Reproduction in Animals - Class 8 : Notes

Reproduction:
Reproduction is the process by which living organisms produce more living organisms of its own kind.

Mode of Reproduction:
There are two main types of reproduction in living organisms.

1. Asexual Reduction: The process of reproduction in which new individuals are produced from a single parent. E.g. microorganisms.
Asexual reproduction is found in the single-celled organism such as the archabacteria, eubacteria etc. Many plants and fungi reproduce asexually as well.

2. Sexual Reproduction: The process of reproduction in which two individuals are involved to produce a new individual. E.g. Human, tiger

Sexual Reproduction:
In animals, males and females have different reproductive parts or organs. The reproductive parts in animals produce gametes that fuse to form a zygote. It is the zygote which develops into a new individual. This type of reproduction beginning from the fusion of male and female gametes is called sexual reproduction.

Reproductive Organs in Humans:

1. Male Reproductive Organs:
A pair of testes (singular, testis), two sperm ducts and a penis, these are the male reproductive organs. The testes produce the male gametes called sperms. Millions of male gametes (sperms) are produced by the testes. Though sperms are very small in size, each has a head, a middle piece and a tail. Sperm is a single cell with all the usual cell components.

Female Reproductive Organs:
A pair of ovaries, oviducts (Fallopian tubes) and the uterus, these are the female reproductive organs. Ovary produces female gametes called ova (eggs). In human beings, a single matured egg is released by one of the ovaries, into the oviduct every month. Uterus is the part where development of the baby takes place. An egg is also a single cell.

Fertilization:
The process of fertilization is fusion of a male gamete (Sperm) with a female gamete (Ovum) is called fertilization. Zygote is formed after fertilization.

Types of Fertilization:There are two types of fertilization in animals, external fertilization and internal fertilization.
1. Internal Fertilisation: When fertilization takes place inside the animal’s body, it is called internal fertilization. Internal fertilization occurs in many animals including humans, cows, dogs and hens.
2. External Fertilisation: In this type of fertilization, the fusion of a male and a female gamete takes place outside the body of the animal’s body is called external fertilization. It is very common in aquatic animals such as fish, starfish, etc.
Example: During spring or rainy season, frogs and toads move to ponds and river. When the male and female come together in water, the female lays the eggs, the male deposits sperms over them. Each sperm swims randomly in water with the help of its long tail. The sperms then come in contact with the eggs.

 

Development of Embryo:Development of embryo takes place in the female reproductive system through following steps:
1. Every month, a single egg comes out of the ovary and reaches the fallopian tube of female reproductive system.
2. During copulation, sperms reach the Fallopian tube where a sperm fertilizes the egg. This results in formation of zygote.

3.The zygote divides repeatedly to give rise to a ball of cells. The cells then begin to form groups that develop into different tissues and organs of the body. This developing structure is termed an embryo.

4. The embryo gets implanted in the wall of the uterus for further gradually developed the body parts such as hands, legs, head, eyes, ears, etc.
5. The stage of the embryo in which 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.

Formation of Egg Shell in Hens:
Internal fertilization takes place in hens. After fertilization, the zygote divides continuously and moves to the oviduct. As it travels down, many protective layers are formed around it. The hard shell in a hen’s egg is one such protective layer. After the hard shell is formed, the hen finally lays the egg. The embryo takes about 3 weeks to develop into a chick. The hen sits on the eggs to provide sufficient warmth. After the chick is completely developed it bursts open the egg shell.

Viviparous and Oviparous Animals:
1. The animals which give birth to young ones are called viviparous animals.  Examples- Human being, dog, cows.
2. Those animals which lay eggs are called oviparous animals. Examples – hen, frog etc.

IVF (In Vitro Fertilisation):
1. A biological process carried out in laboratory is called in-vitro. Thus, fertilization carried out in laboratory is called in-vitro fertilization.
2. Some women are unable to bear babies because of oviducts are blocked in result sperms cannot reach the egg for fertilization. Doctors collect freshly released egg and sperms and keep them together for a few hours for IVF or in vitro fertilization (fertilization outside the body).
3. If fertilization occurs, the zygote is allowed to develop for about a week and then it is transferred in the mother’s uterus. Whole development takes place in the uterus and the baby is born like any other baby. Babies born through this technique are called test-tube babies.

Young Ones to Adults:
1. Direct Development: When the young ones of an animal resemble the adult, then direct development takes place, e.g. hen, man, monkey, etc.

2. Indirect Development: When the young ones of an animal do not resemble the adult, then indirect development takes place, e.g. frog, butterfly, silk moth, etc.

3. Metamorphosis: In case of indirect development, transformation of young ones into adult through drastic changes is called metamorphosis. Larva of butterfly undergoes metamorphosis to become a butterfly. A tadpole undergoes metamorphosis to become a frog.

Asexual Reproduction Methods: The process of reproduction in which new individuals are produced from single parent. Example - Microorganisms.
1. Budding: This reproduction method is seen in those multi cellular animals which are highly simple in structure. A small bud or bulge develops on the body. After developing the bud, it gets detached from the parent’s body to begin life as a new individual. Since new individuals develop from the buds, this type of asexual reproduction is called budding Examples: Hydra and sponges.

2. Binary Fission:This method of reproduction is seen in unicellular animals, example- amoeba. In this method, an organism divides and forms two daughter cells. First the nucleus divides and forms two daughter nuclei. Then the cytoplasm in the mother cell divides into two daughter cells. This leads to the formation of the two daughter cells each having a nucleus and its own cell organelles which then develop into a fully formed adult. Example- paramaecium, leishmania etc.

Cloning:
1. Cloning is process of production of an exact copy of a cell, any other living part, or a complete organism. Cloning process of an animal was successfully performed for the first time by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh, Scotland. They cloned successfully a sheep named Dolly.

2. During the process of cloning sheep named Dolly, a cell was collected from the mammary gland of a female Finn Dorsett sheep. Simultaneously, an egg was collected from a Scottish blackface ewe. The nucleus was removed from the egg. After that, the nucleus of the mammary gland cell from the Finn Dorsett sheep was inserted into the egg of the Scottish blackface ewe whose nucleus had been removed. Then this egg was implanted into the Scottish blackface ewe. Development of this egg followed normally and then finally Dolly was born. It was found to be absolutely identical to the Finn Dorsett sheep from which the nucleus was taken.