Reproductive System and How fertilisation takes place?- Class 8 Science Guide

Reproductive System, Fertilisation, Viviparous Animals and Oviparous Animals- Biology Guide for Class 8

Information about Reproductive System and Fertilisation

Title	Reproductive System and How fertilisation takes place?
Class	Class 8
Subject	Class 8 Biology
Topics Covered	Male Reproductive System Female Reproductive System How fertilisation takes place? How Hens lay eggs? Viviparous and Oviparous Animals Development from Young to Adults

Reproductive Systems

Human reproductive systems includes:

1. Male Reproductive System
2. Female Reproductive System

1. Male Reproductive System

• It includes a pair of testes, a scrotum, two sperm ducts, urethra and penis.
• Testes are oval organs (two in number) (singular-testis) present in a sac called scrotum.
• The sperms develop in the testes. Sperms pass from the testes through the sperm ducts to the urethra.
• As the sperms move, they collect fluids from the glands. This fluid, having sperms in it, is called the semen.
• The last part of the urethra is surrounded by a muscular structure called the penis.
• Penis conducts urine as well as semen; however, the two cannot pass through it at the same time. 

Structure of a sperm

• A sperm is smaller in size than the ovum and can only be seen through a microscope. Each sperm cell has three parts: a head, a middle piece and a tail.
• A structure, at the tip of the head, produces enzymes that help the sperm to penetrate the female ovum (egg).

2. Female Reproductive System

• The female reproductive system consists of a pair of ovaries, a pair of oviducts, uterus and vagina.
• The main functions of the female reproductive system include production of eggs, receiving the sperms and providing the site for fertilisation.
• The growing embryo gets attached to the uterus and the foetus develops there. 
• Ovaries produce eggs. An egg reaches the oviduct; when a sperm fuses with the egg, a zygote is formed.
• It is this zygote that divides and develops to form the young one.
• Ovaries also produce hormones that control the production of ova (eggs). These hormones also help in the maintenance of pregnancy.

How does fertilisation take place?

• Fertilisation takes place in the initial part of the oviduct.
• When the sperm and ovum (male and female gametes) fuse, their fusion (fusion of the sperm and egg) is called fertilisation.
• Since it takes place inside the body of the female, it is an internal fertilisation.
• During this process, the nuclei of the sperm and egg fuse to form a single nucleus. The resultant single cell, formed after fertilisation, is called a zygote.
• The, zygote contains the nuclear part of sperm (from the father) and of the egg (from the mother).
• It is for this reason that a child has some characteristics of the father and some characteristics of the mother. 

Development of the embryo

• The zygote immediately divides into two cells; these cells then divide again and again, over the next couple of days, and the resulting cluster of cells makes its way to the uterus (womb). This cluster of cells is called the embryo.
• The embryo plants itself in the lining of the uterus and continues dividing its cells to make billions of new cells.
• It gradually develops the different body parts, such as the head, eyes, hands, legs, etc.
• When the embryo reaches the stage where all its body parts can be identified, it is called the foetus.
• The foetus, when born, is called an infant.

How do hens lay eggs?

• In hens, fertilisation is internal as it takes place inside the body of the female.
• But unlike humans, the development of their young ones takes place outside the body of the female bird. This is, therefore, an example of internal fertilisation and external development.
• After fertilisation, the zygote starts dividing and moves down the oviduct.
• As it moves down, a number of protective layers get formed around it. The egg shell is one of these protective layers. It is deposited around the egg in the lower part of the oviduct of the hen, just before the egg is laid.
• The egg shell is made of calcite, a crystalline form of calcium carbonate.
• The embryo develops inside the egg for (around) 21 days (the incubation period), after which the chick gets fully developed.
• During the incubation period, the eggs require a specific temperature; this is provided by the hen by sitting on the eggs. (Sometimes, this is done artificially by keeping the eggs in an incubator).
• Finally, the egg shell breaks down and the chick comes out. This process is called hatching. 

Viviparous and Oviparous Animals

Viviparous animals

• Viviparous animals (vivus = alive) are those animals whose females give birth to the young ones.
• The embryo develops inside the body of the mother; the mother gives birth to the young one. This characteristic distinguishes mammals from other groups.

Oviparous animals

• Oviparous animals (ova = egg) are animals that lay eggs; the eggs have little, or no other embryonic development, within the mother.
• The development of the egg takes place in the relevant external surroundings. It is the reproductive method of (most) fish, amphibians, reptiles, birds and insects.

Journey from 'Young ones' to 'Adults'

All young ones grow to form adults regardless of whether they have hatched from an egg or are born directly.

• In many animals the young one, at birth, is a miniature (smaller) version of the adult form. The development of such young ones (into adults), is known as direct development.
• In some animals, the young ones, initially, are very different from their adults.
• For example, a tadpole looks very different from an adult frog.
• Similarly, a caterpillar, or the pupa of a silk moth, or a butterfly, looks very different from its parents.

In fact, there are a number of features in the adult frogs that are not present in the tadpole, or at the larval stage. For example, a frog has lungs but a tadpole has gills for respiration. This type of development is known as indirect development.

• A marked, and more or less abrupt development change in the structure and habits of an animal (as in case of a butterfly or a frog), occurring during its life cycle, is termed as metamorphosis.
• During the life cycle of an amphibian, eggs are laid in water.
• A tadpole then emerges from the egg and swims freely in water. The tadpole has gills, a tail and a small circular mouth.
• The tadpole grows until it begins to undergo metamorphosis.
• Metamorphosis begins with the development of the hind legs followed by the front legs; then the lungs develop.
• The tadpole then begins to swim to the surface of the water to breathe.
• The intestine shortens to adapt the animal to a carnivorous diet.
• In case of frogs, the tail gets absorbed by the body during the last stages of metamorphosis. A tailed tadpole, thereby, changes into a tail-less frog. 

Some Important Points

• In vitro fertilisation (IVF) is a process by which ova are fertilised by sperms outside the female body. The fertilised egg (zygote) is then transferred to the uterus of a woman with the intent to establish a successful pregnancy. Babies, conceived through IVF, are referred to as test tube babies; the term 'test tube' refers to the tube-shaped containers of glass (called test tubes), that are commonly used in science laboratories. The first 'test tube baby', Louise Brown, was born in 1978.
• One sperm can fertilise only one ovum. If two ova (eggs) leave the ovaries at the same time, and are both fertilised, 'non-identical' twins are born. They are like any two brothers, or any two sisters or a brother and a sister.
  If an ovum splits after it has been fertilised into two 'zygotes', identical twins are formed. This is because they have the same set of genes (Genes are special structures in the nucleus that are responsible for different characteristics of a person).