CBSE Class 8 Science Notes Chapter 6 - Reproduction in Animals

Reproduction means 'to reproduce', producing more of the same kind and perpetuating the species. Without reproduction, a species would ultimately die out. Producing new individuals from existing ones to increase the community is called reproduction. Reproduction is the biological process by which existing organisms create new individual organisms that are similar to themselves. Every existing plant, animal or other living organism results from reproduction.

1.0Modes of reproduction

There are two modes by which animals reproduce. These are : 

(i) Sexual reproduction

(ii) Asexual reproduction.

2.0Sexual reproduction

It is a mode of reproduction that involves the fusion of the female gamete and the male gamete.

1. Male reproductive organs

• Male reproductive organs include 
• A pair of testes
• A pair of sperm ducts
• A penis.

(i) Testes: The testes produce the male gamete called sperm; testes lie in a scrotal sac. Millions of sperm (male gametes) are produced within the testes daily. The scrotal sac keeps the testes temperature at 2 °C lower than the body temperature. 

Sperm: A sperm is a single celled male gamete with three parts : 

(a) Head: Contains nucleus. 

(b) Middle piece: Contains numerous mitochondria. 

(c) Long tail: Helps in movement. All the usual cell components are present in the sperm. The sperms are motile because of their long tails. 

(ii) Sperm duct: They are also known as vas deferens. Two sperm ducts through which sperm leave the testes and reach the penis. 

(iii) Penis: The muscular organ having a drainage duct through which urine and sperm are pushed out. 

2. Female reproductive organs 

The female reproductive organ includes 

•  A pair of ovaries, 
•  An oviduct (fallopian tube), 
• A uterus, and 

(i) A pair of ovaries - The ovaries lie in the lower part of the abdominal cavity. Either of the ovaries produces one ovum every month. The egg is also known as ova (singular ovum) or female gamete. This egg passes through the fallopian tube to reach the uterus. An egg is a single cell. 

(ii) A pair of oviducts -  The Oviduct is called the fallopian tube. Each oviduct has a funnel-shaped opening, which helps to pick up and push the released ovum into the oviduct. 

(iii) The uterus is a muscular structure known as the womb. The uterus acts as a life support system. The place where the fertilized ovum (zygote) becomes implanted (fixed), receives nourishment and develops into an infant. 

Fertilization 

• For sexual reproduction to occur, a male gamete should fuse with a female gamete. 
• The male and female gamete fusion to form a zygote during sexual reproduction is called fertilization. 
• When sperm comes in contact with the egg, one sperm may fuse with the egg.  
• During fertilization, the nuclei of the sperm and the egg fuse to form a single nucleus. 
• This results in the forming of a zygote-fertilized ovum or fertilized nucleus. 
• So, the new individual inherits some characters from the father and some from the mother.

Type of Fertilization:

• Internal fertilization: The type of fertilization occurs inside the body of a female. It occurs in many animals, including humans, cows, dogs, and hens. 
• External fertilization: Fertilization that occurs outside the body of a female is called external fertilization. In frogs and toads, reproduction activity starts during spring or the rainy season.

Development of Embryo

• A zygote formed after fertilization begins to develop into an embryo. 
• It starts dividing repeatedly to produce a hollow ball of cells. 
• The cell begins to form a group that develops into different tissues and organs of the body. This developing structure is termed an embryo. 
• The embryo attaches to the wall of the uterus or gets embedded in the wall of the uterus for further development. Inside the womb, the umbilical cord connects the baby with the mother's placenta, which provides nutrition to the fetus. 
• Here, it develops over the nine months. 
• The developing embryo gradually differentiates into body parts such as hands, legs, head, eyes, ears, etc. 
• A fetus is the embryo stage in which all the body parts can be identified.
• Gestation period: This is the period from fertilization to birth. In humans, it is about nine months.

Internal Fertilization in Hens 

• Hens lay eggs, which give rise to chickens as their young ones. 
• After fertilization, the zygote divides repeatedly. 
• At the same time, a protective layer is formed around the developing embryo. 
• The eggs are laid by the hen after the hard shell is formed around the developing embryo. 
• The development of the chick takes place inside the eggshell, and it takes about three weeks. 
• The sitting of a hen on the eggs is to provide warmth to the eggs. 
• As soon as the chick is developed, it bursts and opens the eggshell.

Viviparous and Oviparous Animals 

• Most animals lay eggs while some animals give birth to young ones. The animals which give birth to young ones are called viviparous animals. 
• Those animals which lay eggs are called oviparous.. These are called oviparous animals, for e.g. insects, frogs, snakes, turtles and birds.

3.0Asexual Reproduction 

• There are many ways new organisms are produced from their parents. Some microscopic organisms, like Amoebas and Hydra, reproduce asexually.
• Budding: Budding amongst animals is commonly seen in Hydra. In this type of reproduction, one or more small bulb-like outgrowths or bulges form on the body.

• Binary fission: This mode of reproduction is very common in Amoebas, Paramecium, and bacteria. Amoebas are single-celled organisms. The organism starts the process by dividing the nucleus into two nuclei. The body then divides into two parts, each with a nucleus. 
• Other methods of asexual reproduction : 
• Fragmentation - e.g. Spirogyra 
• Spore formation - e.g. Fungus 
• Vegetative propagation - Through root, stem, leaves (Sweet potato - roots, Ginger, potato - stem, Bryophyllum - leaves). 

Cloning 

• Cloning is a technique used to produce an exact genetic copy of a cell or organism. 
• This process was first successfully carried out by Ian Wilmut and his team at the Roslin Institute in Edinburgh, Scotland. 
• They cloned a Scottish sheep named "Dolly," who was born on July 5, 1996, becoming the first cloned mammal in history. 
• Dolly lived until February 14, 2003, reaching the age of six.