Embryo sac occurs in

The embryo sac occurs in Ovule.
The embryo sac develops within the ovule surrounded by the nucellus, which is in turn surrounded by the integuments. One cell of the nucellus undergoes meiosis to produce four megaspores.
A mature ovule consists of a food tissue covered by one or two future seed coats, known as integuments. A small opening (the micropyle) in the integuments permits the pollen tube to enter and discharge its sperm nuclei into the embryo sac, a large oval cell in which fertilization and development occur.