I. Gut Formation and Derivatives Embryonic endoderm: gut and its derivatives Mesoderm: connective tissues, blood vessels, muscles of gut and associated organs

A. Yolk sac 1. analogous to the archenteron seen in amphibians 2. endoderm forms disk adjacent to yolk or a yolk sac

3. flanks bend together ventrally - closes off endoderm - creates gut - called lateral folding

4. lateral folding enhanced by craniocaudal flexion - driven by rapid extension of neural plate

5. vitelline duct - originally wide opening between gut and yolk sac reduced to narrow passage

6. ends of gut tube are temporarily closed - buccopharyngeal membrane - cloacal membrane

B. Trachea 1. ventral out-pocketing near cranial end of digestive tube 2. pharynx - area between buccopharyngeal membrane and tracheal rudiment

3. tracheal rudiment: forms 2 lateral buds - bronchi - continue to enlarge to form lungs - epithelial lining of respiratory tree (endoderm) - other tissues (cartilage, CT, blood vessels: mesoderm)

C. Pharyngeal pouches 1. bulge out laterally from pharyngeal endoderm 2. displace surrounding mesenchyme 3. pharyngeal cleft - formed as pouch approaches overlying ectoderm

4. pharyngeal arches - columns of tissue that remain between pouches and clefts 5. primitive vertebrates: pouches and clefts fuse to form slits - gills and jaws in primitive fish 6. mammals: no slits - pouches and clefts assume many functions

7. first pouch: gives rise to middle ear bones - maintain connection to pharynx (auditory tube)

8. tympanic membrane - where the first pouch meets the first cleft 9. second pouch: palatine tonsil 10. fish: first pouch becomes primary jaw joint

10. fish: first pouch becomes primary jaw joint - mammal middle ear bones still resemble primitive jaw - retraceable in all mammals - recapitulation

Recapitulation: 1. primitive vertebrates: adult structures similar to embryonic stuctures 2. advanced vertebrates: adult stage is farther removed from embryonic stages

All vertebrate embryos look similar after organogenesis

Vertebrate embryos: - similar basic body plan - notochord and brain rudiments, sense organs, heart, gut - all have similar spatial arrangement

Called phylotypic stage: - stage shared by all species in a phylum or subphylum - also seen in insects and other arthropods - why reinvent the wheel?

D. Gut 1. foregut - esophagus and stomach 2. midgut - small intestine 3. hindgut - colon, rectum, and anal canal - endoderm invaginates to form bladder and urethra (just the epithelial inner lining)

Endoderm and ectoderm give rise to mainly epithelial structures - skin, lining of gut structures Mesoderm: surrounds epithelial layers of internal structures - walls of blood vessels and gut structures Mesoderm forms some epithelia and large amounts of mesenchyme - includes axial, paraxial, intermediate mesoderm, and the lateral plates