Table I.

Parameters that distinguish a mammalian oocyte from an egg. ^*

	Oocyte	Egg
In vivo tissue environment	Ovary	Oviduct
Meiotic stage	Prophase of meiosis I	Metaphase of meiosis II
Defining features by transmitted light	Intact nucleus	Lack of a nucleus;   presence of polar body
Perivitelline space	Minimal	Enlarged
Independent cell volume regulation	Not possible	Possible
Microtubule cytoskeleton	Interphase	Meiotic spindle
Cortical organization	Symmetric	Asymmetric
Normal fertilization	Not possible	Possible

^*Here we distinguish between an oocyte and egg, but when the mammalian gamete is neither at prophase of meiosis I nor metaphase of meiosis II, the precise cell cycle stage should be specified (e.g. cell at pro-metaphase I, metaphase I).