The membrane investing the ovum just outside the membrana granulosa is

The correct option is theca interna.

To understand why this is the correct answer, let us break down the structure of a developing ovarian follicle (specifically a Graafian follicle) from the inside out:
1. Ovum (Oocyte): The innermost cell, which is the female gamete.
2. Vitelline Membrane: The cell membrane (plasma membrane) that immediately invests and surrounds the ovum itself.
3. Zona Pellucida: A transparent, non-cellular glycoprotein membrane surrounding the vitelline membrane of the oocyte.
4. Membrana Granulosa (Granulosa Cells): A layer of follicular cells (granulosa cells) that lines the cavity of the follicle. The cells immediately surrounding the ovum form the discus proligerus (or cumulus oophorus).
5. Theca Folliculi: The capsule or investing envelope of stroma cells surrounding the membrana granulosa. It differentiates into two layers:
    • Theca Interna: The vascular, cellular inner layer located just outside the membrana granulosa.
    • Theca Externa: The fibrous outer protective layer.

Therefore, the membrane investing the ovum situated just outside the membrana granulosa is the theca interna.