UNUSUAL EVOLUTION IN THE VLBI STRUCTURE OF 0735+178

Polarization-sensitive global VLBI observations of the BL Lacertae obj ect 0735+178 have been made at epochs 1987.41, 1990.47, and 1992.44. C omparison of the resulting images with previously published VLBI image s of this source confirms superluminal motion of one component (''K2'' ) at a speed 17.4 h(-1)c. We have identified two additional superlumin al components with apparent transverse speeds greater than or equal to 5.0 h(-1)c and greater than or equal to 4.2 h(-1)c. The images presen ted here span a period of time during which K2 should have collided wi th a nearly stationary component (''K1'') at a distance similar to 5 m as from the core; our images show no evidence of a violent interaction between these components, supporting the possibility that K1 represen ts a local region of enhanced emission in an underlying flow, rather t han a physically distinct ''blob'' of relativistic electrons traveling outward from the core. VLBI polarization images for 0735+178 are now available at four epochs; at three of these epochs, the integrated and milliarcsecond-scale polarization are appreciable, but at epoch 1987. 41, both the integrated and milliarcsecond-scale polarization are very weak. The available VLBI polarization data and integrated monitoring data suggest that this unusual weak milliarcsecond scale polarization was associated with the formation of a new transverse shock, and near cancellation of orthogonally polarized emission from quiescent and sho cked material in the VLBI core.