The complete hypoblast underlies the central regions of the blastoderm. The edges not underlain by hypoblast are called the marginal zone. The blastoderm is now called the epiblast, and all parts of the embryo will emerge from the epiblast. Induction of the primitive streak in the epiblast by the posterior hypoblast initiates gastrulation. The single layer of the epiblast disc must convert itself into a three-layered embryo during gastrulation.
Cells also proliferate more in the streak area.
The endoderm intercalates into the hypoblast. The mesoderm migrates anteriorly and laterally between the endoderm and the epiblast. It becomes a morula, then an early blastocyst, and then a late blastocyst. The blastocyst enters the uterus, hatches from the zona pellucida and begins to implant on day 6. The primitive streak is beginning to form by days 13-14, and the streak is still elongating on day 15 when the first menstrual period is missed.