Introduction

A central problem in understanding the evolution of cellular homeostasis is the recognition that it often involves three systems with no clear inter-relationship in time and space. At some stage, cells produce specific hormone receptors that are linked to particular metabolic pathways and respond to distinctive extracellular molecules. How such functional relationships evolved to cope with a novel situation is a recurrent theme in evolutionary studies and to some extent the same difficulties arise in understanding the development of homeostatic systems in the embryo (Csaba, 1986). In this latter case it is possible to undertake experiments that provide some indication as to how the systems function. This is, however, a relatively recent area of study and the results are frequently unexpected.

Ten years ago, together with Nancy Clark, I attempted to pose some of these problems by studying the onset of calcium homeostasis in the avian embryo. We considered the problem in relation to four general systems: ion diffusion, active transport, passive storage and active resorption and posed a number of fundamental problems (Clark & Simkiss, 1980). In this chapter I will refer to some of the more recent work in these areas taking as examples the problems as they arise chronologically in a developing embryo.

Ion fluxes

Albumen formation

The egg is fertilised at the top of the oviduct and passes down the magnum region where the albumen is secreted.