The mass of cells within the integuments is called:

Nucellus

The mass of cells within the integuments is called the nucellus.  The nucellus is a central region within the ovule of a plant. It is surrounded by protective envelopes called integuments, except at the tip where the micropyle is organized.

The nucellus plays an important role in the development of the ovule and the formation of the embryo sac or female gametophyte. It contains a mass of cells that often have abundant reserve food materials. These reserves provide nourishment for the developing embryo sac and, later on, for the developing embryo after fertilization.

Within the nucellus, a single megaspore undergoes further development and gives rise to the embryo sac. The megaspore undergoes several rounds of mitotic divisions to form a multicellular structure known as the female gametophyte or embryo sac. The embryo sac is the structure where the female reproductive cells, the egg cells, and synergids are located.

The nucellus also provides a protective environment for the developing embryo sac, helping to shield it from external stresses and potential damage. Additionally, it is involved in the nutrient transfer and exchange processes that support the growth and development of the ovule.

Overall, the nucellus, situated within the integuments of the ovule, plays a vital role in the reproductive processes of plants. It houses the developing embryo sac, provides nutrients for its growth, and contributes to the successful fertilization and subsequent development of the embryo within the ovule.