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.

Inhalt
Cell Ageing: Chairman's Introduction.- The Effect of Hydrocortisone on DNA Synthesis and Cell Division during Aging in Vitro.- Proliferative and Functional Impairment of Pancreatic Epithelial Cells Maintained in Vitro.- The Use of Arrested Populations of Human Diploid Fibroblasts for the Study of Senescence in Vitro.- Relationship between Cell Kinetic Changes and Metabolic Events during Cell Senescence in Vitro.- Do Hyperplastoid Cell Lines Differentiate Themselves to Death?.- Time-Lapse Cinemicrophotographic Studies of Cell Division Patterns of Human Diploid Fibroblasts (WI-38) during their in Vitro Lifespan.- Characteristics of Proliferative Cells from Young, Old, and Transformed WI 38 Cultures.- Alterations in Chromatin Functions during Aging in Vitro.- Growth-Promoting Alpha-Globulin and Ageing.- Cell Surface Alterations and in Vitro Aging of Animal Cells.- Changes in Lysosomes during Ageing of Parenchymal and Non-Parenchymal Liver Cells.- The Effect of Age on Mitochondrial Ultrastructure and Enzymes.- Creatinephosphokinase in Human Diploid Cell Lines.- Cytochemical and Cytogenetic Findings in Five Human Leukocyte Long-Term Cultures (LAHL) of Different Origin.- Enzymatic Differences between Short-Lived and Long-Lived Drosophila Strains.- Cell Ageing in the Intestinal Tract.- Studies on the Proliferative Capacity of Mouse Spleen Cells in Serial Transplantation.- On the Ageing of Intermitotic Cells Investigations on Enterocytes and Hepatocytes.- Fat Cells in Ontogenesis.- Ageing and the Loss of Auditory Neuroepithelium in the Guinea Pig.- The Phagolysosomal System of the Retinal Pigment Epithelium in Ageing Rats.- The Role of Retinol in, and the Action of Anti-Inflammatory Drugs on, Hereditary Retinal Degeneration.- Effect of Age on KidneyHyperplasia in the Rat after Unilateral Nephrectomy.- Effect of Age on Kidney Hyperplasia in the Rat during Cold Acclimation.- Age-Related Changes of Cells Involved in Immune Responses.- Observations of Age and Environmental Influences on the Thymus Kept in Tissue Culture.- Fidelity in the Collagen Synthesized and Modified by Aging Fibroblasts in Culture.- Biosynthesis of Collagen during the Life Cycle of Human Diploid Cell Lines.- Ageing Processes in Collagens from Different Tissues of Rats.- The Effect of Nutritional Regimes Upon Collagen Concentration and Survival of Rats.- Bone Aging.- Relations between Development of the Capillary Wall and Myoarchitecture of the Rat Heart.- Functional Capacity of Neonatal Mammalian Myocardial Cells during Aging in Tissue Culture.- Ultrastructural Changes in Senile Muscle.- Denervation, Reinnervation and Regeneration of Senile Muscle.- Morphometrical and Mathematical Analysis of the Ageing Changes of the Muscle-Connective-Tissue-Relation in Smooth Muscles.- Some Histochemical Age Changes of the Smooth Muscle Cells in the Veinous Wall.- Effect of Lysolecithin on the Transport of Plasma Cholesterol to Tissues: Developmental Aspects.- Viral Infection and Interferon in Cell Cultures Aged in Vitro.- Ultrastructure of in Vitro Aged Chick Embryo Cell Cultures in Relation to Viral Infection.- Heritable Cell Cycle Disturbances and Late Recovery in X-Irradiated Murine Lymphoma L5178Y-S Cell Populations in Vitro.- Studies on the Sensitivity of Anticancer Agents of Normal Human Cells in Culture.- Ribonucleases in mouse Ascitic Tumor Cells during Ageing in Vivo and in Vitro.- Proliferation and Morphology of Ascitic Cells as a Function of Age In Cell Culture.