Introduction

Eggs have served as useful model systems for the study of vertebrate growth and energetics of development because they represent a relatively closed system in terms of energy flow. Measurements of growth and energy utilisation can be made easily because, unlike the situation in mammals, the developing embryos are separate from the mother. In addition, metabolic rates of eggs are easy to interpret without the confounding effects of activity, uncontrolled feeding and general stress that are inherent, for instance, in studies of anuran larval development (Feder, 1981). Several reviews are available on the energetics of avian embryos (C. H. Vleck, Vleck & Hoyt, 1980; Bucher & Bartholomew, 1984; Ar et al., 1987; Bucher, 1987; Hoyt, 1987; Vleck & Vleck, 1987). Less information, however, is available for reptile eggs and there are no reviews dealing specifically with reptilian embryonic growth and energetics. Several authors have compared the energetics of development in birds and reptiles (Dmi'el, 1970; Ricklefs & Cullen, 1973; Ackerman, 1980, 1981; Gettinger, Paukstis & Gutzke, 1984) but no recent comparative analysis has been done, despite the growing interest in reptilian egg biology. In this chapter we will discuss the developmental patterns that are evident in bird and reptile eggs. These patterns are compared and an attempt is made to account for them. The problems inherent in making such comparisons between the Classes Reptilia and Aves are discussed.