STUDY OF CLEAVAGE
BLASTULA, GASTRULA

• Cleavage and Blastula Stage
• The development of multi-cellular organisms begins from a single-celled
zygote, which undergoes rapid cell division.
• The rapid, multiple rounds of cell division are termed cleavage.
• After the cleavage has produced over 100 cells, the embryo is called a
blastula.
• The blastula is usually a spherical layer of cells (the blastoderm)
surrounding a fluid-filled or yolk-filled cavity (the blastocoel).
• Mammals at this stage form a structure called the blastocyst, characterized
by an inner cell mass that is distinct from the surrounding blastula.
• During cleavage, the cells divide without an increase in mass; that is, one
large single-celled zygote divides into multiple smaller cells.
• Each cell within the blastula is called a blastomere.

• Cleavage can take place in two ways: holoblastic (total) cleavage or
meroblastic (partial) cleavage.
• The type of cleavage depends on the amount of yolk in the eggs.
• In placental mammals (including humans) where nourishment is provided by the
mother’s body, the eggs have a very small amount of yolk and undergo
holoblastic cleavage.
• Other species, such as birds, with a lot of yolk in the egg to nourish the embryo
during development, undergo meroblastic cleavage.
• In mammals, the blastula forms the blastocyst in the next stage of development.
• Here the cells in the blastula arrange themselves in two layers: the inner cell
mass and an outer layer called the trophoblast. The inner cell mass is also
known as the embryoblast; this mass of cells will go on to form the embryo.
• At this stage of development, the inner cell mass consists of embryonic stem cells
that will differentiate into the different cell types needed by the organism.
• The trophoblast will contribute to the placenta and nourish the embryo.

The typical blastula is a ball of cells. The next stage in embryonic development is the formation of the body plan.
The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as
gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these
layers is called a germ layer, which differentiate into different organ systems.
Differentiation of germ layers: The three germ layers give rise to different cell types in the animal body: the
ectoderm forms the nervous system and the outer layer of skin, the mesoderm gives rise to muscles and
connective tissues, and the endoderm gives rise to the lining of the digestive system and other internal organs.
The three germs layers are the endoderm, the ectoderm, and the mesoderm. The ectoderm gives rise to the
nervous system and the epidermis; the mesoderm gives rise to the muscle cells and connective tissue in the body;
and the endoderm gives rise to columnar cells found in the digestive system and many internal organs.

• THE PLANES OF CLEAVAGE :
a. Meridional plane of cleavage:
• When a furrow bisect both the poles of the egg passing through
the median axis or centre of egg it is called meridional plane of
cleavage. The median axis runs between the centre of animal pole
and vegetal pole.
b. Vertical plane of cleavage:
• When a furrow passes in any direction (does not pass through the
median axis) from the animal pole towards the opposite pole.
c. Equatorial plane of cleavage:
• This type of cleavage plane divides the egg halfway between the
animal and vegetal poles and the line of division runs at right
angle to the median axis.
d. Latitudinal plane of cleavage:
• This is almost similar to the equatorial plane of cleavage, but the
furrow runs through the cytoplasm on either side of the equatorial
plane.

1. Holoblastic or total cleavage:
• When the cleavage furrows divide the entire egg.
• It may be:
a. Equal: When the cleavage furrow cuts the egg into two equal
cells. It may be radially symmetrical, bilaterally, symmetrical,
spirally symmetrical or irregular.
b. Unequal: When the resultant blastomeres become unequal in
size.
2. Meroblastic cleavage: When segmentation takes place only in
a small portion of the egg resulting in the formation of
blastoderm, it is called meroblastic cleavage. Usually the
blastoderm is present in the animal pole and the vegetal pole
becomes laden with yolk which remains in an uncleaved state.
3. Transitional cleavage: In many eggs, the cleavage is atypical
which is neither typically holoblastic nor meroblastic, but
assumes a transitional stage between the two.

TYPES OF BLASTULA
1. Coeloblastula
It is a hollow blastula having a huge spacious blastocoel.
•Equal coeloblastula
The blastula resulting from holoblastic equal cleavage is
called equal coeloblastula. For example, in echinoderms
and amphioxus. In this instance, the blastoderm is single-
layered.
•Unequal coeloblastula
Holoblastic unequal cleavage, as in frogs, causes unequal
coeloblastula. It has a blastocoel displaced in the direction
of the animal pole and a multilayered blastoderm.
2. Stereoblastula
This kind of blastula is composed of an
aggregate of a bigger sized and reasonably
lesser number of cells without or with a very
small blastocoelic space in the center.
Ex: such as insects, some worms like Nereis,
mollusks like Cripidula, Gymnophiona,
amphibians, and certain fishes.

4. Peri blastula
It is a stage in the embryonic development of most
arthropods having centrolecithal eggs. The peri blastula is
a vesicle whose wall consists of one layer of cells and
whose cavity is full of unbroken yolk. It develops as a
result of the surface segmentation of the egg.
3. Disco blastula
Disco blastula consists of a disc-shaped mass of
blastomeres overlying a large yolk mass. This blastula
is the result of meroblastic discoidal cleavage as in
many fishes, reptiles, and birds.
There is no blastocoel, instead, a slit-like cavity called
sub germinal cavity appears in between the blastoderm
and the yolk mass.

A. CLEAVAGE OF AMPHIOXUS
• During the earliest cleavages in Amphioxus, growth does not occur.
• In fact, as nutritive materials are used to power the earliest processes,
the embryo may actually decrease in size. Cleavage planes pass
entirely through the egg.
• Conklin in 1932 studied the fate map of Amphioxus.
•The egg of amphioxus is isolecithal and the cleavage is of holoblastic
type.
• About one hour after fertilization, the egg is divided into 2
blastomeres.
•The plane of this first cleavage is passing through the egg axis from
pole to pole.

A. CLEAVAGE OF AMPHIOXUS
•Next, the cleavage is in a vertical plane, but at right
angles to the first plane.
•In the third series of division, the planes of separation
are horizontal. By the 16 to 32-cell stage, the cells are
crowded together into a compact mass within the zona
pellucida. The embryo is now known as morula.
•Soon the blastomeres arrange themselves in a circular
manner forming a blastula around a central space, the
blastocoel. This marks the end of the cleavage period.

B. Amphioxus (Coeloblastula):
• The blastocoel is large and lies in the centre.
• The blastomeres surrounding the blastocoel
are many in number and small in size.
• It is the type of blastula found mainly in
isolecithal type of egg.
• The blastomeres are not exactly of equal size.
• They get arranged in a single layer.
• Blastomeres towards vegetal pole are larger
than those towards animal pole.

C. Gastrula of Amphioxus:
• Gastrulation:
• In Amphioxus, gastrulation is started when there are about 800 cells
in the embryo, i.e., between 9th and 10th cleavage. The animal and
vegetal poles are the future ectoderm and endoderm respectively.
• It involves two basic types of morphogenetic movements of the
embryonic cells:
• 1. Epiboly:
• It is the expansion of ectodermal cells of the embryo.
• 2. Emboly:
• The blastoderm at the vegetal pole (endodermal plate) becomes flat
and subsequently bends inwards (invagination). Thus, the embryo,
instead of spherical becomes converted into a cup-shaped structure,
having a large cavity, the archenteron, opening outside by a wide
blastopore. Both the processes go on side by side.

1. Cleavage of Frog
In frog's egg the cleavage is holoblastic and
unequal. The cleavage occurs as follows.
1. The first cleavage plane is meridional.
Initially, a furrow appears at the animal pole. It
gradually extends towards the vegetal pole of the
egg. It cuts the egg through its median animal-
vegetal polar axis and results in two equal-sized
blastomeres.
2. The second cleavage furrow is
again meridional. It bisects the first cleavage
furrow at right angles. It is a holoblastic cleavage
affecting both the blastomeres of the first cleavage.
It results in the formation of four blastomeres.

Cleavage of Frog
3. In the next stage a latitudinal furrow is formed
above the horizontal fur-row nearer to the animal
pole. Such a furrow is due to the influence of yolk
concentration in the vegetal pole. The latitudinal
furrow uniformly affects all the blastomeres. It
results in the formation of eight blastomeres. Four
of them remaining in the vegetal pole are large.
They are named as macromeres.
Another four blastomeres remain in the vegetal
pole. They are named as micromeres. The
micromeres are smaller in size than the
macromeres.

4. The fourth set of cleavage planes are meridional and
holoblastic. They are unequal. They divide yolkless
micromeres more rapidly than yolk-rich macromeres.
These cleavages result in the production of 16 blastomeres.
2. Frog (Coeloblastula):
• It develops from telolecithal egg.
• The blastocoel is eccentrically placed and is nearer the
animal pole.
• This is due to unequal cleavage.
• The blastomeres are arranged in a layer surrounding the
blastocoel. It is unevenly thick.

• The blastoderm is multilayered.
• The blastomeres of the animal pole are small and are called micromeres, while those
of vegetal pole are large and are called megameres.
• The ultimate blastula is a ball of blastomeres which have to form different embryonic
body layers and organs of the body.
• The fate of each and every blastomere has been observed and marked. A map
showing various organ forming, areas on the blastula is called the ' fate map'. This
map shows prospective ectoderm, mesoderm and endodermal areas.
• It also shows the ' zone of involution' and ' zone of invagination' for the next stage
of gastrulation.

3. Gastrula of frog:
• It is the stage of embryonic development in which three germinal layers ectoderm,
mesoderm and endoderm are formed.
• In frog this involves peculiar movements of blastomeres and formation of new cavity.
• Gastrula of frog has the following main stages.
• i. Epiboly: First the micromeres from animal pole divide rapidly and spread all over
the embryo except in the region of grey crescent. Here a groove is formed between
micromeres and megameres.
• The opening of this groove is called blastopore, with upper side called dorsal lip. The
cavity enclosed by it is called archenteron.

ii. Emboly: Cells from dorsal lip (micromeres) migrate inside the archenteron
forming roof of archenteron. This process is called Involution.
• They form future notochord and mesoderm.
• The megameres divide and reduce the blastocoel cavity.
• Finally micromeres form ectoderm on the outer side of the embryo while the
blastomeres inside differentiate to form mesoderm and endoderm.

1.Eggs Of Birds(Cleavage)
• Eggs of birds are also telolecithal(In telolecithal
eggs, moderate or large quantity of yolk is present
but the distribution of yolk is not uniform. The yolk
is concentrated more towards the vegetal pole).
• The amount and mass of yolk which they contain is
much greater than that in Amphibian eggs.
Therefore the yolk is not divided.
• The process of segmentation is limited to the small
disc of protoplasm lying on the surface of the yolk
at the animal pole.
• This is referred to as discoidal cleavage.
• The fact that the whole egg is not divided is
indicated by designating the process as partial
(meroblastic) cleavage.
• {(holoblastic) seen in eggs containing less yolk.}

• In the egg of a bird which is about to undergo cleavage, the disc of active
protoplasm at the animal pole (blastodisc) is a whitish, circular area.
• The central portion of the blastodisc is surrounded by a somewhat darker
appearing marginal area known as the periblast.
• It is in the central area of the blastodisc that cleavage furrows first appear.

2. Bird blastulation:
• The morula condition is of short duration.
• Almost as soon as it is established there begins a
rearrangement of the cells for the formation of
the blastula.
• The space thus established between the
blastoderm and the yolk is termed the
segmentation cavity (blastocoele).
• The marginal area of the blastoderm in which
the cells remain undetached from the yolk and
closely adherent to it, is called the zone of
junction.
• The process of blastulation also is modified by
the presence of a large amount of yolk.
• The cells of the central region of the blastoderm
are nevertheless separated from the yolk to form
a small blastocoele.
• The yolk constitutes the floor of the blastocoele.

3. Bird Gastrulation:
Gastrulation includes the following two types of morphogenetic
movements:
a. Emboly(growth):
It involves only the epiblast which contains cells of
ectoderm, mesoderm and notochordal cells. Formation of primitive
streak and head process is due to emboly.
b. Epiboly:
It includes the overgrowth of ectoderm or epiblast and also of
hypoblast (endoderm).
• As a result of gastrulation the chick embryo shows the formation
of primitive streak.
• The primitive streak is formed by movement of cells of blastoderm
towards the caudal end.
• The definite primitive streak is formed by 18 hrs. of incubation.

• In between ectoderm and endoderm, the mesoderm
is formed by involution.
• The embryo shows the lateral sheet of mesoderm
spread over the yolk.
• It is in the form of area pellucida inner to which
area opaca is seen.
• From the caudal end of area pellucida the primitive
streak grows and develops towards the medial part
of blastoderm.
• Primitive streak at its anterior end has swollen,
bulb like region called Henson’s node.