Sentence:  <WSD>Thus</WSD> every part and plane of the picture keeps changing place in relative depth with every other part and plane ; and it is as if the only stable relation left among the different parts of the picture is the ambivalent and ambiguous one that each has with the surface . 
Meaning of the word marked with <WSD> tags:  in the way indicated; (`thusly' is a nonstandard variant)