Reproduction in Animals — NCERT Solutions, AI Tutor & Practice

Understand: Reproduction is the process by which living organisms produce new individuals of their own kind.

Analyze: Animals reproduce in two broad ways depending on the number of parents involved.

1. Sexual reproduction – involves two parents (a male and a female). The male and female gametes (sperm and egg) fuse to form a new individual.
2. Asexual reproduction – involves only a single parent. No fusion of gametes takes place.

Conclude: The two main modes of reproduction in animals are sexual reproduction and asexual reproduction.

Understand: Fertilisation is a key step in sexual reproduction.

Definition: Fertilisation is the process of fusion of a male gamete (sperm) with a female gamete (egg or ovum) to form a single cell called the zygote.

Analyze: In humans, the sperm released by the male enters the female body and swims towards the egg. The fusion occurs inside the female body.

Conclude: In human beings, fertilisation takes place inside the female body, specifically in the oviduct (fallopian tube).

Understand: Fertilisation can occur inside or outside the body of the female.

Analyze — Comparison:

| Feature | Internal fertilisation | External fertilisation | |---|---|---| | Site | Inside the female body | Outside the body, usually in water | | Number of eggs | Fewer eggs | Large number of eggs | | Example | Humans, cows, dogs, hens | Frogs, fish |

Conclude: In internal fertilisation the sperm and egg fuse inside the female body (e.g., humans), while in external fertilisation they fuse outside the body in water (e.g., frogs).

Understand: After fertilisation, a special cell is formed that begins the life of the new organism.

Definition: A zygote is the single cell formed by the fusion of a sperm and an egg during fertilisation. It is the beginning of a new individual.

Analyze — Development:

1. The zygote divides repeatedly by cell division to form a ball of cells.
2. These cells gradually differentiate and form tissues and organs — this stage is called the embryo.
3. When the embryo develops body parts that can be identified, it is called a foetus.
4. When development is complete, the young one is born (in viviparous animals) or hatches from an egg (in oviparous animals).

Conclude: The zygote develops into an embryo, then a foetus, and finally into a new individual.

Understand: Animals can be grouped by how their young ones are produced.

Analyze — Comparison:

| Feature | Viviparous animals | Oviparous animals | |---|---|---| | Meaning | Give birth to young ones directly | Lay eggs | | Development | Embryo develops inside the mother's body | Embryo develops inside the egg outside the body | | Examples | Humans, cows, dogs | Hens, frogs, lizards |

Conclude: Viviparous animals give birth to live young, whereas oviparous animals lay eggs from which young ones hatch later.

Understand: Some animals change their body form drastically as they grow up.

Definition: Metamorphosis is the transformation of a larva into an adult through drastic and abrupt changes in body form.

Analyze — Life cycle of a frog:

1. The female frog lays eggs in water (external fertilisation).
2. Eggs hatch into tadpoles (larvae) that live in water and have gills and a tail.
3. The tadpole gradually develops lungs and legs and loses its tail.
4. It changes into an adult frog that can live on land.

Conclude: The change from a tadpole to an adult frog is an example of metamorphosis.

Understand: Hydra is a small animal that reproduces without gametes, using a single parent.

Analyze — Budding in Hydra:

1. A small outgrowth called a bud develops on the body of the parent Hydra due to repeated cell division at one spot.
2. The bud grows and develops into a small individual, with tentacles and mouth.
3. When fully grown, the new individual detaches from the parent body.
4. It then lives as a separate, independent organism.

Conclude: In budding, a new Hydra develops from a bud on the parent's body and detaches to live independently — this is a type of asexual reproduction.

Understand: Amoeba is a single-celled organism that reproduces asexually.

Analyze — Binary fission in Amoeba:

1. The Amoeba first divides its nucleus into two.
2. Then its body (cytoplasm) divides into two parts.
3. Each part gets one nucleus.
4. The single parent cell thus splits into two daughter Amoebae.

Conclude: In binary fission, a single parent Amoeba divides into two individuals. Since only one parent is involved and no gametes are formed, it is a type of asexual reproduction.

Understand: Cloning is an artificial technique of producing organisms.

Definition: Cloning is the production of an exact copy of a cell, any other living part, or a complete organism.

Analyze: In cloning, the new individual is genetically identical to the single parent from which it is made. This process was achieved artificially by scientists.

Conclude: The first mammal to be cloned successfully was a sheep named Dolly, cloned in 1996 by Ian Wilmut and his colleagues at the Roslin Institute, Scotland.

Understand: In external fertilisation, eggs and sperms are released into water where they are unprotected.

Analyze — Reasons for laying many eggs:

1. In open water, many eggs are eaten by other animals before fertilisation.
2. Sperms may not reach all eggs, so many eggs remain unfertilised.
3. Water movement and harsh conditions may damage or wash away the eggs.
4. Only a few eggs finally survive to develop into young ones.

Conclude: To ensure that at least some offspring survive despite these dangers, animals such as fish and frogs lay a large number of eggs.

Understand: The two modes of reproduction differ mainly in the number of parents and the role of gametes.

Analyze — Comparison:

| Feature | Sexual reproduction | Asexual reproduction | |---|---|---| | Number of parents | Two (male and female) | One | | Gametes | Sperm and egg are formed and fuse | No gametes formed | | Fertilisation | Takes place | Does not take place | | Offspring | Differ from parents | Identical to parent | | Examples | Humans, frogs, hens | Hydra, Amoeba |

Conclude: Sexual reproduction needs two parents and fusion of gametes, while asexual reproduction needs only one parent and no gametes.

Understand: After fertilisation, development proceeds through definite stages inside the mother's body.

Analyze — Correct order of stages:

1. Fertilisation – sperm fuses with egg in the oviduct.
2. Zygote – the single fertilised cell formed.
3. Embryo – the zygote divides many times and gets embedded in the wall of the uterus.
4. Foetus – the embryo develops recognisable body parts.
5. Baby (birth) – after complete development, the fully formed baby is born.

Conclude: The sequence is: $\text{Fertilisation} \rightarrow \text{Zygote} \rightarrow \text{Embryo} \rightarrow \text{Foetus} \rightarrow \text{Baby}$