It looks like cracks that commonly occur around the pith. Not necessarily wind checks, which from memory are around the ring, rather than radial like these are. Wind checks supposedly develop from strong wind, or when a big tree is blown over by wind, or pushed over by a bulldozer. I don't know what causes the type in your stave but they are pretty common in osage and many other trees.
Wood around the pith can be punky, and for some reason if you don't remove early in the process like huisme said it will check deeper as it dries.
These cracks are well centred in the belly and pose no problems with integrity. It looks like you have already sealed the area. As you work the stave into a bow a lot of these cracks will lessen and maybe even disappear. Re seal them when you take a break, because they tend to try and deepen themselves. Some kind of internal stress relief, different to the rest of the piece of wood.
If they are still there near the end of tillering you can fill them with sawdust mixed with epoxy.