Reproduction in Animals Class 8 Notes Science Chapter 6

Introduction

 The processes of digestion, circulation, and respiration are crucial for the survival of every individual organism. Just as these processes are vital for individual survival, reproduction is essential for the continuation of a species. Reproduction ensures that similar kinds of individuals are produced, generation after generation. Without reproduction, life would come to a halt, and that is why it is so important. 

 In this chapter, we will explore the different ways in which animals reproduce. Just like plants have their own methods of reproduction, animals have unique ways of ensuring the survival of their species. 

 Modes of Reproduction in Animals 

 Animals reproduce using two main modes: 

(i) Sexual Reproduction: In this mode, animals produce young ones with the involvement of both male and female parents. The young ones inherit traits from both parents. This process often involves the fusion of male and female gametes (sperms and eggs). 

(ii) Asexual Reproduction: In asexual reproduction, a single parent produces offspring without the involvement of gametes. The young ones are genetically identical to the parent. This mode is less common in animals but can occur in certain species. 

 Sexual Reproduction in Animals 

 Just like plants, animals also reproduce sexually, which means they have different reproductive parts that produce gametes. In animals, males and females have different reproductive organs that produce these gametes. The male and female gametes fuse to form a zygote, which develops into a new individual. This process is called sexual reproduction.  

(i) Male Reproductive Organs

•  The male reproductive system consists of a pair of testes, two sperm ducts, and a penis. 

Male Reproductive Organs

•  The testes are responsible for producing male gametes known as sperm. They generate millions of sperm cells, each of which is a single cell with a head, middle piece, and tail. 

Human Sperm

(ii) Female Reproductive Organs

•  The female reproductive system includes a pair of ovaries, oviducts (also called fallopian tubes), and the uterus.

Female Reproductive Organs 

•  The ovaries produce female gametes known as ova or eggs. 

Human Egg ( Ova/ Ovum)

•  In humans, a matured egg is released into the oviduct by one of the ovaries approximately every month. 
•  The uterus is the organ where fetal development occurs. Like sperm, an egg is also a single cell. 

Fertilisation

Fertilisation is the process where a sperm cell from the father joins with an egg cell from the mother. This is the first step in reproduction. When sperm comes into contact with an egg, one sperm may fuse with the egg, which is called fertilisation. During this process, the nuclei of the sperm and egg combine to form a single nucleus, resulting in a fertilised egg or zygote. The zygote is the beginning of a new individual, inheriting traits from both the mother and father. 

Fertilisation

• Internal Fertilisation: In some animals, such as humans, cows, dogs, and hens, fertilisation occurs inside the female body. This is known as internal fertilisation. 
• External Fertilisation: Fertilisation can also take place outside the female’s body, especially in aquatic animals like fish and starfish. In this process, a female lays eggs in water, and the male deposits sperm over them. This is called external fertilisation. 

Frog Eggs: When a female frog lays eggs, she releases hundreds of them at once. Unlike a hen’s egg, a frog’s egg is not covered by a hard shell. Instead, it is protected by a layer of jelly that holds the eggs together. 

Why Lay So Many Eggs? : Fish and frogs lay eggs in large numbers because not all eggs will be fertilised and develop into new individuals. The eggs and sperm are exposed to various elements like water movement, wind, and rainfall, and there are also other animals in the water that may eat the eggs. 

Test Tube Babies:  In some cases, women have blocked oviducts, making it difficult for sperm to reach the egg for fertilisation. Doctors can help by collecting eggs and sperm and keeping them together for fertilisation outside the body, known as in vitro fertilisation (IVF). If fertilisation occurs, the zygote is placed in the mother’s uterus to develop, and the baby is born like any other. Babies born this way are often called test-tube babies, although they do not actually grow in test tubes. 

Development of Embryo

1. Fertilisation results in the formation of a zygote, which begins to develop into an embryo. 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. The embryo gets embedded in the wall of the uterus for further development. 

(a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells (enlarged); (c) Embedding of the embryo in the uterus (enlarged)(a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells (enlarged); (c) Embedding of the embryo in the uterus (enlarged) 

2. The embryo continues to develop in the uterus. It gradually develops body parts such as hands, legs, head, eyes, ears, etc. 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. 

Foetus in Uterus of Human

Note: Internal fertilisation takes place in hens also. But hens do not give birth to babies like human beings and cows. Instead, they lay eggs. Soon after fertilisation, the zygote divides repeatedly and travels down the oviduct. As it travels down, many protective layers are formed around it. The hard shell that you see in a hen’s egg is one such protective layer. After the hard shell is formed around the developing embryo, the hen finally lays the egg. The embryo takes about 3 weeks to develop into a chick. During this period, the hen sits on the eggs to provide sufficient warmth. The development of the chick takes place inside the egg shell during this period. After the chick is completely developed, it bursts open the egg shell. 

Note:  In animals which undergo external fertilisation, development of the embryo takes place outside the female body. The embryos continue to grow within their egg coverings. After the embryos develop, the eggs hatch. You must have seen numerous tadpoles swimming in ponds and streams. 

Viviparous and Oviparous Animals

• Viviparous Animals: These are animals that give birth to live young ones. Examples include dogs, cats, and cows. 
• Oviparous Animals: These animals lay eggs, which later hatch into young ones. Examples include birds like hens and crows, as well as reptiles like lizards and frogs.  

Young Ones to Adults

 The new individuals which are born or hatched from the eggs continue to grow till they become adults. In some animals, the young ones may look very different from the adults.

Life cycle of Frog4 stages in Life Cycle of Frog

(a) Eggs

(b) Early tadpole

(c) Late tadpole

(d) Adult frog

Metamorphosis

•  The young ones which are born from the eggs grow into adults. However, in some animals, the young ones look very different from the adults. For example, in the life cycle of a frog, there are three distinct stages: egg, tadpole (larva), and adult. The tadpoles look very different from the adult frogs, but they eventually transform into frogs. 
•  The process of transformation from larva to adult with significant changes is called metamorphosis. In humans, the body parts that are similar to those of adults are present from birth, but there are still changes that occur as we grow.

Asexual Reproduction

 Asexual reproduction is a type of reproduction that involves only a single parent. 

(i) Budding in Hydra: In hydra, new individuals develop as outgrowths from a single parent, forming bulges called buds. This process is similar to budding in yeast and is known as asexual reproduction. 

Budding in Hydra

(ii) Binary Fission in Amoeba: Amoeba is a single-celled organism that reproduces by dividing its nucleus into two, followed by the division of its body into two parts, each receiving a nucleus. This process is called binary fission, and it results in the formation of two amoebae from one parent amoeba. 

Binary Fission in Amoeba

Apart from budding and binary fission, there are other methods of asexual reproduction by which a single parent reproduces young ones.  

Story of Dolly, the Clone

• Cloning refers to making an exact copy of a cell, a part of a living being, or a whole organism.
• The first successful cloning of an animal was done by Ian Wilmut and his team at the Roslin Institute in Edinburgh, Scotland.
• They cloned a sheep named Dolly, who was born on July 5, 1996, and became the first mammal to be cloned.

(a) Finn Dorsett sheep (b) Scottish blackface ewe (c) Dolly

• During the cloning process, a cell was taken from the mammary gland of a female Finn Dorsett sheep.
• At the same time, an egg was collected from a Scottish blackface ewe.
• The nucleus of the egg was removed, and the nucleus from the mammary gland cell of the Finn Dorsett sheep was inserted into the empty egg.
• This egg was then implanted into the Scottish blackface ewe, and it developed normally, leading to the birth of Dolly.
• Even though Dolly was born to the Scottish blackface ewe, she was genetically identical to the Finn Dorsett sheep from which the nucleus was taken.
• Since the nucleus of the egg was removed, Dolly did not inherit any traits from the Scottish blackface ewe.
• Dolly was a healthy clone of the Finn Dorsett sheep and went on to have her own offspring through natural means.
• Unfortunately, she passed away on February 14, 2003, due to a lung disease.
• Since Dolly's cloning, there have been many attempts to produce cloned mammals, but many of these clones either die before birth or shortly after, and they often have serious abnormalities.