Embryonic Foldingand Coelom

Key Events of Human Development during the fourth week (week 4) following fertilization or or clinical gestational age week 6, based on last menstrual period.
These notes cover the fourth week of embryonic development, which is the beginning of organogenesis, (specific tissues and systems are beginning to differentiate) from the trilaminar embryo.
On the embryo surface sensory placodes and limb buds appear. Sensory placodes (otic, lens, nasal) will form specific components of the ear, eye and nose. Limb buds form from ectoderm and mesoderm (somite) components and are the "paddle-like" projections from the trunk which will form all the upper and lower limb components.
Within the embryo, this period of organogensis is usually extended to cover until 8 weeks of development. Folding of the embryo continues and the earliest functioning organ is the heart. Other systems such as the circulatory, digestive, urogenital and nervous system begin to all take shape.
As systems are beginning to develop, each page in this section is only a brief summary with additional links to specific notes covering these systems/tissues. Note that most tissue development during week 4 will also be covered in the system development notes.