FHOYLE has suggested (Mon. Not. Roy. Astro. Soc., 107, 4 ; 1948) a criterion for determining the degree of condensation of interstellar material into stars. Observational data for disk-shaped nebulæ, such as M 31 and M 33, suggest that only a small fraction of the total mass is in the form of stars, whereas Oort has found that the elliptical nebula NGC 3115 must be almost entirely composed of stars. The paper is devoted to a consideration of the factors that determine the fraction by mass of a nebula that is condensed into stars, and a new method for attacking this problem is developed which suggests that condensing stars with masses exceeding 0·2 that of the sun must become rotationally unstable before reaching the main sequence stage, and such stars must evolve by ejecting material until their masses are reduced below the critical value. Present available information suggests that the ejected material is thrown off to infinity, and hence stars forming in the main body of nebulæ such as M 31 or M 33 have masses less than 0·2 that of the sun at the time of condensation to the main series state ; but the masses can afterwards increase by accretion of interstellar matter. The application of the criterion shows that we should expect both the nuclei of disk-shaped nebulæ and the elliptical nebulæ to be almost entirely condensed, but the main body of a disk-shaped nebula such as M 31 would be largely uncondensed. The highly condensed state of the nuclei of disk-shaped nebulæ and elliptical nebulæreceives an important application in determining the ages of the extragalactic nebulæ, and Hoyle shows that these are of the order 5 X 109 years.