The endoderm, consisting of the archenteron, will develop into the digestive tract. The three embryonic germ layers have now formed. Once the archenteron reaches the animal pole, a perforation forms, and the archenteron becomes a digestive tract passing all the way through the embryo. Upon reaching the target site, the cells contract to pull the archenteron to fuse with the ectoderm. These cells extend filopodia that use guidance cues to find the future mouth region. Secondary mesenchyme are formed from cells that ingress from, but remain attached to, the roof of the archenteron. Third, secondary mesenchyme pull the tip of the archenteron towards the animal pole. Convergent extension results when cells intercalate to narrow the tissue and move it forward. Second, the archenteron is formed through convergent extension. The arechenteron is elongated by three mechanisms.ДЇirst, the initial invagination is caused by a differential expansion of the inner layer made of fibropellins and outer layer made of hyalin to cause the layers to bend inward. These filopodia then organize into syncytial cables that deposit the calcium carbonate that makes up the spicules (the skeleton of the pluteus larva).Дuring the second phase of gastrulation, the vegetal plate invaginates into the interior, replacing the blastocoelic cavity and thereby forming a new cavity, the archenteron (literally: primitive gut), the opening into which is the blastopore. The cells move by forming filopodia that identify the specific target location. The migration is believed to be dependent upon sulfated proteoglycans on the surface of the cells and molecules on the basal lamina such as fibronectin. The mesenchyme cells divide rapidly and migrate along the extracellular matrix (basal lamina) to different parts of the blastocoel. Some of the cells of the vegetal pole detach and through ingression become primary mesenchyme cells. The vegetal pole begins to flatten to form the vegetal plate. The animal cells divide meridionally and produce mesomeres.ДЄt the beginning of vertebrate gastrulation, the embryo is a hollow ball of cells known as the blastula, with an animal pole and a vegetal pole. The four vegetal blastomeres divide unequally to produce four micromeres at the vegetal pole and four macromeres in the middle of the embryo. Sea urchins deviate from simple cleavage at the fourth cleavage. The illustration, however, depicts the gastrulation of a diploblast, animals with two germ layers. The following description concerns gastrulation in echinoderms, representative of the triploblasts, or animals with three embryonic germ layers. Thus, gastrulation creates all three germ layers of the embryo: ectoderm, mesoderm, and endodermДuring gastrulation, extraembryonic mesoderm forms within the hypoblast or embryonic mesoderm and migrates out to form the blood vessels of the chorion and connect the chorion to the embryo through the umbilical cord.

Gastrulation in mammals is similar to that in birds with the formation of the primitive streak and Hensen's node and the ingression of cells through the primitive groove to form the endoderm and the mesoderm. The embryonic epiblast undergoes gastrulation. The amnion layer forms the fluid filled cavity to surround and protect the embryo during pregnancy. The epiblast further divides into two more layers. The yolk sac is an extraembryonic tissue that produces blood cells similar to the structure that surrounds the yolk in birds. The hypoblast spreads out and covers the blastocoel to form the yolk sac.

As the outer cell mass invades the endometrium, the inner cell mass divides into two layers: the epiblast and hypoblast. In mammals, gastrulation occurs after implantation, around day 16 after fertilization in human embryogenesis. Risk calculators and risk factors for Gastrulation US National Guidelines Clearinghouse on GastrulationДirections to Hospitals Treating Gastrulation Ongoing Trials on Gastrulation at Clinical Д¬linical Trials on Gastrulation at Google Articles on Gastrulation in N Eng J Med, Lancet, BMJ

Blastopore archenteron blastocoel