Why (and when) did the Apostles stop requiring exactly twelve apostles? Why was James not replaced?

In [Acts 1:12-26](http://www.biblegateway.com/passage/?search=Acts%201:12-26&version=NIVUK) we read the story of how Matthias was chosen to replace Judas. This kept the number of apostles at Twelve. Sometime later, James, the brother of John was murdered ([Acts 12:2](http://www.biblegateway.com/passage/?search=Acts%2012:2&version=NIVUK)) . James was also one of the original twelve ([Matthew 4:21](http://www.biblegateway.com/passage/?search=Matthew+4:21&version=NIVUK)) but unlike Judas, there is no record of him being replaced. Why was this? Did the policy change? Alternatively, if he *was* replaced but it was not recorded in the New Testament, when did the "Twelve Apostle" policy end?