How strange! I would definitely think the egg would be full.This is indeed a chicken egg. I would expect a fuller, darker egg at this stage, the veins have just kept me from throwing it out.
What was the temperature and was it consistent...?
With what the other poster's said about eggs hatching late is crazy! I would for sure keep that egg in longer and see what happens. And keep an eye out for any growth.
I wonder if veins could possibly stay and not go away, if its dead? I do think the dark spot is an indicator that its dead, but then there's those veins!