Between or through the mouth(s). Its best to have each piece with a mouth or at least part of a mouth. However the little brain that I ended up with originated as just a bit of flesh on skeleton with no mouth so it is possible to cut anywhere and still end up with a new colony. I've used a wet bandsaw, tile saw and dremal all with diamond blades. I've also heard of using a ring saw or even a hack saw or chisel but those last two would be much more damaging.
A link to this video has been posted a few times on this site
http://www.canreef.com/vbulletin/sho...ighlight=video