Fontanelle (also fontanel), from fontanella, from the Italian word for little spring or fountain: any of the six soft, membranous regions in the the cranium of an infant or fetus where bone tissue formation has yet to take place. The word was coined in Italy in the 13th century.

The term originally referred only to the anterior fontanelle, located on the top of the skull between the parietal bones. It is the largest and the last fontanelle to disappear, usually by the 24th month; in casual usage, it is referred to as the soft spot.

The fontanelles seem unrelated to either fountains or springs and three suggestions, all in reference to the anterior fontaenlle, have been offered to explain the link. In order of decreasing plausibility:

• The natural spring explanation: the ground at the site of a spring often has a slight hollow or indentation associated with it, as may the anterior fontanelle.
  

• The surgical explanation: in the 13th century a blood-letting procedure was developed for the treatment of brain and eye disorders in newborns. The technique involved creating a small opening in the anterior fontanelle so that blood, carrying supposed poisons, could flow out and effect a cure. The welling blood could have reminded one of a fountain.v
  

• The pulsation explanation: a pulse can be felt in the anterior fontanelle, perhaps reminding one of the pulsating flow typically seen in fountains with a vertical water spout. Such fountains would have been a common site in 13th century Italy.
  

The anterior fontanelle in a newborn commons.wikimedia.org