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9. Reproduction in Animals

This chapter talks about reproduction in animals and why it is important for the continuation of the species.

Reproduction is the biological process of production of an offspring from its parent. It is essential for the continuation of a species.

There are two modes of reproduction in animal

  1. Sexual Reproduction
  2. Asexual Reproduction

9.1      Sexual Reproduction

Sexual Reproduction requires the involvement of two individuals or gametes, one each from opposite type of sex. The sexual reproduction process involves the fusion of male & female gametes to form a Zygote. The zygote develops into a new organism. Each gamete carries half the genetic information of the parent.

A gamete is a cell, which fuses with another from the opposite sex during the fertilization process to form a combined cell representing the offspring.

A zygote is a cell formed by the fusion of two gametes

9.1.1      Male Reproductive Organs

The following are the male reproductive organs

  1. A pair of testes
  2. Two sperm ducts
  3. A penis

The testes produce male gametes called sperms. A sperm is a single cell with all the usual cell components. They are very small in size, each having a head, a body and a tail

The sperm ducts are responsible for transferring millions of sperms from tested to penis. Penis is responsible for transferring the same to female reproductive organs, where fertilization occurs.

9.1.2      Female Reproductive Organs

The following are the female reproductive organs

  1. A pair of ovaries
  2. Oviducts (or fallopian tubes)
  3. Uterus

The female gametes called ova (or eggs) are produced by the ovaries. In humans, a single matured egg is released into the oviduct by one of the ovaries every month. The egg is also a single cell; however it is about 20 times bigger than sperm

Uterus is the part is reproductive system, where development of baby takes place.

9.1.3      Fertilisation

During reproduction, millions of male gametes (sperms) are transferred to female body. One of the sperm combines with egg. This process of fusion of sperm & egg is called Fertilisation

Fertilisation is the first step in the process of reproduction. During fertilization, the nucleus of the sperm and the nucleus of the egg fuse to form a single nucleus. This leads to the formation of a fertilised egg or Zygote. As we know, Zygote is the beginning of a new organism.

There are two types of fertilization

  1. When the fertilisation takes place inside the female body, it is called Internal Fertilisation. Internal Fertilisation takes place in humans and animals like cow, dog and hen.
  2. When the of fertilization takes place outside the body of the female, it is called External Fertilisation. External Fertilisation takes place in aquatic animals such as fish, starfish, frogs etc. In rainy season, frogs & toads move to ponds & slow moving streams. The female lays hundreds of eggs. The eggs are not covered by any outer shell and are very delicate. It is surrounded by a layer of jelly which provides protection to the eggs. At the same time, male frogs release sperms in water, which swim upto the eggs using its tail. The sperms come in contact with the eggs and result in Fertilisation.

Only a few of those hundreds of eggs get fertilized, as eggs & sperms are exposed to water movement, wind, rainfall & to other animals which feed on eggs.

9.1.4      Development of Embryo

As a result of fertilization, a zygote is formed, which begins to develop into an embryo. The zygote divides repeatedly to give rise to a ball of cells. These cells then begin to form groups which then develop into different tissues & organs of the body. This developing structure is called an Embryo. This gets attached to the wall of the uterus for further development. It slowly develops body parts such as hands, legs, head, eyes etc. The stage of the development of embryo, when all the body parts can be identified is called a Foetus. When the development of the foetus is complete, the mother gives birth to the baby. The animals which gives birth to young ones are called Viviparous Animals

Although hens reproduce using internal fertilization, they do not give birth to babies like human beings. Soon after fertilization, the zygote divides repeatedly & travels down the Oviduct. As it travels down, many protective layers are formed around it which is the hard, protective shell around the hen’s egg. The hen finally lays the egg. The embryo takes about 3 weeks to develop into a chick. The development of the chick takes place inside the egg shell and hen sits on the eggs to provide warmth. After the chick is completely developed, it bursts open the egg shell. The animals which lay eggs are called Oviparous Animals

9.1.5      Growth of new born organisms

The new individuals born or hatched continue to grow to become adults. In some animals, the young ones look very different from the adults as in the case of silkworms and frogs.

Lifecycle of a Silkworm

Egg -> Larva or caterpillar -> Pupa -> Adult

 

Lifecycle of a Frog

Egg -> Tadpole (Or Larva) -> Adult

The transformation of the larva into an adult with drastic changes is called Metamorphosis.

9.2      Asexual Reproduction

Asexual reproduction only requires the involvement of a single parent. This process of reproduction takes place without the use of gametes and fertilization. Asexual Reproduction can take place by many methods. For example

  1. Budding as in the case of Hydra
  2. Binary Fission as in the case of Amoeba.

9.2.1      Budding

It is an asexual reproduction process, where the new individual develops as an outgrowth from a single parent. For examples, in a hydra, one or more bulges are present. These bulges are called as Buds. New individuals develop from these buds therefore this process is called budding.

9.2.2      Binary Fission

It is also an asexual reproduction process, where the parent animal reproduces by dividing into two individuals. For example, an amoeba begins the process of reproduction by the division of its nucleus into two nuclei. Then the division of the body takes place with each part receiving a nucleus. Finally, two amoebae are produced from one parent Amoeba.

9.2.3      Cloning

Cloning is an artificial process of producing an exact copy of a cell and subsequently, a complete living organism. An experiment was done at the Roslin Institute in Edinburgh (UK) in 1996 to close a sheep. It was created by replacing the nucleus of a fertilized sheep egg with the nucleus of a cell from Dolly’s parent sheep. As you know, all the genetic material is present in chromosomes/genes in nucleus; the new offspring was an exact replica of the parent.