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Egg is a single cell and has a outer hard covering outside inside which there is a cell membrane. Then why isn't the egg shell a cell wall? Is it because no exchange of materials take place through it?

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Egg shells are actually porous so that the organism inside can aquire oxygen and get rid of carbon dioxide as it develops (http://www.scientificamerican.com/article/bring-science-home-chick-breathe-inside-shell/).

Although gametes (eggs and sperm) are single cells, an egg shell (or "wall" if you like) is created by the mother (therefore external to the egg cell) and contains many compartments separated by protein membranes:

enter image description here https://www.exploratorium.edu/cooking/eggs/eggcomposition.html

The initial egg cell is a tiny fraction of a size of the egg visualised here, so it would therefore be incorrect to call the egg shell a "cell wall" as it is a structure independent of the egg cell itself.

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    $\begingroup$ Also worth mentioning is that usually a cell produces its own cell wall but the fertilized egg doesn't produce the shell. It is produced by mineral deposition from the outside. $\endgroup$ – WYSIWYG Sep 9 '15 at 6:28
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Referring to oviparous animals (given that you mention a hard shell), an egg isn't a single cell. It contains a single cell (called the zygote) for a little while, which has its own cell membrane, but that quickly develops into a multicellular organism.

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    $\begingroup$ Since when does a zygote contain a cell wall? Animals lack cell walls entirely. $\endgroup$ – MattDMo Sep 8 '15 at 16:12
  • $\begingroup$ Isn't the egg then more of an analog to an external Uterus and placenta? $\endgroup$ – AMR Oct 8 '15 at 2:41

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