3 Germ Layers from gastrulation:

1. ECTODERM (outer):
• Surface ectoderm → epidermis + skin glands + hair + hoof + horn + claws + mammary gland + lens of eye + inner ear.
• Neural ectoderm → brain + spinal cord (CNS) + neural retina.
• Neural crest → peripheral nerves + ganglia + adrenal medulla + melanocytes + parts of skull.

2. MESODERM (middle):
• Paraxial → vertebrae + ribs + skeletal muscle + dermis.
• Intermediate → kidneys + ureters + gonads.
• Lateral plate → limb skeleton/CT + heart + blood + body wall + serous membranes.

3. ENDODERM (inner):
• Lining of digestive tract (epithelium only).
• Lining of respiratory tract.
• Liver, pancreas.
• Thyroid, parathyroid, thymus epithelium.
• Lining of urinary bladder + much of urethra.

GERM LAYER DERIVATIVES — A COMPLETE MAP

Every adult tissue can be traced to one of the three primary germ layers. Memorising this map is foundational for understanding development.

I. ECTODERM (outer layer):

A. Surface Ectoderm:
• Epidermis of skin + all its derivatives:
– Hair, hoof, horn, claw, beak, scale, feather.
– Sebaceous glands, sweat glands, mammary glands.
– Salivary-gland epithelial origin must be stated by gland and species; it should not be assigned universally to oral ectoderm.
• Lens of eye (from lens placode).
• Cornea epithelium (anterior layer).
• Inner ear (otic placode → membranous labyrinth: cochlea, vestibule, semicircular canals).
• Anterior pituitary (from oral ectoderm — Rathke's pouch).
• Enamel of teeth (from oral ectoderm; dentine, pulp and much craniofacial supporting tissue are neural-crest ectomesenchymal; enamel is ectodermal).
• Tip of GIT epithelium: lining of mouth, pharynx (front part), distal anal canal.

B. Neuroectoderm (Neural Plate / Tube):
• Brain.
• Spinal cord.
• Retina (neural part) + optic nerve.
• Pineal gland.
• Posterior pituitary (neurohypophysis).
• Astrocytes + oligodendrocytes + ependymal cells.

C. Neural Crest:
The 'fourth germ layer' — multipotent cells that pinch off from neural tube edges and migrate widely. Derivatives:
• Peripheral nervous system: cranial + dorsal root ganglia, sympathetic chain, parasympathetic ganglia, enteric nervous system.
• Schwann cells + satellite cells.
• Adrenal medulla (chromaffin cells).
• Melanocytes.
• Much of the facial and anterior cranial skeleton; posterior cranial-vault and cranial-base regions have important mesodermal contributions.
• Dental papilla (dentine, pulp).
• Thyroid C cells arise through the ultimobranchial body and are now considered primarily pharyngeal-endoderm derived in mammals; older texts attributed them to neural crest.
• Smooth muscle of large arteries near heart.
• Aorticopulmonary septum.

II. MESODERM (middle layer):

A. Paraxial Mesoderm → SOMITES
Each somite splits into:
• Sclerotome → vertebrae + ribs + skull base.
• Myotome → skeletal muscle of trunk + limbs.
• Dermatome → dermis of dorsal skin.

B. Intermediate Mesoderm:
• Pronephros + Mesonephros + Metanephros (kidneys; only metanephros persists in adult).
• Ureters.
• Gonads (testes + ovaries).
• Mesonephric and paramesonephric ducts form much of the internal reproductive tract; the vagina has mixed paramesonephric and urogenital-sinus origins, with species-specific proportions.

C. Lateral Plate Mesoderm (splits into somatic + splanchnic):
• Somatic (parietal) mesoderm:
– Limb skeleton (bones, joints, ligaments).
– Limb CT, dermis of ventral body wall.
– Parietal layer of serous membranes (parietal pleura/pericardium/peritoneum).
– Sternum.
• Splanchnic (visceral) mesoderm:
– Heart (myocardium + epicardium + endocardial cushions partly).
– Smooth muscle of gut wall.
– Visceral layer of serous membranes.
– Adrenal cortex.
– Spleen.
– Blood + blood vessels (haematopoiesis from yolk-sac mesoderm initially).

III. ENDODERM (inner layer):

A. GIT lining: entire epithelium + glands of:
• Pharynx (caudal part), oesophagus, stomach, small + large intestine, rectum (cranial 2/3), gall bladder, liver, pancreas.
• Note: muscle + CT of the wall come from splanchnic mesoderm — only the lining is endodermal.

B. Respiratory tract lining:
• Larynx, trachea, bronchi, bronchioles, alveolar lining (Type I + II pneumocytes).

C. Glandular structures:
• Liver (hepatocytes + bile duct epithelium).
• Pancreas (acinar + islet cells).
• Thyroid follicles.
• Parathyroid glands.
• Thymus (epithelial reticular cells; lymphocytes are mesodermal).
• Tonsils (epithelium).

D. Urogenital lining:
• Urinary bladder (most of), urethra (most of).
• Vestibule of vagina.
• Lining of cloaca in birds.

E. Other:
• Auditory tube + middle ear cavity epithelium.
• Tympanic membrane (inner layer).