Introduction

During development, the eggshell supplies the majority of the calcium needed by the fowl (Gallus gallus) embryo, a finding drawn from the cumulative work of many chemical embryologists (Simkiss, 1961) and the 45calcium tracer study of Johnston & Comar (1955). This dependence on the eggshell is particularly evident when embryos are placed ex ovo in longterm shell-less culture (Dunn & Boone, 1977; Tuan, 1980a; Slavkin, Slavkin & Bringas, 1980). These cultures are produced by removing the entire content of a fertilised fowl egg from the eggshell after three days of incubation in ovo, and incubating it in a plastic sac suspended within a ringstand (Fig. 27.1) (Dunn, 1974; Dunn & Boone, 1976; Tuan, 1980a; Dunn, Fitzharris & Barnett, 1981a; Dunn, Chapter 26). These shell-less embryos develop severe systemic calcium deficiency (Tuan, 1980a; Watanabe & Imura, 1983; Narbaitz & Jande, 1983; Ono & Tuan, 1986), since their only available calcium source is the tgg yolk, which constitutes less than 20% of the total calcium found in a hatchling (Packard & Packard, 1984; Romanoff, 1967). The onset of calcium deficiency in the shell-less embryo roughly coincides with the period when shell calcium mobilisation would normally begin, around incubation days 10-12 (Terepka, Stewart & Merkel, 1969; Crooks & Simkiss, 1975; Tuan & Zrike, 1978). The hypocalcaemic state of the embryo is indicated by the significantly lowered serum calcium values (Fig. 27.2).