During the third week of development (days 14-21): 1. The primitive streak forms and gastrulation begins, establishing the three germ layers - ectoderm, mesoderm, and endoderm. 2. Neural induction occurs as the notochord induces the ectoderm to form the neural plate. Left-right asymmetry is also established. 3. Gastrulation involves cell movements that position the germ layers and establish the body plan and central axis of the embryo through structures like the primitive streak, primitive groove/pit, and notochord. The fate map of tissues is also established.

1. Third week of development
Dr Jyoti Chopra
Professor
KGMU, Lucknow

2. Week 3: Days 14-21
• Primitive streak forms
• Gastrulation forms tri-laminar embryo
• Neural induction
• Left-right asymmetry
• 0.4mm - 2.0mm

3. Gastrulation
• Formative process by which three primary embryonic germ layers and
axial orientation is established in embryo:
• Ectoderm: outside, surrounds other layers later in development, generates
skin and nervous tissue
• Mesoderm: middle layer, generates most of the muscle, blood and
connective tissues of the body and placenta
• Endoderm: eventually most interior of embryo, generates the epithelial lining
and associated glands of the gut, lung, and urogenital tracts

4. Formation of Prechordal plate
At one end cubical cells of hypoblast – columnar
Determines central axis of embryo
Future head end

5. Formation of Primitive streak
Epiblast cells- central axis- near tail end- proliferate
Oval- elongated
Cranial end – Primitive node in center primitive pit
Cranio-caudal axis, dorsal-ventral surface, right-left side

6. Primitive groove
Primitive pit

7. Formation of intraembryonic
mesoderm
Formation of endoderm
Formation of ectoderm
All derived from epiblast
Fate
Diminishes in size,
becomes
insignificant in
sacrococcygeal
region- by end of
4th week

8. If the primitive streak fails to regress, multipotent primitive streak cells can develop into
multi-lineage tumors (containing ecto-, meso-, and endodermal tissues).
Sacrococcygeal teratoma

9. Cell movements during gastrulation

10. Buccopharyngeal and
Cloacal membrane

11. Formation of Notochord

15. Notochord degenerates as vertebral bodies are formed
Persists as Nucleus pulposus and Apical ligament of dens
Functions as Primary inductor- induces overlying ectoderm
to form neural plate

16. Fate Map established during
gastrulation