Obviously I need a correcting editor…
Only after third reading I realized; that not only a prefix got lost but a part of the sentence. Here is the corrected paragraph:

…A developing agent fixed to the very layer is employed, with a dye coupled to it. Being of a colour complementary to the colour the adjacent halide layer is sensitive to. This agent being oxidised, after reducing the exposed halide, has become insoluble, whereas the developing agent in the non-exposed areas becomes mobile and migrates with its attached dye into the receptive layer, where they are fixed to form an image in the kind of the imbibition technique...