(b) : Oestrous cycle comprises cyclic changes in female reproductive system of non-primate mammals like cows, dogs, etc. The oestrous cycle consists of a short period of oestrous or 'heat' (e.g., 18 hours in cow ) followed by the rest of period of anoestrous or ' passive'. During oestrous, the female receives the male for copulation. During anoestrus, the female becomes passive and does not receive the male. Although the break down of tissues takes place in the female reproductive tract at the end of an oestrous cycle, yet there is no menstruation.