+i I ♦ q2i I

10/· '1. 01

• T'he· cells in the interior aie loosely connected epithelial layer and only a single layer is
to e.sch other, but at the periphery of the formed, but in birds, only some of the
blustul~ the cells adhere to each other finnly ' blastomeres are incorporated into the superficial
becsuse of cen1enting substance joining the layer, the epiblast. Some cells remain beneath
surfaces of the cells. the superficial layer.
• The amphibian blastuJa can be subdivided into • These cells lie in the space between
three main regions: the superficial layer and the yolk.
1. A reg ion around the animal pole, including • These cells unite to fonn a thin flat epithelium
the cells fom1ing the roof of the blastocoel. called hypoblast. This is separated from the
These cells correspond to the future ectodennal surface layer by a cavity. The space between
b yer. the hypoblast and the yolk is the subgerminal
.

7. A region around the vegetal pole, including

I, 111:llORASTIC (INCOMPLETI) Q£\VAGE
h e large cells in the interior which constitute the .\. Trlol,olhal
1,.._,..throu,..,.,-o1ua,
olk mass. These are the future endodermal cells
I. luiual duvagr
3. A marginal ring of cells in the subequatorial Cq,Aalopod IIIOIJwa

egion of the embryo, including the region of the

rey crescent. Cells of this zone normally form ! Diocoiiuldcmgr
fwi, rcptila, bud!
h e embryonic mesodenn.
ff. ualloloci,llal
IYoltir!.~ afccg)

lastulation in the chick embryo ~~

Mose int«u
;
.I

In some animals such as amphibiaiis, au me

blastomeres become converted into an
. ....
- ... , a( ........ piuen,, "'da.._
 • This cavity does npt appear under the whole of • The cells of the area opaca break down.the
the blastoderm b_u tj n the central part of i~. yolk to supply the embryo with food ~
• No cavity is formed 'under the layer of .. • The subgerminal cavity cannot be taken:as the
·_ blasomeres in the ·p~riphery of the blastodisc.
\ '? • .; • •~
blastocoel because a blastocoel must ha~e a
• The blastodisc is thus subdivided into two 't' roof of cells, the epiblast and a floor of cells,
.-~.·

Parts: ~ the hypoblast. '

\

• The central part of it is called the area • If the subgenninal cavity is considered as a
pellucida whilst the peripheral part is the area blastocoel, it will have an epiblast but no
opaca, which is in contact with the yolk. hypoblast since the floor would only be a mass
• Only the area pellucida contains materials for of yolk.
the formation of the embryo. • The mechanism of the formation of the

(fl) Hypoblast cells fo llowing:

dclaminating
from q,iblast • Delamination: In this case th e cells of the area
pellucida, which originally f onned the roof of
the subgerminal cavity would have a split
occurring in them.
l ?pl hl••I
• The split widens, separating the cells into the
f-lypnblA•t
8,,haJ_-,rmln11t
vnvll)I
epiblast which forms the roof and the
hypoblast which forms the floor.
PATUMAP

···> ,f,,.
.
\· ·
, , ', .
; 1o;..;, 11*' v,i_-,..,_- .,.
~ ,
c,iilCJIICIMlrcl ., -.
• Thus in the chick embryo, at the end of
b lastulation, two cavities are formed;
a) the original sub germinal cavity
L o o k ln a n-c.n1 - •
 b) the newly formed blastocoel coordinated cell and tissue movements wherebv
the cells of the blastula are dramatically .,
Theory of involution
rearranged.
• In this case some of the cells of the area
• It also involves the displacement of parts of
pellucida, at the edges boardering the area
opaca gets lifted turning downwards and the early embryo.
inwards to fqrm a floor, which becomes the • The blastula consists of numerous cells, the
hyp o blast, with the rest of the cells forming positions of which were established during
the epiblast. The hypoblast, which ultimately cleavage.
forms the extraembryonic endoderm possesses • During gastrulation these cells are given new
polarity. positions and new neighbours and the multi-
• The two-layered blastodenn of the chick has layered body plan of the organism is

the frog embryo. established

• The ep iblast is considered equivalent to the • As a result of these movements the cells that
anima l hemisphere whilst the hypoblast shares will form the endodermal and mesodennal
common fea tures with the vegetal hemisphere organs are brought to th e inside of the embryo,
of the frog embryo. while the cells that will form the skin and
GASTRULATION nervous system are spread over its outside
• After cleavage and formation of the blastula, surface.
the em bryo embarks on one of the most • Therefore the three germ layers - outer
critical periods in its development ie. ectoderm, inner endoderm, and interstitial
gas truJation m esoderm are first formed during gastrulation.
• Gastru lation is the process of highly
 10/1/20 19

• Gastrulation involves some combination of the 2. the rate at which the cells divide is reduced
fo llowing types of movements: 3 . growth becomes insignificant
1. Invagination. This is the infolding of a region 4. intensification of oxidation
of cells. s. the nuclei become active in controlling the
2. Involution. The in turning or inward activities of the embryonic cells
movement of an expanding outer layer so that it 6. there is the synthesis of new proteins
spreads over the internal surface of the Fate maps
remaining external cells.
• It has been shown that specific regions of the
3. In g ress ion : The migration of individual cells early embryo contribute to the fo~ation of
from the surface layer into the interior of the different tissues and organs in the adult
embryo.
• It is oossible to stain different oarts of the

4. D elnmination : The splitting of one cellular developing zygote with different dyes.
sheet into two or more parallel sheets. The These dyes do not kill the cells and therefore do
resul ts is the fo m1ation of a new sheet of cells not arrest the process of development
5. E piboly : The movement of epithelial sheets • As each region is stained it is possib le to
(usua lly of ectodermal cells) that spread as a fo llow the fate of each region.
uni t. rather than individually, to enclose the • Having followed the fate of several regions , it
deeper lnyers of the embryo. is possible to construct a fate map i.e project
• During gostrulation, a number of processes back to the blastula, marking out what each
tnkc p ince: region would ultimately develop into.
I. Re-arrnngement of cells by mea ns of • A fate map is a chart showing the fate of each
morphogenctic movements. part of an early embryo, particularly a blastula.
 remains of the blastocoel.
• The different identifiable regions of the fate • As invagination contin~es the bl~stocoel
map are known as the presumptive organ becomes smaller until 1t finally disappear.
forming areas. **** • The external lining consists of the presumptive
G·a strulation in Amphioxus epidermis and presumptive nervous system.
• During gastrulation, the three germ layers of a • The internal lining consists of presumptive gut
triploblastic animal are formed. material ie the endoderm.
• Gastrulation in Amphioxus is initiated with the • The presumptive material of mesodenn and
flattening of the cells in the vegetal notochord initially lie on the rim of the cup,
~emisphere, which subsequently bends but soon they shift to occupy a position on the
1nw~ds, so that the embryo instead of being internal wall of the cup.
spherical becomes converted into a cup-like

• The presumptive notochord, mesoderm and

structure. endodenn disappear from the surface of the
• The flattened cells begin to push inwards in a embryo into the interior, where they belong.
process called invagination. · • The external surface now consists of the
• Invagination continues until a double walled ectoderm.
cup is f onned; an external and internal wall.
• The embryo at this stage is called a gastrula
• The internal wall lines the newly formed and the cavity that results after invagination is
cavity. the primary gut or archenteron.
• The external and internal epithelial layers are • The opening of the archenteron to the outside
continuous with each other. is called the blastopore.
• At this stage there is still a space between the • The b laStopore also marks the pathway by
external and internal wall, represented by the
 which the cells of the mesodenn and
endoderm pass into the interior of the embryo. Gastrulation in Amphioxus
• ~e blastopore, being the opening into the
~nmary gut has been likened to a mouth, its E<todcrm

nms are therefore called the lips of the

b lastopore.
• The blasopore is broad initially, but the lips
begin to contract so that the opening becomes
Endodcrm Archtntfflln
smaller.
• When the presumptive notochord and
mesodenn shift into the interior of the embryo

they change their position relative to each other. the elongation of the embryo.
• During gastrulation the embryo rotates so that • The presumptive material of the nervous
the future dorsal surface comes to lie system becomes stretched into a longitudinal
uppermost , with the ventral side below. band of cells lying over the notochordal
• There is the stretching of the presumptive material.
organ-forming areas in the anterior-posterior Gastrulation in the frog
direction to make the embryo elongated. • Gastrulation in the frog embryo is modified by
• This brings mesoderma1 and notochordal the large yolky cells in the vegetal hemisphere.
materials closer together. • At the beginning of the process a groove or
• The external wall of the gastru1a takes part in depression fonns at the boundary between the
grey crescent and vegetal hemisphere by
 invagination.
• By the time the blastopore has the form of a
• The cells at the bottom begin to stream into ring the cells of the vegetal region fill the
the interior of the embryo toward the space enclosed by the edge of the blastopore.
blastocoel.
• The cells fonn the yolk plug.
• This process marks the beginning of the
formation of the blastopore. • The rim of the blastopore contracts and finally
covers the yolk plug.
• The groove begins to spread transversely; its
lateral ends are prolonged along the boundary • Thus the material in the vegetal hemisphere
disappear into the interior of the embryo.
between the marginal zone and the vegetal
region until they meet, thus encircling the • The yolk plug disappear from v iew and the
vegetal region. blastopore is said to be closed.
• During gastrulation the first group of cells to

• In the animal hemisphere the yolk is not much,

thus the cells there divide faster than those of AUMlul 1'0it (AP) (BJ
the vegetal hemisphere. -AP-...... ( C) Don.al D\CIOdmn

Ard 1c,11,ron
• The cells of the animal hemisphere become Dor-..t
bl.stopore AP
lip Mooocnn
many and therefore begin to flow down
E.ndod,.rm
towards the blastopore.
• They converge at the dorsal lip and move rsal
(E)

inside. 'O<krm

rAI -.:: · Notochord

of
• The process of flowing down of the animal oporc
~- .
)~:- Dorsal
blaaopo,c lip
Dor-..1
blutopo1< lip

ceIIs and their subsequent involution is known ior

crm -·' Ulen!
bl&stoporc lip
1
as epiboly. ,
f
·•••~·rna cndomcaodcrn1 Vc.mral
bLutopur~ lip
\'olk pl'II
IIIOOdcnn

Flgu,- 10.1
Cdl -.11e111, Jun11g fro11 pstru1'1ion. ~ ~•idioulM:<li.<in.U{c' Clll.llu:ci!.d>..!!l.,
 • A number of processes are important .
• In order for neu.raJ induction to occur, the morphogenes .is o f t h 1n
' layer of cells. \n tht
ectoderm overlying the notochordal process
must be able to respond to the inductive l. Epit~elial (oldi~g thi~ is u~ually preceded b'i
stimulus i.e it must be competent. elongation of c~lls 1n a direction perpendicu\ar
• During much of the period of gastrulation in to the exposed surface, a process called
the amphibian embryos, both the dorsal and palisading.
ventral ectodenn have the competence to form • When the folding is inward from the_free
neural tissue when subjected to the influence surface it is called invagination . @=~-_ ;
of inductors. • When the folding is outward from the s~ace·
• Later in gastrulation the ectoderm farthest or lumen, it is called evagination.
from the normal location of nervous tissue __.ft__
2. Detachment of cells from each other. (0)
. begin to lose their capacity to respond to

neural inductors, and late in the neurula stage Epithelial layers or cell m ass may be
most non-neural ectoderm lose their subdivided into parts by the appearance of
competence. gaps/crevices between groups of cells.
• Fol lowing changes leading to the formation of • The cells on the opposite sides of the crevice
1

the neural tube, the mesoderm on either side of lose connect ion with one another.
the notochord splits into longitudinal • The original crevice may be increased by the
d ivisions. secretion of fluid into it, becoming a spacious
• The blocks of mesodenn on either side of the cavity.
notochord beg in to form symmetrical pairs of 3. Thickening of epithelial layer
brick-like masses called somites.
• A very common method of organ formation
• The thickening is brought about by the
 E-......,.
\
· ·-

~-
........ ,,....
-\-
,,_ \
Primary (neural) Induction
• During late gastrulation, the chordamesoderm
pushes cranially just beneath the ectoderm.
~ 1 ,--=::::: • While the forward movement of the
£'"'1)--.,..._.,. ~ E~
notochordal tissue is taking place, the
s.,,,....,_,,,, notochordal cells act on the overlying
-~,._.,,,
! ~ ectodermal cells, causing them to thicken to
.__,! I
form the neural plate.
• This reaction, which initiates the formation of
the central nervous system, causing the central
longitudinal axis of the body to b e established

is called primary induction.

n~ un1 I fo l ds
undergor fus ion • The inducer or inductor is the
chordamesoderm and the responding tissue is
the ectoderm.
• Like other inductive systems, it is essential
that the inductor and responding tissue be at
the right place at the right time.
• Without the presence of the underlying
not o c ho rd
~ notochord, the cells of the dorsal ectoderm do
not form neural tissue but rather continue to
om lte \ qui differentiate as general cutaneous ectoderm.
~• n Jodt~ nn
 elongation in longitudinal direction and NEURULATION
contraction m· the '-ans
u· verse d.1rect1on.
.
• Neurulation is the process whereby the
• The notochordal material becomes ~ . neura\
plate becomes trans1ormed into the dorsal
concentrated on the dorsal side of the embryo. hollow tube, the neural tube.
• Mesoderm: The mesodermal cells roll over
the lateral and ventral lips of the blastopore • The embryo at this stage is called a neurula.
• During this time, the primordia of the brain,
• Inside th e embryo they arrange themselves spinal cord and neural crest become
between the ectoderm and endodenn.
internalized and completely segregated from
• Endoderm: In the blastula, the presumptive the rest of the surface ectoderm.
endodenn is partly found in the marginal zone • The future development of the embryo
and partly in the vegetal region.
depends on the inductive interactions between

• Cells in the vegetal region become covered by some of these newly associated groups of cells.
the contracting rim of the blastopore.
• The primary inductive event is the action of
• The endodermal cells at the bottom are later the chordamesoderm or notochord on the
found in the duodenal region of the embryo. overlying ectodenn, resulting in the
• The endoderm of the vegetal region passes transformation of a band of unspecialized cells
into the interior and comes to lie in the floor of into the primordium of the central nervous
the archenteron. system.
• This layer of endodenn is very thick, reducing • The initial response of the induced ectodenn is
the cavity of the archenteron. to form a plate of thickened cells, but soon
.. 'rh; c, n,:,rrAn T n~rt of the archenteron fonns the
after this the plate becomes transformed into a
1nnrr;hu11n~ 1 oroove and finally into a plate.
 10/1/20

Frog gastrulation. added

. · complexity
but similar mechanisms or archenteron.
2. Involution of Maren
e.na I mne cells • After involution the cells move away from the
(A)
(B)
Animal pole (AP)
blastopore by a process called divergence.
• By the end of gastrulation the blastopore has
Blastocoel
been reduced to a slit.
around dorsal lip
Superficial
cells
The ectoderrn
Dorsal • The material of the animal region, the
blastoporc
\ lip
presumptive epidermis and presumptive
Marginal Zone Cells Bottle cells \ nervous system covers the whole of the
\ inside MZ embryo after the mesoderm and endoderm
outside '.\IZ
have moved inside.

reach the blastopore form the dorsal lip. • The presumptive epidermis expands in all
• Cells of the ventral side also flow towards the directions, but in the case of the presumptive
b las to pore and when they reach, form the . nervous system, the expansion is mainly in the
longitudinal direction.
ventral lip.
• In the transverse direction the presumptive
• Som e of the cells of the dorsal lip flow down
nervous system contracts.
to the sides to form the lateral lips.
Notochord: the cells of the notochord roll over
• Once the blastopore is formed, the cells of the
the dorsal lip ofblastopore into interior of the
presumptive organ-forming areas roll over and
embryo and becomes stretchered along the
mov e inside· this is known as involution.
' dorsal side of the embryo.
• Soon a cavity is fonned, beginning from the • The presumptive notochord undergoes
b lastopore, widening to form the gastrocoel
·epithelial layer flowing toward th .
. e region where of the primitive gut.
th e organ 1s to appear.
• For example, in the formation of the neural • The blastopore decreases in size and finally
closes.
pl~te, the epithelial thickening is towards the
mid-dorsal region of the embryo. • During the latter stages of gastru\ation the
process of primary induction is coming to an
4 . Fus ion of p reviously separated parts
end and the ectoderm which overlies the
• This p lays a cntical
·· - morphogenesis
role 1n notochord has begun to respond hy thickening
• For example, the edges of the neural folds fuse to form the neural l)\ate.
along th e dorsal midline to form the neural • The neural plate assumes the shal)e of a keJ
plate. hole, being wider at the anterior end where the
5 - Breaking up of epithelial layers to produce future brain will develop .

mesenchyme: This process makes materials • The margins of th e neural plate then become~
available for the development of organ thickened and raised to form the neural fo\ds
rudiments. • The neural folds then converge in the dorsa\
• In this case an epithelial layer or a portion of it midline and fuse to form the neural tube.
may break up into mesenchym e so that the • At the same time as the neural folds meet each
epithelium d isappears, or a gap is left in the other , the embryo elongates a\ong the
previously continu ous layer eg the formation anterior-posterior axis.
of the neural crest. • In the interior of the embryo, the primitive gut
Neurulat ion in the Frog becomes surrounded b)' endodermal cells.
• The latter stages of gastrulation are dominated • Surrounding the endoderm is a complete \a;1er
by cell movements that lead to the formation of mesodenn.
 Neural crest
• The notochord at this stage is a discrete rod in
• After the formation of the neural tube, it
the dorsal mid-line.
detaches itself from the overlying epidermis
• On either side of the notochord, the dorsal part
of the mesodennal sheet (epimere) thickens • There remains a group of cells that do not ·
and undergoes segmentation. form part of the neural tube or epidennis.
• The p aired mesodermal segments later become • These cells are the neural crest cells.
the somites. • Initially the neural crest cells may be
• The broad mass of early mesoderm(hypomere) continuous across the midline, but later the
located on the lateral and ventral parts of the mass of cells separate into the left and right
embryo is called the lateral plate mesoderm. halves.
• Later on the lateral plate mesoderm splits into • The neural crest cells have the ability to

two layers: an outer layer attached to the migrate throughout the body.
ectodenn (parietal or somatic mesoderm) and an • In the head region the cells of the neural crest
inner layer surrounding the endoderm known as migrate as a unit, whereas in the trunk the cells
visceral or splanchnic mesoderm move individually.
• Th e somatic mesodenn and the overlyinj!
ectodenn are collectively called the (!!!JI= • The cells leave the neural crest in 2 streams:
• One group is superficially and dorsally
somatopleure.
directed; the other is ventrally directed.
• The splanchnic mesoderm and the underlying
• Cells that follow the dorsal pathway move into
endodem are collectively called
the ectoderm, differentiate into pigment cells
splanchnopleure.
and settle either in the epidermis or dermis.
• The cavity that forms between the 2 layers of
• Cells that move ventrally develop into some
 components of the nervous system.
• Other cells remain close to the original neural
crest but become aggregated into dorsal root
ganglia of the sensory nerves.
• Cells from the cranial region of the neural
crest form much of the musculature and
connective tissue of the face.