Morphology and development of the Drosophila eye II . In vitro development

The morphological changes which take place in the differentiation of the adult Drosophila eye from the larval imaginal disc have been described from a histological and cytological standpoint by Shatoury (1956, 1963) and Shatoury & Waddington (1957). In addition, Gottschewski (1960), Gottschewski & Querner (1961), and Schneider (1964) have studied these processes in eyes and brains in vitro. These latter workers removed eye-antennal anlage with associated structures, and placed them in culture media. Under these conditions, the imaginal discs can apparently undergo most of the normal differentiation processes, finally attaining nearly all of the adult morphology. The imaginal eye disc can be found in the Drosophila larva as early as the larva's emergence from the egg (Kaliss, 1939), and its size is known to increase during the larval instars primarily through cellular division which, in turn, ceases almost entirely shortly after metamorphosis begins (Hadorn, 1965). The onset of differentiation of the eye disc apparently occurs earlier than that of the other imaginal discs (E. Hadorn, personal communication). In the late larval and early prepupal stages, groups of cells are set aside which will eventually become the definitive ommatidia (Chen, 1929; Shatoury & Waddington, 1957). Waddington & Perry (1960) have described, by use of the electron microscope, the morphological changes which occur in the eye components of the late larval and some pupal stages. The various morphogenetic movements of the eye disc and the cerebral hemisphere (and later the optic lobe) have been described by several investigators (Shatoury, 1956; Gottschewski, 1960; Gottschewski & Querner, 1961; Schneider, 1964). The adult size and shape of the eye are reached between 40 and 50 h after puparium formation, while ommochromes first appear at 45 h and pteridines at 65 h when raised at 25 °C (Hanly, Fuller & Stanley, 1967). It is interesting to note that none of these papers describes the differentiation of the bristles (hairs) associated with the eye surface, although Waddington &