Cambridge, Mass. — Outdoor apparel and equipment retailer REI will open an 8,000-square-foot store at Cambridge Crossing, a 43-acre mixed-use development located at the junction of Boston, Cambridge and Somerville. The opening of the store, which is scheduled for this fall, represents the launch of REI’s new small-format prototype. The store will be located in a newly constructed office and retail building. San Francisco-based DivcoWest is the master developer of Cambridge Crossing.