One of the definitive characteristics of chordates (cephalochordates, vertebrates) is the somites, which are a series of paraxial mesodermal blocks exhibiting segmentation. The presence of somites in the basal chordate amphioxus and in vertebrates, but not in tunicates (the sister group of vertebrates), suggests that the tunicates lost the somites secondarily. Somites are patterned from anterior to posterior during embryogenesis. How such a segmental pattern evolved from deuterostome ancestors is mysterious. The classic enterocoel theory claims that chordate mesoderm evolved from the ancestral deuterostome mesoderm that organizes the trimeric body parts seen in extant hemichordates. Recent progress in molecular embryology has been tremendous, which has enabled us to test this classic theory. In this review, the history of the study on the evolution of the chordate mesoderm is summarized. This is followed by a review of the current understanding of genetic mapping on anterior/posterior (A/P) mesodermal patterning between chordates (cephalochordates, vertebrates) and a direct developing hemichordate (Saccoglossus kowalevskii). Finally, a possible scenario about the evolution of the chordate mesoderm from deuterostome ancestors is discussed.

中文翻译：

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胆酸盐分割的进化起源：重新审视肠小肠理论。

脊索动物（头足动物，脊椎动物）的明确特征之一是体节，其是一系列显示分段的近轴中胚层块。在底部的绒毛膜双歧杆菌和脊椎动物中存在体节，但在被膜（脊椎动物的姊妹群）中则没有，这表明被膜继发地失去了体节。在胚胎发生过程中，从前到后的模式形成节段。这种由氘吻口祖先演变成的节段模式是如何神秘的。经典的肠小肠理论认为，date酸盐中胚层是从祖先的氘化子宫体中胚层进化而来的，而祖先的氘化子宫中胚层组织了在现存的半酸盐中看到的三聚体部分。分子胚胎学的最新进展是巨大的，这使我们能够检验这一经典理论。在这篇评论中 总结了关于中碳酸盐中胚层进化的研究历史。接下来是对目前对脊索动物（头足动物，脊椎动物）和直接发育的半酸盐的前/后（A / P）中胚层模式的遗传作图的了解的综述。Saccoglossus kowalevskii）。最后，讨论了有关从氘骨造口祖先进化出氯酸盐中胚层的可能情况。