We report the first systematic observations of precursor effects in Sh
ape transitions of phospholipid-bilayer vesicles in aqueous solution.
Vesicles change abruptly, as temperature T is raised, from a prolate e
llipsoidal shape to a ''budded'' shape consisting of two unequal Spher
es connected by a narrow neck. On the low-T side of this transition, w
e see large thermal shape fluctuations (quasicritical fluctuations) an
d long relaxation times (quasicritical slowing down), which may be int
erpreted, in the context of a phi(6) Landau theory, as the fluctuation
s of a metastable state near its spinodal instability.