You may never be able to "fix" the cracks but you might be able to stop them widening and more cracks appearing.
Wood shrinks across the grain - if the table top is fixed too firmly to the base & legs cracks can appear (my guess is that it has been moved from a damp environment into a dryer place or that the wood is recently cut and has not dried out).
Examine the fixings underneath and try to discover the method of attachment - slightly loosen any screws in slots - they may well be done up too tightly to allow for the natural cross-grain shrinkage of the table top.
Some table tops are fixed with shrinkage plates like the picture here...
http://www.woodfit.com/images/BP310%20Right%20angle%20bracket%20C.jpg
The elongated slot allows for shrinkage.
Peccavi, February 2009