A redefinition of perishing. The first move is terminological: the parts of the Whole "must of necessity perish" (ἀνάγκη φθείρεσθαι) — but at once, TERMλεγέσθω δὲ τοῦτο σημαντικῶς τοῦ ἀλλοιοῦσθαι, "let this be said in the sense of being altered." The word "perishing" is not withdrawn but reinterpreted; the same device as at 9.35 (ἀποβολή = μεταβολή). From there the passage works with ἀλλοίωσις alone, and the fear loses its object.
Theodicy as a disjunction. If dissolution is for the parts both an TERMevil and a necessity, then DOGMAthe Whole is ill administered — which Marcus will not allow. Hence the fork: DOGMAeither nature DOGMAdeliberatelyDOGMA does evil to her own parts, or this happens DOGMAwithout her knowing. ἀμφότερα γὰρ ἀπίθανα — "both are implausible": the first contradicts her goodness, the second her omniscience. An argument by exhaustion: since both branches fall, dissolution is not an evil.
And even without "nature." The second move is a concession to the opponent: let someone drop "nature" as an efficient power altogether and explain everything simply by the way things are constituted (κατὰ τὸ πεφυκέναι). Even then it is γελοῖον — absurd to say in one breath that the parts are constitutionally changeable and in the next to marvel at change as contrary to nature. The argument is not theological but logical: the indignation contradicts the indignant man's own premise.
Where the dissolved goes. The physics: either a TERMscattering of the elements (σκεδασμὸς στοιχείων), or a turning — of the solid (στερέμνιον) into the earthy, of the breath-like (πνευματικόν) into the airy; and both are "taken back into the TERMλόγος of the Whole" (cf. 4.14 — received back into the seminal reason; 6.15 — give the breath back to the element you drew it from). Both Stoic cosmologies are named neutrally: DOGMA"whether consumed by fire at periods or renewed by everlasting exchanges" — ἐκπύρωσις or endless alternation; for the argument Marcus is indifferent.
A body three days old. The last and most unexpected turn: do not TERMimagine (μὴ φαντάζου) that your solid and breath-like parts are yours from birth. All of it was gathered "yesterday and the day before" out of food and inhaled air. TERMWhat changes, then, is what was taken on, not what your mother bore. The very object of attachment is removed: the thing that fears to change was never yours — it is fresh intake, not you. The governing discipline is assent (the judgement whether dissolution is an evil), the sustaining one desire.
A note on Long's version. The closing τὸ ἰδίως ποιόν is a technical Stoic term: the "peculiarly qualified" — the individual as bearer of an unrepeatable quality (as against κοινῶς ποιόν, the generically qualified). Long unfolds it as "that other part, which has the peculiar quality of change," importing change where the point is individuality; Rogovin is closer — «всё своеобразие твоего существа». The modernization restores the term.