Germ Layer Formation

After fertilization, important events that take place in the first week of development-
1. Cleavage  
2. Formation of morula (16-cell stage)
3. Blastocyst formation
4. Migration of the zygote
5. Implantation

2nd week: Formation of bilaminar germ disc
The 2nd week of development is known as the week of 2's:
a) The trophoblast differentiates into 2 layers; the cytotrophoblast and syncytiotropho-blast.
b) The embryoblast forms 2 layers; the epiblast and hypoblast.
c) The extraembryonic mesoderm splits into 2 layers; the somatic and splanchnic layers.
d) 2 cavities form; the amniotic and yolk sac cavities.

3rd week: Formation of trilaminar germ disc
- Gastrulation: The process of formation of three germ layers is called gastrulation.

The notochord is formed by the cells of the – Mesoderm

Reference-langman’s medical embryology/13th/P-86

Spinal cord- Neural tube
Adrenal cortex- Intermediate mesoderm
Ependymal cells- Neural tube

Neuroectoderm derivatives: 
1. Neural Tube: 
   a. Brain
   b. Spinal cord
   c. Neuron
   d. Neuroglia
   e. Ependymal cells

2. Neural Crest:
   a. Craniofacial bones & connective tissue
   b. Dermis in the face and neck
   c. Cranial nerve ganglia
   d. Spinal ganglia
   e. Sympathetic and pre-aortic ganglia
   f. Parasympathetic ganglia of GIT
   g. Glial cell
   h. Schwann cell
   i. C-cells of thyroid gland
   j. Melanocytes
   k. Odontoblast
   l. Adrenal medulla
   m. Leptomeninges (arachnoid & pia matter)
   n. Conotruncal septum in the heart

The process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo is called- Gastrulation

Cells that form the three primitive germ layers are derived from the – Epiblast

Ectodermal placodes are focal thickenings of the cranial ectoderm that generate many different components of the sensory systems of the head. Otic, nasal & lens placodes are found.

The structures representing all three components of embryonic disc is Tympanic membrane

Rathke's pouch and anterior pituitary develop from stomatodaeum (surface ectoderm) 

Mesoderm showing segmentation is - Paraxial