Jamieson, Fausset and Brown on Galatians 3:29

29. and heirs--The oldest manuscripts omit "and." Christ is "Abraham's seed" (Ga 3:16): ye are "one in Christ" (Ga 3:28), and one with Christ, as having "put on Christ" (Ga 3:27); therefore YE are "Abraham's seed," which is tantamount to saying (whence the "and" is omitted), ye are "heirs according to the promise" (not "by the law," Ga 3:18); for it was to Abraham's seed that the inheritance was promised (Ga 3:16). Thus he arrives at the same truth which he set out with (Ga 3:7). But one new "seed" of a righteous succession could be found. One single faultless grain of human nature was found by God Himself, the source of a new and imperishable seed: "the seed" (Psa 22:30) who receive from Him a new nature and name (Ge 3:15; Is 53:10, 11; Joh 12:24). In Him the lineal descent from David becomes extinct. He died without posterity. But He lives and shall reign on David's throne. No one has a legal claim to sit upon it but Himself, He being the only living direct representative (Eze 21:27). His spiritual seed derive their birth from the travail of His soul, being born again of His word, which is the incorruptible seed (Joh 1:12; Ro 9:8; 1Pe 1:23).