On the $\xi$-Distribution of Inclusively Produced Particles in $e^{+}{e}^{-}$ Annihilation

We discuss the momentum distributions of inclusively produced particles in $e^{+}{e}^{-}$ annihilation. We show that the dependence of the position of the maxima of the $\xi =\ln (1\mathrm{/}z)$ spectra on the mass of the produced particles follows naturally from the general definition of fragmentation functions when energy-momentum conservation is correctly incorporated.