I. Organogenesis - germ layers now arranged according to ultimate positions in body - germ layers interact to form organ rudiments

Later: - embryo has formed organ rudiments - basic body plan established - organs not yet functional, yet organogenesis is completed fairly early

Human schedule: cleavage takes about 2 weeks gastrulation takes another week organogenesis takes about 4 more weeks - completed after 6-8 weeks of development - remaining 7 months for functional specialization and growth

A. Neurulation (of neurula) 1. most conspicuous part of development

2. beginning of brain and spinal cord formation 3. best-studied example of organogenesis 4. neural plate: tall plate of cells derived from ectoderm

5. neural tube (hollow): formed by subsequent closure of neural plate in fish is a rod first, then becomes hollow

B. Amphibians (2 phases) - keyhole stage - closure of neural tube 1. neural ectoderm cells move posteriorly (toward blastopore) - part of epiboly 2. they then move dorsal and anterior to begin neurulation - become neural plate 3. neural groove develops along midline 4. neural folds arise

5. neural plate assumes keyhole shape - anterior part: brain - posterior part: spinal cord

6. remaining ectoderm cells become squamous - become epidermis - epidermal ectoderm

7. neural plate grows along axis 8. neural folds curl up and meet in dorsal midline

9. neural tube separates from future epidermis 10. some cells remain in between - neural crest cells - establish their own lineage

C. Birds 1. neural tube forms in “wake” of Hensen’s node 2. anterior always ahead of posterior in neurulation

3. lateral parts of neural plate from epiblast 4. at least some cells in median strip of neural plate derived from Hensen’s node

5. neural tube undergoes convergent extension - becomes longer, narrower, thicker 6. neural plate cells columnar - except those in median strip are wedge-shaped

7. wedge-shaped cells create hinge point (along entire tube) 8. these hinges facilitate closure of neural tube

D. Humans 1. similar to birds 2. anterior closure somewhat delayed

3. closure proceeds anterior to posterior (as in birds) - but begins in neck region 4. more bulk in anterior folds of human

5. anterior and posterior neuropores close a few days later

E. Fish 1. neural plate forms on dorsal half of epiblast 2. cells on both sides of median hinge point grow together 3. rounded rod: neural keel

4. lumen then forms - secondary neurulation

F. Primary neurulation occurs in birds, mammals, reptiles, and amphibians. - localized to head and trunk - land-dwelling vertebrates show secondary neurulation in embryonic tail regions