Life cycle

ARE BORN

GROW

REPRODUCE

DIE

Embryonic development

OVIPAROUS VIVIPAROUS OVOVIVIPAROUS

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EMBRYONICDEVELOPMENT

There are three types, depending on where it takes place

UNIT

4

Natural Science 2. Secondary Education

• These animals hatch from an egg which the mother lays in the environment.

• The embryo feeds on the nutritive stores contained in the yolk of the egg.

• Once it has developed, it hatches, which is when the new individual breaks the shell and emerges from the egg.

•There are two types of eggs:

• Eggs without a shell: these are found in aquatic animals and must be laid in a wet environment or they will dry up.

• Eggs with a shell: they are laid on land so they do not dry up.

Oviparous animals

Toads are oviparous: they lay their eggs in the water, as their eggs do not have shells and would dry up on land.

Embryonic developmentUNIT

4

Natural Science 2. Secondary Education

The structure of the egg

Embryo

Vitelline membrane: this encloses the yolk, which the embryo feeds on.

Amnion: this forms the amniotic sac, which holds the amniotic fluid where the embryo floats.

Membranes: they serve as the egg’s excretory system, collecting waste, and the respiratory system, letting CO2 out and O2 in.

Shell: keeps the embryo from drying out but allows O2 and CO2 to pass through.

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Embryonic developmentUNIT

4

Natural Science 2. Secondary Education

Yolk

• The embryos of these animals develop inside the mother’s uterus.

• The embryo feeds on nutrients that pass from the mother through the placenta.

• When the new individual has developed, it comes out of the mother’s body: this is called birth.

• All mammals, with the exception of monotremes (the platypus and the echidna) and some sharks, are viviparous.

Viviparous animals

Boars are viviparous animals.

Embryonic developmentUNIT

4

Natural Science 2. Secondary Education

The embryo in viviparous animals

Embryo

Placenta: extracts nutrients and oxygen for the embryo from the mother’s blood.

Umbilical cord: connects the embryo to the placenta.

Amnion: forms the amniotic sac, which holds the amniotic fluid where the embryo floats.

Embryonic developmentUNIT

4

Go back to the Start menuNatural Science 2. Secondary Education

• These animals develop inside an egg, but the mother keeps the egg inside her body until it hatches.

• The embryo feeds on the food reserves contained in the egg. When the embryos are fully developed, hatching takes place, just as it does in oviparous animals.

• The young are alive when they come out of the mother.

• Certain sharks, vipers and some insects are ovoviviparous.

Ovoviviparous animals

Many sharks are ovoviviparous.

Embryonic developmentUNIT

4

Go back to the Start menuNatural Science 2. Secondary Education

UNIT

4The reproduction function

Natural Science 2. Secondary Education

POST-EMBRYONICDEVELOPMENT

Post-embryonic development

POST-EMBRYONICDEVELOPMENT

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DIRECT INDIRECT

can be

UNIT

4

Natural Science 2. Secondary Education

• The new-born individual is similar to the adult, only smaller.

• Its development consists of:

• Growth.

• Maturity of the organs.

Direct development

• Boars develop this way.

Post-embryonic developmentUNIT

4

Natural Science 2. Secondary Education

The direct development of the boar

The baby is born with a similar appearance to the adult: the difference is in its size and fur, which has some characteristic lines in the back.

During the first months of its life, the lines disappear and its fur turns a reddish colour.

As it grows, the boar’s fur gets darker becoming brown as an adult.

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Post-embryonic developmentUNIT

4

Natural Science 2. Secondary Education

• In indirect development, the new-born individual, the larva, looks completely different from the adult.

• The transformation from larva to adult takes place through a series of transformations called metamorphosis.

• In metamorphosis, the larva loses some organs and generates others.

Indirect development

• Ladybirds develop this way.

Post-embryonic developmentUNIT

4

Natural Science 2. Secondary Education

The metamorphosis of the ladybird

Fertilisation

After fertilisation the ladybird lays eggs. When the embryo

has completely developed, hatching takes place.

The larvae that are born are very different from the adults.

The larvae form a cocoon where they will complete their metamorphosis.

Once the transformation is complete, an adult individual emerges.

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Post-embryonic developmentUNIT

4

Go back to the Start menuNatural Science 2. Secondary Education