Here are my four spinners on the day they came out of the egg. See the guy on the left. When they injest the yolk, they do it through the umbelical on their belly. They can get quite fat. It's not hard belly though. I have hatched out babies with hard belly and they generally come out pretty thin, at least in my experience. Keep an eye on it, if the belly hardens up then I would start to worry but a lot of healthy babies come out fat like that.
It can be very difficult to deal with hard belly. I think I read there was something like a 95% fatality rate with hardbelly which is pretty consistent with my experience. It can be quite traumatic for you as a keeper so make sure you keep on top of it, but it looks like you have a healthy hatchling there. Just remember you should give her a good rinsing off after she comes out of the egg.
Goodluck!"
Chris.