The document describes key stages in the further development of the embryonic disc, including the formation of the notochord, neural tube, intra-embryonic mesoderm and coelom. It also discusses the yolk sac and folding of the embryo, connecting stalk, allantoic diverticulum, and the effects of head and tail folds on positioning other structures. The timeline provides an overview of developmental events from 15 to 23 days.

2. Further Development of Embryonic Disc
• Highlights.
• Formation of the Notochord.
• Formation of the Neural tube.
• Subdivision of intra-embryonic mesoderm.
• Formation of the Intra-embryonic coelom.
• Yolk sac and folding of embryo.
• Connecting Stalk.
• Allantoic Diverticulum.
• Effect of head and tail folds on position of other
structure.
• Additional points of interest + time table.

3. Highlights
• Primitive knot:
enlarged cranial end of the primitive streak.
• Notochordal process:
a rod-like structure composed of cells from the primitive knot,
multiply and pass cranially, reaching up to the prochordal plate.
• Notochord :
notochordal process undergoes changes (canal, plate and rod-like
structure).
• Nucleus pulposus:
remnant of notochord, of each intervertebral disc.

4. Highlights (continue)
• Neural plate :
ectoderm overlying notochord (future brain and spinal cord).

5. Highlights (continue)
 INTRA-EMBRYONIC MESODERM = 3 subdivisions.
a) Paraxial mesoderm.
b) Lateral plate mesoderm.
c) Intermediate mesoderm.
Intra-embryonic coelom:
It is a cavity appears in the lateral plate mesoderm and splits it
into:
1. Somatopleuric layer (in contact with ectoderm).
2. Splanchnopleuric layer (in contact with endoderm).

6. Highlights (continue)
• Intra-embryonic coelom:
A cavity in the lateral plate mesoderm, later form the pericardial, pleural,
and peritoneal cavities.
 Head and tail folds, + lateral folds appear. Folding results in conversion of
endoderm into a tube (the gut).
 Endodermal tube (gut) is divided into 3 parts,
foregut,
midgut,
and hindgut.
 The gut is closed cranially by buccopharyngeal membrane (prochordal
plate), and caudally by cloacal membrane.
 The umbilical cord develops from the connecting stalk.

7. Highlights (continue)
Umbilical cord contains
two arteries + one vein + vitello-intestinal duct.
Allantoic diverticulum:
Arises from the yolk sac before formation of the gut. After
formation of the tail fold, it is seen as a diverticulum of the
hindgut.

8. Highlights (continue)
Pericardial cavity:
 Derived from intra-embryonic coelom.
 Lies cranial to the prochordal plate.
 Lies ventral to the foregut after formation of the head fold.
 The developing heart lies ventral to the cavity, then dorsal.

9. Highlights (continue)
Septum transversum:
• Is made of intra-embryonic mesoderm.
• Lies cranial to the pericardial cavity then after formation of
the head fold, it lies caudal to the pericardial cavity.
• The liver and diaphragm develop in relation to the septum
transversum.

10. Notochord Formation
• Midline structure,
• Lies between the primitive streak and the prochordal plate.
• Passes through several stages that are as follows:
1. Primitive node, primitive knot or Henson’s node. (thickened cranial
end of primitive streak).
2. Blastophore = central depression in the primitive knot.
3. Notochordal process = head process. -----------------------------------------
Cells in the primitive streak multiply and pass cranially reaching up to the
caudal margin of the prochordal plate. The cells undergo several stages of
arrangement ending in the formation of a Solid cord called the
Notochord.
4. Gives rise tonucleus pulposus in the region of each
intervertebral disc.

12. Neural Tube Formation
Gives rise to the brain and the spinal cord.
 Extends from the prochordal plate to the
primitive knot.
Arises from ectoderm.
 The process is known as NEURULATION.
 FURTHER details will be discussed later. (chapter 17)

13. Intra-embryonic Subdivisions
1) Paraxial mesoderm: --------------------------
the mesoderm on either sides of the
notochord.
2) Lateral plate mesoderm:----------------------
more laterally and thin layer.
3) Intermediate mesoderm: ---------------------
a longitudinal strip between these two.

14. Paraxial mesoderm:
Becomes segmented into cubical masses
called SOMITOMERES.
 SOMITOMERES give rise to SOMITES (metameres or primitive
segments).
The first somites are seen on either side of the
midline. A little behind the prochordal plate.
More somites are formed caudally, on either side of
the developing neural tube.

15. Intra-embryonic Coelom Formation
• During segmentation of paraxial mesoderm, changes occur in the
lateral plate mesoderm.
• Small cavities appear in the lateral plate mesoderm.
• These coalesce to form one large cavity called intra-embryonic
coelom.
• The cavity is horseshoe shape.
• The cavity are two halves (one on either side of the midline), which
are joined together cranial to the prochordal plate.
At first it is a closed cavity but soon it comes to
communicate with the extra-embryonic coelom.

16. Intra-embryonic Coelom Formation (continue)
 With the formation of the intra-embryonic coelom, the
lateral plate mesoderm splits into:
1. Somatopleuric or parietal intra-embryonic coelom (in
contact with ectoderm)
2. Splanchnopleuric or visceral intra-embryonic mesoderm (in
contact with endoderm).
Intra-embryonic Coelom gives rise to 3 cavities:
a. Pericardial cavity.
b. Pleural cavity.
c. Peritoneal cavity.

17. Intra-embryonic Coelom Formation (continue)
• The pericardium is formed from that part of the intra-
embryonic coelom in the midline (cranial to the prochordal plate).
• The heart is formed in the splanchnopleuric mesoderm.
• This area is called the CARDIOGENIC AREA, cardiogenic plate or
heart-forming plate).
• Cranial to cardiogenic area both mesoderm (splanchnopleuric,
Somatopleuric) are continuous with each other.
• The mesoderm here does not split.
• This unsplit mesoderm form a structure called
• SEPTUM TRANSVERSUM.

18. Yolk Sac and Folding of Embryo
• Primary yolk sac is bounded above by cubical endoderm of the
embryonic disc and elsewhere by flattened cells.
• Secondary yolk sac is smaller and lined by cubical cells all around.
• Now there are changes of embryo and yolk sac as follows:
1. Increase the size of embryonic disc.
2. Head and tail ends bulge into the amniotic cavity.
3. The head and tail folds appear.
4. Primitive gut is formed (parts of yolk sac become enclosed within the embryo).
Foregut, midgut and hind gut (according to the site of communication).
Definitive yolk sac = umbilical vesicle (yolk sac becomes small and inconspicuous).
Vitello-intestinal duct (vitelline duct, yolk sac, or omphalomesenteric duct),
narrow channel connects the definitive yolk sac to the gut,
becomes elongate and eventually disappears.

19. Yolk Sac and Folding of Embryo (continue)
5. Lateral folds are formed. The embryo comes to be enclosed all
around by ectoderm except in the region through which the
vitello-intestinal duct passes. This aperture is called the
UMBILICAL OPENING.
5. The amniotic cavity expands greatly, the embryo is
surrounded on all sides and the embryo floats in the
amniotic fluid.

20. Connecting Stalk
• The embryo (along with the amniotic cavity and yolk sac) remains attached to
the trophoplast only by extra-embryonic mesoderm.
• This mesoderm into which the coelom does not extend:
• Is called the CONNECTING STALK.
• It is the only connecting link between the embryo and the
placenta.
• As the embryo grows, the area of attachment of the connecting
stalk becomes relatively smaller.
• The attachment moves to the ventral aspect of embryo, and
attached in the region of the umbilical opening.

21. Connecting Stalk (continue)
• By now Blood vessels have been developed in the embryo ,
and also in the placenta.
• At first there are two arteries and two veins.
• Later the Right vein disappears.
• At this stage the amnion has a circular attachment to the
margins of the umbilical opening and form a wide tube.
• This tube of amnion and the structures within it, constitute
the
• UMBILICAL CORD.

22. Connecting Stalk (continue)
• Umbilical cord contains:
a) Vitello-intestinal duct and the remnant of the yolk sac.
b) Wharton’s jelly (mesoderm is converted to jelly. It protects blood
vessels in the umbilical cord). It is an extra-embryonic mesoderm.
c) Blood vessels that pass from the embryo to placenta. (2 arteries +
one vein).
d) Small part of extra-embryonic coelom.
Umbilical cord progressively increases in length.
At the time of birth it is about 50 cm long and 2 cm diameter.
Too long or too Short = problems during delivery.

23. Allantoic Diverticulum
• Before the formation of the tail fold:
• It is a small endodermal diverticulum arises from the yolk sac near the
caudal end of the embryonic disc.
• It grows into the mesoderm of the connecting stalk.
• After the formation of the tail fold:
• Part of it is absorbed into hind gut.
• It passes from the ventral side of the hind gut into the connecting stalk.
Details with development of the urinary bladder. (chapter 16)

24. Effects of Head and Tail Folds
on position of Other Structures
• Just before formation of head and tail folds;
• If a midline section across the disc at this time (21st day), the
structures seen in the midline are:
I. The septum transversum.
II. The developing pericardial cavity and the heart.
III. The prochordal plate.
IV. The neural plate.
V. The primitive streak.
VI. The cloacal membrane.
The primitive streak is now inconspicuous.
After folding , the relative positions of these structures change.

25. Effects of Head and Tail Folds
on position of Other Structures (continue)
• The important points are as follows:
1) Pericardial cavity comes to lie on the ventral side of the embryo,
ventral to the foregut. The heart lies in the roof of the cavity
(heart was in the floor). The pericardium enlarges rapidly, and form a
conspicuous bulging on the ventral side of the embryo.
2) The septum transversum lies caudal to the heart. (at later stage in
development, the diaphragm and liver develop in relation to the septum
transversum).
3) The prochordal plate forms the buccopharyngeal membrane, (closes the
foregut cranially and when it breaks the foregut communicate with the
exterior).

26. Effects of Head and Tail Folds
on position of Other Structures (continue)
4) The enlarged cranial part of the neural tube becomes the most cranial
part. At this stage there are two big bulging: the developing brain, and a
little below the pericardium, and between the two, there is a depression
called the stomatodaeum or stomodaeum.
the floor of stomatodaeum is formed by buccopharyngeal
membrane.
5) Toward the tail of embryo, the primitive streak is now an inconspicuous,
that gradually disappears.
The distal end of the hindgut is closed by cloacal membrane.
At first, the cloacal membrane is directed caudally, but later it comes to
face ventrally.

27. Effects of Head and Tail Folds
on position of Other Structures (continue)
• We have traced the development of the embryo to a stage when the rudiments
of nervous system, the heart and the gut have been formed.
• What is the next?????????????????
• The development of individual organ system in details.
• Before that, we must study the development of
PLACENTA.

28. Some Additional Points of Interest (you can skip it)
i. In later life, remnant of primitive streak may give rise to
peculiar tumours that contain tissues derived from all three
germ layers. These tumours are seen in the sacral region and
are called SACROCOCCYGEAL TUMOURS.
ii. Experiments have shown that the formation of the neural tube is
induced by the notochord.
iii. Somitomeres are not confined to the region of somites. In the head
region, cranial to somite, somitomeres give origin to some
mesenchyme.
iv. Wharton’s jelly is rich in proteoglycans.

29. Time table of events described in this chapter
Age (in days) Developmental events
15 Primitive streak appears.
Definitive yolk sac is formed.
17 Notochord process appears.
Heart tube is seen in cardiogenic area.
Allantoic diverticulum is seen.
19 Intra-embryonic mesoderm is being formed.
Connecting stalk can be distinguished.
21 Neural groove is seen.
Head folds begins to form.
23 Closure of neural tube is seen.

38. Buccopharyngeal membrane
Lim-1 - head formation
Prechordal plate
Notochord HNF-3 - formation of notochord
Goosecoid - body axis formation
Primitive node Nodal - primitive streak formation
Primitive streak T-gene - movement of cells from primitive streak

41. 1. Future prosencephalon
2. Notochord
3. Neural tube
4. Pericardial cavity
7. Extraembryonic mesoderm
5. Cardiac tube 8. Throat
6. Pharyngeal membrane 9. Septum transversum

42. 1. Notochord
2. Neural fold
3. Amniotic cavity
4. Primitive streak
5. Primary endoderm
6. Cloacal membrane
7. Allantois
8. Body stalk

43. 1. Notochord
2. Neural tube
3. Amniotic cavity
5. Endoderm
6. Cloacal membrane
7. Allantois
8. Body stalk
9. Hind gut

44. Thank you a lot
next lecture
placenta