Sense of Obligation

blurted out.

'Phagocytes, platelets ... leucocytes ... everything seems normal.' Her voice was dull, exhausted, her eyes blinking with fatigue as she stared into the tube.

Anger at defeat burned through Brion. Even faced with failure he refused to accept it. He reached over her shoulder and savagely twisted the turret of microscope until the longest lens was in position. 'If you can't see anything—try the high power! It's there—I know it's there! I'll get you a tissue specimen.' He turned back to the disemboweled cadaver.

His back was turned and he did not see the sudden stiffening of her shoulders, or the sudden eagerness that seized her fingers as they adjusted the focus. But he did feel the wave of emotion that welled from her, impinging directly on his empathic sense. 'What is it?' he called to her, as if she had spoken aloud.

'Something ... something here,' she said, 'in this leucocyte. It's not a normal structure, but it's familiar. I've seen something like it before, but I just can't remember.' She turned away from the scope and unthinkingly pressed her gory knuckles to her forehead. 'I know I've seen it before.'

Brion squinted into the deserted microscope and made out a dim shape in the center of the field. It stood out sharply when he focused—the white, jellyfish shape of a single-celled leucocyte. To his untrained eye there was nothing unusual about it. He couldn't know what was strange—when he had no idea of what was normal.

 

'Do you see those spherical green shapes grouped together?' Lea asked. Before Brion could answer she gasped 'I remember now!' Her fatigue was forgotten in her excitement. 'Icerya purchasi that was the name, something like that. It's a coccid, a little scale insect. It had those same shapes collected together within its individual cells.'

'What do they mean? What is the connection with Dis?'

'I don't know,' she said, 'it's just that they look so similar. And I never saw anything like this in a human cell before. In the coccids, the green particles grow into a kind of yeast that lives within the insect. Not a parasite, but a real symbiote—'

Her eyes opened wide as she caught the significance of her own words. A symbiote—and Dis was the world where symbiosis and parasitism had become more advanced and complex than on any other planet. Lea's thoughts spun around this fact and chewed at the fringes of the logic. Brion could sense her concentration and absorption. He did nothing to break the mood. Her hands were clenched into fists, her eyes staring unseeingly at the wall as her mind raced.

Brion and Ulv sat quietly, watching her, waiting for her conclusions. The pieces were falling into shape at last.

Lea opened her clenched fists and smoothed them on her sodden skirt. She blinked and turned until she saw Brion. 'Is there a tool box here?' she asked.

Her words were so unexpected that it took Brion a moment to answer. Before he could say anything she spoke again.

'No hand tools, it would take too long. Could you find anything like a power saw—that would be ideal?' She turned back to the microscope, so he didn't have any opportunity to question her. Ulv was still looking at the body of the magter and had understood nothing of what they had said. Brion went out into the loading bay.

There was nothing he could use on the ground floor, so he took the stairs to the floor above. A corridor here passed by a number of rooms. All of the doors were locked, including one with the hopeful sign TOOL ROOM on it. He battered at the metal door with his shoulder without budging it. As he stopped to look for a way in he glanced at his watch.

Two o'clock! In ten hours the bombs would fall on Dis.

The need for haste tore at him. Yet there could be no noise—someone in the street might hear it. He quickly stripped off his shirt and wrapped it in a loose roll around the barrel of his gun, extending it in a loose tube in front of the barrel. Holding the rolled cloth in his left hand, he jammed the gun up tight against the door, the muzzle against the lock. The single shot was only a dull thud, inaudible outside of the building. Pieces of broken mechanism jarred and rattled inside the lock and the door swung open.

Lea was standing by the body when he came back, holding up the small power saw with a rotary blade. 'Will this do?' he asked. 'Runs off its own battery, almost fully charged, too.'

'Perfect,' she answered. 'You're both going to have to help me.' She switched into the Disan language. 'Ulv, would you find some place where you can watch the street without being seen. Signal me when it is empty. I'm afraid this saw is going to make a lot of noise.'

Ulv nodded and went out into the bay, climbing a heap of empty crates so he could peer through the small windows set high in the wall. He looked carefully in both directions, then waved to her to go ahead.

'Stand to one side and hold the cadaver's chin, Brion,' 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 a 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 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