Figure 2.

Early histological studies on testicular follicles in butterflies (Lepidoptera) that depict the complex structures of the apical complexes. A: Testicular follicle of the silkmoth Bombyx mori which includes the apical complex (a, b, d). The limited light microscopical resolution caused some misinterpretation concerning the identity of cell types: the central apical cell (a) was considered to be a “germ cell” (“Keimzelle”) with radial extensions. The germline stem cells were described as clumps of protoplasm with nuclei (b, d) (from Verson[7]); B: Testicular follicle of the cabbage white butterfly Pieris brassicae. The relationship between apical cell (Az) (also called Verson cell) and germline stem cells (Ps) is correctly described: The germline stem cells send projections toward the apical cell and their tips penetrate the apical cell. A layer of cells surround the germline stem cells, the cyst stem cell which, however, were not identified as such (from Zick[8]).