Early during onychophoran development and prior to the formation of the germ band, a posterior tissue thickening forms the posterior pit. Anterior to this thickening forms a groove, the embryonic slit, that marks the anterior–posterior orientation of the developing embryo. This slit is by some authors considered the blastopore, and thus the origin of the endoderm, while others argue that the posterior pit represents the blastopore. This controversy is of evolutionary significance because if the slit represents the blastopore, then this would support the amphistomy hypothesis that suggests that a slit-like blastopore in the bilaterian ancestor evolved into protostomy and deuterostomy. In this paper, we summarize our current knowledge about endoderm and mesoderm development in onychophorans and provide additional data on early endoderm- and mesoderm-determining marker genes such as Blimp, Mox, and the T-box genes. We come to the conclusion that the endoderm of onychophorans forms prior to the development of the embryonic slit, and thus that the slit is not the primary origin of the endoderm. It is thus unlikely that the embryonic slit represents the blastopore. We suggest instead that the posterior pit indeed represents the lips of the blastopore, and that the embryonic slit (and surrounding tissue) represents a morphologically superficial archenteron-like structure. We conclude further that both endoderm and mesoderm development are under control of conserved gene regulatory networks, and that many of the features found in arthropods including the model Drosophila melanogaster are likely derived.

中文翻译：

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对中胚层和内胚层发育以及甲胚层胚孔性质的新见解


在 甲细胞发育的早期和生殖带形成之前，后组织增厚形成后坑。在这个增厚的前面形成一个凹槽，即胚胎裂缝，它标志着发育中的胚胎的前后方向。这个裂缝被一些作者认为是胚孔，因此是内胚层的起源，而另一些作者则认为后坑代表胚孔。这个争论具有进化意义，因为如果狭缝代表胚孔，那么这将支持两口学假说，该假说表明双侧祖先中的狭缝状胚孔进化成原吻口术和后吻口术。在本文中，我们总结了我们目前关于甲类内胚层和中胚层发育的知识，并提供了有关早期内胚层和中胚层决定标记基因（如 Blimp、Mox 和 T-box 基因）的更多数据。我们得出的结论是，甲状动物的内胚层形成于胚胎裂隙发育之前，因此狭缝不是内胚层的主要起源。因此，胚胎狭缝不太可能代表胚孔。相反，我们建议后坑确实代表胚孔的嘴唇，而胚胎裂缝（和周围组织）代表形态学上浅表的肠状结构。我们进一步得出结论，内胚层和中胚层发育都受到保守基因调控网络的控制，并且在节肢动物中发现的许多特征（包括模型黑腹果蝇）很可能是衍生的。