This review summarizes the patterns of cell division and the cell lineages of the two types of embryo, namely, the vermiform embryo and infusoriform embryo, of the dicyemid mesozoans, Dicyema japonicum. The infusoriform embryo develops from a fertilized egg. The early cleavages are holoblastic and spiral. At around the 20- to 24-cell stage, cleavages become asynchronous and the cleavage pattern changes from spiral to bilateral. The fully formed infusoriform embryo consists of 37 cells and exhibits bilateral symmetry. These 37 cells are produced after only four to eight rounds of cell division. The vermiform embryo develops asexually from an agamete. Cell division proceeds spirally in the early stages, becoming bilateral from the fifth cell division onward. The fully formed vermiform embryo consists of 23 cells and exhibits bilateral symmetry. These 23 cells are produced after only four to six rounds of cell division. During the development of both types of embryo, a few cells consistently undergo extremely unequal divisions; the much smaller daughter cells eventually degenerate and ultimately disappear during embryogenesis. This disappearance might be an example of programmed cell death. The cell lineages appear to be invariant among embryos, except for the derivation of the lateral cells of infusoriform embryos. In addition, this review describes the patterns of cell divisions and cell lineages of the vermiform embryos of four genera, Conocyema, Dicyema, Microcyema, and Pseudicyema. In the development of vermiform embryos, early stages and cell lineages are identical in four genera. Species-specific differences appear during later stages of embryogenesis.
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