An improved incubator assembly includes a number of movable walls and a circulation system having a fluid guide apparatus adjacent each movable wall. The fluid guide apparatuses generate a first jet of air in a direction generally parallel with a wall in the closed position and a second jet of air in a direction oblique to the wall in the closed position. Whenever one of the movable walls is moved from its closed position to its open position, the jet of air that had been flowing parallel with such wall continues to flow and forms an air curtain.