Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Differentiation of myoblasts, which involves the cessation of division, expression of muscle contractile proteins and fusion to form syncitial muscle fibres, occurs in a proximal-to-distal sequence beginning at stage 25 (Hilfer et al., 1973; Sweeney et al., 1989).
Category: