The Stages of Human Embryonic Development

Stage 19 embryos have a greatest length of 16 to 18 mm and an estimated postfertilization age of approximately 45 to 47 days. The trunk is elongating and straightening. The head no longer forms a right angle with the back and is beginning to elevate. The hand and footplates contain distinct metacarpal and metatarsal condensations, respectively. The auricular hillocks are no longer evident as they have blended to form definitive parts of the external ear. The cochlear duct begins its 2 1/2 turn spiral with the tip turned upward. The mesonephric duct and ureter join close to the urinary bladder and continue to the horn of the bladder as a common excretory duct.