  Many embryonic deformations during development are the global result of local
cell shape changes and other local active cell sheet deformations.
Morphogenesis does not only therefore rely on the ability of the tissue to
produce these active deformations, but also on the ability to regulate them in
such a way as to overcome the intrinsic variability of and geometric
constraints on the tissue. Here, we explore the interplay of regulation and
variability in the green alga $Volvox$, whose spherical embryos turn themselves
inside out to enable motility. Through a combination of light sheet microscopy
and theoretical analysis, we quantify the variability of this inversion and
analyse its mechanics in detail to show how shape variability arises from a
combination of geometry, mechanics, and active regulation.
