Open Fontanel: Congenital Open Fontanel is seen in several toy dog breeds, but most often it's seen in Chihuahuas. Occasionally is will appear in a Cavalier King Charles Spaniel. The fontanel is the area at the top of the skull where the bone plates merge. When babies (human and dog) are born the bony plates of the skull are usually partly soft and separated. Usually this area closes to about the size of a dime by the time the puppy is about one year old, but sometimes the bony plates never completely fuse, leaving a hole in the top of the skull that has a soft mushy feel when you touch it. The affected pup can still live a long, full and active life. There is no treatment for an open fontanel. Since open fontanels are either hereditary or genetic, once a good breeder determines which parent dog carries the gene the parent dog should be retired from the breeding program and spayed or neutered. Dogs with open fontanels should not be bred.