1. Third week of Development

2. view of the germ disc at the end of the second week of development

3. Dorsal aspect of an 18-day embryo
The embryo has a pear-shaped appearance and shows the primitive streak and node at its caudal end on the surface o the epiblast

4. The hypoblast and epiblast are in contact with each other, and the primitive streak forms a shallow groove in the caudal region of the embryo
Primitive streak : a narrow groove with slightly bulging regions on either sides on the surface of the epiblast in the caudal region of the embryo .
Primitive node : slightly elevated area on the cephalic end of the primitive streak surrounding the small primitive pit .

5. Dorsal side of the germ disc from a 16-day embryo indicating the movement of surface epiblast cells
Cells of the epiblast migrate towards the primitive streak and node and move inwards ( invaginate ) through the streak and node . Some of the invaginated cells displace the hypoblast creating the embryonic endoderm . Others come to lie between the epiblast and newly created endoderm to form mesoderm . Cells remaining in the epiblast form ectoderm .

6. Gastrulation
Is the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. This process OCCURS during the third week of development with the appearance of a primitive streak and node on the surface of the epiblast . The epiblast, through the process of gastrulation, is the source of all of the germ layers in the embryo .

7. As more and more cells move between the epiblast and hypoblast layers , they begin to spread laterally and cranially gradually, they migrate beyond the margin of the disc and establish contact with the extraembryonic mesoderm covering the yolk sac and amnion.
In the cephalic direction, they pass on each side of the Prechordal plate

8. The prechordal plate: thickened localized region in the cephalic edge of the endoderm . It forms between the tip of the notochord and the Buccopharyngeal membrane .The prechordal plate will be important for induction of the forebrain and it contributes to the general head mesenchyme .
The buccopharyngeal membrane at the cranial end of the disc consists of a small region of tightly adherent ectoderm and endoderm cells that represents the future opening of the oral cavity.

9. Notochord
A prolongation undercover the ectoderm . It arises from the primitive node and extends cephalically as far as the prechordal plate . This prolongation is known as the notochordal process . The primitive pit may extend into the notochordal process as the notochordal canal .The floor of this channel becomes intercalated in the endoderm and begins to disintegrate allowing communication between the amniotic cavity and the yolk sac ( Neuroenteric canal ) . The roof of the notochordal process form the notochordal plate .

10. B. cross section through the region of the notochordal plate
B. cross section through the region of the notochordal plate. Soon the notochordal plate will detach from the endoderm to form the definitive notochord.
C. Schematic view showing the definitive notochord.

11. The cloacal membrane is formed at the caudal end of the embryonic disc This membrane, which is similar in structure to the buccopharyngeal membrane, consists of tightly adherent ectoderm and endoderm cells with no intervening mesoderm.
allantoenteric diverticulum, or Allantois: the posterior wall of the yolk sac forms a small diverticulum that extends into the connecting stalk. This diverticulum appears around the 16th day of development . in some lower vertebrates, It serves as a reservoir for excretion products of the renal system. In humans, it remains rudimentary but may be involved in abnormalities of bladder development .

12. Fate map for epiblast cells
Notochord-n-: Epiblast cells migrating at the cranialmost part of the node .
Paraxial mesoderm( somitomeres and somites ) -pm- : Epiblast cells migrating at the lateral edges of the node and the cranial end of the streak .
Intermediate mesoderm( urogenital system ) -im- epiblast cells migrating trough the midstreak region
Lateral plate mesoderm ( body wall ) : –lpm-epiblast cells migrating through the more caudal part o the streak
extraembryonic mesoderm (eem; chorion): epiblast cells migrating through the most caudal part . the other source of this tissue is the primitive yolk sac (hypoblast).

13. Growth of the embryonic disc
The embryonic disc, initially flat and round, gradually becomes elongated, with a broad cephalic and a narrow caudal end because of the continuous migration of cells from the primitive streak region in a cephalic direction.
In the cephalic part, germ layers begin their specific differentiation by the middle of the third week, whereas in the caudal part, differentiation begins by the end of the fourth week. This causing the embryo to develop cephalocaudally .
Invagination of surface cells in the primitive streak and their subsequent migration forward and laterally continues until the end of the fourth week. At that stage, the primitive streak shows regressive changes and soon disappears.

14. Clinical Correlates
Teratogenesis Associated with Gastrulation: The initiation of gastrulation at the beginning of the 3rd week is a highly sensitive stage for teratogenic insult. At this time, fate maps can be made for various organ systems, such as the eyes and brain anlage, and these cell populations may be damaged by teratogens) e.g. high doses of alcohol ).
Tumors Associated with Gastrulation : Sometimes, remnants of the primitive streak persist in the sacrococcygeal region. These clusters of pluripotent cells proliferate and form tumors, known as sacrococcygeal teratomas, that commonly contain tissues derived from all three germ layers . This is the most common tumor in newborns. These tumors may also arise from primordial germ cells (PGCs) that fail to migrate to the gonadal ridge.

16. Trophoblast (villous structure)
the trophoblast is characterized by primary villi that consist of a cytotrophoblastic core covered by a syncytial layer .
During further development, mesodermal cells penetrate the core of primary villi and grow toward the decidua. The newly formed structure is known as a secondary villus .
By the end of the third week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system . The villus is now known as a tertiary villus or definitive placental villus.

17. Trophoblast
cytotrophoblastic cells in the villi penetrate progressively into the overlying syncytium until they reach the maternal endometrium. Here they establish contact with similar extensions of neighboring villous stems, forming a thin outer cytotrophoblast shell . This shell gradually surrounds the trophoblast entirely and attaches the chorionic sac firmly to the maternal endometrial tissue.

18. Trophoblast
Stem or anchoring villi: those Villi extend from the chorionic plate to the decidua basalis (decidual plate: the part of the endometrium participate in the formation of the placenta ).
Free (terminal) villi : Those villi that branch from the sides of stem villi . Exchange of nutrients occur through these villi .

19. Chorionic cavity
The chorionic cavity, meanwhile, becomes larger, and by the 19th or 20th day, the embryo is attached to its trophoblastic shell by a narrow connecting stalk . The connecting stalk later develops into the umbilical cord, which forms the connection between placenta and embryo.

20. Thank You
Next lecture is Embryonic peroid