|Surface view of embryo of a rabbit. |
arg. Embryonic disk.
pr. Primitive streak.
|Gray's||subject #6 47|
Formation of the Archenteron
As 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. 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 archeteron 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 blastopore lip in amphibians and fish, and the primitive streak in birds and mammals. Each is controlled by the dorsal blastopore, and primitive node, respectively.