“In laboratory parlance, and even in print, the oocyte …, ovum, zygote, morula and blastocyst

are frequently referred to indiscriminately as the ‘egg’” (Perry 1981:321).

What’s wrong with eggs?  As the quote indicates, the word egg is ambiguous.  It can refer either to a female gamete by itself, the female gamete plus maternally derived tissues (e.g. a chicken egg), or to an early conceptus, as when we say an egg implants in the uterus.   In other words, the definition depends completely on context.

Thus, using the word ‘egg’ conflates a female’s gametes with product of conception.  This allows the two evolutionary very separate entities to be treated as the same thing.   This conceptual error may prevent us from seeing the different selection pressures on an ovum versus a zygote.

What can you do?  When talking about a female’s gametes you could use ova or ovum.  When talking about the product of conception you could use zygote, blastocyst, morula, or conceptus.

Perry, J.S.  1981.  The mammalian fetal membranes.  Journal of Reproduction and Fertility 62:321-335.