Many crustacean groups show stereotyped cleavage patterns during early ontogeny. However, these patterns differ between the various major crustacean taxa, and a general mode is difficult to extract. Previous studies suggested that also copepods undergo an early cleavage with a more or less stereotyped pattern of blastomere divisions and fates. Yet, copepod embryology has been largely neglected. The last investigation of this kind dates back more than a century and the results are somewhat contradictory when compared with those of other researchers. To overcome these problems, we studied the early development of a so far undescribed calanoid copepod species, Skistodiaptomus sp., applying histochemical staining, confocal laser scanning microscopy, and bifocal 4D microscopy. The blastomere arrangement of the four-cell stage of this species varies to a large degree. It can either form a typical radial pattern with the four blastomeres lying in one plane or a tilted orientation of the axes connecting the sister cells of the previous division. In both cases, a stereotyped division pattern is maintained inside each quadrant during subsequent cleavages. In addition, we found two types of blastomere arrangements with a mirror symmetry. Most divisions within the quadrants follow the perpendicularity rule until the eighth cleavage. Deviations from this rule occur only in the narrow regions where the different quadrants touch and near the site of gastrulation. Gastrulation is initiated around the descendants of one individually identifiable blastomere of the 16-cell stage. This cell divides in a specific manner forming a characteristic cell arrangement, the gastrulation triangle. This gastrulation triangle initiates the internalization process of the gastrulation and it is encircled by another characteristic cell type, the crown cells. Our observations reveal several similarities to the early development of Calanus finmarchicus, another calanoid species. These relate to blastomere arrangements and divisions and the pattern of gastrulation. As Calanoida represent a basal or near basal branch of the copepod tree, this description will provide the ground for reconstruction of the cleavage pattern of the last common ancestor of Copepoda.

中文翻译：

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颅骨co足类的卵裂模式-一个案例研究。

许多甲壳类动物在个体发育早期就表现出定型的卵裂模式。但是，这些模式在各种主要的甲壳类生物分类之间是不同的，并且一般模式很难提取。先前的研究表明，co足类也经历了早期分裂，其卵裂球分裂和命运或多或少具有定型模式。然而，co足类胚胎学已被大大忽略。上一次此类研究的历史可以追溯到一个多世纪之前，与其他研究人员相比，结果有些矛盾。为了克服这些问题，我们研究了迄今未曾描述的类cal足类pe足动物Skistodiaptomus的早期发育等，应用组织化学染色，共焦激光扫描显微镜和双焦点4D显微镜。该物种的四细胞阶段的卵裂球排列有很大的不同。它可以形成一个典型的径向图案，其中四个卵裂球位于一个平面上，或者可以形成连接上一个分区的姊妹细胞的轴的倾斜方向。在这两种情况下，在随后的切割过程中，每个象限内均保持定型划分模式。此外，我们发现了两种镜面对称的卵裂球排列。象限内的大多数划分都遵循垂直性规则，直到第八次分裂。与该规则的偏差仅发生在不同象限接触并在胃形成部位附近的狭窄区域。在16个细胞阶段的一个可单独识别的卵裂球的后代周围开始放胃。该细胞以特定的方式分裂，形成一个特征性的细胞排列，即胃三角形。该气化三角形启动了气化的内部化过程，并被另一种特征性细胞类型（冠状细胞）围绕。我们的观察揭示了与早期开发的一些相似之处Calaus finmarchicus，另一类类人猿。这些与卵裂球的排列和分裂以及胃气形成的模式有关。由于Calanoida代表了co足类树的基部或近基部分支，该描述将为重建Co足类的最后共同祖先的分裂模式提供基础。