Reproduction In Animals
Reproduction: The process by which an organism produces its offspring is called reproduction.
Modes of Reproduction
There are two modes of reproduction in animals, viz. sexual and asexual reproduction.
- Sexual Reproduction: When two parents are involved in reproduction and gamete formation is necessary for reproduction, it is called sexual reproduction. Male gametes are produced by the male parent, and female gametes are produced by the female parent. Fusion of male and female gametes is an important step in this type of reproduction.
- Asexual Reproduction: When a single parent is involved in reproduction and gamete formation does not take place, it is called asexual reproduction.
Human Reproductive System
Male Reproductive Organs:
The male reproductive organs are a pair of testes, two sperm ducts and a penis. Testes produce the male gametes which are called sperms.
Structure of Sperm: A sperm is very small in size, unicellular, and is composed of a head, middle piece and a tail.
Female Reproductive Organs:
The female reproductive organs are a pair of ovaries, a pair of oviducts (fallopian tubes) and a uterus. The female gametes, i.e. eggs are produced by ovaries.
Fusion of a male gamete with a female gamete is called fertilization. During fertilization, the fusion of nuclei of sperm and egg takes place. The zygote is formed after fertilization.
Types of Fertilization:
There are two types of fertilization in animals, viz. external fertilization and internal fertilization.
- External Fertilization: When fertilization takes place outside the animal’s body, it is called external fertilization. Most of the aquatic animals show external fertilization, e.g. fish and frog. A large number of eggs need to be fertilized in this method because most of the fertilized eggs get washed away by water or are eaten by predators. A large number of eggs ensures the survival of some of the offspring.
- Internal Fertilization: When fertilization takes place inside the animal’s body, it is called internal fertilization. Internal fertilization is seen in most of the terrestrial animals. Unlike external fertilization, there is no need to produce a large number of eggs in internal fertilization.
Conception and Child Birth:
It takes place in the female reproductive system through following steps:
- A single egg comes out of the ovary every month and reaches the fallopian tube.
- Sperms reach the fallopian tube where a sperm fertilizes the egg. This results in the formation of zygote.
- The zygote undergoes several rounds of cell division to become a ball of cells.
- This ball of cells moves to the uterus and gets implanted in the wall of the uterus. This process is called conception. Conception marks the beginning of pregnancy.
- The embryo develops inside the uterus to become a baby.
- Gestation: The period from conception to childbirth is called gestation period. The gestation period in humans is of 40 weeks.
- Embryo: After conception, the ball of cells undergoes further rounds of cell division. During the process, the cells begin to differentiate into different types of tissue. Tissue level stage after conception is called embryo.
- Foetus:When the embryo begins to resemble a human being, it is called a foetus. At this stage, various external organs; like legs, head, ear, eyes, etc. become apparent.
- IVF (In Vitro Fertilisation): Any biological process carried out in the laboratory is called in-vitro. Thus, fertilization carried out in the laboratory is called in-vitro fertilization. After in-vitro fertilization, the zygote is allowed to develop into a mass of cells and is then transferred into the uterus for subsequent development. Many women may fail to conceive due to certain problems. In-vitro fertilization helps such women to bear a child. A baby which is born with the help of this technique is often called the ‘test tube baby’.
Formation of Egg Shell in Hens:
We know that birds and many other animals lay eggs. Eggs of these animals have a tough shell of calcium carbonate. When zygote travels down the fallopian tubes, it undergoes several rounds of cell division. Many protective layers are formed around the zygote during this process. These layers subsequently develop into the hard shell of the egg. The hen lays an egg only when the shell is completely formed. The embryo inside the egg of a hen takes about 3 weeks to develop into a chick.
Based on their ability to lay eggs or to give birth to young ones, animals are divided into two types, viz. oviparous and viviparous animals.
- Oviparous Animals: Animals which lay eggs are called oviparous animals, e.g. birds and most of the reptiles.
- Viviparous Animals: Animals which give birth to young ones are called viviparous animals, e.g. most of the mammals and some fish.
- Direct Development: When the young ones of an animal resemble the adult, then direct development takes place, e.g. hen, man, monkey, etc.
- 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.
- Metamorphosis: In case of indirect development, the transformation of young ones into the adult is called metamorphosis. Larva of butterfly undergoes metamorphosis to become a butterfly. A tadpole undergoes metamorphosis to become a frog.
Types of Asexual Reproduction
Budding: This method is seen in multicellular animals which are highly simple in structure. A small bud develops on the body. The bud develops and starts resembling its mother. After that, the bud gets detached from the mother’s body to begin life as a new individual. Examples: Hydra and sponges.
Binary Fission: This method is seen in unicellular animals, e.g. amoeba. The cell of amoeba divided into two daughter cells. Each daughter cell becomes a new individual.
Cloning: When a new cell or a part of an organism or the whole organism is produced from a cell, it is called cloning. A sheep was the first mammal to be cloned. It was named as Dolly. Ian Wilmut and his colleagues at Roslin Institute in Edinburgh (Scotland) cloned Dolly in 1996. For this, the scientists took a cell from mammary gland from a sheep and took an egg from another sheep. They removed the nucleus of the egg and replaced it with the nucleus of the cell from the mammary gland. After that, the egg was transplanted into the uterus of sheep. The egg subsequently developed into a clone in due course of time.