25 h -Appearance of eye; vertebral column visible; embryo begins to turn on left side (6 somites).
28 h -Ear begins (7 somites).
30 h -Amnion (embryonic membrane around embryo) begins. Primary function is to protect embryo against shock and sticking; also responsible for some albumen absorption. Chorion (embryonic membrane that fuses with allantois) begins; heartbeat begins (10 somites).
38 h -Cranial flexure and torsion evident; heartbeat moves blood (16 to 17 somites).
42 h -Thyroid begins.
48 h -Anterior pituitary and pineal glands begin to develop.