Figure 2. The late-stage vermiform embryos of four species of dicyemids. Scale bar represents 10 µm. Cilia are omitted. See text for explanations of cell division notations. Other abbreviations: AG, agamete; AX, axial cell; DP, diapolar cell; MP, metapolar cell; PR, propolar cell; PP, parapolar cell; S, syncytium; UP, uropolar cell. Microcyema vespa: (a) a late-stage embryo (sagittal optical section)—note an agamete (5a2) in the cytoplasm of an axial cell (5a1); (b) a late-stage embryo (sagittal optical section); (c) formed embryo (sagittal optical section)—pairs of 6B11, 6B12, and 6B21 cells form a syncytium (S) that is more conspicuously stained with hematoxylin. Conocyema polymorpha: (d) a late-stage embryo (sagittal optical section)—note an agamete (6a2) incorporated in the cytoplasm of an axial cell (6a1); (e) a late-stage embryo (lateral view); (f) formed embryo (lateral view)—pairs of 4B11 and 5B21 cells form propolar cells (PR) that are more conspicuously stained with hematoxylin. Dicyema apalachiensis: (g) 13-cell stage—note an anaphase figure of 4B12 cell and a metaphase figure of 4B12 cell; (h) 15-cell stage—the 5B111 and 5B121 pairs form the propolar cells (PR), while the 5B112 and 5B122 pairs form another type of polar cell, the metapolar cell (MP); (I) formed embryo (lateral view)—propolar cells and metapolar cells are more conspicuously stained with hematoxylin. Pseudicyema nakaoi: (j) 22-cell stage—note a metaphase figure in 4B12 cell—the plane of this division, in contrast to the divisions of 4B12 pair (Fig. 2g), are oblique to the anterior-posterior axis, and as the result, cells of the propolar tier alternate with cells of the metapolar tier (see Fig. 2k, 1); (k) a late-stage embryo (lateral view); (l) formed embryo (lateral view)—propolar cells and metapolar cells are more conspicuously stained with hematoxylin.
