I am a little confused, I find sites on the internet that say that the yolk of an egg is a macrocell, and another says that it is not. So... is it a macrocell or not?
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1$\begingroup$ Related post with no useful answer: how is chicken egg a single cell. $\endgroup$– Jiminy Cricket.Commented Feb 4, 2022 at 0:46
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2$\begingroup$ links to the sites themselves, with the contradictory quotes, might be helpful. $\endgroup$– Maximilian PressCommented Feb 4, 2022 at 3:38
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1 Answer
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No, the yolk is not part of the egg cell in chicken and ostrich eggs. Although I found many websites claiming that the yolk is part of the egg cell, none of them were reputable. I think that it is more of a myth than a fact. Although in some animals the yolk is enclosed within the cytoplasm of the cell, in larger species like chickens and ostriches the yolk is separate from the egg cell.
Wikipedia quotes:
The yolk is mostly extracellular to the oolemma [cell membrane], being not accumulated inside the cytoplasm of the egg cell (as occurs in frogs), contrary to the claim that the avian ovum (in strict sense) and its yolk are a single giant cell.
[...]
The yolk mass, together with the ovum proper (after fertilization, the embryo) are enclosed by the vitelline membrane, whose structure is different form a cell membrane.
This means that in the chicken egg, the egg yolk is mainly held outside of the cell membrane (of the egg cell/embryo) and hence is not considered part of the cell. However,
[in many small species] such as some fish and invertebrates, the yolk material is not in a special organ, but inside the egg cell (ovum).
So although the yolk is part of the cytoplasm of the egg cell in some species, this is not true for chickens or for ostriches.
