Human Body

Reproduction and Human Development

What are some major developmental events during the embryonic period?

At the end of the embryonic period (eighth week of development), all of the major external features (ears, eyes, mouth, upper and lower limbs, fingers, and toes) are formed and the major organ systems are nearing completion.

Major Developmental Events during the Embryonic Period
Time Period Major Developments
Week three Neural tube, primitive body cavities and cardiovascular system form
Week four Heart is beating; upper limb buds and primitive ears visible; lower
limb buds and primitive eye lenses appear shortly after ears
Week five Brain develops rapidly; head grows disproportionately; hand plates develop
Week six Limb buds differentiate noticeably; retinal pigment accentuates eyes
Week seven Limbs differentiate rapidly
Week eight Embryo appears human; external ears are visible; fingers, toes lengthen;
external genitalia are visible, but are not distinctly male or female

