The line down the middle is normal - that's the pith line.
It's those innocuous looking light grey patches. If you get the odd one or two in small areas, or around knots it's normally absolutely fine, but when it's spread out like in your photos and not radiating from any particular flaw in the timber it's likely to be hard hat time.
Saying that, it's English yew. It's the most ridiculous bow wood in the world. You can get 100lb out of stuff with holes and rot and worms inside, and yet have 8 staves in a row of the most pristine, beautiful timber you'll ever see explode one after the other on the tiller... Can you tell I'm speaking from experience?
If you've got your mind set on a heavy bow and you're not particularly bothered about losing the wood then go for it. You might be surprised. If you *need* a bow and can't afford to lose the wood, personally I'd keep it under 50lb.