The notochord would almost appear to arise as a third median and dorsal diverticulum of the archenteron (fig. 180 ch).
They form the rudiment of the notochord, which, as in Amphioxus, is derived from the dorsal wall of the archenteron.
In ontogeny the notochord is a derivative of the dorsal wall of the archenteron.
It soon becomes shut off from the slit-like posterior part of the archenteron.
The other part of the archenteron wall make the Allantois, the hind gut and the bladder.
In the tail the notochord is shewn at ch, the muscles at m, and the solid remnant of the ventral wall of the archenteron at dd.
The mesoblast is developed from the walls of hollow outgrowths of the archenteron, the cavities of which become the body cavity.
It has already been stated that the lateral walls of the archenteron in the tail give rise to muscular cells.
While the above external changes take place the archenteron undergoes considerable modifications.
They form the walls of a new cavity, the enterocœl, which is to be regarded as a nipped-off diverticulum of the archenteron.
archenteron ar·chen·ter·on (är-kěn'tə-rŏn', -tər-ən)