At first, the mesenteron freely communicates with the exterior by the opening of the blastopore.
After the separation of those parts the remainder of the hypoblast forms the wall of the mesenteron.
Ventrally the stomodum is prolonged for a considerable distance under the anterior part of the mesenteron.
The general arrangement of the parts, at the time when the hind end of the mesenteron is first closed, is shewn in fig. 27.
The stomodum and proctodum are both formed before the mesenteron.
Immediately behind the stomodum is placed the branchial region of the mesenteron.
The splanchnopleure also envelops it, so that, morphologically speaking, the yolk lies within the mesenteron.
The next changes which take place lead to the formation of the mesenteron, and to the enclosure of the yolk-cells by the epiblast.
The main branches of the latter enter the mesenteron just behind the fifth pair of cephalic appendages.
Rudiments of the proctodum and stomodum have appeared, but neither of them as yet communicates with the mesenteron.
mesenteron mes·en·ter·on (měz-ěn'tə-rŏn', měs-)