Gastrulation is the stage of development during which three germ layers
(ectoderm, mesoderm, and endoderm) are formed from the blastula or
blastocyst (in mammals).
Figure 5.1: The formation of tissues in human embryos. The
blastocyst consists of trophoblast and inner cell mass. The latter first splits
into two layers: hypoblast and epiblast. The hypoblast develops into yolk sac
which, as indicated by the dashed line, may also contribute to the
formation of the extraembryonic mesoderm . The epiblast develops into the three germ layers
which constitute the embryonic tissues. The rest (amnionic ectoderm, yolk sac,
extraembryonic mesoderm and syncytiotrophoblast) forms the extraembryonic
During gastrulation, cells continue to divide and involve dramatic cell movements
The blastocyst implants in the uterine wall about 8 to 9 days after fertilization
and the three germ layers are formed by
the end of the third week.