| The splanchnopleure also envelops it, so that, morphologically speaking, the yolk lies within the mesenteron. |
| Ventrally the stomodum is prolonged for a considerable distance under the anterior part of the mesenteron. |
| The constriction shown by a longitudinal section of the transition area between the mesenteron and the proctodaeum is the pyloric valve. |
| Rudiments of the proctodum and stomodum have appeared, but neither of them as yet communicates with the mesenteron. |
| After the separation of those parts the remainder of the hypoblast forms the wall of the mesenteron. |
| The mesenteron consists at first of a simple hypoblastic tube, which however becomes enveloped by a layer of splanchnic mesoblast. |
