DEVELOPMENT OF THE INFUSORIFORM EMBRYO OF DICYEMA-JAPONICUM (MESOZOA, DICYEMIDAE)

The cleavage pattern and cell lineage of the infusoriform embryo of the dicyemid mesozoan Dicyema japonicum were studied in fixed material with the aid of a light microscope. The early cleavages are holoblastic and spiral. At the 16-cell stage, the animal pole consists of four mesomeres, the equatorial region consists of four macromeres with four alternating sub-macromeres, and the vegetal pole is composed of four micromeres. At around the 20- to 24-cell stage, cleavage becomes asynchronous and its pattern changes from spiral to bilateral. The four micromeres, namely, the presumptive germinal cells, do not divide further and are finally incorporated into the cytoplasm of four urn cells, which are generated after divisions of the sub-macromeres. The blastomeres situated in the animal hemisphere give rise to ciliated cells that cover the posterior part of the embryo. Two blastomeres (2a2 and 2d2) undergo extremely unequal divisions and the much smaller sister blastomeres degenerate and ultimately disappear during embryogenesis. The fully formed embryo consists of 37 cells. These cells are produced after only four to eight rounds of cell division. The cell lineage appears to be invariant among embryos, apart from the derivation of the lateral cells.