Menstrual cycle is the reproductive cycle in all primates and begins at puberty (menarche). In human females, menstruation occurs once in 28 to 29 days. The cycle of events starting from one menstruation till the next one is called the menstrual cycle. During the middle of the menstrual cycle, one ovum is released (ovulation). The cycle starts with the menstrual flow (3 to 5 days), caused due to the breakdown of the endometrium of the uterus. Blood vessels in liquid state are discharged, but this occurs only when the ovum is not fertilised. It is followed by the follicular phase where the primary follicles mature into Graafian follicles. This causes the regeneration of the endometrium. These changes are brought about by ovarian and pituitary hormones. In this phase, the release of gonadotropins (LH and FSH) increases. This causes follicular growth and the growing follicles produce oestrogen. The LH and FSH are at their peak in the middle of the cycle (14^th day) and cause the rupture of the Graafian follicles to release ovum. This phase is called the ovulatory phase. The remains of the Graafian follicles get converted into the corpus luteum, which secretes progesterone for the maintenance of the endometrium. In the absence of fertilisation, the corpus luteum degenerates, thereby causing the disintegration of the endometrium and the start of a new cycle.

Placenta. It is temporary structure formed by the union of the foetal and uterine tissue for purpose of nutrition, respiration and excretion of the embryo .Although the blood vessels of the embryo and the mother come close but are kept separated by some barriers between them.The useful substances pass from maternal blood to foetal blood while the wastes(excretory products and \({ CO }_{ 2 }\)) are passed from the foetal blood to maternal blood.
Functions of placenta. The placenta serves primarily as an organ that permits the interchange of meterials carried  in the blood of mother and foetus.The main functions are:
1.Nutrition. Supply of nutrient materials to foetus.
2.Respiration. Supply of \({ O }_{ 2 }\) to foetus and receives \({ CO }_{ 2 }\) back from it.
3.Excretion.Fluid nitrogenous waste products excape through the placenta.
4.Barrier. The placenta is a barrier-like semipermeable membrane.
5.Storage. The placenta stores fat, glycogen and iron for the embryo before the formation ofliver.
6.Hormonal function. The placenta secretes extra ovarian hormones-estrogen and progestrone in female during pregnancy that serves to maintain foetus.

Structure of Epithelio-chorial placenta. Epithelio-chorial placenta is the simple and basic type of placenta as formed in cow, goat etc. In this case some of the components are derived from the uterine wall of mother while the other parts are derived from the foetus itself.

The maternal components of placenta are:
1. Mucous membrane of the uterine wall.
2. Uterine connective tissue.
3. Maternal endothelium (wall of maternal blood capillaries).
Foetal components of placenta are:
1.Wall of foetal blood capillaries.
2. Foetal connective tissue.
3. Foetal chorionic epithelium

a) Structure of ovum. Most animal eggs are spherical or oval, non-motile but on close examination

it is noted that one pole is different from the other. The pole from which polar bodies are given off is called animal pole while the opposite is termed as vegetal pole. Thus it is said to have polarity. Thus various
cytoplasmic substances are distributed along the axis in unequal manner. The nucleus also called (germinal vesicle) having chromatin network is surrounded by nuclear membrane. It also contains prominent nucleolus. All animal eggs contain some reserve material to provide food called yolk. The cytoplasm of ovum is called ooplasm. It lacks centrosome but contains cortical granules derived from Golgi bodies in its outer region termed cortex.
(b) Function of Zona pellucida. During fertilisation as a result of cortical reaction, zona pellucida hardens and prevents the entry of additional sperms (polyspermy).

Similarities between Spermatogenesis and Oogenesis. 
1. Both the process starts with primordial germ cells derived from inside the gonads.
2. They occur inside the gonads.
3. Both the processes are completed in three phases i.e. multiplication, growth and maturation.
4.Both the processes show meiosis in the maturation phase.
5.Both the processes produce haploid gametes.

Hormonal Control of Menstrual Cycle.
Gonadotropin-releasing hormone (GnRH) also called Gonadotropin-releasing Factor (GnRF), is secreted by the hypothalamus of the brain, which stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).FSH stimulates the ovarian follicles to produce oestrogens during proliferative phase.LH stimulates the corpus luteum of the ovary to secrete progesterone.
1.Menstrual phase is caused by the reduction of progesterone and oestrogens.
2.Proliferative phase is caused by the increased production of oestrogens.
3.LH causes ovulation.
LH also causes transformation of empty Graafian follicle into corpus luteum by luteinisation inside the ovary. LH also stimulates corpus luteum to secrete progesterone hormone to help in implantation, placentation and maintenance of pregnancy.
4.Secretary phase is caused by increased production of progesterone.

(i) A-Spermatids, undergo spermiogenesis
(ii) A-Spermatids, B-Spermatogonium.
Spermatogonium (B) are diploid and have 46 chromosomes in humans, while spermatids (A) are haploid and have 23 chromosomes in case of human male.
(iii) E-spermatozoa, are motile cells
(iv) F-Sertoli cell, it provides nutrition to the germ cells
(v) Seminiferous tubule.