The document summarizes key aspects of embryonic development in animals, including: - The three germ layers (endoderm, mesoderm, ectoderm) and what tissues each produces. The endoderm forms digestive tract linings, the mesoderm forms muscles and circulatory tissues, and the ectoderm forms the epidermis and nervous system. - Cleavage patterns in early embryos, including holoblastic vs meroblastic cleavage. Cleavage planes like meridional, vertical, equatorial and latitudinal are also defined. - Fate maps show the positions of presumptive tissues in early embryos, like those of amphioxus which mark areas that will form

1. II B.Sc.(Zoology)
Developmental Biology
Topics: Planes and Types of Cleavage Pattern,
Fate Map, Blastulation and Gastrulation in
Amphioxus and Frog and Organogenesis
E-Learning Study Material Prepared by
Dr. M. PAVUNRAJ

2. Cleavage
• Cleavage is initiated by the appearance of a
grooves or constriction called cleavage
furrow. The furrow appears first at one point
of the eggs. For example, in Amphioxus, the
first furrow appears at the animal pole. The
furrow then deepens and extends downward
on both side.

3. Difference Between an Animal Pole and
a Vegetal Pole
• The animal pole consists of small cells that divide
rapidly, in contrast with the vegetal pole below it. In
some cases, the animal pole is thought to differentiate
into the later embryo itself, forming the three
primary germ layers and participating in gastrulation.
• The vegetal pole contains large yolky cells that divide
very slowly, in contrast with the animal pole above it.
In some cases, the vegetal pole is thought to
differentiate into the extraembryonic membranes that
protect and nourish the developing embryo, such as
theplacenta in mammals and the chorion in birds.

6. • The two ends meet at the vegetal pole. The
furrow then extends inwards radially, finally
constricting the egg into two blastomeres.
• The cleavage furrows divide the egg at different
angles or planes. There are four main planes of
cleavage. They are as follows:
• a. Meridional Plane
If the cleavage furrow bisects both the poles of
the egg passing through the polar axis*; the
clevage plane is said to be meridional (Fig 8.5A).
[*Polar axis is the imaginary line passing from the
centre of animal pole to that of vegetal pole]

7. • b. Vertical Plane
It resembles the meridional plane because the
furrow tends to pass from the animal pole to
the vegetal pole. But it does not pass through
the median axis of the egg; it appears on one
side of the axis (Fig 8.5D).
• c. Equatorial Plane
The equatorial plane of cleavage bisects the
egg at right angles to the median axis and half
way between the animal and vegetal poles.
This type of cleavage plane is exhibited by Sea
urchin (Fig. 8.5B).

8. • d. Latitudinal Plane
Latitudinal plane cuts the egg at right angles
to the median axis; but it passes either above
(near the animal pole) or below (near vegetal
pole) the equator of the egg (Fig. 8.5C)
PATTERNS OF CLEAVAGE
There are mainly two types of cleavage. They
are holoblastic cleavage and meroblastic
cleavage.

10. • Holoblastic Cleavage
In holoblastic cleavage, the entire egg divides, It
is otherwise called total or complete cleavage. In
holoblastic cleavage, when the blastomeres are
equal in size, the cleavage is said to be equal.
• When the blastomeres are unequal, the cleavage
is called unequal
• Meroblastic Cleavage
In meroblastic cleavage, only a portion of the egg
divides. It is otherwise called partial of
incomplete cleavage. It is characteristic of
telolecithal and megalecithal eggs.

11. FATE MAP IN AMPHIOXUS
Fate map is a chart showing the fate of each cell
of any embryo. It shows the positions of the
various presumptive organs in the early embryo
itself. The fate map is essential for the correct
interpretation of gastrulation. In amphioxus, the
presumptive organ forming areas can be marked
clearly in the blastula stage.
1. All the micromers develops into the endodermal
lining of alimentary canal. Hence, the
macromeres are said to be presumptive
endoderm cells. They are all situated at the
vegetal pole in the blastula.

12. 2. The micromeres at the animal pole develops into
skin epidermis. Hence, the micromeres are called
presumptive epidermal ectoderm.
3. There is a crescent-like area on one side of the
blastula just above the equator. The cells or this
area develops into the mesoderm.
4. Opposite to the mesoderm cells, there is another
creascent like areas, the cells of which develops
into the notochord.
5. Between the notochordal cells and the epidermal
ectoderm, there is creascent like areas which
develops into the nervous system. Hence, these
cells are clled neurectoderm cells.

13. Fate Map in Frog

15. Blastulation and Gastrulation in Frog

22. • A germ layer is a primary layer of cells that forms
during embryonic development. The three germ layers
in vertebrates are particularly pronounced; however, all
eumetazoans (animals more complex than the sponge)
produce two or three primary germ layers. Some animals,
like cnidarians, produce two germ layers
(the ectodermand endoderm) making them diploblastic.
Other animals such as chordates produce a third layer
(the mesoderm) between these two layers, making
them triploblastic. Germ layers eventually give rise to all
of an animal’s tissues and organs through the process
of organogenesis.
Organogenesis - Germ layer

23. Gastrulation of a diploblast: The formation of germ layers from a (1) blastula to a
(2) gastrula. Some of the ectoderm cells (orange) move inward forming the
endoderm (red).

24. Endoderm
• The endoderm is one of the germ layers formed during animal embryonic
development. Cells migrating inward along thearchenteron form the inner
layer of the gastrula, which develops into the endoderm.
• The endoderm consists at first of flattened cells, which subsequently
become columnar. It forms the epithelial lining of the whole of
the digestive tract except part of the mouth and pharynx and the terminal
part of the rectum (which are lined by involutions of the ectoderm). It also
forms the lining cells of all the glands which open into the digestive tract,
including those of the liver and pancreas; the epithelium of the auditory
tube and tympanic cavity; the trachea, bronchi, and alveoli of the lungs; the
bladder and part of the urethra; and the follicle lining of the thyroid gland
and thymus.
• The endoderm forms: the pharynx, the esophagus, the stomach, the small
intestine, the colon, the liver, the pancreas, the bladder, the epithelial parts
of the trachea and bronchi, the lungs, the thyroid, and the parathyroid.

25. The endoderm produces tissue within the lungs, thyroid,
and pancreas.

26. Mesoderm
• The mesoderm germ layer forms in the embryos of triploblastic animals.
During gastrulation, some of the cells migrating inward contribute to the
mesoderm, an additional layer between the endoderm and theectoderm. The
formation of a mesoderm leads to the development of a coelom. Organs formed
inside a coelom can freely move, grow, and develop independently of the body
wall while fluid cushions and protects them from shocks.
• The mesoderm has several components which develop into tissues: intermediate
mesoderm, paraxial mesoderm,lateral plate mesoderm, and chorda-mesoderm.
The chorda-mesoderm develops into the notochord. The intermediate mesoderm
develops into kidneys and gonads. The paraxial mesoderm develops into cartilage,
skeletal muscle, and dermis. The lateral plate mesoderm develops into the
circulatory system (including the heart and spleen), the wall of the gut, and wall of
the human body.
• Through cell signaling cascades and interactions with the ectodermal and
endodermal cells, the mesodermal cells begin the process of differentiation.
• The mesoderm forms: muscle (smooth and striated), bone, cartilage, connective
tissue, adipose tissue, circulatory system, lymphatic system, dermis, genitourinary
system, serous membranes, spleen and notochord.

27. The mesoderm aids in the production ofcardiac muscle, skeletal
muscle, smooth muscle, tissues within the kidneys, and red blood cells.

28. Ectoderm
• The ectoderm generates the outer layer of the embryo, and it forms
from the embryo's epiblast. The ectoderm develops into the surface
ectoderm, neural crest, and the neural tube.
• The surface ectoderm develops into: epidermis, hair, nails, lens of
the eye, sebaceous glands, cornea, tooth enamel, the epithelium of
the mouth and nose.
• The neural crest of the ectoderm develops into: peripheral nervous
system, adrenal medulla, melanocytes, facial cartilage,dentin of
teeth.
• The neural tube of the ectoderm develops into: brain, spinal
cord, posterior pituitary, motor neurons, retina.
• Note: The anterior pituitary develops from the ectodermal tissue
of Rathke's pouch.

29. The ectoderm produces tissues within the epidermis, aids in the
formation ofneurons within the brain, and constructsmelanocytes.

30. Neural crest
• Because of its great importance, the neural
crest is sometimes considered a fourth germ
layer. It is, however, derived from the
ectoderm.

31. Development of Brain
• The mammalian central nervous system (CNS) is derived from the ectoderm—the
outermost tissue layer of the embryo. In the third week of human embryonic
development the neuroectoderm appears and forms the neural plate along the
dorsal side of the embryo. The neural plate is the source of the majority of neurons
and glial cells of the CNS. A groove forms along the long axis of the neural plate
and, by week four of development, the neural plate wraps in on itself to give rise to
the neural tube, which is filled with cerebrospinal fluid (CSF). As the embryo
develops, the anterior part of the neural tube forms three brain vesicles, which
become the primary anatomical regions of the brain:
the forebrain (prosencephalon), midbrain (mesencephalon),
and hindbrain (rhombencephalon). These simple, early vesicles enlarge and further
divide into the telencephalon (future cerebral cortex and basal
ganglia), diencephalon (future thalamus and hypothalamus), mesencephalon (futur
e colliculi),metencephalon (future pons and cerebellum),
and myelencephalon (future medulla).[ The CSF-filled central chamber is
continuous from the telencephalon to the spinal cord, and constitutes the
developing ventricular system of the CNS. Because the neural tube gives rise to the
brain and spinal cord any mutations at this stage in development can lead to fatal
deformities like anencephaly or lifelong disabilities like spina bifida. During this
time, the walls of the neural tube contain neural stem cells, which drive brain
growth as they divide many times. Gradually some of the cells stop dividing and
differentiate into neurons and glial cells, which are the main cellular components of
the CNS. The newly generated neurons migrate to different parts of the developing
brain to self-organize into different brain structures. Once the neurons have
reached their regional positions, they extend axons and dendrites, which allow
them to communicate with other neurons via synapses. Synaptic communication
between neurons leads to the establishment of functional neural circuits that
mediate sensory and motor processing, and underlie behaviour.

32. Development of Eye
• Eye formation in the human embryo begins at approximately three weeks into
embryonic development and continues through the tenth week. Cells from both
the mesodermal and the ectodermal tissues contribute to the formation of the
eye. Specifically, the eye is derived from the neuroepithelium, surface ectoderm,
and the extracellular mesenchyme which consists of both the neural
crest and mesoderm.
• Neuroepithelium forms the retina, ciliary body, iris, and optic nerves. Surface
ectoderm forms the lens, corneal epithelium andeyelid. The extracellular
mesenchyme forms the sclera, the corneal endothelium and stroma, blood
vessels, muscles, andvitreous.
• The eye begins to develop as a pair of optic vesicles on each side of the forebrain
at the end of the 4th week of pregnancy. Optic vesicles are outgrowings of the
brain which make contact with the surface ectoderm and this contact induces
changes necessary for further development of the eye. Through a groove at the
bottom of the optic vesicle known as choroid fissure the blood vessels enter the
eye. Several layers such as the neural tube, neural crest, surface ectoderm,
and mesoderm contribute to the development of the eye.

33. • Eye development is initiated by the master control gene PAX6, a
homeobox gene with known homologues in humans (aniridia), mice
(small eye), and Drosophila (eyeless). The PAX6 gene locus is a
transcription factor for the various genes and growth factors
involved in eye formation.Eye morphogenesis begins with
the evagination, or outgrowth, of the optic grooves or sulci. These
two grooves in the neural folds transform into optic vesicles with
the closure of the neural tube. The optic vesicles then develop into
the optic cup with the inner layer forming the retina and the outer
portion forming the retinal pigment epithelium. The middle portion
of the optic cup develops into the ciliary body and iris. During
the invagination of the optic cup, the ectoderm begins to thicken
and form the lens placode, which eventually separates from the
ectoderm to form the lens vesicle at the open end of the optic cup.
• Further differentiation and mechanical rearrangement of cells in
and around the optic cup gives rise to the fully developed eye.

34. Development of Heart
• Heart development (also known as cardiogenesis) refers to the prenatal
development of the human heart. This begins with the formation of
two endocardial tubes which merge to form the tubular heart, also called
the primitive heart tube, that loops and septates into the
four chambers and paired arterial trunks that form the adult heart. The
heart is the first functional organ in vertebrate embryos, and in the
human, beats spontaneously by week 4 ofdevelopment.
• The tubular heart quickly differentiates into the truncus arteriosus, bulbus
cordis, primitive ventricle, primitive atrium, and the sinus venosus. The
truncus arteriosus splits into the ascending aorta and pulmonary artery.
The bulbus cordis forms part of the ventricles. The sinus venosus connects
to the fetal circulation.
• The heart tube elongates on the right side, looping and becoming the first
visual sign of left-right asymmetry of the body. Septa form within the atria
and ventricles to separate the left and right sides of the heart.