Chemical Abundances and Yields from Massive Stars
  Stellar rotation produces an internal mixing of the elements due to shear instability and meridional circulation. This leads to observable $N/C$ enhancements in massive stars above about 7--9 $M_{\odot}$. Rotation also favours mass loss by stellar winds. Mass loss effects dominate for masses above 30 $M_{\odot}$, while mixing dominates below that limit.   The effects of mixing are also much larger at lower metallicity $Z$, because the internal $\Omega$--gradients are steeper. This appears to be in agreement with observations in the SMC.   At very low $Z$ and Z=0, mixing between the He--burning core and the H--burning shell leads to the production of primary N in intermediate mass stars.   Such enrichments increase the metallicity of the rotating star, also massive Z=0 stars with moderate initial velocities currently reach break--up velocity during a fraction of the MS phase. Both effects favour mass loss in Z=0 stars, which have ejecta with abundance anomalies very similar to those of C--rich very metal poor stars. 
Andr\'e Maeder, Georges Meynet & Raphael Hirschi