Apple Removes X11 in OS X Mountain Lion, Shifts Support to Open Source XQuartz
The X Window System has an extensive open source history, with development on the Mac side being handled under the XQuartz project. Apple's X11 application has been based on this effort, although as with many open source projects bundled for use in commercial software, X11 has generally been somewhat behind the latest XQuartz releases. The current version of X11 is 2.6.3, while XQuartz is currently available in version 2.7.0.
With Mountain Lion, Apple seems to eliminating its dedicated support for the X11 application, instead redirecting users to the open source XQuartz project, which it will continue to support.