Abstract
In the maternal-effect embryonic lethalmat(3)6, although cell formation occurs only at the poles, posterior blastoderm cells give rise to a posterior midgut rudiment (PMG) that undergoes extension movements similar to those in normal embryos (Rice and Garen 1975). Inmat(3)6 embryos, PMG cells retain cytoplasmic continuity with the yolk sac during early extension, and a microfilament system is present in the yolk sac beneath and anterior to the PMG. This correspondence between normal and mutant embryos in what we have postulated to be essential structural components of the morphogenetic system (Rickoll and Counce 1980) supports our interpretation that the yolk sac has a causal role in early germ band extension. Further, extension movements in these mutant embryos provide evidence that neither large-scale changes in cell shape nor cell interactions are essential for PMG extension and invagination.
