Transovarial transmission of bacterial symbionts in Xiphinema brevicollum was examined by means of electron microscopical- and fluorescence microscopical observations of intra-uterine and freshly laid non-embryonated eggs, as well as of embryonated eggs. The symbionts are mainly aggregated near one of the egg poles and this is observed in non-embryonated as well as in embryonated eggs. The bacteria are present in intestinal cells of first stage juveniles that are still enclosed by the egg-shell. In subsequent juvenile developmental stages their number increases and the intestinal cells are filled with the symbionts. Bacteria were not present in the genital primordium. It is hypothesised that the aggregated symbionts at one of the egg poles become enclosed in the E founder cell during embryogenesis and subsequently are distributed to the endodermal daughter cells. The bacteria become enclosed in the ovarial wall through an unknown mechanism only during the later stages of gonad development.
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