The germ disc in a bird (a disc because its a chordate) is a mystery to me. The ovum (yolk) is a huge mass of nutrients. It then gets impregnated. Then, what? What does the initial cell divisions look like (they can't be symmetrical since yolk is clearly pushed to one end of the whole thing. )
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1$\begingroup$ I'm pretty certain the ovum isn't the yolk. Ova are way smaller than that. Human embryos also have a yolk sac, and it develops during early embryonic development. According to wikipedia, in birds it's a specialized structure made in the mother's body and is part of the ovum in very small animals like some fishes or invertebrates, not birds. $\endgroup$– OosakaCommented Jun 3, 2020 at 20:31
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$\begingroup$ I'm just interested in what is true. I've kept hearing the meme that the yolk in birds is a single cell, and it looks plausible but always sounded a bit weird. The human yolk sac isn't homologous with birds yolk sac, at all. It develops from the germ disc long after fertilization during the formation of the gut tube. Overall phylotypical stages in embryogenesis are hard to master because there is so much variation, one thing that looks identical is achieved by a completely different procedure, is common. $\endgroup$– IanMalcolmCommented Jun 3, 2020 at 20:40
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2$\begingroup$ The yolk of birds may well be a single cell, but it doesn't mean that cell is the ovum. (actually I read down the wikipedia page, and it has a section on chicken eggs that explicitly says the claim you're describing is false "in strict sense". I know I know, wikipedia, there's a reason this is a comment and not an answer, but the citations might be useful). I didn't know the human yolk sac wasn't homologous to other yolk structures, thanks for that! $\endgroup$– OosakaCommented Jun 3, 2020 at 20:47
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$\begingroup$ Overall, how the embryo gets to different stages differs a lot while the stages look the same. It's pretty confusing until you get that the processes aren't homologous. The mammal yolk sac is a part of the gut tube, it forms as the initial germ disc (that exists to conform to notochord organizational axis) bends into a cylinder. $\endgroup$– IanMalcolmCommented Jun 3, 2020 at 20:53
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1$\begingroup$ @IanMalcolm It is not controversial that there is one and only one cell present before post-fertilization devisions (which occur before the full egg is developed). What is a bit unclear is what you call a "single cell" after that in the early stages of development. I'm not an avian development (or any development) expert and people tend to pick up chickens as model organisms after the first day or so after many many cell divisions (frogs are much nicer models for the first few divisions), so the earliest steps of avian development are often left out of textbooks. $\endgroup$– Bryan Krause ♦Commented Jun 3, 2020 at 22:29
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2 Answers
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This is covered well in almost any text that talks of early embryonic development in birds. Basically, the cleavage divisions look roughly symmetrical if you look end-on from the animal pole. But, as mentioned in the question, the process is quite asymmetrical in profile, with the blastoderm being at one end and the huge yolk occupying the other.
The yolk later gets invested by a layer of cells (the splanchnopleure) to form the yolk sac while the blastoderm forms the embryo proper.
You could see this page for more details. (Not just this page, this site in general is an excellent resource for learning embryology.)
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The question is based on the false belief that the ovum contains the yolk as a single cell. This is a belief that is very common, and often spread in the literature. Grau and Wilson said in 1963 in Avian Oogenesis and Yolk Deposition: The yolk of the hen's egg is often given as an example of a huge cell containing a food supply for the developing chick embryo. Current views hold that oocytes grow by removing nutrients from the blood and the follicle cells which surround them. Thus yolk deposition is thought of as primarily an intracellular phenomenon. We believe that this viewpoint is inadequate, and wish to propose the following alternative hypothesis.
Of course, if the yolk is not inside the ovum, then there is no mystery to the early cell divisions.
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$\begingroup$ Hi, it's me, the person telling you the bird yolk isn't the ovum! I told you wikipedia confirmed, and I let it at that initially because it was just a comment, I didn't feel the need to do deep research, but I saw the question continued to spark debate so I actually went to look at wikipedia's sources. And from what I was able to read of "Atlas of chick development", a 2005 book, I'm thinking... the wikipedia page was maybe wrong and you were right all along? But also, the book is detailed enough (and chapters free to read on google books) that it might answer your questions regardless. $\endgroup$– OosakaCommented Jun 4, 2020 at 0:17
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$\begingroup$ The yolk is both part of the oocyte and yet separate from the cytosol of the cell including the nucleus, mitochondria, and all the other normal cellular stuff. Grau and Wilson are merely arguing (without much evidence) that the oocyte doesn't play much of an active role in growing itself. $\endgroup$– Bryan Krause ♦Commented Jun 4, 2020 at 0:21
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$\begingroup$ @Oosaka You were probably right, I always thought the claim that the yolk was a single cell seemed wrong. But it seems like a very common myth, myths are good at capturing attention so it took me a while to get that the yolk is extracellular and secreted by cells surrounding follicle lumen. That's what Grau and Wilson said, seem as credible as anyone else, and it fits with my own reasoning. $\endgroup$ Commented Jun 4, 2020 at 0:35
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1$\begingroup$ @IanMalcolm Grau's credentials are great but that book being from 1963 less so; it's why I pointed out "Atlas of Chick development" being from 2005. I don't know if the sources actually disagree though, I would have to see what both say. $\endgroup$– OosakaCommented Jun 4, 2020 at 1:01