Introduction

Two basic problems which have to be addressed in order to understand the molecular basis of development are the temporal and spatial control of gene expression. An adult bird develops from a single haploid ovum and sperm. All the genetic information is, therefore, contained within the two genomes. The development of the embryo involves cell division and cell differentiation. From an initial undifferentiated cell, specialised cells arise at different times and different positions within the developing embryo. Since all the cells have the same DNA, the main problems is to understand how different genes are expressed in different cells at different times. During the earlier part of this century, the avian embryo was one of the most studied species by embryologists, and the morphological stages of development were described in detail (Lillie, 1919). The avian embryo has many advantages over other vertebrates. It is a convenient size and can be developed in an incubator, so all stages are readily accessible. However, when it comes to understanding the molecular basis underlying these morphological changes, Drosophila is the species of choice. Its genome had been studied in more detail, and a wide range of developmental mutants are available, which made it the species of choice in which to study temporal and spatial gene expression. Work on a number of vertebrates has shown that many of the basic control mechanisms found in Drosophila are common to vertebrates.