Fig. 5

Details of oogenesis and early embryogenesis in Crisiella producta (a‒f Colony branches showing autozooids cross-sectioned on different levels and gonozooids sectioned in their lower part) (light microscopy). a Two autozooids and gonozooid containing ovary with mature oligolecithal oocyte (arrow). b‒f Consecutive stages of embryonic growth and placental development. b Gonozooid (upper left) with early primary embryo surrounded by massive multilayered ‘envelope’ presumably originated from follicular cells. c Gonozooid with early primary embryo showing loose arrangement of blastomeres and surrounded by multilayered ‘envelope’. d Gonozooid with early primary embryo surrounded by multilayered ‘envelope’ transforming to nutritive tissue. e Gonozooid with primary embryo surrounded by placental analogue (nutritive tissue). Non-altered membranous sac wall is still visible as a thin ‘nuclei-bearing’ line (arrowheads) either adjacent to or somewhat removed from the multilayered ‘envelope’ or nutritive tissue in b‒e; plugged communication pore in interzooidal wall shown by arrow in e. f Gonozooid with late primary embryo starting fission; membranous sac (arrowheads) is partially free and partially incorporated into nutritive tissue. In all gonozooids the pseudocoel (space between placental analogue/membranous sac and cystid wall) is filled with a network of large and irregular, darkly-stained ‘mesothelial’ cells. c, coelom; cw, external cystid wall; dp, degenerating polypide; e, primary embryo; g, gut; iw, interzooidal wall; ms, membranous sac; nt, nutritive tissue; ov, ovary; pc, pseudocoel; t, tentacle; ts, tentacle sheath; uc, ‘upper cell complex’