  We study an XXX open spin chain with variable number of sites, where the
variability is introduced only at the boundaries. This model arises naturally
in the study of Giant Gravitons in the AdS/CFT correspondence. We show how to
quantize the spin chain by mapping its states to a bosonic lattice of finite
length with sources and sinks of particles at the boundaries. Using coherent
states, we show how the Hamiltonian for the bosonic lattice gives the correct
description of semiclassical open strings ending on Giant Gravitons.
