More bone structure to the spine/tail.
Hearts beating stronger.
I stayed up till 1:00 waiting for these little guys to hatch. (in the dark with no pumps running)
Needless to say they didn't hatch.
Tonight perhaps.
I keep my display tank at 78-79.
Cooler temps slow the rate that they develop/hatch.
I've had eggs take as long as 12 days before.