The Prospective Mother, a Handbook for Women During Pregnancy. J. Morris Slemons

we have arranged that the water changes constantly. In terms of this illustration, the water corresponds to the mother's blood, rich in oxygen, mineral matter, and all other kinds of essential nutriment; and the fingers are the villi. The blood-vessels in the fingers, to go a step farther, represent the blood-vessels which exist within the villi, connecting with the umbilical cord, and passing by that route to the body of the child. The blood which thus circulates through the villi, it is important to emphasize, is the child's blood; it cannot escape through the coating of the villi, just as our blood cannot escape through the skin of the fingers. Similarly, the mother's blood cannot enter the child; the two circulations are absolutely separate and distinct.