Morphology of segments and genital primordia of M. corti during early segmentation. All specimens were stained with TO-PRO-3. A. Tetrathyridium beginning segmentation. B. Close-up of early cell accumulations in segment primordia (arrows) with central genital primordia. C. Developed adult worm. D. Posterior region of the same adult worm as in C; anterior is to the bottom; notice the lack of segmentation of the posterior most region. E. Early segments of adult worm, showing early genital primordia (arrows). F. Later segment of adult worm showing late genital primordium (arrow) and testis primordia (asterisks). G. Later segment of adult worm showing cirrus pouch (arrowhead) and developing uterus (ut). H. Schematic drawing of a transverse section of a segment with late genital primordium. cp, cortical parenchyma; g, genital primordium; iml, inner muscle layer; mp, medullary parenchyma; nc, main nerve cords; st, sub-tegumental region with sub-tegumental muscle layer; t, testes primordia; teg, tegument (which consists of a syncytium that covers the body in a continuous sheet connected to perikarya (tegumental cells) that lie below the sub-tegumental muscle layer ). Bars represent 200 μm in A, 100 μm in B and F, 1000 μm in C and D, and 50 μm in E and G.