The second virial coefficient A2 was determined for atactic oligo- and polystyrenes in cyclohexane below, at, and above THETA in the range of weight-average molecular weight M(w) from 3.70 x 10(2) to 4.00 x 10(4). It is found that A2 increases significantly with decreasing M(w) at any temperature T for M(w) < 5 x 10(3), while this dependence almost disappears for larger M(w) below theta, in agreement with the literature results. It is then shown that such molecular-weight dependence of A2 may be explained quantitatively by the Yamakawa theory that takes account of the effect of chain ends. An analysis gives values of the effective excess binary-cluster integrals beta1 and beta2 associated with the chain end beads and also the binary-cluster integral beta between intermediate identical beads as functions of T, all of them except for beta above theta being quadratic in tau = 1 - theta/T. With these values of beta, the conventional and scaled excluded-volume parameters z and z below theta are recalculated. The results for the part A2(HW) of A2 without the effect of chain ends give a single composite curve below theta when A2(HW)M(w)1/2 is plotted against z, being consistent with the two-parameter theory prediction. The deviation of the A2M(w)1/2 vs z plot or the A2 vs tau plot from this prediction previously reported arises not only from the effect of chain ends but also from the previous assumption of the proportionality of beta and hence z to tau. The present and literature findings of the M(w) independence of A2 itself (except for small M(w)) below THETA are due to a cancellation of the M(w) dependence of A2(HW) by that of the effect of chain ends. The composite curve above and also that of the gyration-radius expansion factor alpha(S) vs z below theta are found to be close to the respective first-order perturbation theory values. With these results, a supplementary discussion of the coil-to-globule transition is also given.