Sentence:  In their very first collages , Braque and Picasso draw or paint over and on the affixed paper or cloth , so that certain of the principal features of their subjects as depicted seem to thrust out into real , bas-relief space - or to be about to do so - while the rest of the subject remains imbedded in , or flat upon , the <WSD>surface</WSD> . 
Meaning of the word marked with <WSD> tags:  the outer boundary of an artifact or a material layer constituting or resembling such a boundary