The yolk of any bird's egg is a single cell which also contains a
small bit of "active" cytoplasm and the nucleus. The albumin (the white
of the egg) is added to the outside of this cell as it migrates through the
oviduct. Ultimately the shell is added. But the entire yolk, cytoplasm,
and nucleus of the egg is within a single cell membrane. Therefore, the
entire egg is not a single cell (since the albumin is extracellular), but the
yolk (along with the tiny bit of cytoplasm and nucleus) certainly is.
But it would seem that the largest cell in the world belongs to
the genus Caulerpa, a single-celled alga. This was described by
William Jacobs in Scientific American (December 1994, pp. 100-105).
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