The electrons that are added progressively on moving from $\mathrm{Ce}$ to $\mathrm{Lu}$ enter the

Lanthanoids (Z = 58 to 71) are the elements in which the antepenultimate $4f$ subshell is being filled. The configuration is $4f^{1-14}5d^{0-1}6s^2$. Hence the added electrons enter $4f$.