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

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

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

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

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

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

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

8. 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)

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

10. 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

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

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

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

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

16. All vertebrate embryos look similar after organogenesis

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

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

19. 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)

20. 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