This document outlines the key stages of mammalian embryonic development from formation of the germ layers through early organ development. It describes how the morula forms three germ layers - endoderm, ectoderm, and mesoderm. It then discusses formation of structures like the notochord, neural tube, neural crest cells, and subdivision of mesoderm. It details how the embryo folds, how branchial arches form and their fate, and early development of structures like the face and palate.

2. contents:
 FORMATION OF GERM LAYERS
 FORMATION OF NOTOCHORD
 FORMATION OF NEURAL TUBE
 NEURAL CREST CELLS
 SUBDIVISION OF INTRA EMBRYONIC
MESODERM
 FORMATION OF INTRA EMBRYONIC
COELOM
 FOLDING OF EMBRYO
 BRANCIAL ARCHES
 DEVELOPMENT OF FACE
 DEVELOPMENT OF PALATE
 DEVELOPMENT OF TONGUE

3. FORMATION OF GERM LAYERS
Mammalian development involves a
phase of rapid proliferation and migration of
cells, with little or no differentiation. This
proliferative phase lasts until three germ layers
have formed
1. ENDODERM
2. ECTODERM
3. MESODERM

4. After fertilization of ovum, a series of cell
division
give rise to an egg cell mass known as the
MORULA. Fluid seeps into morula, and its cells
realign themselves to form fluid filled hollow ball, the
BLASTOCYST. Its consists of
1) Inner cell mass / Embryoblast
2) Trophoblast
The inner cell mass seperates into two layers
a. Epiblast embryo proper
b. Hyboblast
The trophoblast cells are associated
with implantation of the
embryo and formation of placenta.

6.  Some cells of inner cell
mass differentiate into
flattened cell, that come
to line its free surface.
These constitute the
ENDODERM First
germ layer
 The remaining cell of
inner cell mass become
columnar. These cells
form the second germ

7. A space appears between the
ectoderm and the trophoblast. This is
AMNIOTIC CAVITY, filled by amniotic fluid/liquor
amnii.
Flattened cells arising from the
endoderm spread and line the inside of the
blastocystic cavity. In this way, a cavity called
PRIMARY YOLK SAC is formed.

8. The cells of
trophoblast give origin to
mass of cells called
the EXTRA EMBRYONIC
MESODERM /
PRIMARY MESODERM.
Small cavities appear in the extra
embryonic mesoderm which join together to form
large cavity called the EXTRA EMBRYONIC
COELOM / CHORIONIC CAVITY. With its
formation extra embryonic mesoderm is split into
two layers
1. Parietal / Somatopleuric extra embryonic
mesoderm

9. Extra embryonic
coelom does not extend
into that part of extra
embryonic mesoderm
which attaches the wall of
amniotic cavity to the
trophoblast. This
mesoderm forms a
structure called the
CONNECTING STALK.
With the
appearance of the extra
embryonic mesoderm, and

10. At one circular area near
the margin of the disc, the
cubical cells of the endoderm
becomes columnar. This area
is called the PROCHORDAL
PLATE.
Soon after the
formation of prochordal plate
some of the ectodermal cells
lying along the central axis,
near the tail end of the disc,

11. The cells that
proliferate in the region of
the primitive streak pass
sideways, pushing
themselves between the
ectoderm and endoderm.
These cells form the
INTRA EMBRYONIC
MESODERM /
SECONDARY
MESODERM. THIRD
GERM LAYER
The process of

12. The intra
embryonic mesoderm
spreads throughout the
disc except in the
region of prochordal
plate. Which remains
relatively thin, and later
forms the bucco-
pharyngeal membrane.

13. As the embryonic
disc enlarges in size, and
also elongates, the
connecting stalk becomes
relatively small, Some intra
embryonic mesoderm
arising from the primitive
streak, passes backward
into the connecting stalk.
In this region the ectoderm
and endoderm remain in
contact, and forms the
CLOACAL MEMBRANE.

14. FORMATION OF NOTOCHORD
 NOTOCHORD :
 It is a midline structure, that
develops in the region extending from the cranial
end of primitive streak to the caudal end of
prochordal plate.

15. STAGES:
1. Cranial end of
primitive streak
becomes thickened,
This thickened part of
the streak is called
the primitive knot /
primitive node /
Henson’s node.

16. 2. A depression appears in the centre of the
primitive knot. This depression is called the
blastopore.
3. Cells in the primitive knot multiply and pass
cranially in the middle line, between the
ectoderm and endoderm, reaching up to the
caudal margin of the prochordal plate. These
cells form a solid cord called the notochordal
process / head process.

18. 4. The cavity of the blastopore now extends into
the notochordal process, and converts it into a
tube called the notochordal canal.
5. The floor of the notochordal canal begins to
break down. Gradually the whole canal comes
to communicate with the yolk sac and the
amniotic cavity. Thus, at this stage, the
amniotic cavity and the yolk sac are in
communication with each other.
6. Gradually the walls of the canal becomes
flattened to form the notochordal plate.
7. However, this process of flattening is soon

19. FORMATION OF NEURAL TUBE
 The nervous system develops as a thickening
within the ectodermal layer at the rostal end of
the embryo. This thickening constitutes the
neural plate, which rapidly forms raised margins
called the neural fold.
 These folds in turn encompass and delineate a
deepening midline depression, the neural
groove.
 The two edges of neural plate come nearer each
other and eventually fuse, thus converting the
neural groove into the neural tube.
 The process of formation of neural tube is called

21. THE NEURAL CREST :
 The band of specialized cells from the neuro
ectoderm that lies along the
outer surface of each side of the neural tube in
the early stages of embryonic development.
 These cells have the capacity to migrate and
differentiate with in the developing embryo.

23. NEURAL CREST
REGIONS:
1. CRANIAL
2. CARDIAC
3. VAGAL
4. TRUNK

25. SUBDIVISION OF INTRA EMBRYONIC
MESODERM:
 Cranial to the
prochordal plate the
mesoderm of two sides
meets in the midline.
 At the edges of the
embryonic disc, the intra
embryonic mesoderm is
continuous with the extra
embryonic mesoderm.

26. The intra embryonic
mesoderm now becomes
subdivided into three
parts:
a. Paraxial mesoderm
b. Lateral plate
mesoderm
c. Intermediate
mesoderm

27. the paraxial
mesoderm now
segmented into cubical
masses called
somitomeres. Which give
rise to somites /
metameres / primitive
segments.

28. Formation of intra embryonic
coelom:
 Small cavities appear in the lateral plate
mesoderm. These coalesce to form one large
cavity called intra embryonic coelom

29. • At first, this is a closed cavity but soon it
comes to
communicate with the extra embryonic
coelom.

30. • With the formation
of the intra embryonic
coelom, the lateral
plate mesoderm splits
into:
a. Somatopleuric /
parietal
intra embryonic
mesoderm
b. Splanchnopleuric /
visceral

31. • The intra embryonic coelom gives rise to
pericardial, pleural, and
peritonial cavities.
• Pericardium is formed from that part of intra
embryonic coelom which lies cranial to prochordal
plate.
• The heart is formed in the
splanchnopleuric
mesoderm forming the floor
this part of the
coelom. This is, therefore,
called the
cardiogenic area.

32. • Cranial to the cardiogenic area the
somatopleuric and
splanchnopleuric mesoderm are continuous
with each other
This unsplit mesoderm forms a structure
called the septum
transversum.

33. Folding of embryo:
 After formation of the
secondary yolk sac, there is
progressive increase in the
size of the embryonic disc.
 With further enlargement,
the embryonic disc
becomes folded on itself, at
the head and tail ends.
These are called the head
and tail folds.

34.  The head fold is critical to the formation of
primitive stomatodeum / oral cavity.
 Through this fold ectoderm comes to line the
stomatodeum, with the stomatodeum seperated
from the foregut by the buccopharngeal
membrane, but this soon breaks down so that the
stomatodeum communicates directly with the
foregut.
 Laterally the stomatodeum becomes limited
by the first pair of pharyngeal / branchial arches.

36. The pharyngeal arches:
 The branchial arches form in the pharyngeal wall
as a result of proliferating lateral plate mesoderm
and subsequent reinforcement by migrating
neural crest cells.
 Six cylindrical thickening thus form. That expand
from the lateral wal of pharynx, pass beneath the
floor of the pharynx, and approach their anatomic
counterparts expanding from the opposite side.
 The arches progressively separate the primitive
stomatodeum from the developing heart.

37. • Arches are seperated
externally by small clefts
called the branchial
groove / ectodermal clefts.
And internally by small
depressions called
pharyngeal pouches /
endodermal pouches.

38. Branchial arches:
 At first there are six arches. The fifth arch
disappears and only five remain.
 The first arch is also called the mandibular arch,
and the second arch, the hyoid arch.
 Each pharyngeal arch contains
1. a skeletal element
2. striated muscle
3. nerve of the arch
4. arterial arch

39. A SKELETAL ELEMENT:
• The neural crest mesenchyme condenses to form
a bar of cartilage, the arch cartilag.
• The cartilage of first arch is called Meckel’s
cartilage, and that of second arch is called
Reichert’s cartilage.

40. MUSCLES AND NERVES:
• Some of the mesenchyme sorrounding the
cartilagenous bar develops into striated muscle.
• The nerve consists of two components,
1. motor supply muscles of arch
pretrematic branch
2. sensory
post trematic branch

41. PRE TREMATIC BRANCH:
Supply the
epithelium that covers the
anterior half of the arch.
POST TREMATIC
BRANCH:
Supply the
epithelium that covers the
posterior half of the arch.

44. Fate of ectodermal cleft:
 The dorsal part of first cleft
develops into the epithelial
lining of external acoustic
meatus.
 The pinna is formed from a
series of swellings or hillocks,
that arise on the first and
second arches, where they
adjoin the first clefts.
 The second, third and fourth
grooves normally are
obliterated by over growth of
the second arch forming a
cervical sinus that sometimes
persists and opens into the
side of the neck ( branchial

45. Fate of endodermal pouches:

47. Development of face:
 After the formation of
head fold, the
developing brain and
the pericardium form
two prominent bulgings
on the ventral aspect of
the embryo.
 These bulgings are
seperated by the
stomatodeum. The
floor of stomatodeum
is formed by the
buccopharyngeal

48. • Mesoderm covering the
developing forebrain
proliferates, and forms a
downward projection that
overlaps the upper part of
the stomatodeum. This
downward projection is
called the frontonasal
process.

49. • The first branchial arch, which forms the lateral
wall of stomatodeum, gives a bud from its dorsal
end, called the maxillary process.
• It grows ventro-medially cranial to the main
part of the arch which is now called the
mandibular process.

50. • At about 28 days, localised thickenings develop
within ectoderm of the frontonasal process, just
rostal to the opening of the stomatodeum. These
thickenings are called the nasal placodes.
• The placodes soon sink below the surface to
form nasal pits. The edges of each pit are raised
above the surface: the medial raised edge is
called the medial nasal process and the lateral
raised edge is called the lateral nasal process.

51. • The medial nasal processes of both sides,
together with the frontonasal process, give rise
to the middle portion of the nose, middle portion
of the upper lip, anterior portion of the maxilla,
and the primary palate.
• LOWER LIP:
The mandibular processes of the two
sides grow towards each other, and fuse in the
midline to form the lower lip, and the lower jaw.

52. UPPER LIP:
• Each maxillary process now grows medially and
fuses, first with the lateral nasal process and then
with the medial nasal process. the medial and
lateral processes also fuses with each other. In
this way the nasal pits are cut off from the
stomatodeum.

53. • The frontonasal
process becomes much
narrower from side to
side, with the result that
the two external nares
come close together.
• The deeper part of the
frontonasal process
ultimately forms the
nasal septum.

54. • Mesoderm becomes
heaped up in the median
plane to form the
prominence of nose.
• A groove appears
between the regions of
the nose and the bulging
forebrain called the
forehead.
• As the nose becomes
prominent the external

55. CHEEKS:
After formation of upper lip and lower
lip, the stomatodeum is very broad. In its lateral
part, it is bounded above by the maxillary process
and below by the mandibular process. These
processes undergo progressive fusion with each
other to form the cheeks.

56. EYE:
• The region of the eye is first seen as an
ectodermal thickening ,the lens placode, which
appears on the ventro- lateral side of the
developing forebrain, lateral and cranial to the
nasal placode.
• The developing eye ball produces a bulging
which , at first directed laterally and lie in the
angles between the maxillary processes and the
lateral nasal processes.
• With the narrowing of the frontonasal process
they come to face forwards.

57. • The eyelids are derived from folds of
ectoderm that arre formed above and below
the eyes, and by mesoderm enclosed with in
the folds.

58. EXTERNAL EAR:
• The external ear is
formed around the
dorsal part of the first
ectodermal cleft.
• A series of mesodermal thickenings ( called
tubercles or hillocks ) appear on the mandibular
and hyoid arches where they adjoin this cleft.
• The pinna is formed by fusion of these
thickenings.

59. • The pinna first lies caudal to the developing
jaw. Later it is pushed upward and backward to
its definitive position due to great enlargement of
the mandibular process.

60. Development of palate:
 The palate as a whole forms from two primordia which
can be classified as
1. the primary palate.
2. the secondary palate
 At around the sixth week of development the primary
palate begins to take shape, arising from the medial
nasal process.
 The formation of secondary palate comencess
between seventh and eight weeks from two palatine
processes and completes around the third month of
gestation.

61. • Three outgrowths appear in the oral cavity, the
nasal septum grows downward from the
frontonasal process along the midline, and two
palatine processes, one from each side, extend
from the maxillary processes towards the midline.
• The shelves are directed first downward on
each side of the tongue. After the seventh week
of develoment, the tongue is withdrawn from
between the shelves, which now elevate and fuse
with each other. Their fusion begins anteriorly and
proceeds backwards.

62. • Each palatal process
fuses with the posterior
margin of the primitive
palate.
• The medial edges of the
palatal processes fuse with
the free lower edge of the
nasal septum, thus
seperating the two nasal
cavities from each other
and from the mouth.

63. • At a later stage, the
mesoderm in the palate
undergoes intra
membraneous
ossification to form the
hard palate. However,
ossification does not
extend into the most
posterior portion, which
remains as the soft
palate.

64. Development of tongue:
 The development of tongue starts at fourth
month of intra uterine life.
 The tongue develops in relation to the
pharyngeal arches in the floor of developing
mouth.
 The medial most part of the mandibular arches
proliferate to form two lingual swellings.
 The lingual swellings are partially seperated
from each other by another midline swelling
called tuberculum impar.

65. • Immediately behind
tuberculum impar, the
epithelium proliferates to
form a down growth called
thyroglossal duct from which
the thyroid gland develops.
• The site of this down
growth is subsequently
marked by a depression
called the foramen ceacum.
• Another midline swelling
seen in relation to medial

66.  The anterior two third of tongue is formed by
fusion of
a. The tuberculum impar, and
b. The two lingual swellings.
 The second arch mesoderm gets buried
below the surface. The third arch mesoderm
grows over it to fuse with the mesoderm of the
first arch. The posterior one third of the tongue
is thus formed by third arch mesoderm.
 The posterior-most part of the tongue is
derived from the fourth arch.
 The musculature of the tongue is derived
from the occipetal myotomes.

68. Parts of tongue Embryonic part from
which derived
General
sensation
Taste Motor
Epithelium over
anterior two- third first arch Mandibular
lingual br
facial
chordtympani
Epithelium over
posterior one third second arch glosso-
pharyngeal
glosso-
pharyngeal
Epithelium over
posterior most part third arch
superior
laryngeal br
of vagus
superior
laryngeal br
of vagus
MUSCLE
occipital
myotomes
hypoglossal

69. REFERENCES:
1. HUMAN EMBRYOLOGY : INDERBIR SINGH
2. ORBAN’S ORAL HISTOLOGY AND
EMBRYOLOGY: G S KUMAR
3. TENCATE ORAL HISTOLOGY:

70. Thank you