Phases of Embryonic Development
Abdelalim Gadallah (Ph.D.)
Lecturer of Comparative Anatomy and Embryology
Zoology Department, Faculty of Science,
MANSOURA UNIVERSITY
EGYPT
7 steps:
1. gametogenesis
2.
3.
4.
5.
6.
7.
fertilization
cleavage
blastulation
gastrulation
neurulation
organogenesis
1.Gametogenesis
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is a process by which the diploid germ cells
undergo a number of chromosomal and
morphological changes to form mature haploid
gametes.
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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.
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Structure of sperm
Structure of ovum of sea urchin
Common terms
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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 located.
2.Fertilization:
is the process whereby two sex cells (gametes) fuse together
to create a new individual with genetic potentials derived
from both parents.

Have two separate activity:
1.
Combining of genes derived from the two parents.
2.
Creation of new organisms.
Thus
The first function is: Transmit genes from parents to
offspring.
The second is : initiate reactions in the egg cytoplasm that
proceed development. Also,
Restoration of the diploid number of chromosomes reduced
during meiosis.
It have 4 major steps:
1.
2.
3.
4.
Sperm contacts the egg
Sperm or its nucleus enters the egg, and
regulation of sperm entry .
Sperm and egg nuclei fuse
Egg becomes activated and developmental
changes begin
3. Cleavage
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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.
Figure 47.8x Cleavage in a frog embryo
4. Blastulation
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The result (end period) of cleavage.
The production of a multicellular blastula
Blastula cells are called blastomeres.
A cavity forms within the ball of the cells called
the blastocoel.
Blastula of frog
Sea urchin blastula
Human blastula
Starfish development, unfertilized egg.
16 blastomeres.
2 blastomeres.
32 blastomeres.
morula
4 blastomeres.
Starfish development,
nonmotile
blastula.
5. Gastrulation
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The morphogenetic process called gastrulation
rearranges the cells of a blastula into a threelayered (triploblastic) embryo, called a
gastrula, that has a primitive gut.
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 into the hole; this forms
the gastrula
Characterized by cell movement.
Blastocoel is gradually disappear and a new
cavity is formed Gastrocoel.
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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 invaginate, initiating
gastrulation.
Gastrulation in a Frog Embryo
Figure 47.10 Gastrulation in a frog embryo
6. Neurulation
Chordates Only
formation of a dorsal, hollow neural tube
by
 ectodermal cells flatten into neural plate
 the center of the plate sinks forming neural groove
 edge of plate is elevated to form neural folds
 neural folds fuse and form neural tube
 anterior end develops into brain
 posterior end develops into spinal cord
Neurulation
The
Neural
Crest
 The neural crest is a

critical structure that
guides formation of several
organ systems
The neural crest forms on
either side of the point of
fusion
 Its
cells migrate to form the
dorsal root ganglia, the
postganglionic sympathetic
neurons, many sense organs
and all pigment-forming
cells
Organogenesis
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Organogenesis is the formation of the organs
The layers are germ layers; they have specific fates
in the developing embryo:
Endoderm
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The innermost layer
Goes on to form the gut
Mesoderm
The middle layer.
Goes on to form the muscles, circulatory system, blood
and many different organs
 Ectoderm
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The outermost
Goes on to form the skin and nervous system
Organogenesis Begins With
Development of the Nervous System
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The nervous system is the first
organ system to develop.
The notochord grows and
induces overlying ectoderm to
form the neural plate.
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Cells of the neural plate fold to
form the neural groove and
the surrounding neural folds
fuse to form the neural tube.
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The anterior portion forms the
brain; the rest forms the spinal
cord.
7. 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.
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Figure 47.11 Organogenesis in a frog embryo
Somatic &Germ Cells
Somatic Cells:
Found in all body tissues except gonads.
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Contain diploid numbers of chromosomes (2N).
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Replacement of dead cells
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Reproduce by mitotic division.
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Functions:
1.
Responsible for formation of different system and
organs.
2.
Have other specific functions
e.x.: muscular system have myoplast for contraction and
relaxation .
nervous system have neurons for transmission of
impulses.
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digestive system have secretory cells for secretion of
enzymes for digestion.
bone have osteoplasts make hardening of bone.
Lung have cells for respiration.
Germ Cells:
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Found only in gonads (testes & ovary)
Contain haploid number of chromosomes (1N)
Reproduce by meiotic division (meiosis).
Function : Formation of gametes (male & female)
Primordial germ cells
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Appears in the wall of the endodermal layer of the
yolk sac due to their large size and high content of
alkaline phosphatase , and migrate by amoeboid
movement toward the hind gut epithelium and
then through dorsal mesentery reach to the
primordia of the gonads ( primitive sex glands).
Become recognizable at 24 days post-fertilization.
Invading the genital ridges in the 6th week of
development.
The primordia of the gonads give a chemostatic
attractant for the germ cells.
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Also may be the germ cells directed toward the
gonads by the mature of cellular and non-cellular
microenvironment that surround them.
Or by tissue rearrangement that occur in the early
embryos.
If they fail to reach the ridges, the gonads will not
develop.
So, it have the inductive influence on the
development of gonads into ovary and testes.
Determine sex of the embryo.
They will form different stages of spermatogenesis
and oogenesis in the future.
Found in adult in gonads (testes and ovary)
Basic Developmental Vocabulary
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Fertilization – activates egg & brings together the
nuclei of the egg and sperm.
 Cleavage partitions the zygote into many smaller
cells.
 Gastrulation rearranges the blastula to form a
three-layered embryo with a primitive gut, the
archenteron.
 Organogenesis is the process by which the organs
in the animal body form from the three embryonic
germ layers.
Basic Developmental Vocabulary
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Blastula – a hollow sphere of cells (128 cells) formed by
cleavage of the morula. The blastula contains the blastocoel
that is fluid-filled. The blastopore is the place where
gastrulation begins.
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Gastrulation – the process leading to the creation of the
primitive gut or archenteron. Invagination at the blastopore
results in the gut.
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Gastrula – transformation of the blastula into an embryo
possessing 3 germ layers, ectoderm, mesoderm, endoderm.
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Morula a solid mass of 16-64 cells formed by cleavage.
Thank you for your time.
Questions?
Dr. Abdelalim Gadallah