Experimental investigation of the endoderm in early stages of development shows that the endodermal organ rudiments, like those derived from other germinal layers, are not initially determined, but that the endodermal cells destined to participate in the formation of the various organ rudiments are no more determined for their respective fates than are the cells of the other germinal layers. [...]
The relation between the archenteron and the definitive alimentary canal is very different in vertebrates with complete cleavage from that in vertebrates with incomplete (meroblastic) cleavage. The Relation in Lower Vertebrates: The archenteron in Amphioxus is originally lined with presumptive endoderm, presumptive mesoderm, and presumptive notochordal cells. When the notochord and the mesoderm are segregated from the endoderm, the latter [...]
In all vertebrates, the mouth opening appears rather late in embryonic life after all the primary organ rudiments have already been formed. The mouth opening breaks through where the anterior end of the endodermal part of the alimentary canal touches the ectodermal epidermis beneath the front end of the neural tube. Here the ectodermal epidermis sinks in to form a [...]