Sentence:  Most significant is the observation that the vaporous for the more volatile <WSD>end member</WSD> occupies a decreasingly smaller portion of the diagram as the other <WSD>end member</WSD> becomes less volatile. Ultimately, when the less volatile <WSD>end member</WSD> is essentially involatile, the vaporous of the volatile <WSD>end member</WSD> may become microscopically small. 
Meaning of the word marked with <WSD> tags:  A member at one end of a range or series made up of similar members.