The occurrence and proportions of petrologic types among ordinary chondrites have been explained by internal heating of three (H,L,LL) parent asteroids of ~100-km radius (e.g., Miyamoto et al, 1982). This conventional (''onion-shell'') model predicts an inverse relation between petrologic type and metallographic cooling rate, but none has been observed. Scott and Rajan (1981) devised a ''metamorphosed-planetesimal'' model to explain this discrepancy, whereby metamorphism occurs in planetesimals a few kilometers in radius, which then accrete to form r~100 km parent bodies. Metallographic cooling rates are then controlled by burial depth. Thermal and collisional constraints on the metamorphosed-planetesimal model are examined here, and the model is found to be applicable only to highly insulating, 26Al-rich planetesimals that can accrete intact without being shattered by impact or to shattered planetesimals whose fragments are not widely dispersed over the target surface. An alternative model is presented here, in which onion-shell parent bodies are collisionally fragmented during metamorphism and then gravitationally reassembled. If reassembly times are short (~days), then metallographic cooling rates would be determined by burial depth in the reaccreted parent body. This model, unlike previous ones, can explain both coherent and incoherent cooling rates of breccia clasts, by collisions during or after metamorphism, respectively.