Drosophila germ-band extension (GBE) is an example of
the convergence and extension movements that elongate
and narrow embryonic tissues. To understand the collective
cell behaviours underlying tissue morphogenesis, we have
continuously quantified cell intercalation and cell shape
change during GBE. We show that the fast, early phase of GBE
depends on cell shape change in addition to cell intercalation.
In antero-posterior patterning mutants such as those for
the gap gene Krüppel, defective polarized cell intercalation
is compensated for by an increase in antero-posterior cell
elongation, such that the initial rate of extension remains the
same. Spatio-temporal patterns of cell behaviours indicate
that an antero-posterior tensile force deforms the germ band,
causing the cells to change shape passively. The rate of anteroposterior cell elongation is reduced in twist mutant embryos,
which lack mesoderm. We propose that cell shape change
contributing to germ-band extension is a passive response to
mechanical forces caused by the invaginating mesoderm.