A sonic hedgehog - dependent signaling relay regulates growth of diencephalic and mesencephalic prim

A holding device for holding a laminar photographic material, particularly in production of photographic enlargements, has a supporting member with a supporting surface arranged so that a laminar photographic material can be placed on said supporting surface from outside, and an ejecting device associated with the supporting member. The ejecting device is movable between an operative position in which it projects outwardly beyond said supporting surface in a direction transverse to the latter so as to lift the laminar material from the supporting surface, and an inoperative position in which the ejecting device does not project outwardly beyond the supporting surface in the transverse direction. The ejecting device may be formed as a two-armed ejecting member whose one arm is arranged to be urged by the user, whereas the other arm lifts the laminar material from the supporting surface in response to such urging.