Gastrulation is a critical event whose molecular mechanisms are thought to be conserved among vertebrates. However, the morphological movement during gastrulation appears to be divergent across species, making it difficult to discuss the evolution of the process. Previously, we proposed a novel amphibian gastrulation model, the “subduction and zippering (S&Z) model”. In this model, the organizer and the prospective neuroectoderm are originally localized in the blastula’s blastocoel roof, and these embryonic regions move downward to make physical contact of their inner surfaces with each other at the dorsal marginal zone. The developmental stage when contact between the head organizer and the anterior-most neuroectoderm is established is called “anterior contact establishment (ACE).” After ACE, the A-P body axis elongates posteriorly. According to this model, the body axis is derived from limited regions of the dorsal marginal zone at ACE. To investigate this possibility, we conducted stepwise tissue deletions using Xenopus laevis embryos and revealed that the dorsal one-third of the marginal zone had the ability to form the complete dorsal structure by itself. Furthermore, a blastocoel roof explant of the blastula, which should contain the organizer and the prospective neuroectoderm in the S&Z model, autonomously underwent gastrulation and formed the complete dorsal structure. Collectively, these results are consistent with the S&Z gastrulation model and identify the embryonic region sufficient for construction of the complete dorsal structure. Finally, by comparing amphibian gastrulation to gastrulation of protochordates and amniotes, we discuss the gastrulation movement evolutionarily conserved among chordates.

原肠胚形成是一个关键事件，其分子机制被认为在脊椎动物中是保守的。然而，原肠胚形成过程中的形态运动似乎在不同物种之间是不同的，因此很难讨论该过程的演变。此前，我们提出了一种新的两栖动物原肠胚形成模型，即“俯冲和拉链 (S&Z) 模型”。在这个模型中，组织者和预期的神经外胚层最初位于囊胚的囊胚腔顶部，这些胚胎区域向下移动，使其内表面在背侧边缘区域相互物理接触。头部组织者与最前部神经外胚层之间建立接触的发育阶段称为“前部接触建立 (ACE)”。ACE 后，AP 体轴向后伸长。根据该模型，体轴源自 ACE 背缘区的有限区域。为了研究这种可能性，我们使用非洲爪蟾胚胎揭示了背侧三分之一的边缘区能够自行形成完整的背侧结构。此外，囊胚的囊胚腔顶部外植体（在 S&Z 模型中应包含组织者和预期的神经外胚层）自主进行原肠胚形成并形成完整的背部结构。总的来说，这些结果与 S&Z 原肠胚形成模型一致，并确定了足以构建完整背部结构的胚胎区域。最后，通过比较两栖动物的原肠胚形成与原脊索动物和羊膜动物的原肠胚形成，我们讨论了在脊索动物中进化上保守的原肠胚形成运动。