DAVID SPEARS LTD / SCIENCE PHOTO LIBRARY DAVID SPEARS LTD / SCIENCE PHOTO LIBRARY
Macrophotograph of a 15.5 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, which is mostly formed by water, protects the foetus against the pressure of the internal organs of the mother and from any injury from the mother's movement. The embryo of a rat attaches to the uterine epithelium about 5 days after ovulation and the pregnancy lasts roughly 22 days. Magnification unknown.
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