what is an embryo sac?
Definition of EMBRYO SAC. : the female gametophyte of a seed plant consisting of a thin-walled sac within the nucellus that contains the egg nucleus and other nuclei which give rise to endosperm on fertilization.See embryo sac defined for kids.ADVERTISEMENT. the female gametophyte of a seed plant consisting of a thin-walled sac within the nucellus that contains the egg nucleus and other nuclei which give rise to endosperm on fertilization. See embryo sac defined for kids. ADVERTISEMENT.