Acanthor is the first larva of Acanthocephala (Acanthocephala/Reproduction, Acanthocephala/Figs. 2 and 3). During the first equal cell divisions after fertilization, two polar bodies usually appear at the end of the embryo that will become the anterior end of the acanthor. Further, equal and unequal divisions show a kind of spiral cleavage resulting in micromeres and macromeres. In a later stage, the central nuclear mass (inside the central syncytium) appears. However, there is no formation of a digestive tract at any phase of development. In addition, the very early embryo attains a syncytial organization. Thus, it is difficult to decide what is ectoderm, endoderm, or mesoderm. During the course of development, the embryo detaches from the floating ovary, and the single eggshell differentiates into the different envelopes.