At 24 hrs. incubation period
the chick embryo is oval in shape. The primitive streak is seen. From the hensen's node a notochord is developed.
In the 24 hrs. chick embryo four pairs of somities are present.
On either side of mid dorsal line the ectoderm becomes thick. It forms longitudinal folds. They are called neural folds. They include a space called neural groove, this neural groove is deep in anterior
region, and the depth decreases gradually posterior
region. Near the first somite region the folds will startrfusing. It will start after 24 hours of incubation. Posteriorly diverse. This area is called sinus rhomboidalis. In this region regressing primitive streak is developed.
At this incubation stage
foregut is formed. Mid and hind guts are not formed. The opening of foregut into midgut is called anterior intestinal portal on either side of this the splanchanic mesoderm becomes thick.
MESODERM: It is present between ectoderm and endoderm. It occupies the entire blastoderm.
BLOOD ISLANDS: Around the area pellucida, area opaca is present. In this opaca region groups of mesenchyme cells will present. They are called blood islands. They produce embryonic blood vessels.
AREA VASCULOSA & AREA VITELLINA: In the area opaca the region which contain blood islands, is called area vasculosa. The peripheral area without blood vessels is called area vitellina.