When instructional designers develop content-targeted pedagogical situations, their practice can be theorized as engineering students' development of conceptual schemes. To account for the contributions of students' prior schemes and situated experiences towards their development of conceptual schemes, I suggest a distinction between the evocation of schemes and enactment of situations. The former suggests that the design of an instructional situation can activate students' prior schemes. The latter suggests that the structure of students' activity in an instructional situation determines the nature of newly constructed schemes. I contextualize my views using empirical data from a case study of early fraction instruction. [For the complete proceedings, see ED585874.]