I keep a subset of my photographs in Photos on my Mac, and I would like to sync these with Photos on my iPhone. I used to be able to do so, but the option seems to have gone away. The 'Sync Photos' checkmark in iTunes is now off, and if I turn it on, the only option is to sync the phone with a selected directory.
What's going on here? How can I enable Photos on the phone to sync with Photos on the Mac?