(i) ‘a’ is Ovum (Secondary oocyte). 

(ii) ‘c’ is Zona pellucida; the changes occurring in it when a sperm comes in contact with it, block any other sperm entering the ovum, i.e. it ensures the entry of only one sperm into the ovum. 

(iii) ‘d’ is corona radiata. 

(iv) The follicle stimulating hormone (FSH) stimulates the growth and development of ovarian follicles; it also stimulates the follicle cells to secrete estrogens, which are also necessary for the process of oogenesis.

• The levels of luteinizing hormone (also from pituitary) and FSH increase and reach their peaks around the middle of the menstrual cycle, i.e. about the 14th day; it is called LH surge.
• LH induces ovulation, i.e. rupture of the mature follicle and the release of ovum (secondary oocyte); it also induces the formationof corpus luteum from the ruptured follicle and secretion of progesterone from it. 

(v) The parts of sperm are Head, Neck, Middle piece and Tail. Mitochondria is present in middle piece.