Finally, the remainder is split between the introduction and the discussion. After you now know about how long each chapter of your dissertation proposal must be, you can begin tentatively with the issue of division (sub-chapters). If you have for example 10 pages for the introduction and then 25 sub-chapters to come, there is certainly something that went wrong.
Remember: in the dissertation (and each of its chapters) one must include only what the things that you need to help you display your ‘message’.
You also have to quickly be clear and certain about what your message is . This will result necessarily from the factual part of your experiments’ (and the successful ones!) obtained results. In the beginning (still before you wrote your first sentence!) the first step is the inspection of your “hard-copy data” (original curves, blots, photomicrographs, “raw” charts, etc.). Ask yourself critically, what is new about it (hopefully everything, which no one else has ever posted before you) and what is the contribution to general knowledge (i.e., something that no one has known before your paper was written). The answer to the latter question can be difficult, but it is essential also for your entire thesis: it is to be written exactly around this point, or it is going to be worthless.
If you have the good fortune (or mis-fortune), many of these new findings you have to be really new, then you must go sort them out now: one (or very few) is the really important thing (= the message), the others will have the effect (if they fit into it) of working as a “decoration” that is aligned or (if they do not match the message ) – crossed out of the dissertation. Of course, such things get published occasionally, but then not everything you have ever thought of, or found in this document, must be included.
If you have now defined the core of your thesis, you can finally define the outline of your work.