[chris (OP)]

I was asked today by someone why some eggs are "double yolkers"; embryologically, I suspect that the gene programme that drives the development of the egg tissues plays out in two tissues side by side; these tissues are derived from some founder tissue that, rather like a monozygotic twin, spilts up earlier in the developmental pathway.

But I'd be interested in the insights and wisdom - and accurate answers - of others.

Chris

[CliffordK]

The egg is essentially a single cell.

I was thinking it was like monozygotic twins.  However, I believe the differentiation of the yolks comes prior to fertilization.  Whereas in mammals, at least for monozygotic twins, the separation comes after fertilization.

I would be curious about a comparison of the genetic material between the yolks.

[chris (OP)]

The yolks are just a fat-rich energy store. The eggs we eat are unfertilised, as a rule, so the genetics argument ought not come into it - both yolk structures and related tissues should be genetically identical, I would think.

chris

[RD]

I was thinking it was like monozygotic twins.

It seems it's dizygotic twins ...
http://www.poultry.allotment.org.uk/Chicken_a/Chicken_Egg_Excess/double-yolk-eggs.php

[CliffordK]

It seems it's dizygotic twins ...
http://www.poultry.allotment.org.uk/Chicken_a/Chicken_Egg_Excess/double-yolk-eggs.ph

That would make a lot of sense.
After meiosis, one would end up with haploid cells, which would be unlikely to divide again until fertilization. 

[chris (OP)]

Thank you; that's very helpful. A double ovulation.