Class 8 Science
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 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, transformation of young ones into 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 egg from another sheep. They removed the nucleus of the egg and replaced it with nucleus of the cell from 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.