Because of the large amount of yolk present in the hen's egg, cleavage, morphogenesis and differentiation are confined to the blastoderm. Initially the blastoderm becomes several cell layers thick and a cavity, called the subgerminal cavity , is formed under this layer. This stage of the embryo is comparable to the starfish morula . This embryo stage corresponds to the starfish blastula and the cavity separating these 2 layers is called the blastocoel . Development to this stage takes place while the egg is still in the oviduct of the hen.
Involution is the curling inward and ingrowth of a group of cells.
These involuted cells will form the mesoderm germ layer. As gastrulation progresses, the anterior end of the streak moves posteriorly, so the anterior region of the embryo is formed first. The primitive streak is functionally the same as the blastopore of the starfish gastrula. The coelom results from a splitting of the lateral mesoderm . The primitive streak is the area where the cell involute to form the mesoderm germ layer.