Oocytes, early development, and gonad structure in Wirenia argentea. A, B. Adult specimen bearing eggs in the gonad (arrowheads) and pericardium (pd). A. Scale bar 500 μm; B. Arrow indicating posteriormost part of the gonad containing white sperm, scale bar 250 μm; C. Gonad of W. argentea, histological cross section. Oocytes of different developmental stages with large nuclei (n) containing two dark-staining nucleoli are attached to a median diaphragm (arrows) and surrounded by spermatogonia (sc); bw: body wall, mg: midgut; scale bar 50 μm; D, E. Scanning electron micrographs of an uncleaved embryo (D) and a 32 cell stage (E) covered by the smooth egg hull, scale bars 30 μm; F-H. Light micrographs of 2-cell stage (F), 4-cell stage (G), and 8-cell stage (H), macromeres conventionally marked, the arrow in F points to the egg hull. Note that the CD, D and 1D blastomeres are larger than the other blastomeres; scale bar 40 μm; I. Larva 12 hours after hatching, light micrograph, scale bar 40 μm; J. Schematic drawing of a pericalymma larva depicting apical cap and trunk, stomodaeum (st), protonephridium (pn), as well as the main ciliary bands; at: apical tuft, fc: foot ciliation, ot: oral troch, pro: prototroch, tt: telotroch.