EMBRYONIC DEVELOPMENT

UNIT

4The reproduction function

Natural Science 2. Secondary Education

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