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What causes the embryo in a chicken egg to develop from around the centre of the yolk? (lemmy.blahaj.zone)
submitted 2 weeks ago* (last edited 2 weeks ago) by birdwing@lemmy.blahaj.zone to c/askscience@lemmy.world
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I watched a few vids of chickens developing in eggs without shells (see here for an example, somewhat graphic), and got to wonder.

You'd think that all the yolk is uniform and therefore it could develop anywhere, but is there an underlying mechanism that could cause the primitive streak and everything to develop near the centre? Maybe a sort of yolk density mechanism, that it starts where the yolk is densest? The furthest from oxygen exchange at the shell?

Or does such a mechanism not exist?

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[-] southsamurai@sh.itjust.works 2 points 2 weeks ago

Wait, are you asking where the actual fertilized egg cell is? I think that's what you're wanting, but could be wrong.

The yolk isn't actually the "egg" in the same way a human ovum is. I mean, it kinda is, but there's a spot in/on it called a blastodisc. That is what rooster sperm fertilizes, and where all development starts from. Once fertilized, it's called a blastoderm. Not the blastodisc is the direct equivalent of a mammalian ovum.

That's why the embryo develops in roughly the same place, it's all starting from one place; and because of how yolks form, that growth is going to be roughly towards the center.

this post was submitted on 17 Apr 2026
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