1. The document discusses critical periods of human embryonic development from the first through eighth weeks. During this time, organ systems develop from the three germ layers and major malformations can occur if development is disrupted. 2. Specific processes discussed include gastrulation, neurulation, heart development and septation, palate formation, and limb bud emergence. 3. Teratogens acting during critical periods can cause defects such as neural tube defects, holoprosencephaly, cardiac defects, and limb abnormalities. Disorders from disrupted development of neural crest cells are also examined.