We predict stable geometrical structures and interaction energies of Ni clusters using non self-consistent electron density functional based corrected effective medium (CEM) and MD/MC-CEM methods. A plot of the reaction energies for the atomic ejection process, Ni-N+1-->Ni-N+Ni, for Ni-24-Ni-55 displays a number of informative characteristics : (a) peaks and valleys represent internal structural rearrangement in which the number of core atoms increases by at least one and; (b) a plateau at N=50-54 is associated with the closing of the second MacKay icosahedron at Ni-55. The lowest energy structures of Ni-N clusters for N=24-55 are dissimilar generally to those of both rare gas clusters and fragments of the bulk crystal lattice except where a stable icosahedral or bulk core is present. The growth scheme for N<50 is determined by the stability and structure of the changing number of core atoms. By contrast, the growth scheme for 51 less than or equal to N less than or equal to 55 is determined by the addition of surface atoms to a very stable and invariant 13-atom icosahedral core. The theoretical predictions are compared to available model growth schemes and experimental data.