Usage of 卵 vs. 玉子 when talking about eggs

Question

While reading the first manga of 「3月のライオン」, I came across an interesting usage of kanji for eggs. Until then, I've only seen 卵 used for eggs, or its readings using hiragana or katakana. The protagonist of the story received a text message on his phone, that asked him to drop-by the convenience story on the way to the sender's home, in order to buy some groceries. 「あと、玉子も！！」was at the end of it.

I'm wondering why these kanji were used and if there are general guidelines when to use one or the other.

Answer 1

The short of it is that 卵 is used for the general concept of eggs (usually in a scientific context) ー like animals laying eggs ([産卵]{さん・らん}), an ovum ([卵子]{らん・し}), an ovary ([卵巣]{らん・そう}), ovulation ([排卵]{はい・らん}), etc.

玉子 is used to talk about eggs in the context of food/eating. That is why 玉子 was used by the character when speaking of groceries to purchase.

Answer 2

Well, if you're talking about food, chicken/bird eggs (and any egg you could make scrambled eggs with) are 玉子｛たまご｝. All other eggs, e.g. salmon roe, are 卵｛らん｝ (e.g.　fish eggs are 魚卵).

And, as the other answer says, all other usages of "egg", e.g. the human egg, translate to 卵｛らん｝.

卵 could also be read たまご, but it means strictly chicken/bird eggs only when using this reading. e.g. 卵｛たまご｝焼｛や｝き is made only with chicken eggs, never fish eggs.