The document discusses the four main types of extra-embryonic membranes that develop in birds during embryonic development - the yolk sac, amnion, chorion, and allantois. Each membrane has a distinct structure and function. The yolk sac surrounds and digests the yolk. The amnion forms a fluid-filled cavity that protects the embryo. The chorion forms the outermost layer where gas exchange occurs. The allantois acts as a respiratory organ and aids in waste removal. Together these membranes provide protection, nutrition, respiration and excretion to support the developing embryo.

1. Shri Shankaracharya Mahavidyalaya, Junwani , Bhilai
Extra –embryonic membrane in chick
Dr. Sonia Bajaj
(Head of Department)

2. Extra embryonic membrane: These are the membranes which do not
form any part of the embryo proper but performs various functions which assist in
the development of the embryo. These are discarded at the time of hatching.
These membranes formed outside the embryo .
Amniotes: These are the vertebrates group whose eggs contain extra
embryonic membranes for protecting the embryo. They lay eggs on the land.
Therefore ,in chick and other amniotes the following extra embryonic membrane
develop in them for the protection, respiration and nutrition of the embryo.
Example: Reptiles, Birds and Mammals.
An amniotes: These are the vertebrates group whose eggs do not contain
extra embryonic membranes during embryonic development. They lay eggs in the
water. Example: Fish, Amphibian.

3. Extra embryonic membranes in chick
In chick, the presence of an enormous amount of yolk and embryonic life to
be spent within a shell is correlated with the development of extra-embryonic
membranes. Original blastoderm is a small disc, which spreads by peripheral
expansion and eventually covers the entire surface of the egg. But only the
most central region is directly connected with the formation of the embryo
proper. All the remainder of the blastoderm is extra-embryonic.
There are four types of extra embryonic membranes in birds-
1. Yolk sac.
2. Amnion.
3. Chorion.
4. Allantois

4. Extra embryonic membrane:
These are the membranes which do not form any part of the embryo proper but
performs various functions which assist in the development of the embryo. These
are discarded at the time of hatching. These membranes formed outside the
embryo.

5. 1. Yolk sac: It is the first structure to develop among the embryonic
membrane. In 16 hours stag, the embryonic entrones is situated over the
yolk in the form of a flat and circular space.`
• Formed from splanchnopleure (inner endoderm and outer mesoderm).
• Well developed in the animals with megalecithal egg as reptiles, birds and
Prototheria.
• Formed completely on the 9 th day of incubation. In human it is vestigial.
• The yolk –sac gradually grow over yolk to completely surround it.
• The yolk is used up with the increase in size of the embryo and the yolk –sac
gradually reduce in size.

6. Functions of Yolk sac:
1.It surrounds the yolk. Its main function is in digestion. It
serve as extra embryonic gut.
2. It help in digestion of yolk and transfer the digested
material to the developing embryo.
3. First respiratory organ in the embryo.
4. Form yolk sac placenta in the marsupials.

7. Development of yolk sac-
In reptiles and birds, the somatopleure and splanchnopleure develop from the
periphery of the blast disc. These usually spread peripherally over the yolk mass.
Soon afterwards, the embryo undergoes series of folds, which appear all around
the body of the embryo. These folds are termed as the body folds. The extra
embryonic splanchnopleure (splanchnic mesoderm + endoderm) constantly
spreads over the yolk mass and eventually yolk sac encloses the mass of yolk in a
large measure. The yolk sac, however, not surrounds the yolk fully. A small
passage is left on the ventral side for the embryo to absorb the remains of
albumen at a later stage. Immediately with the formation of the yolk sac, the intra
embryonic splanchnopleure is subjected to fold resembling with the more
superficial body folds i.e., the intra embryonic folds. The intra embryonic folds
give rise to walled digestive tract, or gut, in the body of the embryo. The middle
of the embryonic gut remains open to the yolk beneath. At this level, yolk sac is
connected to the digestive tract by a constricted yolk stalk.

8. 2. Amnion:
Formed of somatopleure (inner ectoderm and outer mesoderm). It surrounds the
embryo. In 13 somite stage of the embryonic development of chick, It appears
after 30 hours of incubation. A amniotic cavity is present between the amnion
membrane and the embryo, which filled with the amniotic fluid. In this fluid filled
cavity embryo floats.

9. Function of Amnion:
The fluid filled in the amniotic cavity forms a sort of pool for the embryo and
therefore, performs the following functions-
1. It prevents the embryo from drying.
2. Protection of the embryo from the mechanical injury and desiccation.
3. It protects the embryo from external shock.
4. It prevents the embryo from sticking to the shell or embryonic membranes.
5. It also helps in the absorption of the albumen.
6. The amniotic fluid allows freedom of motion to the embryo .
7. It is working as a barrier of outer organisms.
8. In most mammals of a part of the chorion forms finger-like out-growths called
chorionic villi. That penetrates into corresponding crypts or depressions in the wall
of the uterus. It is at this site where exchange of substances occurs found in the
inner cell mass begin to spread along the inner surface on the trophoblast and
surround the internal cavity of the blastocyte in the same way as the endodermal
cell in reptiles and birds surrounded the mass of uncleaved egg.

10. 3. Chorion:
Formed of somatopleure (outer ectoderm and inner mesoderm). It forms the
outermost boundary. Space between amnion and chorion is called chorionic cavity
which further provides protection to the embryo.
Function of chorion:
1. It is outer layer, therefore, it is the place where exchange of substance occurs
between embryonic tissue in the maternal environment.
2. It acts as a respiratory organ and is in contact with the allantois.
3. In most mammals it develops villi which penetrate into the tissue of uterus wall
and increase the absorptive area of chorion.
4. Blood vessels are developing after the formation of allantochorion and this is
working as a lung. It takes O2, from atmosphere and removes CO2, to the
atmosphere.
5. It provides calcium for skeleton of developing embryo by transportation of Ca by
shell.

11. Development of Amnion & Chorion-
The amnion and chorion are developed jointly as upward projecting folds, the
amniotic folds of the extra embryonic somatopleure. The amniotic folds are
named according to their location. They are amniotic head fold and amniotic tail
fold. The amniotic fold first appears as a transverse fold in front of the head. It
is called amniotic head fold. It grows upwards and then bends backwards, over
the anterior end of the head and covers it as with a hood. Another fold develops
behind the embryo. It is termed as amniotic tail fold. All these folds finally
cover an embryo in two sheets of somatopleure. The inner somatopleuric sheet
becomes the amnion and the outer, the chorion.
The amnion consists of a layer of extra embryonic ectoderm on the inside and a
layer of extra embryonic somatic mesoderm on the outside whereas, the chorion
is made up of a layer of extra embryonic ectoderm on the outside and a layer of
extra embryonic somatic mesoderm on the inside. The cavity between the
amnion and the embryo is called the amniotic cavity. In between, the amnion
and the chorion is the chorionic cavity or extra embryonic coelom.

12. 4. Allantois:
Formation of Allantois membrane is starting in the 29 somatic stage of after
60 hours incubation. Formed of splanchnopleure (inner endoderm and
outer splanchnic mesoderm). Connected with the hindgut of the embryo.
Develops on the third day of incubation from the floor of the hind gut as a
outgrowth.

13. Development of allantois -The allantois arises as a ventral outgrowth of the
splanchnopleure from the hindgut on the third day of incubation. It slowly enlarges
as holosac and expands inside exocoel. Its walls are formed of an outer splanchnic
mesoderm and inner endoderm. The proximal part of the allantois forms a slender
neck or the allantoic stalk with which it remains connected with the hindgut of the
embryo. The distal part of the allantois expands and penetrates between the amnion
and the yolk sac on one side and the chorion on the other side. By the middle of the
incubation period, the allantois spreads all around the egg underneath the chorion.
The mesoderm on the external surface of the allantois fuses with that of the chorion
forming a conjoined chorio-allantoic membrane.

14. Function of allantois:
The allontois is highly muscular from the beginning, As a result of the
formation of the allanto-chorion the blood vessels become dispersed over the
inner surface of the porous shell. Thus, it performs the following functions –
1. It acts as a respiratory organ. It is working as a lung and provides
atmospheric O2, to the embryo by porous shell. It is working of the
exchange of gases because it is situated near the porous shell.
2. It collects the embryonic villi and working as a excretory organ (urinary
bladder) of embryo.
3. Also helps in digestion and nutrition from albumen and calcium of the shell.
4. It helps in removal of waste products, e.g, CO2, urea, etc. of metabolism
from the embryo.
5.

15. Importance of foetal membranes:
• The presence of amnion, allantois and chorion is an adaptation
for terrestrial life of animals. Their presence enables the embryo
and adult to live on land.
• The presence of yolk-sac is for different purposes. It acts as a
digestive and absorptive surface through which the embryo
gets the yolk.

16. References-
Modern text book – R.L.Kotpal
Jantu Vigyan- S.M. Sexsena
Jantu Vigyan- Dr.H.N. Baijal