CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 13

(a) When implantation has occurred: the thick, spongy lining of the uterus, richly supplied with blood, is retained (so there is no menstruation) and nourishes the embryo. A special tissue, the placenta, develops in the uterine wall: villi on the embryo's side lie in blood spaces on the mother's side, so glucose and oxygen pass from the mother's blood to the embryo and the embryo's wastes pass into the mother's blood. The uterus enlarges as the embryo develops, over about nine months. (b) When the egg is not fertilised: the thickened lining is no longer needed, so it slowly breaks down and comes out through the vagina as blood and mucus. This is menstruation; it lasts about 2 to 8 days and occurs roughly every month.