The document summarizes embryonic membrane development in birds, reptiles, and mammals. It discusses the formation and functions of the amnion, chorion, yolk sac, and allantois membranes. It also describes how mammals are divided based on their embryonic development methods into prototheria, metatheria, and eutheria. The types of placentas and twins are summarized, including dizygotic, monozygotic, conjoined, and parasitic twins.

1. Implantation, Embryonic membrane and
Twinning
‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬
EMBRYOLOGY
BIO 415 LECTURE 7

2. ‫الطيور‬ ‫في‬ ‫الجنينية‬ ‫األغشية‬ ‫تكوين‬
Embryonic membrane Development
Embryonic membranes are new additional
structures that appear on the embryonic
development of birds, reptiles and mammals.
1-The Amnion Membrane :
• It is first membrane surrounds the embryo.
• The membrane form during the formation of the
two amniotic folds (front and hind folds)
consists of the two layer the ectoderm and
mesoderm.where these two folds are fused
above the embryo.
• It contain a liquid known as amniotic fluid, the
amniotic fluid works to protect the embryo from
shocks during flipping and movement

3. Embryonic membrane Development
2-The Chorion membrane:
• The chorion membrane is also formed
during the two amniotic folds but to the
outside of the embryo, and surrounding
the amniotic membrane form outside
Later.
• The chorion membrane works to
exchange gases between the fetal
vessels and the shell pores of the egg.

4. Embryonic membrane Development
3- The Yolk Sac membrane:
The yolk sac membrane consists of
the endoderm and visceral
mesoderm where it surrounds the
yolk and its cell works to digest
the yolk and then move through
the blood vessels to the rest of the
body.

5. 4- The Allantois membrane:
• the allantois membrane is formed as
protrusion in the lower back part of the
embryo in the form of a sac.
• Consists of two layers of endoderm and
mezoderm, which acts as the urinary
bladder of the embryo and stores the
output materials of the embryo.
• And then in the late stages of fussion
with the chorion to be the chorion
allanotic membrane of the embryo.
Embryonic membrane Development

8. Mammals divided according to the type of embryo
development
Mammals are called because the feed their new born by mammary glands (secrete
milk).
They are divided into 3 group in terms of the method of embryonic formation:
1- Primitive mammals (Prototheria):‫البدائية‬ ‫الثدييات‬
• Are mammals that lay eggs and do not give birth and resemble the method of
embryonic formation of birds and reptiles.
• Their embryos have the same embryonic membranes found around the embryo of
birds.
• In some species, during the reproduction period, a small bag on the female's
abdominal surface is incubated until the hatching stage, and examples as the ant-
eating mammals (Echidna aculatea). ‫اكل‬‫النمل‬

9. 2- Marsupials or (Metatheria) : ‫الثدييات‬‫الكيسيه‬
That give birth to not fully developed
embryo and continue its growth in a sac on
the abdominal side of the mother where the
fetus completes the stages of his
development by feeding from the breast of
the mother, as Kangaroo.‫الكنغر‬
Mammals divided according to the type of embryo
development

10. 3- Placental mammals or (Euotheria): ‫الثدييات‬‫الحقيقية‬
• It is the one that does not have in its eggs quantities of yolk ,but
the embryo depends on its nutrition since the beginning until its
birth on the mother,
• where it implanted in the lining of the uterus by the placenta,
which is a source of nutrition in the uterus and get rid of waste
and so called placental mammals.
• Most mammals from rodents , Lions, wales, to humans are also
called mammals because they feed their young milk from their
mammary glands.
Mammals divided according to the type of embryo
development

11. The reproductive system in the female mammals is composed of two ovaries and
two oviduct tube with two openings, in front of the ovary and the other opening
connected to the uterus, in which the placenta is formed and is considered as a
incubating of the fetus.
The uterus in mammals have different forms:
1- Simple uterus ‫البسيط‬ ‫الرحم‬ consists of one room (as in the Primates and
humans
2 - Duplex uterus (‫المزدوج‬ ‫الرحم‬) consists of two separate tubes just like in
Marsupials such as kangaroo
3 - Bicornate uterus: ‫المزدوج‬ ‫القرني‬ ‫الرحم‬ consists of two branches slightly
conjoined before they unite with a single opening in the vagina as in cattle and
carnivores
4-Bipartite uterus: ‫المزدوج‬ ‫الفصي‬ ‫الرحم‬ is a separate lobe, but it opens with
one opening in the vagina as in rodents.
The uterus Types in mammals

12. Types of uterus in mammals

13. • The Blastula consists of the nutritional tissue (Trophoblast), which nourishes the
fetus and its implantation in the lining of the uterus and contributes to the
formation of the placenta later with the uterine layer.
• The trophoblast tissue surrounds the inner cell mass (ICM) that makes up the
embryo's body and takes a side position in the blastula. It also contains the
cavity of the blastula or (Blastoceol).
• The blastula implanted of the in the lining of the uterus, where the process of
the gastrulation start and the formation of the three embryonic layers begins.
• It is characterized on the internal surface of the inter cell mass (ICM) facing the
blastula cavity, a layer of flat cells represents the beginning of the appearance of
the internal layer (endoderm), which is initially confined to the lower surface
only of the inner cell mass(ICM) and then spreads with remarkable speed to line
the inner wall of the entire germinal vesicles.
Blastula structure

14. • Then the endoderm layer extends on both sides of the blastula cavity until it is
surrounded from the bottom so it is called the cavity of the old intestine or
(Archentron) and resembles the area of the yolk sac of the bird embryo
membrane.
• The inner cell mass (ICM) in all the embryos of mammals covered by an external
layer ( nutritive ectoderm) known as the layer of Raubers,
• The Raubers layer begins to recede from the ICM so the embryo disk is fully
exposed to the outside in some types of mammals as in the embryos of rabbits
and cattle, ant-eater, pigs and lemurs
• The others (e.g. bats, monkeys and humans) the layer of roper remains
surrounding the embryonic disc, and therefore also the way in which the structure
of the amnion membrane surrounding the fetus is different.
• In the first type: the amnion is formed by the appearance of the amnion folds
that soon rise above the fetal disc to conjoin over the fetus to surround it.
Cont.

15. ‫الجنين‬ ‫إنغراس‬Embryo Implantation
• Implantation begins with the invasion of the cells of the trophoblast tissue of
the blastula, invading the cells of the outer layer of the uterine linning tissue,
and around the twelfth day of the human embryo, for example, the blastula is
completely enclosed inside the lining of the uterus.
• During the second week the embryo is characterized by the inner cell mass
of the blastula. The embryo at this stage has two-layer epiblast (the upper
layers or outer ectoderm layer )and hypoblast and the internal endoderm.
• During the third week of the human embryo, start the formation of the
primitive streak, as the same way that the mesoderm or the middle layer
formed in the birds embryo is formed.
• Thus, the gastrula layers is complete with the formation of the three
embryonic layers(ectoderm, mesoderm and endoderm).

16. • At the end of the day 4-5, the blastula stage inters the uterine region, where
the cells of the blastula begin to emerge from the zona pellucida membrane,
(known as hatching of Blastula),from the membrane that surrounded it .
• and their trophoblat cells begin to be implanted in the lining of the uterus
during the day 5-7 or more depending on the type of female.

21. ‫المشيمة‬ ‫أنواع‬:Placenta Types
• The placenta is a tissue structure consisting of the tissues of the mother's uterus
and fetal tissues, which is abundant in blood vessels that work to attach the fetus
to the uterus and feed it and provide it with oxygen and the get rid of waste
materels of the fetus.
• There are several types of placenta by type and quantity of tissues that make up
the placenta and share the uterus and fetal tissues:
1- Yolk sac placenta:
in marsupial mammals the placenta is produced from the union of the chorion
membrane and the yolk sac of the embryo with the endometrium tissues and
consists of folds on the surface so it is called yolk sac placenta, which is the simplest
type of placenta and it is implanted in the wall of the uterus.

22. 1-‫المشيمة‬‫السلية‬‫المحية‬(Yolk sac placenta

23. 2. Allantoise placenta:
• The allantoise placenta is found in most mammalian species and produces the
allantosis placenta from the union of the allanttoise membrane with the chorion
membrane and from this union the finger-shape villi are formed towards the
uterine wall.
• They are closely connected by syncytiotrophlast, which surrounds the blastuola
from the outside and lose the membranes that bind together to form bloody
islands (Lacunae).
• It connect what is absorbed by previous cells by other cells that form the link
between these cells and the fetus called cytiotrophoblast.
• The blood circulation of the fetus never directly contacted with the mother's
blood, but the placental tissue acts as a barrier between them, where food and
oxygen are filtered from the mother's blood to the placenta and then into the
blood of the fetus, while the discharge material from the fetal blood is filtered into
the placenta and then to the mother's blood.
Placenta Types

25. 1- Diffuse placenta: ‫المشيمة‬‫المنتشرة‬ where a large part of the chorine meet with a
large area of the uterine wall as in the pig where the placenta covers the bulk leaving a
small part of the chorine on the edge
2 - The Cotyledonary placenta: ‫المشيمة‬‫الفلقية‬ in the cotyledonary placenta is the
connection between the mother's tissues and the tissues of the embryo through many
areas of disk-shaped communication that appear as blisters spread on the surface of
chorine and because each tablet consists of two parts, one of the mother and the other of
the fetus, it is similar in terms of form. It's called the cotyledon placenta. They are found in
sheep and cows.
3- The Zonary placenta or circular placenta: in which the area of fusion of fetal
membranes to the uterine wall is limited to a belt in the middle of the cell membrane,
which is the type found in cats and dogs
4- Discoidal placenta the disc placenta consists of the disc placenta when the fusion is
between two types of tissues defined by an oval-shaped disc. This type of discus placenta
is found in humans and some types of rodents
Forms of the alintosian placenta

26. Ex: pig EX: sheep
Ex: Cat Ex: human

28. Twins:
The term twins is called animals, which often give one embryo during the period of each
pregnancy, if it gives more than one fetus is considered as twins as in humans, sheep, the
twins are either not similar or similar.
There are to type of twins:
1. dizygotic twins (the are mostly not similar) because the female produce two ova and
each ova were fertilized and give two embryos.
2. monozygotic twins were the female produce only one ova and during the cleavage stage
they separated into two embryos ,these embryo are similar in shape and sex.
Multi-birth:
• Many mammals that give birth more than one newborn at a time pregnancy and this
phenomenon is known as multiple births (as in rodents, carnivores and pigs).
• The embryos are produced from the ovulation of more than many ova during the
female reproductive cycle and each ova is fertilized with one sperm.
• These embryos are distributed within the wall of the uterus.
Twins and Multiple births

29. Asymmetric twins and non-identical twins:
• Are similar to multiple births i.e. there is more than one ovum that came down
from the ovary during the female reproductive cycle and then each oocytes
was fertilized with a sperm
• The twins may be of the same sex or may be of different sexes (female and
male) so they are not the same and Non-similar twins can be activated by
reproductive hormones (FSH,LH) to give more than one ova during a single
reproductive period, so they are known as dizygotic twins.

30. Identical twins, Siamese and parasitic twins:
• Identical twins are produced from a single oocytes that is fertilized with a
single sperm, but this ova during the stages of its formation separated into
more than one embryo and each part of the blastomears form a fetus so the
are similar and they are exactly the same and of they have same sex.
• Therefore, it is sometimes known as mono zygotic twins:
accordingly each fetus may have a placenta if the separation of the fetus in the early
stages of the cleavage.
• for example in the phase of the two embryos and separated each cell and formed an
independent embryo.
• If the separation is in the phase of the blastula, the twins are one or both placenta,
with each embryo retaining its own amniotic membrane.

31. • But when the separation of the embryo cells occurs at a late stage such as
that occurs in the germinal disc stage, this leads to the formation of two
embryos surrounded by a single amnion membrane, which may lead to the
formation of conjecated twins or Siamese twins) this is because the
separation between the two fetuses was not complete and they may share
parts of the body.
• Sometimes the separation in parts of the fetus, for example in the front of
the head or in the back or part of the fetus such as limbs and the newborn
appears as part of a fetus that is carried next to the other child, known as
parasitic twins, which is the result of incomplete formation of one of the
twins
Cont.

34. conjecated twins or Siamese twinsparasitic twins