ERIKSSON, BJ*; TAIT, NN; AKAM, M; University of Cambridge; Macquarie University, Sydney; University of Cambridge: Early development in the velvet worm Euperipatoides kanangrensis (Onychophora: Peripatopsidae).

While early development in the yolk-free eggs of viviparous onychophorans is reasonably well documented, our knowledge of the equivalent stages in the yolky eggs of ovoviviparous species is sketchy. From a saddle of blastomeres, cells proliferate and spread over the surface of the egg. They begin to aggregate in two parallel streams at the future posterior end. These will later become the germ bands. At the same time, a `mouth-anus� furrow is formed between the two germ bands on the future ventral side. The furrow elongates as cells invaginate and then migrate laterally and dorsally just below the ectoderm of the embryo. It is believed that these cells function as vitellophages that later form the anterior midgut. The yolk is exposed in the centre of the ventral furrow. The furrow eventually closes, leaving an opening in each end. These openings will become the mouth and anus. The blastopore is located posterior to the anus and is believed to contribute cells that form the mesoderm and posterior midgut. The assumption that the endodermal cells function as vitellophages during development is supported by their ultrastructure, which shows yolk granules of different sizes in the cytoplasm and cellular protrusions towards the side opposite the yolk. These protrusions are presumed to form a surface extension, facilitating transport of nutrients to cells further from the yolk. The early germ disc with its ventral furrow is variously orientated in relation to the long axis of the egg, as has been described for a South African species. This suggests that the anterior-posterior axis is not pre-fixed in the egg by maternal factors as in e.g. Drosophila melanogaster. A light and electron microscopical study of early development in an ovoviviparous species of onychophoran is presented to reassess and extend previous studies.