1. In young gastrulae of Triton alpestris the median part of the organization centre immediately in front of the blastopore was excised and replaced after reversal of its anterior-posterior axis. Completely normal embryos developed in many cases, but in others there was some degree of microcephaly or spina bifida.
2. Similar normal embryos can develop after exchange of the anterior and posterior regions of the organizer, either with normal or reversed orientation.
3. The spina bifida which frequently appears is a consequence of the impediment offered by the graft to the normal gastrulation movements.
4. Microcephaly may also result if the presence of the graft prevents the proper development of the anterior part of the archenteron roof.
5. It may also be caused when the posterior organizer region is brought into the anterior region, if the host fails to convert it into anterior material. This occurs the more frequently the older the grafted posterior material is.
6. Even at the end of gastrulation (slit yolk-plug stage) the regional character of the archenteron roof is not finally determined. A fairly normal embryo (with over-thick mesoderm) may develop if an extra archenteron roof is added with reversed orientation between the normal archenteron roof and the presumptive neural plate.
7. The determination of the regional structure of the archenteron roof and the neural plate is discussed. Attention is drawn to the spatio-temporal factors involved in the production and diffusion of the evocator with the dynamic system of the gastrulating egg.