A nice picture here of the little comb that was there earlier. It has grown quite a bit to about 5 inches across. They will continue expanding this heart shaped wax fin until it fills a cross section of the hive.
They "know" just how to do this without any directions! Young bees produce wax between the segments on their abdomens which they mold using their mandibles to shape into comb. A long process that takes many bees a short time.
I've also noticed that the last jar of syrup I fed them is only half eaten. I'm hoping that is a sign that they've had their fill of sugar and are getting enough nectar naturally.