The extra embryonic membranes include amnion, yolk sac, allantois, and chorion. They protect the embryo from desiccation, mechanical shock, absorption of nutrients, gaseous exchange and placental formation.

(i) Ovaries are the primary female sex organ producing eggs (ovum).
(ii) They are located one on each side of lower abdomen (pelvis).
(iii) The ovary is attached in pelvic wall uterus by an ovarian ligament called mesovarium.
(iv) It is an elliptical structure of 2-4 cm long
(v) Each ovary is covered by thin cuboidal germinal epithelium encloses ovarian stroma.
(vi) Below germinal epithelium is a dense connective tissue called tunica albuginea.
(vii) The stroma is differentiated into an outer cortex and an inner medulla.
(viii) The cortex is dense and granular due to follicular cells at varying development stages.
(ix) The medulla is a loose connective tissue with blood vessels, lymph vessels and nerve fibres.

Structure of human spermatozoan:
(i) The human sperm is a microscopic, flagellated and motile gamete. The whole body of the sperm is enveloped by plasma membrane and is composed of a head, neck and a tail.
(ii) The head comprises of two parts namely acrosome and nucleus.
(iii) Acrosome is a small cap like pointed structure present at the tip of the nucleus and is formed mainly from the golgi body of the spermatid.
(iv) It contains hyaluronidase, a proteolytic enzyme, popularly known as sperm lysin which helps to penetrate the ovum during fertilisation.
(v) The neck is very short and is present between the head and the middle piece. It contains the proximal centriole towards the nucleus which plays a role in the first division of zygote and the distal centriole gives rise to the axial filament of the sperm.
(vi) The middle piece possesses mitochondria spirally twisted around the axial filament called mitochondrial spiral or nebenkern. It produces energy in the form of ATP molecules for the movement of sperms.

(vii) The tail is the longest part of the sperm and is slender and tapering.
(viii) It is formed of a central axial filament or axoneme and an outer protoplasmic sheath. The lashing movements of the tail push the sperm forward.

Menstrual cycle / Ovarian cycle
(i) The menstrual or ovarian cycle occurs approximately once in every 28/29 days, during the reproductive life of the female from menarche (puberty) to menopause except during pregnancy.
(ii) Menstrual cycle comprises of the following phases:
1. Menstrual phase
2. Follicular or proliferative phase
3. Ovulatory phase
4. Luteal or, secretory phase
(i) Menstrual phase: (Occurs and last for 3-5 days)
1. Menstrual flow is due to the breakdown of endometrial lining of the uterus, and its blood vessels due to decline in the level of progesterone and oestrogen.
2. Menstruation occurs only if the released ovum is not fertilized.
3. Absence of menstruation may be an indicator of pregnancy
4. However, it could also be due to stress, hormonal disorder and anaemia.
(ii) Follicular (or) proliferative phase: (extends from the 5th day of the cycle until the time of ovulation)
1. During this phase, the primary follicle in the ovary grows to become a fully mature Graafian follicle and simultaneously, the endometrium regenerates through proliferation. These changes in the ovary and the uterus are induced by the secretion of gonadotropins like FSH and LH, which increase gradually during the follicular phase. lt stimulates follicular development and secretion of oestrogen by the follicle cells.
(iii) Ovulatory phase (about 14th day):
1. Both LH and FSH attain peak level in the middle of the cycle.
2. Maximttm secretion of LH during the mid-cycle is called LH surge induces the rupture of the Graafian follicle and the release of the ovum (secondary oocyte) from the break down of ovary wall into the peritoneal cavity. This process called ovulation occurs at late luteal phase.
(iv) Luteal phase or secretory phase;
1. During luteal phase, the remaining part of the Graalian follicle is transformed into a transitory endocrine gland called corpus luteum at the end of ovulation.
2. The corpus luteum secretes a large amount of progesterone which is essential for the maintenance of the endometrium during pregnancy.
3. If fertilization takes place, it paves way for the implantation of the fertilized ovum.
4. The uterine wall secretes a nutritious fluid in the uterus for the foetus.
5. So, this phase is also called as secretory phase. During pregnancy all events of menstrual cycle stop and there is no menstruation.
6. In the absence of fertilization, the corpus luteum degenerates completely and leaves a scar tissue called corpus Albicans.
7. It also initiates the disintegration of the endometrium leading to menstruation, marking repetition of the next cycle.

Relaxin  
(i) It is a hormone secreted by the placenta and also by the corpus luteum during pregnancy.
(ii) It promotes parturition by relaxing the pelvic joints and by dilation of cervical ligaments with continued powerful contractions.
Oxytocin:
(i) It causes the "Let-Down" reflex.
(ii) The actual ejection of milk from the alveoli of the mammary glands.
(iii) During lactation, oxytocin also stimulates the recently emptied uterus to contract, helping it to return to pre-pregnancy size..