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The egg of a large modern bird is a very large cell, not the largest possible biological cell since there are very long nerves in squid, but it's likely the most massive modern cell.

But, what about the cells of prehistory? Many dinosaurs were far larger than modern birds, but I had trouble finding information on just how large their eggs could be. I was also surprised that the eggs were not bigger than those of modern birds. It seems the ratio of egg size to dinosaur size was much smaller... or has that even been studied?

Maybe the large eggs were all destroyed?

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    $\begingroup$ I was also surprised that the eggs were not bigger than those of modern birds. It seems the ratio of egg size to dinosaur size was much smaller. - where have you read this? $\endgroup$
    – rg255
    Jan 2, 2016 at 18:52
  • $\begingroup$ I suppose that you define "large" in terms of volume. Note that there will have some issue of definition about what a cell is when you consider cases where cells of an entire tissue vanished to end up with more or less a big single cell with many nucleus. $\endgroup$
    – Remi.b
    Jan 2, 2016 at 19:22
  • $\begingroup$ I didn't read it. It's an observation I'm having a hard time confirming or invalidating. Let's just use mass for largest. Volume is confusing. $\endgroup$
    – futurebird
    Jan 2, 2016 at 23:12

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The largest known dinosaur eggs were the size of a basketball. The egg of an ostrich is also very large, comparable in size to dinosaur eggs.

The reason why even the very large dinosaur laid relatively small eggs is diffusive ventilation of oxygen and "breathing" through the eggshell.

The rate of ventilation is proportional to the eggshell surface area while the required amount of fresh oxygen is proportional to the volume of the egg. The surface area is proportional to the square of its radius while the volume to the cube of the radius. As the size (=radius) of the egg increases the ratio between the incoming oxygen through the shell to the amount of oxygen needed is decreasing, making too large eggs unsustainable.

Another consideration, also related to oxygen diffusing through the eggshell is that a very large egg requires a thick shell, and too thick shell prevent oxygen diffuse through it to the embryo.

References:

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