Primitive Gut Tube = endodermal tube from buccopharyngeal membrane → cloacal membrane.

3 Regions:
• Foregut: pharynx → cranial duodenum (up to bile duct opening). Branches: liver, gallbladder, pancreas, lungs, thyroid, thymus, parathyroid.
• Midgut: caudal duodenum → jejunum, ileum, cecum and species-specific proximal colonic segments. Forms a ventral loop with vitelline duct → herniates into umbilicus → returns + rotates.
• Hindgut: species-specific distal colonic segments → cloaca → rectum + anus.

Liver + Pancreas: Endodermal buds from foregut.

Tongue: From pharyngeal arches (1st + 3rd + 4th).

Teeth: Enamel = ectoderm (oral); dentine + pulp + cementum = mesoderm/neural crest.

DEVELOPMENT OF THE DIGESTIVE SYSTEM

I. PRIMITIVE GUT TUBE:
The endoderm rolls into a tube from cranial to caudal as the embryo folds. It is divided into 3 regions, defined by their blood supply:

1. FOREGUT (supplied by celiac artery):
• Pharynx + oesophagus + stomach + cranial half of duodenum (up to opening of bile duct) + liver + gall bladder + pancreas + respiratory primordium.

2. MIDGUT (supplied by cranial mesenteric artery):
• Caudal half of duodenum + jejunum + ileum + cecum and species-specific portions of the ascending/proximal colon.
• Connected to yolk sac by vitelline duct (which normally closes; if not → congenital anomaly).
• Loop herniates into umbilicus (physiological umbilical hernia), then withdraws + undergoes species-specific rotation as it returns. Failure of return produces omphalocele, distinct from an umbilical hernia.

3. HINDGUT (supplied by caudal mesenteric artery):
• species-specific distal colonic segments + descending colon + rectum + cranial part of anal canal + most of urinary bladder + urethra.
• Ends in CLOACA — common chamber that splits into rectum (dorsal) + urogenital sinus (ventral) by the urorectal septum.
• In birds, cloaca PERSISTS as the common cloacal opening.

II. STOMACH DEVELOPMENT:
• Begins as a simple dilatation of foregut.
• Differential growth and species-specific rotation establish the adult stomach; the classic human 90° description must not be applied unchanged to all domestic mammals.
• In ruminants: stomach undergoes massive expansion + sub-divides into 4 compartments from gastric primordium (rumen, reticulum, omasum, abomasum). The forestomachs derive from the cardiac region; the abomasum is the homolog of the simple stomach.

III. LIVER & PANCREAS:
• Hepatic diverticulum: endodermal bud from ventral foregut at duodenum level.
– Forms hepatocytes, bile duct system, gall bladder.
– Mesoderm from septum transversum forms the CT, Kupffer cells, haematopoietic cells.
• Pancreatic primordia: TWO endodermal buds from duodenum:
– Dorsal pancreatic bud → most of pancreas + accessory duct.
– Ventral pancreatic bud → uncinate process + main duct.
– Buds fuse during gut rotation.

IV. TONGUE:
• Develops from pharyngeal floor + arches:
– Anterior 2/3 = body of tongue → derived from 1st pharyngeal arch (lateral lingual swellings + tuberculum impar).
– Posterior 1/3 = root → from 3rd + 4th arches (copula + hypobranchial eminence).
• Innervation reflects origin:
– Anterior 2/3: general sensation = CN V (mandibular); taste = CN VII (chorda tympani).
– Posterior 1/3: general sensation + taste = CN IX.
– Epiglottic region: CN X.
• Muscles of tongue: from occipital somites → motor by CN XII.

V. TEETH:
• Tooth bud develops from oral ectoderm + neural crest mesenchyme.
• Layers:
– Enamel organ (ectodermal) → ameloblasts → secrete ENAMEL (the only ectodermal hard tissue).
– Dental papilla (mesenchymal — neural crest origin) → odontoblasts → secrete DENTINE + PULP remains.
– Dental sac (follicle) → cementum + periodontal ligament.
• Diphyodont (deciduous + permanent) in mammals.

VI. SALIVARY GLANDS:
• Endodermal/ectodermal epithelial buds invade mesenchyme; develop into compound tubuloalveolar glands.
• Parotid: ectodermal origin.
• Mandibular & Sublingual: endodermal origin.

VII. PHARYNGEAL POUCHES (4 well-defined pairs; a rudimentary fifth component may join the fourth):
1. → Auditory tube + middle ear + tympanic cavity.
2. → Tonsillar fossa.
3. → Thymus + external parathyroid in domestic mammals.
4. → Internal parathyroid + ultimobranchial body (source of thyroid C cells, now considered primarily pharyngeal-endoderm derived in mammals); a rudimentary caudal component is sometimes historically called a fifth pouch.