In animals, reproduction takes place by two methods:

1. A Sexual Reproduction
2. Sexual Reproduction

There are different ways of A sexual Reproduction some are given below:

In this type of asexual reproduction, a single parent cell directly splits into two or more daughter cells after replication of genetic material i.e. DNA. This method of reproduction is common in Protozoans and Bacteria.

First of all, the nucleus stretches and assumes a Dumb- Bell shape. A complete nucleus is not present (as in Bacteria) then the DNA separates into two equal halves. The dumb-bell shaped nucleus stretches further and finally splits into two daughter nuclei. Later on cytoplasm divides in the same way and thus two daughter cells are formed. This type of fission is called Binary Fission.

In some conditions, the nucleus divides into many small fragments, each fragment is released with some cytoplasm, thus a number of smaller cells are formed. This is called Multiple Fission. Examples are found in Amoeba and Plasmodium etc.

In this type of asexual reproduction, the outer surface of the animal grows outward forming a small branch. This outgrowth soon develops other accessory structures of the animal and ditches to become a new individual. Examples are found in Hydra and Sponges.

In this type, an unfertilized egg develops into a young one without being fertilized by a male gamete i.e. sperm. Examples are found in certain arthropods like Termites.

Generally an animal has male or female reproductive organs capable of producing only one type of Gametes. Such individuals are called Unisexual Animals. However there are certain animals in which both type of organs are formed on the same individual, such animals are called Hermaphrodite. Examples are Earthworms and Nematodes.

Unisexual animals reproduce by means of gametes. These gametes are haploid i.e. having half of the original number of chromosomes in their nuclei. The haploid nuclei of gametes are generally called Pronuclei.

Fertilization may take place inside or outside the body or an animal, therefore the process of fertilization is divided into two types:

1. External Fertilization
2. Internal Fertilization

In this type of fertilization fusion of male and female gametes takes place outside the body of animal i.e. A female lays eggs over which the male sheds its sperms. This is commonly found in aquatic animals i.e. Amphibians and different fishes.

In this type of fertilization, fusion of male and female gametes take place inside the body of female animal in specialized organs called female reproductive organs. For this type of fertilization male introduces its sperms in the female reproductive tract by means of a specialized organ called Penis. In animals which are devoid of Penis introduction of sperms in female tract is carried out by direct contact of anal opening of male with the female. Example of internal fertilization are found in Mammals, Reptiles and Birds.

A developing embryo is delicate structure, which needs protection from environment and nutrition through out the period of development. Different animals have developed different methods and strategies to protect and nourish then embryos. For examples Amphibians and fishes lay their eggs in large number in water. These eggs are covered with a gelatinous substance, which keeps them in a cluster called Spawn. Therefore there is no loss of moisture and thus eggs are not covered with hard shells.

In Reptiles and birds, eggs are covered with a hard shell, which protects the contents from drying. The yolk provides necessary nourishment to the developing embryo.

In Placental mammals zygote completely develops inside mother’s body in a specialized organ called Uterus. Nourishment is supplied by the mother through a highly vascular Placenta.

In higher animals like In higher animals like "Cordates" protective membranes develop surrounding the embryo, these membranes are called Amnion and Chorion. Amnion lies closer to the embryo and completely surrounds it. A cavity lies between amnion and embryo called Amniotic Cavity. It contains a fluid called amniotic fluid. This fluid baths the embryo and protects it from Physical Trauma as well as provides space for movement.

Another membrane i.e. Chrion lies externally in close contact with the egg shell or uterus. In reptile and birds, an extension of hind gut called Alantois is also found which merges with a part of chorion to form chorio-alantoic membrane. This structure is highly vascular and serves as a respiratory organ. It cavity serves to accumulate waste products.

The egg of Prototherians contains a nutritional substance called yolk. This yolk is rich in proteins and fats and provides sufficient and effective nutrition to the embryo during the development. After hatching the young one is fed on milk produced by the mammary gland of the mother. Examples are Duck Bill.

In pouched mammals, eggs do not contain sufficient yolk for the developing embryo therefore a primitive placenta is found to provide maternal nourishment to the embryo. However young ones are born at a premature stage after which they are carried into the abdominal pouch of the mother where they are fed on milk. For example Kangaroo.

In Eutherians egg does not contain any yolk or in a very small amount insufficient to provide nutrition to the embryo. Instead of yolk they develop a special and unique structure which attaches the embryo with the mother’s uterus. This vascular structure is called Placenta. In placenta a closely lying network of embryonic and maternal blood vessel is found in which selective exchange of nutrients and waste between mother’s and embryo’s blood takes place. After birth the young one is fed on milk. Examples are Dog, Cat and Human beings.

Female reproductive system is composed of following organs:

1. Pair of Ovaries
2. Fallopian tubes
3. Uterus
4. Vagina

The part of female reproductive system, which communicates to the exterior, internally remains moist and slimy. It is the part where male deposits its sperms.

Male reproductive system is composed of following organs:

1. A pair of testes
2. Vas Deferens
3. Prostate Gland
4. Penis

From seminal vesicle sperms are liberated in a tube called Urethra. It runs downward and outward and when comes out from the body, it is then covered with muscular erractile organ called Penis.

It is the main copullatory organ of Male reproductive system. It serves to deposit sperms in the vagina of the female.

The length of each menstrual cycle is 28 days in average. The whole period is also called menstrual calendar starting from 1^st day of bleeding till the repetition of next episode. Ovulation i.e. discharge of ovum from the follicle takes place on the 14^th day of cycle.

Different events of menstrual cycle are as follows:

It lasts for 4 to 5 days dated 1^st to 5^th of Menstrual Calendar. During this stage, Endometrium sheds off.

Each human ovary is composed of two types of cells i.e. Germ cells and Supportive or Follicular Cells. Each germ cells grows and gives rise to an egg supported by follicular cells, which surround the egg and constitute the Graffian Follicle. The period of maturation of each egg corresponds to follicular phase of cycle.

On 14^th day of cycle the mature graffian follicle bursts and liberates the egg called ovulation. The emptied follicle now fills with a yellowish mass of cells which constitutes the corpus Leuteum. It starts to secrete hormones, which cause proliferation of Endometrium. After a week corpus leuteum began to decay if the egg is not fertilized. Due to lack of Hormonal support the endometrium sheds off and menstruation occurs.

Secretion of Hormones from Corpus Leuteum causes rapid proliferation of endometrium due to which endometrium becomes thick and spongy. New blood vessels are formed in the endometrium and blood tends to accumulate in the vessels. Later on Corpus Leuteum began to shrink and decay by which Hormonal support finally cuts off. Endometrium ultimately sheds off and menstruation occurs. With the start of new cycle endometrium again repeats whole process.

Pituitary Gland secretes a number of hormones which regulates different phasis of cycle. They are as follows:

This hormone reaches its peak level on day 14^th of cycle by which it causes release of egg from the follicle.

Prolactin or Leutothophic hormone has no effect on Menstruation directly but it stimulates mammary glands to secret milk for the young one.

Ovary itself also acts as an Endocrine gland and during maturation of follicles and later on following hormones are released.

After ovulation, the empty Graffian follicle forms a yellowish mass of fibria, which eventually develops into a glandular structure, called Corpus Leuteum. It secretes Progesterone, which stabilizes endometrium and further induces Proliferation.