The cytoplasmic architecture of the egg cell ofSmittia spec. (Diptera, Chironomidae) : I. Anterior and posterior pole regions.

1. The egg of the Chironomid midgeSmittia spec. has been studied by light and electron microscopy. The present paper describes the fine structure of the anterior and posterior pole regions before pole cell formation. These regions were selected because of their functional involvement in body pattern determination and pole cell formation. 2. In the anterior cytoplasm (region I), 3 subregions can be recognized. A thin outer layer (Ia) which resembles the more equatorial periplasm (region II) but contains fewer organelles, covers a layer rich in mitochondria (Ib). This in turn borders a central cone of cytoplasm (Ic) which protrudes into the anterior face of the yolk endoplasm and frequently contains a cytaster-like structure but no chromatin. 3. The posterior cytoplasm (region III) includes a germ plasm or oosome similar to the type found in other lower dipterans. It is lens-shaped and contains a 3-dimensional network of electron-dense material. This material is probably granular, but may appear fibrous due to the spatial arrangement of the granules. 4. A series of organelles of multivesicular or lysosome-like appearance is described. These may be involved in the formation or utilization of proteid yolk. 5. Special structures or organelles restricted to the anterior pole region were not found. This might indicate that the role of this region in the switch from head formation to tail formation after UV irradiation could be due rather to differences in quantity or arrangement of ubiquitous structures than to qualitative differences between this and other egg regions. However, qualitative singularities cannot be excluded. They are obvious in the posterior pole region which contains the oosome.