"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 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 has 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 away. 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. 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?"