The gastrulating embryo

A. Gastrulation is the process by which the epiblast and hypoblast layers of the bilaminar embryonic disc are transformed into the trilaminar embryonic disc, consisting of the three germ layers (ectoderm, mesoderm and definitive endoderm).

B. Gastrulation begins as the primitive streak, formed in the anterior epiblast on day 15 of human development, develops a deep midline groove with a rounded primitive node situated at the cranial tip of the primitive streak. The primitive node contains a depression called the primitive pit. The primitive pit is continuous with the primitive groove. 

C. The first cells to migrate through the streak are endoderm progenitor cells originating in the anterior epiblast.  As the cells ingress into the primitive streak, they displace the hypoblast layer, forming the definitive endoderm. 

D. Later, more posterior waves of ingressing mesoderm progenitor cells form the mesoderm layer between the epiblast and the definitive endoderm. After mesoderm formation is complete, cell migration into the primitive streak ceases and the remaining epiblast layer is called ectoderm.