All the other gut-living plants have to enter
the host and establish themselves as outsiders, permitted to remain
as long as they are useful. The green-fly and its yeast plant have a
permanent symbiotic relationship that is essential to the existence
of both. The plant spores appear in many places throughout the fly's
body--but they are _always_ in the germ cells. Every egg cell has
some, and every egg that grows to maturity is infected with the
plant spores. The continuation of the symbiosis is unbroken and
guaranteed."

"Do you think those green spheres in the magter's blood cells could
be the same kind of thing?" Brion asked.

"I'm sure of it," Lea said. "It must be the same process. There are
probably green spheres throughout the magters' bodies, spores or
offspring of those things in their brains. Enough will find their
way to the germ cells to make sure that every young magter is
infected at birth. While the child is growing, so is the symbiote.
Probably a lot faster, since it seems to be a simpler organism.
I imagine it is well established in the brain pan within the first
six months of the infant's life."

"But why?" Brion asked. "What does it do?"

"I'm only guessing now, but there is plenty of evidence that gives
us an idea of its function. I'm willing to bet that the symbiote
itself is not a simple organism, it's probably an amalgam of plant
and animal like most of the other creatures on Dis. The thing is
just too complex to have developed since mankind has been on this
planet. The magter must have caught the symbiotic infection eating
some Disan animal. The symbiote lived and flourished in its new
environment, well protected by a bony skull in a long-lived host.
In exchange for food, oxygen and comfort, the brain-symbiote must
generate hormones and enzymes that enable the magter to survive.
Some of these might aid digestion, enabling the magter to eat any
plant or animal life they can lay their hands on. The symbiote might
produce sugars, scavenge the blood of toxins--there are so many
things it could do. Things it must have done, since the magter are
obviously the dominant life form on this planet. They paid a high
price for the symbiote, but it didn't matter to race survival until
now. Did you notice that the magter's brain is no smaller than
normal?"

"It must be--or how else could that brain-symbiote fit in inside
the skull with it?" Brion said.

"If the magter's total brain were smaller in volu