Cell Impairment in Aging and Development Papeback -

lar aging, to which this model contributes, has grown. Apart from reports on work in this almost "classical" diploid cell system, the symposium presents studies using different biological systems with results that have been rewarding as information is obtained on patterns of change that are common to more than one experimental system. Indeed, in recent years much more has been learned about the fate of all different types of intermitotic and postmitotic cells in situ. The symposium has also presented contributions dealing, not directly with aging but with early ontogeny; such information on early developmental changes should certainly shed light on some of the mechanisms involved in aging. We are cognizant of the fact that environmental influences resulting from the complexities of modern civilization may have results that only occur much later, and profoundly affect the lifespan of the organism. There remain, of course, many unanswered questions. Whether there is "physiological" as opposed to "pathological" aging; whether "old" cultures living in unchanged, although not exhausted, medium, are degenerating, not aging; what is involved when "old" fragment cultures regenerate after excision by filling the wound with "young" cells; why some tumor cells in vivo as well as in vitro die while others live; all are questions eserving of our attention.