Fig. 5. The development of the Drosophila ovary. (A) At puparium formation, the ovary is an anteroposteriorly stratified structure, consisting of apical cells (light gray), presumptive terminal filament (TF) cells (green), germ cells (dark gray), somatic cells (magenta) and presumptive basal stalk cells (blue); anterior is to the top and lateral to the left in all diagrams. About 20 ovarioles are formed as the TF cells intercalate laterally into stacks of elongated cells in register anteroposteriorly, which then separate into stacks of discs (A, parts a-d, B). As they do so, the apical cells form an epithelial sheet and move posteriorly between the stacks of discs, separating them into TFs (A, parts a-d). (C) The posteriorward (arrows) invasion of the apical cell epithelium then separates the germ cells and associated somatic cells and, finally, basal stalk cells into ovarioles (D,E). (F) The apical cell epithelium with an underlying basal lamina (red) separates the basal stalk cells into arrays of several cells in diameter (part a). These cells intercalate transversely to form a longer, narrower array, thus forming the elongated basal stalks and completing the separation of the ovarioles (parts b,c). A similar process occurs among the somatic cells associated with the germ cells, to elongate the interfollicular stalk, which separates the newly formed follicles from the germarium. Anterior is to the top and posterior to the bottom in all figures. Adapted, with permission, from Godt and Laski (Godt and Laski, 1995).