Answer: Embryo sac is the megaspore mother cell.
Step-by-step explanation:
The bisporic embryo sac can be defined as the comprises of the nuclei that is obtained from two units of the megaspore tetrad on the other hand, the tetrasporic embryo sac comprises of four units of megaspore tetrad. The megaspore mother cell undergoes meiosis to produce megaspore tetrad or four haploid megaspores.