Download Extra Embryonic Membranes - Directorate of Higher Education, Tripura

Extra embryonic membrane
formation in Chick
and
their functions
By
Dr. R. C. Nath; Associate Professor
Department of Zoology
Govt. Degree College, DMR.
Fig: Chick’s egg with different regions
Definition of Extra Embryonic Membrane:
The blastoderm in birds , reptiles
and mammals not only gives rise to
embryo, but also to certain structures that
lie outside the embryo. These extra
embryonic structures are called Extra
Embryonic Membrane.
Fig: Extra Embryonic Membranes:
Types of extra embryonic
membranes
There are basically four types of extraembryonic membranes in Chick• Amnion
• Chorion
• Allantois and
• Yolk sac
Fig: Four extra-embryonic membranes
1.Amnion:
It is innermost fold of somatopleur (inner
ectoderm and outer mesoderm) above the
embryo. Between the amnion and embryo,
there is amniotic cavity filled with amniotic fluid
secreted by both embryo and amnion. Amnion
protects the embryo while amniotic fluid acts as
shock absorber and also prevents desiccation of
embryo.
Functions of the Amnion :
 Amnion protects the embryo from
shock and injury.
 Amniotic fluid prevents its
desiccation.
2.Chorion:
It is outermost fold of somatopleur and
surrounds the embryo. In reptiles, birds and
prototherians, allantochorion acts as extra
embryonic lung and helps in exchange of gases.
But in primates including human beings, only
chorion forms the placenta (chorionic placenta)
while in other eutherian, allantochorion forms
allantoic placenta.
Functions of the Chorion :
 It protects the foetus.
 Provides place for the growth of
allantois.
 Helps in the formation of the
placenta.
3.Allantois:
It is a fold of splanchnopleur developed
from the hind gut of the embryo. It is well
developed in amniotes with polylecithal egg (e.g.,
reptiles, birds and prototherians) and stores the
nitrogenous wastes of the embryo so acts as
extra embryonic kidney. In most of eutherian, it
combines with chorion to form allantochorion
which takes part in placenta formation (Allantoic
placenta). It is reduced in human beings.
Functions of the Allantois :
 Store insoluble nitrogenous waste matter, uric
acid.
 Functions as extra embryonic lung .
 Gaseous exchange taking place between
blood and external air through it.
 Carries on excretion,respiration and nutrition.
 Allantois function as a soft, elastic cushion for
protecting the embryo from shock .
 Allantois helps in a formation of umbilical
chord.
4.Yolksac:
It is formed of splanchnopleur (inner
endoderm and outer mesoderm) and is well
developed in reptiles, birds and prototherians
having poly lecithal egg. It is mainly digestive in
function so acts as extra embryonic gut. It also
absorbs the dissolved yolk and passes it to
developing embryo. In human beings, it is
vestigial.
Functions of the Yolk Sac :
 Digest the yolk .
 Transfer the products of digestion to the
embryo.
 Digestive surface increased by force off the
walls of the yolk sacs called yolk sac septa .
 In Mammals yolk sac is less nutritive organ
then Reptiles and Aves .
In the development of embryo, amnion
and chorion are closely associated, Amnion is
bag like covering over the embryo, it separates
the embryo from internal environment,
Amnion is developed from somato-pleuric
amniotic folds. These folds are head fold, lateral folds and tail folds.
a) At about 30 hours of incubation, in front of
the head of embryo a head fold is developed, it
is called amniotic head fold.
b) At about third day of incubation amniotic tail
fold is developed. It grows opposite to head fold.
c) Mean while lateral folds will develop, they
grow dorso-medially.
d) After some time head fold, lateral folds, and
tail fold will fuse near posterior end of a
embryo.
e) At 72 of incubation they are still not fused.
They show an opening called amniotic umblicus,
afterwards they unite.
f) After their union at the point of union "seroamniotic raphae" is present. It is a fold.
g) Because of this union outer chorion inner
amnion will form, because it is developed from
somatopleure. In chorion ectoderm is present
out side and mesoderm is present inside. In
amnion ectoderm is inside, mesoderm is out
side. Hence the space between amnion and
chorion is called exocoel or extraembryonic
coelome.
Fig: Chick’s developing embryo with E.E.M.
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