The density of the cytoplasm is less than that of the underlying yolk. The egg is fertilized internally and begins cleavage as it travels through the oviduct. A subgerminal space forms beneath the blastoderm cells during cleavage. The blastocoel lies between them in the center of the blastodisc. This is the structure of the egg at the time it is laid.
Gastrulation begins after egg laying.
Formation of the primitive streak signals the beginning of gastrulation. The primitive streak originates from the posterior marginal zone, and marks the posterior axis of the embryo. The primitive streak is a thickening of the blastodisc caused by the convergent migration and ingression of epiblast cells. In the center of the pimitive streak is a depression called the primitive groove. Ingressing cells that take up a position between the epiblast and hypoblast become mesoderm.