UNIT 3 PART 4: EMBRYOLOGICAL
DEVELOPMENT
• In the early stages of
development the
organism is called an
embryo.
• The basic processes
of development are
always the same in
animals.
• Development involves
cleavage, growth, and
differentiation.
1
Fertilization
• Fertilization occurs
when the sperm
nucleus fuses with the
egg nucleus.
• A fertilization
membrane forms
around the egg once a
sperm has entered to
prevent other sperm
from entering.
2
Cleavage
• After fertilization the
zygote starts to divide
by a special type of
mitosis called
cleavage to form an
embryo.
• During cleavage cells
divide like in mitosis,
but there is no growth
between divisions.
• The cells get smaller
with each division.
3
2 & 4 Cell
Embryo
• The first division
results in a 2 celled
embryo.
• Each cell in the
embryo then divides to
form 4 cells.
• Notice that all the cells
are still inside the
fertilization membrane.
4
8 & 16 Cell
Embryo
• The cells continue to
divide and form 8 and
16 cell stages.
• Note that all
structures are the
same size as the
original unfertilized
egg; the cells
produced by
cleavage have not
grown.
5
• After several rounds
of cleavage, a solid
ball of cells, called a
morula, is formed.
Morula
• The cells on the
inside have trouble
getting water &
oxygen and getting
rid of wastes.
6
Blastula
• The cells on the
inside of the morula
push their way out
and form a hollow ball
called a blastula.
• Cells still get smaller
with each round of
cleavage.
• The hollow space
inside is called the
blastocoel.
blastocoel
7
Gastrula
• A gastrula forms
when one side of the
blastula surface
pushes in at a point
called the
blastopore.
• This forms a tube that
will become the lining
of the digestive
system.
• The blastopore is the
future anus of the
embryo.
Blastopore
8
Late Gastrula
ectoderm
• By the end of the
gastrula stage, the cells
start to grow between
divisions.
mesoderm
• A third layer of cells
begins to grow between
the inner and outer
layers forming three
primary germ layers.
• The cells in these germ
layers will differentiate
to become different
endoderm
types of cells.
9
DIFFERENTIATION
• All embryos form from one cell, the zygote, by
the process of mitosis.
• This means that all the embryo’s cells are alike
- they all have all the same genes.
• As development progresses groups of cells will
become different and make tissues which form
the different organs.
• These tissues and organs are different because
one type of cell uses different genes than
another. Example: stomach cells use different
genes than eye cells.
10
The Primary Germ Layers
• Ectoderm:
– Cells on the outside
– Forms the skin and &
nervous system
Mesoderm
• Endoderm:
– Cells on the inside
lining the tube
– Forms the ling of the
digestive system
• Mesoderm:
– Cells that form
between these layers
– Forms the muscles,
bones, and organs
Mesoderm
11
Review
• How does the zygote
change after
fertilization?
• Label the cleavage
stages on the right.
• What is the blastopore?
• What is ectoderm &
what does it form?
• What is endoderm &
what does it form?
• What is mesoderm &
what does it form?
12
Starfish
Development
Composite 13