Macrophotograph of a 10.2 day old foetus of a rat. The foetus lies inside a fluid filled sac, known as the amniotic cavity, which is enclosed by a double membrane formed by two layers called the amnion (inner) and the outer chorion. The fluid protects the foetus against the pressure of the internal organs of the mother and from any injury from the mother's movements. At top left the placenta (red) is also visible. The placenta is firmly attached to the uterus and is connected to the foetus through the umbilical cord (not visible here). The embryo attaches to the uterine epithelium about 5 days after ovulation and the pregnancy lasts 22 days. Magnification unknown.