Mouse foetus. Coloured scanning electron micrograph (SEM) of a 23-day-old mouse foetus (Mus musculus). The embryo is at full-term and the head, body, limbs and tail can be clearly distinguished. The embryo attaches to the uterine epithelium about five days after ovulation and the pregnancy lasts for approximately three weeks. Within the uterus the embryo lies inside a fluid- filled sac, known as the amniotic cavity, which is enclosed by double membranes. The fluid protects the embryo against the pressure of the internal organs of the mother and from any injury from the mother's movements.