The Spine is where you will find the speakers, microphone and display. And the Heart is where you will find the Puzzlephone's battery. The modular handset is being built by Finnish startup Circular Devices. A forked version of Android will drive the phone. Eventually, the device will support other platforms, like Windows Phone, Sailfish and Firefox OS.
Puzzlephone is expected to launch sometime during the second half of 2015, priced the same as a mid-range smartphone. You might not get the full customization of Project Ara, but you still will have choices to make. One decision you'll have to make, is whether you want a modular phone without the Google Play Store.
Modular handset, Puzzlephone
source: Puzzlephone via PhoneBloks, AndroidandMe