The gamete formation in females is a cyclic activity that takes about 28 days and involves changes in the structure and function of the entire reproductive system. It is called menstrual cycle.
 The menstrual cycle occur only in primates.
(1) Period : The length of menstrual cycle varies widely in women, but on average it is completed in 28 days (mensem means a month). In a female, successive cycles may vary in length by 1 to 2 days. It is absent during pregnancy, may be suppressed during lactation and permanently stops at menopause.
(2) Phase : Menstrual cycle is divided into four phases �

(i) Follicular (Proliferative) phase or Post-menstrual or Pre-ovulatory phase : It follows the menstrual phase and lasts for about 10 � 12 days (from 5th to 14th day of menstrual cycle). It involves following changes :
(a) Under the stimulation of FSH-RF of hypothalamus, there is increased secretion of FSH from anterior pituitary.
(b) FSH stimulates the change of a primary follicle of the ovary into a Graffian follicle.
(c) Follicular cells of Graffian follicle secrete estrogens. Estrogens have the following effects :
(1) Stimulate the growth, maintenance and normal functioning of secondary sex organs.
(2) Uterne endometrium becomes thick (about 2-3 mm thick), more vascular and more glandular cork-screw shaped. So it prepares itself for implantation.
(3) The epithelium of Fallopian tubes becomes thick and densely ciliated to conduct the ovum / zygote.
(4) Inhibit the secretion of FSH and stimulate the secretion of luteinising hormone (LH) from the anterior pituitary.
(5) Glycogen and fat accumulate in the endometrial cells.

(ii) Ovulatory phase or fertility phase : It involves the ovulation from the Graffian follicle of ovary. The mature graffian follicle rises to the surface of the ovary and ruptures to release ovum. The phenomina is called ovulation. It occurs midway between two menstrual cycles on 14th day of the onset of the menstrual cycle. it is caused by increasing turgidity and contraction of smooth muscles fibres around the Graffian follicle. Ovum is received by the fimbriad of the fallopian tube. Ovum is viable for two days. Ovulation is controlled by the increased level of LH in the blood. Egg at that time is in the secondary oocyte state. LH also starts the change of empty Graffian follicle into corpus luteum and secretion of progesterone from corpus. There is no much change in uterine endometrium during ovulatory phase. In animals the ovulation follow three patterns �
(a) Fix or spontaneous ovulators : In these animals ovulation takes place a fix time in the midway of cycle. There is no need of coitus for ovulation. e.g. Primates (Human, Ape and Monkey)
(b) Induced or reflex ovulators : In these animals copulation or ciotus is necessary for ovulation. e.g. � Rabbit.
(c) Seasonal ovulators : Ovulation occur in breeding season e.g. � Frog.

(iii) Luteal or progestational or Pre-menstrual or Secretory or Post-ovulatory phase : It lasts for about 12 � 14 days and extends from 16th to 28th day of menstrual cycle. It is characterised by following changes �
(a) Corpus luteum (Yellow body) : Formed from empty Graffian follicle, increase in size, so is called luteal phase.
(b) Corpus luteum begins to secrete hormone called progesterone. The latter reaches its peak about 22nd day after the beginning of cycle.

The oestrous cycle consists of a few days of oestrus or "heat" followed by a few days of anoestrus of "quiescence".
 During oestrus, the female is sexually responsive, allows a male to copulate, eggs are released and pregnancy is possible.
 During anoestrus, the female become passive and does not accept a male.
 The oestrus occurs in most species of mammals. Many mammals reproduce in the breeding season only. The oestrus cycles run only during the breeding season in these mammals and anoestrus spreads over the entire non breeding season.
 Except primates, oestrous cycle is found in all mammals.
 Some mammals, such a cow and buffalo experience oestrous cycles throughout the year. They have no specific breeding season.
 In rabbit the oestrous cycle is of 7 days.
 Oestrous cycle of rat is of 5 days only.
 Dog has one cycle per year.
 Oestrous is also found in the new-age monkeys.
 During the oestrous-cycle, the wall of uterus does not dissolve i.e. no bleeding takes place.
 Mono � oestrous animals : In the breeding period of some animals only one oestrous cycle is present. e.g. Rabbit, Hare, Dog, Fox, Bat, Deer etc.
 Poly � Oestrous animals : In many animals many oestrous cycles are found in the breeding period. e.g. � Rats squirrels, Cow, Sheep, Pig, Horse etc.