THE DEVELOPMENT OF THE VERMIFORM EMBRYOS OF 2 MESOZOANS, DICYEMA-ACUTICEPHALUM AND DICYEMA-JAPONICUM

The pattern of cell division and the cell lineage of the vermiform emb ryos of dicyemid mesozoans were studied under the light microscope usi ng fixed and stained specimens of two species, namely, Dicyema acutice phalum, which has 16 to 18 peripheral cells, and Dicyema japonicum, wh ich has 22 peripheral cells. An agamete first divides into two apparen tly equivalent daughter cells which remain in contact with one another . One of these cells becomes the mother cell of the head of the embryo . The other cell divides again equally to produce the prospective axia l cell and the mother cell of the trunk and the tail of the embryo. Th e division proceeds spirally in the early stages but becomes bilateral from the fifth cell division onward. The embryo finally exhibits appa rently bilateral symmetry. In two lines of cells, namely, those descen ded from the prospective axial cell and those from the mother cell of the head, extremely unequal divisions occur and the resultant, much sm aller cells from each unequal division degenerate and ultimately disap pear during embryogenesis. At the thirteen-cell stage, peripheral cell s surround the prospective axial cell. At the final stage of embryogen esis, the prospective axial cell divides into two daughter cells. The anterior one is the axial cell itself and the posterior one is incorpo rated into the axial cell to form an agamete. Differences in numbers o f peripheral cells are due to the number of times that divisions of th e mother cells occur. The cell lineage of the calotte differs between D. acuticephalum and D. japonicum.