Chabad.org — English translation + Rashi
Torah Temimah points to Chullin 49b.
E-Daf.com — Chullin 49b
HebrewBooks / Shas PDF
English: Rabbi Akiva says: “and all the fat that is on the innards” — just as the covering fat is under a membrane and peels, so all that is under a membrane and peels.
Definition by membrane and peelability — forbidden chelev is identified by being membrane-covered and peelable.
Forbidden chelev is identified by being membrane-covered and peelable. Rabbi Akiva analogizes from the covering fat (which is known to lie under a membrane and to peel away) to every deposit of fat that shares those two physical traits. The result is a practical criterion for distinguishing the biblical chelev of the innards from ordinary fat.