1. Animal Development Chapter 47
Bozeman Tutorial -- Development: Timing and Coordination (13:41)

2. Three Stages of Embryonic Development
Embryonic development is a succession of mitotic divisions which result in the single cell zygote becoming a complete organism consisting of trillions of cells.
1. Cell division: increase in the number of cells.
2. Differentiation: development of specialized cells that are organized into tissues and organs.
3. Morphogenesis: physical processes that give shape to an animal's body and organs.

3. Gastrulation
Gastrulation forms a three-layered embryo with a primitive gut
Three layers produced by gastrulation are tissues called embryonic germ layers.
Ectoderm (nervous system and outer layer of skin).
Endoderm (lining of the digestive tract and liver and pancreas).
Mesoderm (kidneys, heart, muscles, inner layer of the skin, and most other organs).

4. “It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.” ~~ Lewis Wolpert
Animals are classified on the basis of how these tissues are organized.
Acoelomates – no body cavity between the gut and the body wall (flatworms).
Pseudocoelomates – have a body cavity, but not completely lined with mesoderm (roundworms).
Coelomates – have a “true coelom”, a body cavity completely lined with mesoderm; body cavities may develop differently.

7. Gastrulation

8. Morphogenesis
In humans, all major organs have developed from the three germ layers in the first 3 months of pregnancy.
Development of cells depends on:
1. Polarity of the embryo -- anterior-posterior, dorsal-ventral, left-right; established by morphogens (proteins accumulate in cells).
2. Homeotic genes – specify where body parts will develop; contain a “homeobox sequence” (Hox, for short) which is present in many life forms; Hox genes make proteins which turn other genes on/off during development.