[Experiments on the uncleared egg ofPimpla turionellae L. (Hymenoptera) for the functional analysis of the oosome...

In endoplasm close to the posterior pole of the egg ofPimpla one finds conglomerated oosome material, rich in RNA. Investigations after various operations in the oosome region (10% of egg length), before cleavage were intended to show whether pole cells develop, how many segments form and if gonads contain primordial germ cells.Oosome material was squashed with a blunt glass needle. The uninjured part of the egg in front of the oosome region develops blastoderm but no pole cells. It gives rise to a fully segmentated larva with germ cells in the gonads.After ligation of up to 15% of egg length complete embryos with germ cells can develop. The smaller the anterior isolates, the more abdominal segments are missing.By ligation and invagination of the hindpole of eggs with a blunt glass needle, anteriorly material from the oosome region is combined with ooplasm situated more. Translocation of only a small amount of ooplasm results in the same number of abdominal segments in the anterior isolate as in ordinary ligated eggs. Translocation of much ooplasm yields a significantly greater number of abdominal segments. It is immaterial for the metameric segmentation of the embryo whether the oosome is situated before or behind the ligature or is destroyed. But the depth of the invagination and how many segments result do not seem to be correlated.A completely segmentated embryo can develop also after extirpation of the oosome provided care is taken not to injure the hindpole-plasm. No pole-cells result when the complete oosome is missing and the hindpole-plasm is present; loss of part of the oosome results in the development of only a few pole-cells. Thus oosome material is a necessary and quantitative condition for pole-cell differentiation. In one favourable case pole-cells developed in the extraovate because the oosome was followed after some hours by endoplasm and cleavage nuclei.Functions of the oosome are discussed: together with cleavage nuclei it is responsible for pole-cell development. As pole-cells are not invariable precursors of germ-cells, the oosome cannot contain determinants for them. Possibly it includes postembryonic growth modifiers or it could be active in gametogenesis later on. As an egg without oosome-region is able to develop an embryo, this region does not or exclusively contain an activation-center (e. g.Platycnemis), or special hind-pole factors (e. g.Euscelis). In any case the oosome itself does not include these factors. A greater number of segments in the anterior isolate after translocation of ooplasm could be due to its special quality, as inEuscelis andBruchidius whose metameric organisations originate from a bipolar ooplasmic reaction system. Also it could depend only on the increase of ooplasm competent for differentiation-factors in the middle and anterior egg parts.