NERVOUS SYSTEM:
• Neural plate → neural folds → NEURAL TUBE (closes from middle outward; cranial then caudal).
• Tube → 3 vesicles (forebrain, midbrain, hindbrain) → 5 vesicles (telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon).
• Spinal cord = caudal portion of neural tube.
• Neural crest → PNS + ganglia + adrenal medulla + melanocytes.

SENSE ORGANS:
• Eye: optic vesicle (forebrain) + lens placode (ectoderm) + surrounding mesenchyme.
• Ear: otic placode → otic vesicle → membranous labyrinth (cochlea + vestibule + semicircular canals).

MUSCULOSKELETAL:
• Bone: from somites (sclerotome) + lateral plate mesoderm (limbs).
• Skeletal muscle: from somites (myotome).
• Limbs: limb bud (mesoderm + ectoderm) develops with progressive proximodistal growth.

ENDOCRINE:
• Pituitary: anterior from oral ectoderm (Rathke's pouch), posterior from neural ectoderm (infundibulum).
• Thyroid: from foramen cecum of tongue floor.
• Adrenal: cortex from mesoderm; medulla from neural crest.

DEVELOPMENT — NERVOUS, SENSORY, MUSCULOSKELETAL, ENDOCRINE

I. NERVOUS SYSTEM:

A. Neurulation (recap):
• Neural plate → neural folds rise → neural tube forms by zipping shut from middle.
• Closure points: cranial + caudal neuropores.
• Failure of cranial closure → anencephaly; failure of caudal → spina bifida.

B. Brain development:
• Neural tube cranial end forms 3 PRIMARY VESICLES (~day 25):
– Prosencephalon (forebrain).
– Mesencephalon (midbrain).
– Rhombencephalon (hindbrain).
• These split into 5 SECONDARY VESICLES:
– Telencephalon → cerebral hemispheres + lateral ventricles + basal nuclei.
– Diencephalon → thalamus + hypothalamus + 3rd ventricle + posterior pituitary.
– Mesencephalon → midbrain + cerebral aqueduct.
– Metencephalon → pons + cerebellum.
– Myelencephalon → medulla oblongata + part of 4th ventricle.

C. Spinal Cord:
• Caudal portion of the neural tube.
• Three layers form the wall: ventricular zone (inner) → mantle zone (middle, becomes grey matter) → marginal zone (outer, becomes white matter).
• Mantle organises into dorsal horn (sensory) + ventral horn (motor) + lateral horn/intermediolateral autonomic region in species-dependent thoracolumbar segments.

D. Neural Crest derivatives (already detailed in germ-layer entry):
• PNS, autonomic ganglia, Schwann cells, adrenal medulla, melanocytes, etc.

II. SENSE ORGANS:

A. Eye:
• Optic vesicle outpouches from forebrain (diencephalon) → cups inward → forms optic cup (inner = neural retina; outer = pigmented retina).
• Lens placode (surface ectoderm) thickens, invaginates → lens vesicle → mature lens.
• Neural-crest mesenchyme contributes choroid, sclera and corneal stroma; extraocular muscles arise from cranial paraxial/preotic mesoderm.
• Cornea epithelium + conjunctiva = surface ectoderm.
• Optic nerve fibres grow from retina back into brain through optic stalk.

B. Ear:
• Otic placode (surface ectoderm beside hindbrain) thickens → otic pit → otic vesicle (otocyst).
• Otocyst splits into:
– Dorsal utricular part → utricle + semicircular canals.
– Ventral saccular part → saccule + cochlear duct.
• Surrounding mesenchyme → bony labyrinth.
• Middle ear: 1st pharyngeal pouch → tympanic cavity + Eustachian tube.
• Ossicles: from 1st (malleus, incus) + 2nd (stapes) pharyngeal arch cartilages.
• External ear derives chiefly from first-cleft ectoderm and arch tissues; the tympanic membrane has ectodermal, mesodermal and endodermal layers.

III. MUSCULOSKELETAL SYSTEM:

A. Skeleton:
• Vertebrae, ribs and much of the skull base arise from sclerotomes; the sternum arises from somatic lateral plate mesoderm.
• Vault of skull + facial bones → from neural crest.
• Limb skeleton → from limb-bud lateral plate mesoderm.
• Bones form by:
– Endochondral ossification — cartilage model first (most bones).
– Intramembranous ossification — directly in mesenchyme (flat bones of skull, mandible).

B. Skeletal Muscle:
• Body muscles → from MYOTOMES of somites.
• Limb muscles → from migratory cells leaving lateral somite into limb bud.
• Head muscles (mastication, expression) → from pharyngeal arches.
• Tongue muscles → occipital somites.

C. Limbs:
• Limb bud appears as a thickening on lateral body wall (~day 24-28 in chick; varies by species).
• Apical ectodermal ridge (AER) drives proximodistal growth.
• ZPA (zone of polarizing activity) controls anteroposterior patterning (Sonic hedgehog signaling).
• Limb develops in proximal-to-distal sequence (stylopod → zeugopod → autopod).
• Apoptosis between digit primordia separates fingers/toes (failure → syndactyly).

IV. ENDOCRINE GLANDS:

A. Pituitary: Dual origin.
• Adenohypophysis: Rathke's pouch (oral ectoderm).
• Neurohypophysis: infundibulum (downgrowth of diencephalon).

B. Thyroid:
• Foramen cecum at root of tongue → endodermal evagination descends through neck → settles below larynx.
• Thyroglossal duct remnant may persist as a cyst.
• C cells (calcitonin) arise through the ultimobranchial body, now considered primarily pharyngeal-endoderm derived in mammals; older texts attributed them to neural crest.

C. Parathyroid:
• External parathyroid → 3rd pharyngeal pouch.
• Internal parathyroid → 4th pharyngeal pouch (descends with thymus).

D. Adrenal:
• Cortex → mesoderm (similar to gonads).
• Medulla → neural crest (chromaffin cells = modified sympathetic neurons).

E. Pancreatic Islets: from endodermal pancreatic bud cells; differentiate into α, β, δ, PP cells.