Egg

Eggs were a method of reproduction in which the foetus developed outside the mother's body.

The air of Belepheron reportedly smelled like bad eggs. (PROSE: The Last Dodo)

Brian Williams suggested, among other theories, that the millions of cubes that suddenly appeared on Earth were alien eggs. (TV: The Power of Three)

Egg-laying species
A wide variety of creatures laid eggs, including fish, amphibians, reptiles, birds, monotremes, arachnids and insects. Some specific species included:
 * Aubertide (PROSE: Human Nature)
 * Bane (TV: The Lost Boy)
 * Centuripede (TV: The Last Oak Tree)
 * Chelonians (PROSE: The Highest Science)
 * Chimeron (TV: Delta and the Bannermen)
 * Earth Reptiles
 * Gappa (PROSE: Snowglobe 7)
 * Gastropod (TV: The Twin Dilemma)
 * Ice Warrior (PROSE: Lucifer Rising)
 * Krill (PROSE: Storm Harvest)
 * Mayflies (TV: Reset)
 * Nostrovite (TV: Something Borrowed)
 * Racnoss (TV: The Runaway Bride)
 * Raxacoricofallapatorians (TV: Boom Town)
 * Process (PROSE: Cat's Cradle: Time's Crucible)
 * Saraani (PROSE: Dry Pilgrimage)
 * Silurians (TV: Sarah Jane's Alien Files episode 6)
 * Viperox (TV: Dreamland)
 * Wirrn (TV: The Ark in Space)

Food source
Eggs were prepared and eaten as food. On Earth, chicken eggs were eaten commonly.

Barbara Wright ate some food that had been processed by the First Doctor's food machine to taste like bacon and eggs with brown sauce. (AUDIO: 1963)

The Seventh Doctor cooked fried eggs, bacon, sausages and fried potato slices for breakfast at Smithwood Manor for Justine and Vincent Wheaton. (PROSE: Cat's Cradle: Warhead)

Upon taking Rose Tyler to see the Earth's destruction in the year five billion, the Ninth Doctor jokingly mentioned eggs as one of the things humanity thought would cause their extinction. (TV: The End of the World)

Sarah Jane Smith tried and failed to make scrambled eggs on several occasions. When she tried to make some before Luke Smith went to university, she accidentally set her kitchen on fire. (TV: The Nightmare Man)

Walker ordered a breakfast of eggs, bacon, toast, coffee, and rough-cut marmalade. (TV: The Sea Devils)

While trapped in a room in the Dalek Asylum for almost a whole year, Oswin Oswald claimed she was making soufflés. The Doctor asked where she got the eggs from on multiple occasions. This was eventually resolved - she was in reality a Dalek and she had created her own reality in her mind in which she was still human and making soufflés. (TV: ''Asylum of the Daleks)