Mean masses of galaxies in clusters, derived from velocity dispersion through the virial theorem, are systematically larger by one and a half orders of magnitude than masses derived from the rotation of individual objects Several hypotheses have been put forward to account for this discrepancy: (1) elliptical and lenticular galaxies have extremely high mass4uminosity ratios; (2) a high density of intergalactic stars and/or diffuse matter is present in clusters; (3) clusters are not in statistical equilibrium and are rapidly expanding. These hypotheses are re-examined and discussed mainly in terms of the apparent mean mass per galaxy, , and apparent mean space density, p*. Available data on velocity dispersion in groups and clusters are reviewed; in all cases, for small groups as well as for large clusters, log = 12.2 j 0 2 (H = 75 km ). A relation between maximum apparent density and radius, log PM = -19 (log R - 7 0) (in c g s. units), is found to apply in the observed range 1 <R < cm. The negative result of a search for intergalactic stars in the Coma Cluster indicates that, to account for the apparent mass of the cluster, such stars must have the improbable mass4uminosity ratio f > 1250; other observable forms of intergalactic matter also seem to be ruled out. A mass-luminosity ratio J 160 in ellipticals and lenticular galaxies, which might account for the apparent mass of the Coma Cluster, fails to account for the masses of other groups and clusters by large factors; it is totally inadequate for systems that have no elliptical members. Carpenter's density restriction between maximum population and cluster radius is re-examined and found to be also in agreement with the conclusion that most groups and loose clusters of galaxies are unstable and evaporate with lifetimes of the order of a few billion years.