You can poke bits of short sinew soaked in hide glue into the cracks, using the point of a knife or a needle. Just don't do like I used to....fill 'em with glue! All that does is add a lot of weight with no benefits.
And then when it is fully cured, you hear lots and lots of cracks and pops when you are first excercising it in for final tiller! Bad for the old ticker, trust me!