part of part of Danio rerio embryonic structure gastrula archenteron primitive endodermal cavity BTO:0001696 AAO:0010430 gastrocoel UBERON:0004735 The primitive gut that forms during gastrulation in the developing blastula. It develops into the digestive tract of an animal. As primary mesenchyme cells detach from the vegetal pole in the blastula 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 narrowing and lengthening of the archenteron is 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 a late gastrula 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 gastrula has a functional digestive tube. The indentation that is actually formed is called the lip of the blastopore in amphibians and fish, and the primitive streak in birds and mammals. Each is controlled by the dorsal blastopore, and primitive node (also known as Hensen's node), respectively The cavity of a gastrula forming a primitive gut. XAO:0000092 Wikipedia:Archenteron mesenteron presumptive gut