  We present a deep image of the radio galaxy MRC 1138-262 taken with the
Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to
have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized
structure, identified with a protocluster. The morphology shown on the new deep
HST/ACS image is reminiscent of a spider's web. More than 10 individual clumpy
features are observed, apparently star-forming satellite galaxies in the
process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago.
There is an extended emission component, implying that star formation was
occurring over a 50 times 40 kpc region at a rate of more than 100 M_sun/yr. A
striking feature of the newly named ``Spiderweb galaxy'' is the presence of
several faint linear galaxies within the merging structure. The dense
environments and fast galaxy motions at the centres of protoclusters may
stimulate the formation of these structures, which dominate the faint resolved
galaxy populations in the Hubble Ultra Deep Field. The new image provides a
unique testbed for simulations of forming dominant cluster galaxies.
