When does the anterior endomesderm meet the anterior-most neuroectoderm during Xenopus gastrulation?

During amphibian gastrulation, the anterior endomesoderm is thought to move forward along the inner surface of the blastocoel roof toward the animal pole where it comes into physical contact with the anterior-most portion of the prospective head neuroectoderm (PHN), and it is also believed that this physical interaction occurs during the mid-gastrula stage. However, using Xenopus embryos we found that the interaction between the anterior endomesoderm and the PHN occurs as early as stage 10.25 and the blastocoel roof ectoderm at this stage contributed only to the epidermal tissue. We also found that once the interaction was established, these tissues continued to associate in register and ultimately became the head structures. From these findings, we propose a new model of Xenopus gastrulation. The anterior endomesoderm migrates only a short distance on the inner surface of the blastocoel roof during very early stages of gastrulation (by stage 10.25). Then, axial mesoderm formation occurs, beginning dorsally (anterior) and progressing ventrally (posterior) to complete gastrulation. This new view of Xenopus gastrulation makes it possible to directly compare vertebrate gastrulation movements.