How are double yolk eggs made?

I have a hen egg that only has yolk shawdow but a blood vessal ring around it. is it fertile or not?

• theres this hen comes 2 my ouse and i feed her everyday.She was hinding in my rose bush and she made a nest there and i thought she laid eggs.I checked the next day and there no eggs.About 2 weeks later my mom was mowing our lawn and she found 2 hen eggs burried! She ran 1 over and inside was a baby chick embyo! She gave me the other 1 and i candled it and iall i saw was the yolk shadow. I looked again and i saw a ring around it i think its called the blood vessal but thats all.If 1 egg was fertalized the second has to be fertalized right? im so confused becouse youre spose 2 the embryo and veins but all see are the bleed vessals and no embryo. btw this all happend today.Im keeping in warm in a heating pad at 99degress and i sprayed it just in case it be alive. But is it possible to the blood vessals and no other thing but the embryo?

• Answer:

  If the other egg had an embryo, that one should too, as hens start sitting on the eggs so their hatching date is about the same. The ring you may see could be a bacterial ring. These are rotten eggs and should be thrown immediately. Two weeks is a long time to try incubating again. They can be held at room temp without any development, but once started if something were to happen, within 24 hours the embryo would've passed away. Also, just because one egg was fertile doesn't mean the other was. She may only have been bred once or twice.