Editing Archenteron (section)

==Formation in sea urchins==
{{further information|Gastrulation}}
As [[primary mesenchyme]] cells detach from the [[vegetal pole]] in the gastrula and enter the fluid-filled cavity in the center (the [[blastocoel]]), the remaining cells at the vegetal pole flatten to form a [[vegetal plate]]. This buckles inwards towards the blastocoel in a process called [[invagination]]. The cells continue to be rearranged until the shallow dip formed by invagination transforms into a deeper, narrower pouch formed by the [[gastrula]]'s [[endoderm]]. This pouch narrows and lengthens to become the archenteron, a process driven by [[convergent extension]]. The open end of the archenteron is called the [[blastopore]].

The [[filopodia]]—thin fibers formed by the mesenchyme cells, found in late gastrulation—contract to drag the tip of the archenteron across the blastocoel. The endoderm of the archenteron will fuse with the [[ectoderm]] of the blastocoel wall. At this point gastrulation is complete, and the embryo has a functional digestive tube.