Human Development

1. Human Development Ch 28

2. Gastrulation rearranges the blastula to form a three-layered embryo with a primitive gut • Gastrulation rearranges the embryo into a triploblastic gastrula. Primary Germ Layers Ectoderm Endoderm Mesoderm

3. Fates of the Primary Germ Layers • Ectoderm • hair, nails, epidermis, brain, nerves • Mesoderm • notochord (in chordates), dermis, blood vessels, heart, bones, cartilage, muscle • Endoderm • internal lining of the gut and respiratory pathways, liver, pancreas

4. Body Cavities coelom Mesoderm Eucoelomate- body cavity completely lined with mesoderm gut Ectoderm Endoderm

5. The Formation of Primary Germ Layers

7. Gastrulation

8. Extraembryonic Membranes • Amnion: epiblast cells form a transparent sac filled with amniotic fluid • Provides a buoyant environment that protects the embryo • Helps maintain a constant homeostatic temperature • Allows freedom of movement and prevents parts from fusing together • Amniotic fluid comes from maternal blood, and later, fetal urine amnion

9. Extraembryonic Membranes • Yolk sac: a sac that hangs from the ventral surface of the embryo • Forms part of the digestive tube • Source of the earliest blood cells and blood vessels Yolk sac

10. Extraembryonic Membranes • Allantois: a small outpocketing at the caudal end of the yolk sac • Structural base for the umbilical cord • Becomes part of the urinary bladder • Chorion: helps form the placenta • Encloses the embryonic body and all other membranes Allantosis Chorion

16. Neurulation A: Embryonic disc accomplished gastrulation - ectoderm thickens B: Neural plate forms neural folds and neural groove

17. Neurulation C: Neural folds close D: Neural tube detached from surface ectoderm

18. Embryonic Folding