Figs 15–26.

Halteria grandinella, conjugants during first (15–18) and third (19–21) maturation division, reciprocal fertilization (22), and first synkaryon division (23–26) after protargol impregnation. 15. Ventral view of pair during prophase stage 5 (“parachute” stage). Arrow marks elevated frontal plate of peristomial field. The oral primordia commence to develop in the cleft between the conjugants. 16. Ventral view of pair during metaphase. 17, 18. Ventral view of pairs during anaphase and telophase. In both partners, the buccal portion (arrows) of the adoral zone of membranelles starts to degenerate, and the macronucleus has split into usually two fragments (18). 19, 20. Ventral view of pairs during prophase and metaphase. A darkly impregnated pycnotic derivative (arrowheads) is invariably posterior to the dividing derivative, while the position of the faintly impregnated one (arrows) is more variable. The buccal membranelles disappeared in both partners. Note the faintly impregnated granules (probably somatic dikinetids) in the posterior portion of the metaphase couple (20). 21. Ventral view of pair during telophase. 22. Ventral view of pair during exchange of the migratory pronuclei. 23–26. Ventral view of pairs during prophase, metaphase, anaphase, and telophase. Note the disappearance of the argyrophilic vesicles and the spreading of the membranelles in the oral primordium. AV – argyrophilic vesicles, B – bristle kineties, BM – buccal membranelles, CM – collar membranelles of right conjugant, DMA – degenerating vegetative macronucleus and its fragments, MD – maturation derivatives, MI – micronucleus derivatives, MP – migratory pronuclei, OP – oral primordium, SP – stationary pronuclei, SYK – synkaryon. Scale bar 10 μm.