Epithelio-mesenchymal relationships in the regional differentiation of the digestive tract in the amphibian embryo.

1. Endodermal pieces taken out of the presumptive gastric and intestinal regions of the early neurula were explanted together with ecto- or endomesodermal fragments removed from various regions of neurula ofTriturus pyrrhogaster. 2. The future development of the endodermal piece was found to be shifted anteriorwards to produce branchial and/or oesophago-gastric epithelium when the endoderm was left to develop within a mesenchymal environment. The endodermal piece showed predominantly a development into intestinal structures when it was combined with lateral mesoderm and left to develop within a mesenchyme-free environment. 3. The anteriorward shift of the future development of the explanted endoderm by niesenchyme occurred with different grades, according to the source from which the mesenchyme was derived. The mesenchyme derived from the cranial mesectoderm was found to be most effective, and the efficiency falls gradually in the mesenchyme derived from more lateral or posterior regions. 4. Considerable differences were noted between the endodermal pieces tested, in their reactivity to the effect from mesenchyme. The explanted gastric piece can be shifted anteriorwards more easily than can the explanted intestinal piece. 5. The developmental paths of the undetermined endoderm, in the explantation condition, appeared to be decided by two factors, one residing in the mesenchyme and another in the endoderm.