Here are a couple of paragraphs from a current draft of a paper of mine. It touches on a point I have been concerned to make in more than one place of late, a point about the structural similarity of TLP and PI.
In TLP and PI, the concentration of metaphilosophical remarks occurs in the dialectical middle (a middle not necessarily the same as its paginal middle): the 4s in the TLP and in 89-133 in PI. Rhetorically, each of the books is a large epanados, a chiasmus. That is, each of the books is organized spatially around a center or middle. Each book has the structure, roughly, of a large ‘x’, with the metaphilosophical remarks stationed at the crux of the ‘x’. (A handy example of a small epanados is Unamuno’s false but memorable sentence, “Martyrs create faith, faith does not create martyrs.”) The similar chiastic structures of the two books has not been stressed as it should be.
Placing the remarks as Wittgenstein does is a broad hint about how not to understand them. It is a broad hint not to think, among other things, that the remarks can be understood in isolation from the other non-metaphilosophical remarks that stand to the left of them and to the right of them. I like to think of PI as unified by both a centripetal and a centrifugal energy, the first the movement of the left-hand and right-hand remarks inward toward the center, and the second the movement of the center outward toward both the left-hand and right-hand remarks.