Dr. Douglas Jones, a pediatrician at Johns Hopkins University Hospital, received the phone call on Tuesday. The caller was a doctor from a hospital in Salisbury, Md., and he had an unusual message: Siamese twins had been born at his hospital; could he transfer them by ambulance to Johns Hopkins? "Yes, of course," Jones replied.

Four days later, after extensive testing and hours of study of medical literature, eight surgeons operated for 10 hours on tiny Emily and Francesca Selvaggio, who were joined at the abdomen and shared one liver. Today, the Selvaggio twins lie in separate cribs at the hospital, where the newborns are in serious but stable condition.

Only about 22 other sets of Siamese twins ever have been separated, according to surgeons at Johns Hopkins. Siamese twins occur once in every 100,000 births, the result of the egg failing to divide completely to form identical twins.

Surgeons at the neonatal center of the huge Baltimore medical complex said they were pleased with their accomplishment and expect the twins to lead normal and healthy lives.

A relative of the twins' parents, Charles and Carol Selvaggio of Dagsboro, Del., said they were "elated" about the successful operation. Charles Selvaggio, a music teacher, and his wife, Carol, a nurse, also have a 1-year-old daughter.

The Selvaggio twins were described by surgeons as "beautiful, beautiful girls" who have dark hair and weigh 7 1/2 pounds each.

From the time the twins arrived at the hospital Tuesday, the surgeons, anesthesiologists and nurses in the hospital's neonatal intensive care unit had been examining them with great care.

First there were extensive tests to determine the position of the twins' organs and which organs they shared. As it turned out, the twins shared only their liver. "We were fortunate that the major organs were duplicated; each had her own circulatory system," said Dr. Jones.

As a result of that finding, the surgeons decided that it was safe to operate. But the question was when. Siamese twins usually are not separated until weeks--or even months or years--after birth, according to surgeons. Time was of essence in this case, the surgeons decided, because Baby A, who was later named Emily, had an obstruction in her intestine that would require an operation within a few days anyway.

The day before surgery, the surgeons and anesthesiologists had a dress rehearsal in the operating room. Using two identical baby dolls that were tied together at the waist with rubber bands, doctors tried to figure out how they would anesthetize the infants. "The problem was that they faced each other and normally no one is on top of a person when you give anesthesia," said anesthesiologist Dr. James Pepple.

Pepple decided that the infants would be placed on their sides on the operating table, and that one infant would be held so that her face was as far away from the other's as possible.

During the rehearsal, it was also decided that Francesca would wear a special surgical gown with a flap so that, once she was separated from her sister and moved to another operating table, the flap could go over her abdominal wound to keep it free from bacteria.

"The problem," said Dr. Alex Haller, "was how do you move her and not break the sterility of the operating area?"

At 1:15 p.m. Saturday, the surgeons made their first incision, cutting the skin and soft tissue joining the abdomens of the babies. Eventually, they divided the commonly shared liver. "The liver had grown so intimate to both babies that there was no clear cleavage plane to separate it," said Dr. Haller. "It was only after considerable anguish that we felt we had determined a plane where we could separate it."

After Francesca was moved to another operating table, surgeons continued operating on both infants, closing the raw edges of the liver, repairing and reconstructing the intestines and closing the large abdominal wound.

This was a special operation in many respects, with surgeons and anesthesiologists caring for Francesca wearing white surgical gowns and those caring for Emily wearing blue flower-print gowns to avoid confusion about which baby they were monitoring.

"From a surgical point of view, this is something we've never seen before (at Johns Hopkins)," Jones said yesterday at a press conference. "You have to be very imaginative and creative and a little lucky in the operating room to bring it off."