This document describes the three types of embryonic development: - Oviparous development occurs when the embryo develops outside the mother's body in an egg she lays (e.g. birds, reptiles). - Viviparous development occurs when the embryo develops inside the mother's body, receiving nutrients from the placenta (e.g. mammals). - Ovoviviparous development is a combination, with the embryo developing in an egg inside the mother's body until hatching (e.g. some sharks, snakes).

1. EMBRYONIC DEVELOPMENT

2. Embryonic development
OVIPAROUS VIVIPAROUS OVOVIVIPAROUS
EMBRYONIC
DEVELOPMENT
There are three types, depending on
where it takes place

3. • 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 development

4. 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 development
Yolk

5. • 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 development

6. 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 development
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7. • 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 development
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