We have analyzed the cell lineage of larval and imaginal cells in the thoracic ectoderm of the early embryo of Drosophila melanogaster, by homotopic transplantation

In holometabolous development, higher insects pass through two stages, the larva and the imago. Both larval and imaginal cells are derived from the cells of the blastoderm stage. After the final embryonic wave of mitosis, however, only the imaginal cells remain diploid and capable of division, proliferate massively and do not differentiate. The development of the imaginal discs in Dacus tryoni and Drosophila melanogaster has been followed back to the late embryo (Anderson, 1963; Bate and Martinez-Arias, 1991). In this stage, very small groups of cells can be seen to separate from the larval cells. Even in the largest imaginal disc, the wing disc, Bate and Martinez-Arias counted only 24 cells in the late embryo of Drosophila melanogaster, whereas Madhavan and Schneiderman (1977) had found about 38 cells in the newly hatched larva. The difference is probably due less to additional cell divisions than to mesodermal precursors of the adepithelial cells (Janning et al., unpublished data) which have become associated with the primordium (Bate et al., 1991). Postembryonic mitosis in the imaginal discs does not begin until the end of the first larval instar (Madhavan and Schneiderman, 1977; Garcia-Bellido and Ripoll, 1978), but then the cells proliferate exponentially, reaching a final number of as many as 50,000 cells by the end of pupation (Garcia-Bellido and Merriam, 1971; Steiner, 1976). Very little is known about the origin of the imaginal structures in the blastoderm. Are the cell lineages of larval and imaginal cells separate? That is, are the cells of the larva 1107 Development 118, 1107-1121 (1993) Printed in Great Britain © The Company of Biologists Limited 1993