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?
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.