In the 3rd week:

1. Intraembryonic mesoderm forms
2. Notochord forms
3. Chorionic villi form

Intraembryonic mesoderm

This results in the formation of trilaminar germ disc from bilaminar germ disc. This is known as gastrulation

Gastrulation results in the formation of :

1. Ectoderm (faces amniotic cavity)
2. Mesoderm
3. Endoderm (faces yolk sac)

In summary:

1. Ectodermal cells in caudal end of bilaminar germ disc
2. Form primitive streak, growing cranially
3. Ectodermal cells insinuate themselves between ectoderm and endoderm
4. Forming the intraembryonic mesoderm

Formation of notochord

1. Primitive node and primitive pit form – thickening of ectoderm at the cranial end of primitive streak, where a pit forms in the middle
2. Cells proliferate and a solid rod of cells grows in cranial direction between ectoderm and endoderm – known as notochordal process
3. Notochordal process canalizes to form notochordal canal
4. Notochordal canal fuses with underlying endoderm to form notochordal endodermal plate
5. Which breaks down to form neurenteric canal – temporary connection between amniotic cavity and yolk sac
6. Remaining part of notochordal plate folds along longitudinal axis. forms a solid rod of cells, known as definitive notochord
7. Acts as a temporary axial skeleton

In summary

1. Primitive node and primitive pit
2. Notochordal process
3. Notochordal canal
4. Notochordal endodermal plate
5. Neurenteric canal
6. Notochordal plate folds
7. Definitive notochord

NB: The midline region, prochordal plate and cloacal membrane do not contain mesodermal cells and remain bilaminar.

The prochordal plate will later become the buccopharyngeal membrane (oral membrane)

Formation of chorionic villi

1. Primary villi: Syncytiotrophoblast – cytotrophoblast
2. Secondary villi: Syncytiotrophoblast – cytotrophoblast – extraembryonic mesoderm
3. Tertiary villi: Syncytiotrophoblast – cytotrophoblast – extraembryonic mesoderm – afferent and efferent capillaries

There are two types of villi:

• Anchoring villi – penetrates into decidua basalis and anchor chorion vesicle to wall of uterus
• Absorbing villi – branches in intervillus space

Anchoring villi: