In the early 1950's, during the near ava-lanche of discoveries, rediscoveries, and redefinitions of subcellular components made possible by electron microscopy, those prospecting in this newly opened field were faced with the problem of what to do with their newly acquired wealth. It could be increased by extending the inquiry on the horizontal to manyother cell types prepared by many other techniques; it could be extended in further depth, instrumental resolution permitting (" ultra" was the preferred prefix of the period); or it could be used as a guide to monitor cell fractionation procedures of thetype previously developed by Claude (1). The last alternative seemed particularly attractive since the small dimensions of many of the