The prompt γ-radiation emitted in 235U thermal fission has been investigated in a multiparameter experiment, γ-yield, time-distribution, energy spectrum and nuclear anisotropy of the emission have been measured, especially γ-yield and anisotropy as a function of fragment mass and γ-energy. The mass dependence of the primary spins of the fragments can be deduced from these measurements. A value of 7 h̶ for the average primary spin and an increase of the primary spins within each fission product group is obtained. This dependence is explained within an adiabatic model of scission due to the mass dependence of the deformation of the fragments.