A typical chimera starts as two fertilized egg cells. For birds and egg-laying snakes, the chimera must hatch from a double-yolked egg. I had a bullsnake that in three clutches dropped 1-3 eggs per clutch twice the size of the other eggs. These large eggs were probably double yolked. (I did not open them to see.) Such eggs hatched out one large but otherwise normal looking baby. And a Burmese python breeder buddy of mine often had one or two eggs in a clutch that contained twins. So double yolk eggs are not terribly uncommon.
Double yolk eggs also occur in chickens, too. http://www.abc.net.au/radionational/...lk-egg/7625330
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