Jamieson, Fausset and Brown on 1 Corinthians 15:8

8. One born out of due time--Greek, "the one abortively born": the abortion in the family of the apostles. As a child born before the due time is puny, and though born alive, yet not of the proper size, and scarcely worthy of the name of man, so "I am the least of the apostles," scarcely "meet to be called an apostle"; a supernumerary taken into the college of apostles out of regular course, not led to Christ by long instruction, like a natural birth, but by a sudden power, as those prematurely born [Grotius]. Compare the similar image from childbirth, and by the same spiritual power, the resurrection of Christ (1Pe 1:3). "Begotten again by the resurrection of Jesus." Jesus' appearance to Paul, on the way to Damascus, is the one here referred to.