Hydatidiform mole. Gross specimen of placental tissue showing the formation of a benign tumour: an hydatidiform mole. This disorder, which resembles a bunch of grapes, develops from the placental tissue of a pregnancy that has failed to develop normally. The membrane surrounding the embryo (the chorion) degenerates, forming a collection of fluid-filled sacs. The cause of this degeneration is unknown but it can be detected using ultrasound scanning. There is a risk that cells from the mole may go on to form a choriocarcinoma, an extremely rare and malignant cancer.