for quantifying the availability of water in substrates. Cunningham et al. used a qualitative approach and did not report the water content of substrates used to incubate eggs in their studies, while Fitch & Fitch expressed the water content of substrates gravimetrically. Neither of these methods is based upon thermodynamic principles. Thus, the studies have limited value in elucidating either the patterns or the mechanisms of water exchange by incubating eggs of reptiles (Packard & Packard, 1988a).