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?
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.
That’s what I wondered, yeah. I thought the yolk as a whole was the equivalent of the ovum, but apparently not, now that you’ve told. TIL!
But then, how does the blastodisc form?
To the best of my understanding, it works similar to how our reproductive cells do. Basically, a splitting up of genetic material followed by formation of the initial cell. Then, upon fertilization, you get the same process of cellular multiplication as mammals undergo.