HS-LS1-4 Use a model to illustrate the role of cellular division (mitosis)
and differentiation in producing and maintaining complex organisms
Phases of Embryonic Development
1-Explain that different organisms have similar embryo physical
characteristics.
2-Explain how one cell becomes many cells
• •
3-How cell differentiation allows for different cells despite the
common DNA.
Warm up
Compare the embryo development in different living
things
https://www.youtube.com/watch?v=LNY6Gagxvl4
Watch the video, take active notes, trace the path of sperm reaching egg
and the names of membranes involved
Figure 47.2
EMBRYONIC DEVELOPMENT
Sperm
Zygote
Adult
frog
Egg
Metamorphosis
Blastula
Larval
stages
Gastrula
Tail-bud
embryo
6 steps of embryonic development :
1. gametogenesis
2. fertilization
3. cleavage
4. blastulation
5. Gastrulation
6. organogenesis
1- Refer to the power point and research on your
assigned stage of embryonic development
2- Discuss with your group the stages of embryonic
development
3- Support your answer with illusrtrations whenever
possible
6- Every group will swap the poster and write your
feedback on the other group work .You may add
information ,illustration or suggest ways for
improvement.
1.Gametogenesis

is a process by which the diploid germ cells
undergo a number of chromosomal and
morphological changes to form mature haploid
gametes.

Animals produce gametes directly through meiosis
in organs called gonads. Males and females of a
species that reproduces sexually have different
forms of gametogenesis:
spermatogenesis (male) in testes produce sperms.
oogenesis (female) in Ovary produce ova.


Structure of sperm
2.Fertilization:
Fertilization is the formation of a diploid zygote from a 
haploid egg and sperm.
Thus
The first function is: Transmit genes from parents to
offspring.
The second is : initiate reactions in the egg cytoplasm that
proceed development
Sperm travel through an outer layer of cells to 
reach the zona pellucida, the extracellular matrix
of the egg
When the sperm binds a receptor in the zona 
pellucida, it triggers a slow block to polyspermy
(the entry of multiple sperm nuclei into the egg )

so only one sperm fuses with the egg plasma

membrane
No fast block to polyspermy has been identified in
mammals
In mammals the first cell division occurs 1236 
hours after sperm binding
© 2011 Pearson Education, Inc.

Figure 47.5
Zona pellucida
Follicle cell
Sperm
basal body
Sperm
nucleus
Cortical
granules
3. Cleavage
Fertilization is followed by cleavage, a period of rapid cell division
without growth
 Is the process of repeated rapid mitotic cell divisions of the zygote
(unicellular structure) to form the Blastula (multicellular structure).
-The produced cells named Blastomeres.
 During this stage the size of the embryo does not change, the
blastomeres become smaller with each division.
 The type & pattern of cleavage differ from species to species.
 continues divisions to form a ball of 32 cells called the morula.
 The morula continues divisions to form the hollow blastula with up to
several hundred cells.
 The cavity of the blastula is the blastocoel.
 https://www.youtube.com/watch?v=Glk0vVdOSXE

blastula
morula
4. Blastulation


The result (end period) of cleavage.
The production of a multicellular blastula(128
cells)
The blastula is a ball of cells with a fluid-filled 
cavity called a blastocoel
 A cavity forms within the ball of the cells called
the blastocoel.
After cleavage, the rate of cell division slows and
the normal cell cycle is restored
Morphogenesis, the process by which cells
occupy their appropriate locations, involves
Gastrulation, the movement of cells from the
blastula surface to the interior of the embryo
Organogenesis, the formation of organs
5. Gastrulation





The morphogenetic process called gastrulation rearranges the cells
of a blastula into a three-layered embryo, called a gastrula
It means rearrangement of blastula cells that transforms the
blastula into a gastrula.
The blastula develops a hole in one end and cells start to migrate
and fold inward into the hole; this forms the gastrula
Characterized by cell movement.
Blastocoel is gradually disappear and a new cavity is formed
Gastrocoel.
The newly formed cavity is called the archenteron 
This opens through the blastopore, which will become the anus

Gastrulation in human Embryo
https://www.youtube.com/watch?v=dAOW
QC-OBv0&t=8s








The gastrula is a three-layered embryo
The formation of three primary embryonic germ layers
 Endoderm (inner)
 Mesoderm (middle)
 Ectoderm (outer)
The pattern of gastrulation is affected by the amount of yolk.
The cells at the vegetal pole initiate gastrulation.
Animal Pole: the pole (end) of the egg where yolk is least concentrated.
Animal hemisphere: the hemisphere of the egg where animal pole is located.
Vegetal pole: the pole (end) of the egg where yolk is the most concentrated.
Vegetal hemisphere: the hemisphere of the egg where vegetal pole is location
6. Organogenesis
Development of organs from three primary germ layers
Ectoderm forms:
skin and associated glands, nervous
system.
 Mesoderm forms:
muscles, skeleton, gonads, excretory
system, circulatory system.
 Endoderm forms:
lining of digestive tract, liver,
pancreas, lungs.

Figure 47.8
ECTODERM (outer layer of embryo)
• Epidermis of skin and its derivatives (including sweat glands,
hair follicles)
• Nervous and sensory systems
• Pituitary gland, adrenal medulla
• Jaws and teeth
• Germ cells
MESODERM (middle layer of embryo)
• Skeletal and muscular systems
• Circulatory and lymphatic systems
• Excretory and reproductive systems (except germ cells)
• Dermis of skin
• Adrenal cortex
ENDODERM (inner layer of embryo)
• Epithelial lining of digestive tract and associated organs
(liver, pancreas)
• Epithelial lining of respiratory, excretory, and reproductive tracts
and ducts
• Thymus, thyroid, and parathyroid glands