," she said.
"Hold it firmly so the head doesn't shake around when I cut. This
is going to be a little gruesome. I'm sorry. But it'll be the
fastest way to cut the bone." The saw bit into the skull.

Once Ulv waved them into silence, and shrank back himself into the
shadows next to the window. They waited impatiently until he gave
them the sign to continue again. Brion held steady while the saw
cut a circle completely around the skull.

"Finished," Lea said and the saw dropped from her limp fingers to
the floor. She massaged life back into her hands before she finished
the job. Carefully and delicately she removed the cap of bone from
the magter's head, exposing his brain to the shaft of light from
the window.

"You were right all the time, Brion," she said. "There is your alien."

XVI

Ulv joined them as they looked down at the exposed brain of the
magter. The thing was so clearly evident that even Ulv noticed it.

"I have seen dead animals and my people dead with their heads open,
but I have never seen anything like that before," he said.

"What is it?" Brion asked.

"The invader, the alien you were looking for," Lea told him.

The magter's brain was only two-thirds of what would have been its
normal size. Instead of filling the skull completely, it shared the
space with a green, amorphous shape. This was ridged somewhat like a
brain, but the green shape had still darker nodules and extensions.
Lea took her scalpel and gently prodded the dark moist mass.

"It reminds me very much of something that I've seen before on
Earth," she said. "The green-fly--_Drepanosiphum platanoides_--and
an unusual organ it has, called the pseudova. Now that I have seen
this growth in the magter's skull, I can think of a positive
parallel. The fly _Drepanosiphum_ also had a large green organ, only
it fills half of the body cavity instead of the head. Its identity
puzzled biologists for years, and they had a number of complex
theories to explain it. Finally someone managed to dissect and
examine it. The pseudova turned out to be a living plant, a
yeastlike growth that helps with the green-fly's digestion. It
produces enzymes that enable the fly to digest the great amounts
of sugar it gets from plant juice."

"That's not unusual," Brion said, puzzled. "Termites and human
beings are a couple of other creatures whose digestion is helped
by internal flora. What's the difference in the green-fly?"

"Reproduction, mainly.