Apple today seeded the first beta of OS X 10.10.5 to developers, two weeks after releasing OS X 10.10.4 to the public. Coming just a few months ahead of the public release of OS X 10.11 El Capitan, OS X 10.10.5 may be one of the last significant updates to Yosemite.
The new beta can be downloaded fro the Apple Developer Center.
It is not yet clear what’s new in the OS X 10.10.5 beta, but like 10.10.4 and earlier betas, it likely includes bug fixes and performance enhancements.
OS X 10.10.4, the current version of OS X Yosemite, introduced multiple bug fixes and under-the-hood performance improvements, including the removal of the problematic Discoveryd process.
Source: MacRumors