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Summary
In newly hatched larvae of Bacillus rossius (Insecta Phasmatodea) viewed with light and electron microscopes, ovarioles appear as bag-like structures; during the first instar they initially assume a bell-shaped appearance. Afterwards, they begin to elongate and their vitellarium essentially consists of a row of oocytes with a clear growth gradient. This typical ovariole morphology becomes more evident in the subsequent instars up to the fourth.
The germarium appears as a region interposed between growing oocytes of the vitellarium and somatic cells of the terminal filament. Light and electron microscope observations indicate that the structure of the germarium does not markedly change during the different instars: it contains, besides somatic prefollicular cells, germ elements arrested in the “diffuse” stage which precedes diplotene of growing oocytes. Only in newly hatched larvae, squash preparations of the germarium also show germ cells in earlier meiotic prophase (zygo-pachytene).
The observations here reported indicate that in B. rossius the remarkable elongation and size increase of the ovarioles from the second larval instar onwards can be ascribed to differential growth of germ cells already present in the vitellarium by the end of the first instar rather than to new release of oocytes from the germarium.
The absence of intercellular bridges interconnecting clustered germ cells in the germarium is also discussed.