The interaction between a laminar boundarylayer and an open cavity is investigated experimentallyfor medium range Reynolds numbers. Flowvisualizations are carried out for three differentobservation directions in order to understand the spatial development of dynamical structures. In particular, synchronized visualizations in two parallel planes picture the transverse development of the flow. The study is conducted by changing the cavity aspect ratio, the Reynolds number and therefore the flow patterns inside the cavity. The issue is to emphasize the 3-D development of the flow. In particular, we show that the dynamical structures are not due to secondary shear layer instabilities.