Tongue and esophagus begin; embryo separates from yolk sac; allantois grows through amnion; contractions occur in amnion wall; adrenal development begins; pronephros (nonfunctional kidney) disappears; metanephros (definitive or final kidney) begins; proventriculus, gizzard, ceca, large intestine begin. Pigment visible in eye (dark eye).
Day 5: Reproductive system and differentiation of sex appear; thymus, bursa of Fabricius, duodenal loop begin; chorion and allantois begin to fuse; mesonephros begins to function; first cartilage present.
Day 7: Digits appear; comb growth begins; egg tooth begins; melanin produced; absorption of mineral from shell begins. Chorioallantois is attached to inner shell membrane and growth around the inner surface is progressing.