Egg

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

According to Martha Jones, the air of Belepheron smelled like bad eggs. (PROSE: The Last Dodo)

Brian Williams suggested, among other theories, that the millions of Shakri 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: Arcturians, (PROSE: Only a Matter of Time) Aubertides, (PROSE: Human Nature) Bane, (TV: The Lost Boy) Centuripede, (TV: The Last Oak Tree) Chelonians, (PROSE: The Highest Science) Chickens, (TV: City of Death) Chimeron, (TV: Delta and the Bannermen) Gappa, (PROSE: Snowglobe 7) Gastropods, (TV: The Twin Dilemma) Ice Warriors, (PROSE: Lucifer Rising) Krill, (PROSE: Storm Harvest) Kiseibya, (AUDIO: Enemy of the Daleks) Mayflies, (TV: Reset) Nostrovites, (TV: Something Borrowed) Raxacoricofallapatorians, (TV: Boom Town) the Process, (PROSE: Cat's Cradle: Time's Crucible) Saraani, (PROSE: Dry Pilgrimage) Silurians, (TV: SJAF 6) Viperox (TV: Dreamland) and Wirrn. (TV: The Ark in Space)

Food source
Eggs were prepared and eaten as food. On Earth, chicken eggs were eaten commonly. Eggs could be prepared a number of ways: boiled, fried,  as an omelette,  pickled,  and scrambled.

Ian Chesterton and Barbara Wright ate some food blocks that had been processed by the First Doctor's food machine to taste like bacon and eggs. (TV: "The Dead Planet")

Barbara ate some food blocks processed to taste like bacon and eggs, this time with brown sauce added. (AUDIO: 1963)

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)

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 Eleventh Doctor wondered where she got the eggs needed for baking on multiple occasions. It turned out that Oswin was a Dalek who had created a reality in her mind in which she was still human and cooked soufflés. (TV: Asylum of the Daleks)