Final answer:
A gymnosperm megagametophyte produces egg cells and archegonia, while an angiosperm megagametophyte produces seven cells with eight nuclei and no archegonia.
Step-by-step explanation:
A gymnosperm megagametophyte produces egg cells and archegonia (structures that house the egg cells). An angiosperm megagametophyte, commonly referred to as the embryo sac, produces seven cells with eight nuclei and no archegonia. The seven cells include one egg cell, two synergids, three antipodal cells, and one large central cell containing two of the eight nuclei. This central cell is unique as it participates in the process of double fertilization, forming the triploid endosperm upon fertilization.
In gymnosperms, such as conifers, the female gametophyte is much larger and can consist of thousands of cells, while in angiosperms, the female gametophyte is extremely reduced in size. These differences reflect the evolutionary changes in seed plants regarding the protection and development of the female gametophyte, with angiosperms optimizing the process of reproduction with a highly reduced and efficient structure compared to their gymnosperm predecessors.