Origin of Animal diversity considered from the early development of chaetognaths

为了阐明发育的命运，单卵裂球的四细胞和八细胞胚胎Paraspadella gotoi注射谱系示踪染料。观察了标记在幼体中的分布。四细胞胚胎由动物和植物的横沟细胞组成，呈四面体排列，其中一个植物的横沟细胞通常含有种质。当在四细胞期注射单个细胞时，观察到四种标记模式（D、V、L和R）。注射在八细胞阶段揭示了八个标记模式，这代表了在四细胞阶段注射的幼雏中观察到的四种模式的子集。V型可能是由含有胚质的卵裂球产生的。我们发现，在四细胞阶段的卵裂球的位置对应于未来的身体轴，这是类似于经典的spiralians和修改的spiralians，如甲壳类动物。此外，我们证实，第二次分裂发生在leiotropic的方式，这是看到在第二次分裂的经典spiralians。在八细胞阶段的标记区域也检查了原肠胚和孵化的相同的胚胎。大部分的身体肌肉是由原肠壁产生的。这一结果支持了毛颚类中胚层形成的经典观点。第三次卵裂将胚胎分成动物和植物两半。由于原肠胚形成发生在植物区，动物和植物细胞分别产生身体的前部和后部。卵裂球的排列与体轴一致，胚层的假定区域沿动物-植物轴沿着。这些都是三胚层动物的共同特征。

To elucidate developmental fates, single blastomeres of the four-celled and eight-celled embryos of Paraspadella gotoi were injected with a lineage-tracing dye. The distribution of the labels was observed in the hatchlings. The four-celled embryo consists of the animal and vegetal cross-furrow cells in a tetrahedral arrangement and one of the vegetal cross-furrow cells typically contains the germ plasm. When single cells were injected at the four-cell stage, four labeling patterns were observed (D, V, L and R). Injection at the eight-cell stage revealed eight labeling patterns which represent subsets of the four patterns observed in the hatchling injected at four-cell stage. The V pattern is probably generated from the blastomere containing the germ plasm. We found that the positions of the blastomeres at the four-cell stage correspond to the future body axes which is similar to classic spiralians and modified spiralians such as crustaceans. Furthermore, we confirmed that second cleavage occurs in a leiotropic fashion which is seen in the second cleavage of the classic spiralians. Labeled regions at the eight-cell stage were also examined at the gastrula and the hatchling of the same embryos. Most of body muscles were generated from the wall of the archenteron. This result supports the classical idea on the mesoderm formation of chaetognaths. The third cleavage divides the embryo into animal and vegetal halves. As gastrulation occurs from the vegetal region, the animal and vegetal cells produce the anterior and posterior regions of the body, respectively. The arrangement of the blastomeres corresponds to the body axis and the presumptive areas of the germ layers arrange along the animal-vegetal axis in chaetognaths. These are common characteristics among the triploblastica.