1. Development
Ch 29b

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 Eucoelomate- body cavity completely lined with mesoderm
gut
Ectoderm
Endoderm

5. The Formation of Primary Germ Layers

6. Gastrulation

7. 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

8. 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

9. 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

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

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

15. Embryonic Folding
The rapid growth of the neural tube is thought to cause the embryo to fold and bring the developing primitive heart caudal to the brain plate. Early in the third week of development, the germ disk has the appearance of a flat oval disk and is composed of two layers: the epiblast and the hypoplast. The first faces the amniotic cavity and the latter faces the yolk sac. A primitive groove, ending caudally with the primitive pit surrounded by a node, first appears at approximately 16 days of development and extends half the length of the embryo.
The primitive groove serves as a conduit for epiblast cells that detach from the edge of the groove and migrate inwards toward the hypoblast and replace it to form the endoderm. After the endoderm is formed, cells from the epiblast continue to migrate inwards to infiltrate the space between the epiblast and the endoderm to form the intraembryonic mesoderm. After this process is complete, the epiblast is termed the ectoderm