My System Study Group Chapter 1 Part 5: Pawn Hunting
This covers the last small part of chapter 1.
It starts on Page 10 bullet 7, and 7a.
Nimzo is saying that taking pawns in the opening for the gain of a pawn is not a good policy. Then he states that the exception is center pawns, but not for the pawns themselves rather for control of the center itself. Flank pawns should be left alone. (Modern play has shown there are exceptions to these rules. Still in general it is good policy unless you have studied a specific line and understand the exception).