As far as I know, veins are a GOOD sign. When my (single successful hatched duck) egg was about 24 of 28 days, I could see veins like that. In fact, that's all I could see moving anymore, so much of the egg was full.
When I have candled chicken eggs (I have not successfully hatched any yet, but have a better incubator now that will hold a steady temp/humidity), when they die I can tell because the veins don't look nice and dark and 'full' anymore. They fade and get indistinct...then the blood pools in a blood ring.
As long as those veins look nice and thick and full of moving blood, I reckon it's a good thing.
