During the third week of development, gastrulation occurs where the three germ layers (ectoderm, mesoderm, endoderm) are formed. The notochord also begins developing from epiblast cells that ingress through the primitive streak and primitive node. These cells form the notochordal process which then fuses with endoderm and detaches to form the definitive notochord cord between the ectoderm and endoderm. The mesoderm organizes into three segments - paraxial, intermediate, and lateral plate mesoderm - which will give rise to muscles, skeleton, urinary/genital systems, and other tissues.

1. 3rd WEEK OF
DEVELOPMENTMuhammad Ishaque Abbasi
Lecturer
Department of Eastern Medicine
University of Balochistan

2. Third week of development:
 Gastrulation.
 Formation of Notochord

3. Gastrulation: 3 Germ Layers Formed
Ectoderm (forms from
epiblast)
Nervous system
Epidermis
Endoderm (forms from
hypoblast)
Mucosae (eg: GI-tract
Associated glands
Mesoderm
Everything else

4. Gastrulation:
 During the third week of gestation is gastrulation start.
 The Gastrulation is process that establishes all three layers (ectoderm,
mesoderm, & endoderm) in embryo.
 The Gastrulation begins with formation of the primitive streak on
surface of epiblast.
 At the end of 16 day a small circular swelling appears at the cranial end
of Primitive streak this swelling called primitive node, which show small
depression called primitive pit.
 Cells of the epiblast migrate toward the primitive streak & upon the
arrival in the region of the streak, they become flask-shaped, detach
from the epiblast and slip beneath it and this inward movement is known
as invagination.

5.  Once the cells have invaginated some displace the hypoblast, creating the
embryonic endoderm, and others come to lie between the epiblast and
newly created endoderm to from mesoderm. Cell remaining in the
epiblast then form ectoderm.
 These 3 layers will give rise to all of the tissues and organs in the embryo.
 The earliest epiblast cells that invaginated through the cranial most part
of the primitive node form a compact mass of cells called prechodral
plate.
 Others epiblast cells that ingress through the cranial part of the primitive
node give rise to a midline, elongated structure called notochord.

6. Formation of the Notochord:
 The notochord is a cellular cord
that defines the primitive axes
of the embryonic body.
(vertebral column develop
around the notochord in later
development).
 The formation (development) of
Notochord occurs in two stages.
1. Formation of notochordal
process.
2. Formation of notochord.

7.  Formation of notochordal process: those epiblast cells which invaginated
into the cranial part of the primitive pit are known as prenotochordal
cells, these cells give rise to a tubular structure called notochordal
process. This lies between the embryonic ectoderm and endoderm. The
cavity present within the notochordal process is call notochordal canal.
 Formation of notochord: at day 18th the development the floor of the
notochordal process fuses with the underlying endoderm. The fused layer
soon degenerate and disappear, resulting in the loss of floor of the
notochordal process. After the disappear floor, the roof of the notochord
process remain in the form of a curved plate which is called notochodal
plate. This plate lies in the roof of the yolk sac and along its both margins,
its cells fuse with the endodermal cells lining the roof of the yolk sac.
Later these cell detach from the endodermal cells, proliferate and
gradually roll up to form solid cord of cells called definitive notochord. It
is b/w ectoderm and endoderm.

8. Organization of the intraembryonic
mesoderm:
Mesoderm develop b/w ectoderm and
endoderm and becomes organized
into three segments
Paraxial mesoderm.
Intermediate mesoderm
Lateral plate mesoderm

9.  Paraxial mesoderm:
 The mesoderm immediately lateral to the notochord thickens to form a
longitudinal column.
 Derivatives of the paraxial mesoderm include axial skeleton, voluntary
musculature, and part of the dermis of skin.
 Intermediate mesoderm:
 This is a relatively thinner column of mesoderm which lie b/w the paraxial
mesoderm and lateral plate mesoderm.
 This segment gives rise to the urinary system and some parts of the
genital system.

10.  Lateral plate mesoderm:
 This is lateral to the intermediate mesoderm and this is thin and sheet-
like. It also known as lateral mesoderm.
 The right and left parts of the LPM are joined to each other in the cranial
part of the embryonic disc by the cardiogenic plat which lies anterior to
the oropharyngeal membrane.
 At the margins of the embryonic disc, the LPM makes contact with the
extraembryonic mesoderm ( which covering the amniotic cavity and yolk
sac).
 The lateral mesoderm splits into two layers b/w which a cavity appears
which is called intraembryonic coelom .