  We use laser hole-drilling to assess the mechanics of an embryonic epithelium
during development - in vivo and with subcellular resolution. We ablate a
subcellular cylindrical hole clean through the epithelium, and track the
subsequent recoil of adjacent cells (on ms time scales). We investigate dorsal
closure in the fruit fly with emphasis on apical constriction of amnioserosa
cells. The mechanical behavior of this epithelium falls between that of a
continuous sheet and a 2D cellular foam (a network of tensile interfaces).
Tensile stress is carried both by cell-cell interfaces and by the cells' apical
actin networks. Our results show that stress is slightly concentrated along
interfaces (1.6-fold), but only in early closure. Furthermore, closure is
marked by a decrease in the recoil power-law exponent - implying a transition
to a more solid-like tissue. We use the site- and stage-dependence of the
recoil kinetics to constrain how the cellular mechanics change during closure.
We apply these results to test extant computational models.
