Week 2 = 2 cell types of embryoblast: epiblast (--> amniotic sac) & hypoblast (--> yolk sac). In other words, it's a bilaminar disk.
Gastrulation (3 germ cell layers), neurulation (form notochord, which induces ectoderm to become neuroectoderm --> neural plate). Also, blood islands form from mesodermal (mesenchymal) cells this week.
Heart begins to beat, limb buds form. Pronephros degenerates after Week 4.
Fetal movement; fetus looks like baby.
External genitals look male or female.
Fate of thyroglossal duct, which connects thyroid to tongue -- the location from which thyroid arises (i.e. floor of pharynx)
Foramen cecum... or, if not obliterated, then pyramidal lobe of thyroid.
If parts of duct persist: thyroglossal duct cyst = moves w/swallowing, unlike persistent cervical sinus.