Gastrulation is the process by which a single-layered blastula develops into a multi-layered structure called a gastrula through a series of cell movements. These movements include invagination of endoderm cells to form the inner layer, convergence and divergence of surface cells toward the blastopore, and involution of prechordal and mesoderm cells between the endoderm and ectoderm. By the end of gastrulation, the gastrula contains three germ layers - an outer ectoderm layer, an inner endoderm layer, and a middle mesoderm layer.

2. What is gastrulation ?
Gastrulation is the process of development of the single layered blastula into
a double or triple layered embryonic stage called gastrula.
The three layers of cells developed in gastrula is termed as germ layers. They
are ectoderm , endoderm and mesoderm.
GASTRULA

5. Morphogenic movements : The movements of cells during gastrulation which bring out a definite shape to
embryo.
Morphogenic
movements
Epibol
y
Embol
y
Convergenc
e
Extension
Invagination
Involution
Divergence
Epiboly is the movements of cells on the
surface of the embryo.
Emboly is the movement of cells
into the embryo.

7. EVENTS IN GASTRULATION
1. INVAGINATION OF ENDODERM ( infoldings or inpushing of layer of cells into the blastocoel )
Invagination is an active infolding of blastomeres. During
invagination, few blastomeres near grey crescent are pushed
inward to form a slit or groove.
The opening of this groove is called as blastopore and the cavity
is called as archenteron. The blastopore gradually assumes a
crescentic shape. Finally it becomes circular.
The region dorsal to the blastoporal opening is called the ‘dorsal
lip’. The lower edge called the ‘ventral lip’.
Due to enlargement of archenteron, blastocoel is gradually
reduced.
The endoderm cells, enclosed by the blastopore is called the
yolk plug ( yolk plug stage)

9. 2. CONVERGENCE AND DIVERGENCE
The cells on the surface area ( the prechordal cells, the notochordal cells and the mesodermal cells) migrate towards the blastopore
and the movement is called convergence.
Once the cells reach inside the it continue the movements in the interior of the embryo.
In the interior of the embryo, the cells move in different directions from the blastopore and this movement is called divergence.

10. 3. INVOLUTION OF PRECHORDAL AND NOTOCHORDAL CELLS
Involution refers to 'rolling in' of cells.
In the beginning of gastrulation, the prechordal plate cells are situated on the dorsal lip. Infront of them, the notochordal
cells are found.
The prechordal plate materials involute over the dorsal lip of the blastopore into the interior of the embryo and move
forward along the roof of the archenteron.
Notochord materials follows.
They both together form a mid-dorsal strip on the roof of the archenteron

11. 4. INVOLUTION OF MESODERM CELLS
The mesoderm cells converge towards the dorsolateral lips of the blastopore. On reaching the blastopore lips the mesoderm cells
migrate inwards by involution, over lateral and ventral lips of the blastopore.
Once they enter inside the mesoderm cells move in an anterior direction as sheets of cells, penetrating between ectoderm and
endoderm.
On the dorsal side, the mesodermal sheet makes an close contact with the notochord.
Chordomesoderm – a continuous sheet of the notochord and mesoderm cells.
The mesoderm moves from posterior end to the anterior end ( not to the extreme) . The mesoderm free area gradually diminishes
as gastrulation process continues, where the moth is formed later.

12. 5. EPIBOLY
Epiboly means growth of one layer of cells over another.
During epiboly, micromeres of animal pole divide rapidly and move over the macromeres of vegetal pole. This layer forms
ectoderm. As a result of these morphogenetic movements, three primary germ layers are formed.
The cells which cover the gastrula externally form ectoderm.
Those involuted cells into the roof of archenteron give rise to mesoderm
Cells of sides and floor of the archenteron will develop into endoderm.
Some other internal changes are also taking place along with those morphogenetic movements. As the archenteron is enlarging, the
yolky megameres are pushed out towards the blastopore. This structure is called as yolk plug.

14. • 6. CONSTRICTION AND CLOSURE OF BLASTOPORE
• The yolk plug cells gradually withdraws into the interior. As the yolk plug cells move inward the
diameter of the blastopore steadily contracts until the blastopore is reduced into a narrow slit. The
ectoderm still continues to converge until the blastopore is closed completely.
• 7. ROTATION OF GASRULA
• As the ectoderm expands, the dorsal lip of the blastopore is shifted to an angle at 90 0 . At the same
time, the entire gastrula rotates within the fertilization membrane in a horizontal axis. As a result of
these movements, the blastopore is brought to the original vegetal pole.

16. GASTRULA CHARACTERISTICS
Gastrula is an embryo developed from the blastula.
The gastrula is oval in shape and it contains three germ-layers ,namely ectoderm,
endoderm and mesoderm.
Internally the gastrula has a cavity called the archenteron.
It opens to the exterior by the blastopore
A mass of endoderm cells protrudes through the blastopore. Thesse cells constitute the Yolk plug
The floor and the lateral sides of the archenteron are formed of the endoderm.
The roof of the archenteron is formed of chordomesoderm.
Externally the gastrula is covered by ectoderm
The ectoderm that lies on the mid dorsal line develops into The Nervous system and hence it is called
neurectoderm .
The remainder of the ectoderm is the epidermal ectoderm

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