"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. 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 volume than normal it could fit into the remaining space in the cranial hollow. But the brain is full-sized—it is just that part of it is missing, absorbed by the symbiote."
"The frontal lobes," Brion said with sudden realization. "This hellish growth has performed a prefrontal lobotomy!"
"It's done even more than that," Lea said, separating the convolutions of the gray matter with her scalpel to uncover a green filament beneath. "These tendrils penetrate further back into the brain, but always remain in the cerebrum. The cerebellum appears to be untouched. Apparently just the higher functions of mankind have been interfered with, selectively. Destruction of the frontal lobes made the magter creatures without emotions or ability for really abstract thought. Apparently they survived better without these. There must have been some horrible failures before the right balance was struck. The final product is a man-plant-animal symbiote that is admirably adapted for survival on this disaster world. No emotions to cause complications or desires that might interfere with pure survival. Complete ruthlessness—mankind has always been strong on this anyway, so it didn't take much of a push."