Although the egg is not mentioned at all in the Exodus account, it has become a prominent symbol for Passover as indicated by its place on the seder plate although it is not specifically mentioned in the Haggadah. No one really knows when the egg became a part of Passover. It most likely was introduced after the Second Temple Period.
The egg is one of the richest symbolic items of Passover. In some respects, it possesses similar meaning to the karpas. Its ovalish shape represents the life cycle. The fact that a chick must break open the egg to emerge into life is also symbolic of the renewal of life that takes place in springtime when Passover is held. This symbol has a logical extension to the emergence of the Israelite people as a free nation when they were released by Pharaoh. As with the karpas, some communities dip an egg in salt water just before the Passover meal reminding them of the tears that were shed while in captivity. This egg is hard-boiled in contrast to the egg on the seder plate which is roasted, suggesting that just as an egg becomes harder the more it is boiled, Jews will become stronger and more resolved as persecution increases. This idea has become so prominent as to become a midrash.
Not only does dipping the hard-boiled egg in salt water represent mourning, but the shape and texture of the roasted egg does as well. The egg is completely smooth and has no opening. It is thus compared to the mourner who bears his or her grief in silence and therefore appears composed and unaffected on the surface. It was traditional for mourners to eat an egg upon returning from a funeral as well as on the 9th of Av in remembrance of the destruction of the Second Temple. Thus, Rabbi Moses Isserles, in his authoritative notes on the Shulhan Arukh, writes that the egg at the Seder commemorates the destruction of the Temple, after which the Paschal sacrifice was discontinued.
Klein, Mordell ed. Passover (Philadelphia: Jewish Publication Society
of America, 1973) 57-58
Silverman, Rabbi Morris, ed. Passover Haggadah: New Translation with explanatory
notes and original readings (Hartford: Prayer Book Press, 1959) 35
Davis, Rabbi Avrohom. The Metsudah Linear Passover Haggadah (Hoboken:
KTAV Publishing House, 1993) 49
 Isserles To Shulhan Arukh, Orah Hayyim 476:2