Dictionary.net

• introduction during embryonic development of characters or structure not present in the earlier evolutionary history of the strain or species (as addition of the placenta in mammalian evolution)

2006 - WordNet 3.0
By Princeton University

• introduction during embryonic development of characters or structure not present in the earlier evolutionary history of the strain or species (as addition of the placenta in mammalian evolution)

2011 - English Dictionary Database
By DataStellar Co., Ltd