Mass fraction of helium in the stellar core Y_c and at the surface Y_s depicted for stellar models of 100, 300, and 500  M_⊙ and rotation rates explained in the figure key. For a given rotation rate, initially more massive stars increase their surface helium abundance more quickly. Every 10⁵ yr the models are highlighted by filled circles. Chemically homogeneous evolution is indicated by equal changes in both mass fractions and corresponds to a slope unity. At Y_c ≥ 0.8, all calculated stellar evolution models show an evolution toward the line of equal Y_c and Y_s, caused by mass loss.