Page 193 - Innovator's Science-8

• In birds and mammals, fertilisation happens inside the female’s body.
The sperm is deposited inside, where it meets the egg.

After fertilisation:

• In birds, the fertilised egg is laid outside the body. The
Air cell
embryo grows inside the egg and hatches after a period
The chalazae
The
eggshell of time. The egg contains all the food the embryo needs
during its development.
The egg yolk
• In mammals (including humans), the embryo develops
The egg inside the mother’s body. The baby receives food and
white/albumen
The chalazae oxygen through the mother until it is born.
Shell membranes
Each of these methods has its own benefits. Laying eggs
Structure of an egg allows many offspring to be produced at once, but they

need protection. Giving birth ensures the developing baby is safe and well-
nourished inside the parent’s body.

WHAT ARE THE THREATS TO LIFE ON EARTH?

Life on Earth depends on a delicate balance between living things and their
environment. But human activities are now disrupting this balance. Even
small changes in temperature, oxygen levels, or the ozone layer can

seriously affect life on our planet.

Today, the biggest environmental problems we face are climate
change, biodiversity loss, and pollution. Together, these are
known as the triple planetary crisis.

When we burn fossil fuels like coal, oil, and gas, they release
greenhouse gases such as carbon dioxide and methane. These
gases trap extra heat in the atmosphere, causing the Earth to

Burning of fossil fuels warm up— a process known as global warming .
Normally, trees, plants, and even tiny ocean plankton absorb carbon
dioxide. But burning fossil fuels adds too much carbon into the atmosphere
more than the Earth can handle. As a result, the planet heats up faster. Even

a small increase in temperature can melt polar ice, raise sea levels, cause
floods in coastal areas, create extreme weather, and threaten the survival
of many species. Over time, these changes in temperature and weather

patterns are called climate change .
When forests, wetlands, and other natural habitats are destroyed, plants
and animals lose their homes and may go extinct. This breaks the balance of

nature. For instance, if grass disappears, animals that eat it will also suffer,

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