Position of mitotic cells in serial sagittal sections of M. corti at the beginning of segmentation. A. Schematic drawing of one analyzed specimen, indicating the orientation and direction of the successive planes of section. Numbers of planes correspond to illustrations in part B. B. Scale drawings of some of the analyzed sections from that specimen, indicating the position of mitotic figures (red dots). The approximate distance between sections is indicated to the right. In sections 2 and 3, the posterior-most region (which, due to the curvature of the specimen, is not continuous in these sections with the anterior-most region) has no mitotic figures and is not illustrated. The only mitotic cell found in the cortical region (in over 600 identified mitoses) is illustrated in section 4. Notice the proximity of this cell to an accessory excretory pore. cp, cortical parenchyma; ec, excretory ducts; ep, excretory pore; gp, genital primordium; mp, medullary parenchyma. The bar represents approximately 200 μm.