The pituitary produces FSH (Follicle Stimulating Hormone.

FSH causes a cell in the epithelium of the ovary to develop into an egg cell.

There are thousands of cells in the epithilium that could develop into an egg cell. These are called primordial germ cells.

Once a cell has started to develop it is called a primary oocyte.

FSH also causes the ovary wall to begin producing oestrogen.