Morphotrophic

This is an excerpt from the novel Morphotrophic by Greg Egan, first published in 2024. All rights reserved.

As soon as Marla opened her eyes and saw the patch of sunlight on the wall of her bedroom, she knew that something was amiss. The light from the east-facing window above her bed had fallen in more or less the same place the morning before – low on the wall, stretching partway across the floor – so if she’d woken at the usual time again, there was nothing about its location that should have surprised her. But after a moment she realised that the illuminated surfaces were glistening, catching the sunlight and casting it back with an uncharacteristic sheen. It was as if she’d mopped the floor and the linoleum was still wet – except that she hadn’t, and even if water had leaked into her room from a spill elsewhere, the same slickness continued up across the wall in a thoroughly gravity-defying manner. So it was more like a layer of varnish than water, but that made even less sense.

Marla turned beneath the sheets and began to rise, steadying herself with her right hand on the mattress. As she sat on the edge of the bed, prepared to step off it, she looked down and saw that her left arm was absent from elbow to fingertips, and her left leg had vanished from the knee to the toes.

She bellowed with alarm, then squeezed her eyes shut, willing the whole scene to be an invention of her sleep-addled brain. Without the sight of the amputations to distress her, she managed to observe that she was suffering almost no discomfort; her gut was tight from panic and her skin was tingling, but there was nothing like the kind of pain she would have expected if some animal or human assailant had torn any part of her limbs away.

She opened her eyes just as her mother, Silvia, flung the door open, asking impatiently, “What is it? What’s wrong?” Marla couldn’t bring herself to speak, but she assumed that once the appearance of her body hit home, it would answer the question at least as well as she could have answered it herself.

Silvia froze, then swept her gaze across the room, infuriating Marla with the implication that anything could be worth more attention than her own mutilated state. But then she grasped the significance of her mother’s interest in the details of her surroundings. “Your flesh is deserting you,” Silvia concluded grimly. “Just keep still. I’ll bring some syrup and bandages.”

Marla still couldn’t frame a reply, but her mother didn’t wait around to hear one. She’d barely been gone when Marla’s youngest sister, Fiona, stepped through the open doorway and gaped at her, her expression caught between sympathy, disbelief and amusement. “What happened?” she asked.

“I don’t know,” Marla managed to respond.

“Does it hurt?”

“Not really.”

Fiona took a couple of steps towards her, as if to offer some comfort, then noticed the shiny coating on the floor and retreated. “What’s that?”

“I think it’s my arm and my leg,” Marla admitted glumly.

“Why did they do that?” Fiona demanded.

“I have no idea.”

Silvia returned, shooing Fiona away. Marla drank a whole cup of syrup, so sweet it almost made her retch, then sat, numb and uncomplaining, as her mother wrapped the weeping stumps in bandages. With the top sheet pulled aside, Marla could see several small piles of flaky, translucent debris sitting on the bed; they reminded her of the scales that were shed from a moth’s wings if you tried to catch one in your hand and grabbed it too tightly.

“I’ll call the doctor,” Silvia said, straightening up and standing back to inspect her handiwork. She left the room, and Marla heard her on the phone.

Joanna and Emily came in to gawk. “Aren’t you eating properly?” Joanna asked accusingly.

Marla stared at her, bewildered. “I eat the same as you do.” They sat at the same table for breakfast and dinner, and took the same packed lunches to school.

But Joanna was committed to her hypothesis. “If you’re not starving your cytes, why would they give up on you?”

“You don’t know what you’re talking about,” Marla decided. “Can you just leave me alone?”

Joanna ignored her, but before Marla had a chance to repeat the demand, Silvia returned and ejected both sisters. “The doctor will be here soon,” she told Marla. “But she wants us to try using Geraldine, straight away.”

“All right.” Marla had fed Geraldine and played with her since the family had acquired her as a tiny piglet, and she’d watched her mother use Geraldine’s salve to help stave off some of the maladies of ageing, as well as treating some of the children’s more stubborn wounds. But no one in the family had ever lost this much flesh before, either through an accident or outright desertion.

Silvia went to fetch the pig. Marla sat staring at the floor, wondering why the scent of all the syrup she’d ingested wasn’t enough to lure her erstwhile constituents back into the coalition. What had she done, to make them think they’d have a better life as solitary scavengers? It couldn’t be a question of food alone; once they’d crept out of the house and made their way into the garden, every morsel they consumed would need to be hunted down laboriously, and they’d be competing with both the long-term locals and their one-time comrades for every scrap. If they’d left her for someone richer and better fed, she could have understood the attraction. But scattering to the soil to eke out a living on bacteria, when they could have been sharing three home-cooked meals a day, made no sense to her.

Her mother brought Geraldine in, on a leash. Geraldine waddled up to Marla and nuzzled her good leg affectionately; Marla leaned down and scratched her head.

“Don’t play with her,” her mother scolded. She knelt beside Geraldine and ran a rag across her belly, soaked in the foul-smelling liquid that elicited the expression of salve. Marla gathered that the concoction was a by-product of butchering other pigs; they were hijacking the natural response her body would have made if her own young had been injured.

As the glutinous grey fluid began dripping out of Geraldine’s teats, Silvia collected it in a small bowl, then added a few drops of vinegar and whisked it, which apparently robbed the cytes of any lingering memory of their old pig-specific habits. Then she partly unwrapped the bandage around Marla’s elbow, and daubed some salve onto the stump.

“Why would they join me, when the others are leaving?” Marla asked.

“Maybe it’s just one lineage that’s gone bad,” her mother suggested. “These should be better at cooperating.”

Marla hoped that was true. If the fault was not in her body as a whole, but in the deserters themselves, an influx of new recruits might fill the gap. She gazed down at her arm; the salve seemed to be adhering, at least, rather than just falling away, or visibly fleeing.

The doorbell rang. “That will be Dr. Gimbel,” Silvia said, and went to let her in.

When Dr. Gimbel saw Marla’s condition for herself, she made no attempt to downplay the scale of the problem. “The salve probably won’t be enough to stem the outflow,” she said.

“What if we got hold of some more replenishers?” Silvia asked. Their closest neighbours relied on Geraldine, but Marla knew of at least two other animals kept for the same purpose living nearby.

“That would just increase the risk that all these new cytes might fail to coordinate.” Dr. Gimbel sat on the bed and examined Marla’s wounds, then took hold of her right hand and prodded it at various points on the palm. “Is that tender?” she asked.

“Yes,” Marla complained, forcing herself not to pull free.

“I think there’s a breakout coming here as well,” Dr. Gimbel predicted. “The skin’s holding the line for now, but it might just be a matter of time before it gets recruited into the exodus.”

Marla had been hoping for a better strategy than standing around lamenting her cytes’ shifting loyalties. “Can’t you do something to get rid of the traitors,” she asked, “instead of waiting for them to take everyone else along with them?”

“I could make an incision,” Dr. Gimbel replied. “Then they could get out faster, without needing to sway any of the rest. But a rapid loss of cytes can cause its own problems, and we really don’t know what proportion is already committed to leaving.”

And if the loss was too rapid, the proportion too high? In biology lessons, Marla’s teachers had made it sound as if death always came first: when the body suffered some catastrophic failure, the cytes would abandon it and seek new homes, like people fleeing a city levelled by an earthquake or a fire. But she couldn’t recall anyone raising the possibility that so many citizens might simply grow disgruntled and leave that an otherwise-thriving metropolis could cease to function.

Dr. Gimbel said, “At this point, I don’t think we have any choice but to use the whole pig.”

“I understand,” Silvia replied.

So did Marla, but she didn’t want to hear this. “You can’t kill Geraldine,” she wailed. “I won’t let you!”

Silvia was firm. “This is what she’s for. I always told you not to get too close to her.”

Marla started sobbing. She didn’t want to die, but what else had they tried? A few daubs of salve? “There must be some medicine to keep the cytes inside me. Or some food, like the syrup. Something they can’t resist.”

“Have you been skipping meals?” Dr. Gimbel asked her sternly.

“No!”

“Then nothing you eat now is going to turn this around,” she declared. “Sometimes a whole subpopulation gets switched onto a pathway that leads to dispersal. We don’t know why it happens, and we don’t know how to stop it. The only thing that works is to replace them as fast as possible; when you’ve already lost this much flesh, you can’t risk waiting to see if what remains will still be viable.”

Marla lowered her gaze and surrendered. She couldn’t stop weeping, but she was too afraid now to keep arguing.

“We’ll do it the right way,” Silvia said gently. “She won’t feel any pain.”

Marla nodded mutely, hoping this was true. When she looked up, the room was empty.

She sat and watched the deserting cytes. Their motion was almost imperceptible, but there was a kind of anti-ripple moving slowly across the floor, a visible depletion where the exodus from her body had been interrupted by the bandages. It hadn’t lasted long; when she looked closely at her elbow she could see a shimmering exudation escaping around the edges of the salve.

Still, the salve itself was staying put, so it had to be taking its cues from its better-behaved neighbours. Chemical signals diffusing out from the honest cooperators would summon the newcomers to wherever they were needed, then more detailed messages passing through the gap junctions where the cytes met wall-to-wall would spell out their new roles. Marla had witnessed the effects of salve on small wounds ever since she was an infant, and when her classes had finally shed light on the process she’d felt as if the whole mystery of injury and repair had been solved. But that just seemed as naïve now as imagining that putting out a campfire with a handful of sand meant she could walk unharmed out of a burning forest.

Fiona appeared in the doorway again. “What are they doing in the bathroom?” she asked.

“I don’t know,” Marla lied. She’d heard a motor whirring, but she’d tried to push the sound to the back of her mind, along with the precise moment she’d last heard a grunt of protest from the victim. At least there had been no squeals of pain or distress; Dr. Gimbel would have come prepared, with anaesthetic.

“They won’t let me in,” Fiona complained.

“What do you need to do? Wash your hands? Clean your teeth?”

“No. I want to see what they’re doing.”

“I promise you, you really don’t. Have Joanna and Emily left for school yet?”

Fiona said, “No.”

“Then go and pester them.”

“I didn’t do anything wrong!” Fiona protested.

Marla said, “Do I look like I’ve got nothing better to do than answer your stupid questions?”

Fiona took affront at this, but she seemed unable to choose between defiance and fragility. “You’re horrible,” she declared.

“Then imagine how horrible the puddle you’re standing in must be,” Marla replied.

Fiona lifted one foot and examined the sole, then walked away muttering complaints about all the injustices she was suffering.

Silvia returned, with Dr. Gimbel. They were both wearing surgical masks, rubber gloves and plastic aprons. Marla couldn’t see any stains on the aprons, but they looked as if they’d been hastily wiped clean.

“Do I get a mask?” she wondered. Or did they want the material they were trying not to inhale themselves to enter through her nose, as well as every other route it could find?

“No, but we’ll blindfold you if you like,” Silvia offered.

Marla considered this. “If I can’t see anything, that will just make me panic.”

“All right.” The two women approached and helped her off the bed. Marla had underestimated how hard it would be to move; even with this much assistance, it was worse than a three-legged race. She hopped down the passageway, wishing that the stench of vinegar wafting towards her from the bathroom could have been even stronger, so it might have drowned out the musky scent of her old pet.

What the bathtub contained didn’t quite look like salve, but Marla did her best to pretend there was no difference. Her mother took the bandages off, then helped her undress and lie down in the slurry. It felt like lukewarm porridge. A black plastic bag sat in a corner of the room; there was no discernible shape to its contents, but hopefully it included everything sharp or gritty. On TV police shows, when the murder victim’s cytes had fled, the remnants of their bones turned to dust but the cutting surfaces of their teeth usually kept their shape well enough to be compared to dental records.

“Make sure your arms and legs are fully submerged,” Dr. Gimbel instructed her. Marla shuddered with revulsion, but complied. She couldn’t stop picturing herself wrestling with Geraldine in the straw of her pen; Geraldine had always kept her strength in check, and taken care not to use her weight to endanger her companion. Silvia had once told Marla, “You know, if you died she’d happily eat your body before a tenth of it got away?” And maybe that was true, but it didn’t make Marla feel any better.

She closed her eyes and tried to stay focused on what was at stake. If Geraldine’s cytes chose a snug new home over the deserters’ plan of going it alone, her flesh might hold together even as the traitors continued streaming out. The newcomers’ abilities were not in question; it hadn’t been their choice to give up their old tasks. They’d been forced to forget them, but that wouldn’t stop them learning new tricks in exchange for food and shelter. And if that deal no longer cut it for all of Marla’s own cytes, she couldn’t see how the pros and cons of it had changed – so long as the departing ex-participants didn’t turn their lack of confidence into a self-fulfilling prophecy.

Her teeth started chattering. She wished she could just face the whole ordeal with equanimity, accepting the treatment as her best chance, but the more starkly she acknowledged exactly what was happening, the more keenly she felt the disgust and fear that seemed inseparable from the hope of surviving.

“Are you all right?” Silvia asked.

Marla nodded. “Are you sure I won’t end up as a pig?” she joked.

“A cyte is a cyte is a cyte,” her mother claimed. Marla had been told as much many times, but surely that depended on how thoroughly their old allegiances had been disrupted. Maybe instead of being replenished by Geraldine, Geraldine would be replenished by her: the undead pet she was marinating in would digest her for spare parts, then reconstitute and charge out of the bathroom, fatter, stronger and smarter than ever.

After a while, Marla opened her eyes and lifted her left arm out of the gruel. The end had taken on a spongy appearance; she wasn’t sure what she’d been hoping for, but at least it wasn’t growing any shorter.

Dr. Gimbel bent down and inspected the stump. “This looks promising,” she said. “See all those tiny pores? There are unobstructed paths here for cytes to come and go, while still making sure all the blood vessels stay closed.”

Marla was glad of any hint of good news, but trying to make sense of it left her vertiginous. There wasn’t a single cyte here that possessed, by itself, the slightest inkling of what a healthy pig, a healthy person, or a person’s body in her current predicament needed. The body that was telling the newcomers what to do for its own benefit was built entirely from their more-or-less identical cousins, but the precise way it had tweaked and joined and organised its own constituents somehow allowed it to maintain this kind of knowledge. If the knowledge was only stored collectively, though, how much would be lost as the deserters filed out?

“Where do I remember the shape of my left hand?” she asked.

“No one knows, exactly,” Dr. Gimbel admitted. “But it’s not just in the limb itself. People regrow arms that have been lost to the shoulder, and legs that have been lost to the hip.”

“How quickly?”

“It can take a few months. You should ... ” Dr. Gimbel gestured to Marla to immerse her arm again.

Silvia said, “That’s a good sign, isn’t it?” She pointed to a slick trail of cytes climbing up the bathroom wall, away from the tub. “If they’re still getting away, but the flesh isn’t shrinking?”

“It is,” Dr. Gimbel affirmed cautiously.

“Can’t you catch a few in a bottle and ... do some tests on them?” Marla suggested. If they didn’t learn what had gone wrong, how would they keep it from happening again?

Dr. Gimbel hesitated, but then either she decided to humour her patient, or she concluded that there was some merit in the idea. She took a cotton swab from her bag and scraped it across the wall, then placed it in a small glass vial. “With auto-ablation, it’s always hard to pin down the cause,” she warned.

Marla started shivering, though the slurry wasn’t especially cold. “I’ll bring in a heater,” her mother said, and left to fetch it.

“When will we know for sure if this is working?” Marla pleaded.

“You should stay immersed for about eight hours,” Dr. Gimbel replied. “I wouldn’t expect any further change after that.”

Marla was about to protest that this didn’t answer her question, but then she realised she was meant to read between the lines. If she lay here soaking in liquid pig for the next eight hours, and was still alive at the end of it, that would mean the treatment was a success. And if her body was going to fall apart, forgetting every detail of its own shape and function, it would happen right here in the stench and sludge. She’d meet her demise with all the knowledge of who she was flowing out of her in a torrent of memories, and vanish from the world like a library succumbing to a mudslide.

Ruth was in the middle of filing an amended tax return for one of her clients when the email appeared in her inbox, so she disciplined herself and left it unread until she’d finished the task at hand. The vagaries and glitches of the online portal required all her attention to keep the process from becoming derailed, and if she allowed herself to be distracted she risked having to start again from scratch.

When the lodgement was finally completed and confirmed, she closed the client’s file and opened the email. The night’s match-ups had been scheduled and the venues chosen – at the last minute, as always. Zaleh had agreed to an exchange with her, at nine p.m. in an abandoned shoe factory in Kindred Hills.

Ruth replied to the email, then called her sister. “Can you make it?” she asked Sarah. “I need you as a witness.”

“Are you sure you want to do this?” Sarah pressed her. “That woman’s terrifying.”

“Everyone’s terrifying if you don’t know what they’re made of,” Ruth countered. “But I’ve been watching her for years. I know her inside out.”

Sarah was not convinced. “You do understand that your ‘system’ is about as foolproof as the ones people use for picking horse races?”

Ruth didn’t have time for a long debate about her cyte-tracking methods. “Can you make it, or not? I’m doing it either way.”

“But you’d like to have someone to take your teeth home in a bag, if it comes to that?”

“I’d like to have someone I trust looking out for me. Is that too much to ask?”

“I’ll be there,” Sarah agreed, reluctantly. “Just promise me you’ll think about it. You can still change your mind.”

“Yeah, yeah. See you at the station, around half past eight.”

Ruth went to the kitchen and started cooking. It had been almost a year since she’d first written down a recipe for a pre-match meal to fortify herself in an encounter with Zaleh, but she’d been scrupulous about keeping it up to date, and she’d already bought all the ingredients she’d need on the chance that her latest offer would be accepted.

She sorted the eighteen vegetables into three piles, depending on whether the nutrients she needed from them were most efficiently obtained from the raw plant, after steaming, or after frying. Then she fed them one by one into her dicing machine, nibbling samples of the raw ones, and dropping those needing cooking into the pan or steamer.

When the meal was ready she sat down before the polychromatic mountain on her plate and began eating slowly, chewing every mouthful thoroughly. She had covered all the favourite vitamins, fats and amino acids of those of her cytes that might otherwise be tempted to leave her for Zaleh, along with similar delicacies for those of Zaleh’s cytes she was hoping to lure. She was under no illusion that Zaleh’s own diet would skimp on the latter, but at least this would even out the landscape. So long as the two competitors offered similar nutrient profiles, their cytes would be free to stay or go based on the company they’d be keeping at their destination, while taking the quality of the catering for granted.

By the time she finished she was more than sated, so instead of rushing to clear away the dishes she just sat and let the meal settle and digestion take its course. For all the bravado she’d shown Sarah, it was impossible not to feel a little nervous; at the very least, she risked emerging from the match-up without any of the acquisitions she was hoping for. No one doubted that Zaleh’s cytome offered a multitude of benefits to all of its inhabitants; commentators indulged in fanciful comparisons to thriving polyglot city-states at the height of the Renaissance. Ruth knew she lacked the same dazzling variety, but that didn’t preclude a few of Zaleh’s lineages recognising an advantage in her own distinctive milieu. Nobody could even come close to guessing what reassortment of all the cyte populations in all the multicellular bodies on the planet would be truly optimal – however that was defined – but the whole point of the Swapper movement was to give the cytes themselves an opportunity to decide.

Ruth glanced at her watch; it was almost seven. She changed clothes, grabbed everything she needed, and made it to the station with two minutes to spare before the southbound train came through. Compared to the express service she usually took on the same line, the parade of minor stations along the way felt interminable, but she knew she was getting close to her destination when the lights from the buildings around her began to grow sparser. Kindred Hills had been a hub for artisans and industry a century before, but for the last few decades it had been left to decay into a ghost town of disused warehouses and factories.

Sarah was waiting for her on the platform. “I keep forgetting how few trains bother to stop here,” she said. “But then I wonder why there are any at all.”

Ruth embraced her. “I’m glad you could make it. Sorry if I sounded ungrateful on the phone.”

“I was worried that you’d get mugged if you went alone.”

“Mugged for what?” Ruth joked, gesturing at her threadbare grey tracksuit and a body entirely devoid of jewellery.

Sarah snorted. “Just because your phone is five years old doesn’t mean it can’t be pawned.”

“Eight years old,” Ruth corrected her, as they set out from the station.

“Okay, that’s impressive,” Sarah conceded.

“Besides, if anyone hassles me I’ll just melt their eyeballs.” Ruth took the can out of her pocket and shook it; Sarah flinched back and held a hand up, though the lid was firmly locked in place.

“Put that away,” she said angrily.

Ruth complied. “Sorry.”

Sarah looked back along the street, but there was no one in sight. “I shouldn’t flash it around in public,” Ruth admitted. Just carrying the liquid could bring three months in prison.

Away from the railway line there were no working streetlights, and if there were squatters’ lamps in the old industrial buildings they passed, they were confined to the interiors far from the windows.

“Is that real?” Sarah wondered, looking to a giant faded advertisement painted on the side of a warehouse.

Ruth struggled to make out the details in the moonlight; there was a smiling woman in an old-fashioned bathing costume that hugged her skin from ankles to chin, and a caption that read ENJOY THE DIP, BE SURE THEY DON’T SKIP! “If it’s not real, what else could it be?”

“Someone’s post-ironic artwork,” Sarah suggested.

“I have no idea what ‘post-ironic’ means,” Ruth confessed. “But with all the, err, nutrients they dumped into the water back then, it really could draw some cytes out.”

“Cytes eat sewage?”

“No, but they eat some of the organisms that eat sewage.”

As they drew closer to the venue, Ruth spotted a few other pedestrians in the shadows, converging on the shoe factory. At the rusty, spiked gate, people hung back, pretending not to share any common purpose, let alone form any kind of queue, while still managing to approach the entrance in the order in which they’d arrived.

When it was her turn, Ruth stepped up to what had once been a security guard’s post and spoke her invitation code into the intercom. There wasn’t so much as a crackle of static in reply, but she gestured to Sarah and they squeezed through a gap in the fence together. Their entry would have been unimpeded whether they’d given the code or not, but anyone crossing the infrared beam here without the right preamble would have triggered the swift, silent evacuation of everyone already present.

They trudged across the courtyard, over broken paving stones and some disconcertingly sticky patches whose precise composition Ruth could not discern. Following the instructions in the email, they clambered onto a stack of crates beside one of the buildings and entered through a window, filthily cobwebbed but still fully glazed, that had been left unlatched.

“I hope no one was coming here tonight to try to fix their knees,” Sarah grumbled, as they jumped down onto the wooden floor.

“You can do that with salve.”

“Yeah, sorry. I keep forgetting that minor ailments aren’t even on the map here; it’s go immortal, or go home.”

Ruth laughed. “I just want the best set of compatible cytes. Why put up with whatever random shuffle I got dealt at birth?”

Sarah stopped to inspect the soles of her shoes. “Why couldn’t you collect something normal, like trading cards? It’s a miracle I can still recognise you – and I’m probably the last remaining relative who wouldn’t just walk right past you in the street.”

Ruth let the subject drop; she needed to stay focused. “This way,” she said, gesturing towards a hint of artificial light in the distance.

Once they entered the room, it was almost painfully bright, with half a dozen LED lamps sitting by the walls. A couple of organisers were still unrolling plastic sheeting across the floor, while the half dozen participants stood around with their accompanying witnesses, keeping to themselves but exchanging appraising glances.

“There’s Zaleh,” Sarah said quietly. “Good grief, what do you think she weighs now?”

“She was a hundred and twenty-six kilograms at the end of her last match-up.” Ruth hadn’t been there in person, but all the statistics were up on the forums. “That really doesn’t work in her favour, though; if size was all that cytes were looking for, there’d just be one giant woman alone in this city.”

“It might not work in her favour, but I’d say it’s a pretty good sign of net inflow.”

“She usually gets what she’s after,” Ruth conceded. “But there are lineages I wouldn’t mind losing, so long as I get a few that I want in return.”

“With trading cards, you could leave all the ones you didn’t want to lose at home in a safe,” Sarah said. “And I know, you keep saying you’re committed to accepting the cytes’ verdict on where they want to live, but ... seriously? What if they decided that humans weren’t working out for them at all, and that they’d have more fun as elephants, or butterflies?”

“We’re the most successful experiment they’ve tried,” Ruth argued. “We feed and shelter them far better than any other morphotype. The only question is how the individual collaborations should be fine-tuned. It’s like ... a hit play that ends up with hundreds of productions being staged worldwide, but if you don’t have the right chemistry among the cast and crew in each of the local versions, you’re not doing justice to the work.”

Sarah smiled. “As a set designer, I applaud half of your metaphor, but if theatre companies started meeting like this for a night of spur-of-the-moment personnel swaps, I can confidently say that the result would not be a vast enhancement in their harmony, efficiency or artistry.”

One of the organisers struck a metal gong. “All right, people! The first match-up tonight is Tonia and Xue.”

The women stepped forward, disrobed, and stood on the scales. Each gave their verbal consent to the exchange, for what that was worth. Ruth had heard it suggested that if the worst happened, any testimony from the onlookers that supported an absence of coercion could lead to a reduced sentence for the surviving participant. A video recording of the whole thing might be even more persuasive to a court, but the consensus seemed to be that the risk of it falling into the hands of the police after a match-up that would otherwise have escaped their notice more than outweighed any good it might do under other circumstances.

The witnesses for the participants joined them, making an oath to aid them as required. Then they handed over the cans of dermalyse they’d brought, swapping them between the two parties; Tonia sprayed Xue’s back with Xue’s supply, and vice versa. Ruth could see their skin glisten in the lamplight then grow dull, as the liquid was absorbed and began its work.

The two women approached each other, bowed slightly, then turned to face in opposite directions. They were very similar in height and build, and when they came together there was no awkward shifting and manoeuvring needed to bring their skin into contact.

From Ruth’s vantage she could only see Tonia’s face, which remained characteristically impassive. Ruth had had a match-up with her a couple of years before, and it had seemed to be beneficial for both of them; certainly Ruth had felt more vigorous in the weeks that followed, and when she saw Tonia again she was glowing with good health. It was a shame there was no objective, public record of the results of each exchange, beyond the crude before-and-after weights, but people seemed afraid that any systematic disclosures like that would put them at a disadvantage. Swapping was meant to help everyone – and in an ideal world, with perfect knowledge of the cytes’ needs and behaviour, no exchange would take place unless the outcome could be predicted and both participants were sure to end up better off than they started. But that was never going to happen while it was outlawed, and any medical scientist trying to study the practice risked prison as surely as the Swappers themselves.

Tonia began to grimace with discomfort. Undulations appeared in the skin around her shoulders and rib cage, and over the next few minutes she grew noticeably more gaunt. It was painful to see the flow so unbalanced, but the exchange wasn’t over yet; sometimes the cytes preparing to leave each body negotiated a definite order for their migrations, with one population deferring to the other to avoid congestion along what would have been shared routes. Ruth watched closely, waiting for some compensating movement to begin. But Tonia just continued to be hollowed out.

“Call it off,” Sarah muttered, though not loudly enough for anyone to imagine she thought that was a real proposition. But a couple of minutes later, the cytes themselves had had enough; Ruth could hear the sucking sound as the women’s skin peeled apart.

Tonia fell forward and staggered. Her witness stepped up and steadied her, then helped her onto the scales. She’d lost eight kilograms.

Sarah was dismayed. “You think that’s fine?”

“She can regain the weight just by eating,” Ruth replied. “And maybe she’s better off with the lineages that remained; for all we know, they could be an optimal combination for her.”

“Ah, the wisdom of the cytes!” Sarah proclaimed sarcastically. “Nothing they’ve tried in the last billion years ever gave them their ideal living arrangements, but all we need to do is dissolve our pesky skin and they’ll sort it all out in ten minutes.”

“Who said ‘sort it all out?’” Ruth protested. “But so long as they have a chance to improve things, that’s better than having no chance at all.”

When the organiser called for her by her pseudonym – “Next, we have Deborah and Zaleh!” – Ruth tried to empty her mind of all the objections Sarah had raised, and all her own defences. She was committed to this now, and it didn’t matter what either of them believed.

She stood on the scales, and the organiser read out her weight. “Sixty-eight point three kilograms.” She ceded the machine to Zaleh. “One hundred and nineteen point seven.”

Ruth knew the statement of consent by heart, but she read from the written copy for the sake of formality. Sarah’s declaration of her own role was decidedly less rote; she’d done this many times, but she stumbled on the words and made no effort to conceal her displeasure.

When the dermalyse struck Ruth’s skin, she felt the sting of it, but then numbness took over. She sprayed Zaleh’s back, shielding her eyes from falling droplets with her forearm as she raised the can above her head.

It was clear that the only way she could align her body with this mountain of a woman was by sitting. They faced each other, bowed, turned, and then lowered themselves to the floor. As the lighter of the two, Ruth felt obliged to move first; she bent her knees then pressed her bare feet against the plastic, but she had to clutch at the sheet as well to keep from sliding the wrong way as she straightened her legs. When her back made contact with Zaleh’s, it was the jolt to her whole body that confirmed it; the skin itself was thoroughly anaesthetised, as if it no longer belonged to her at all.

She was too low to see anyone’s face as she looked across the room at the spectators, but she kept her gaze on Sarah, who cut a much more reassuring figure with her scowl of perpetual misgiving temporarily out of sight. Ruth could still taste the meal she’d consumed, and feel its heavy, satisfying presence; she pictured her gut as a welcoming stove-fire her cytes could gather around, to sup from the simmering stew. She just had to hope that the competing scent wafting over from Zaleh’s kitchen wasn’t so strong as to overwhelm the aroma of her own offerings.

When the flow began, the sensation was so diffuse it barely registered above the stronger signals from the surface of her body: the slick plastic against the back of her legs and buttocks, the sweat trickling down the left side of her face. Even the air crossing her sinuses on its way in and out of her lungs felt far more tangible. But all the internal sensory nerves whose normal roles were to judge the disposition of her joints and muscles or the distension of some part of her digestive tract were able to combine into an ad hoc coalition, sharing a thousand tiny cues in aid of a different kind of observation, like a network of seismometers revealing the movement of traffic. Cytes were leaving her muscles, her bones, her viscera. It was a thin outflow so far, not a stampede. And that the organs sourcing it were so widespread was no reason for alarm; a cyte’s lineage did not dictate its role, and the kidney cells, muscle cells, liver cells, marrow cells heading off to their new home might well be close cousins, not a rabble of wholly unrelated emigrants all giving up their allegiance at once.

A few minutes later the counterflow began, heralded by a sharp burning sensation deep in the muscles of her back, all the more acute in contrast to the silence from her anaesthetised skin. Her body was advising her to pull away as fast as possible from the source of the invasion, but Ruth had long grown accustomed to this stage, and she waited it out with no urge to move at all. As the incoming cytes negotiated with the locals – declaring their intentions to join the collective project, then demonstrating their willingness to take up their assigned roles – the pain gradually dissipated, replaced by an exhilarating warmth suffusing through her body, like the afterglow of her meal but purer and more intense. The new cytes were bringing their own resources to the never-ending task of maintaining this city, promising not just growth and repair but a new kind of vigour.

It would take time to know for sure if these benefits were real, but Ruth was already feeling as relieved and optimistic as she dared. Zaleh might easily have left her as emaciated as Tonia, or worse; instead there’d been a modest, even-handed exchange.

Ruth closed her eyes, surprised at how tired she was. Maybe the newcomers were burning through her blood sugar faster than she was used to, as they adjusted their expectations to their new home. She opened her eyes and looked down at her torso, to reassure herself that she hadn’t lost more flesh than she’d realised, but she appeared no different than before. She was, undeniably, a little dizzy though; when this was over, she’d drain the flask of orange juice she’d brought to ward off any risk of hypoglycaemia.

She gave in to the heaviness of her eyelids, then thought of how that might look to Sarah and struggled to raise them again. How much longer would the exchange go on? She tried to gauge the outflow, but she’d lost the necessary acuity. It had taken her years to hone her sense of what was happening to her body, but if it had deserted her before, it was usually from a momentary panic, not this peculiar lethargy.

“Can I ... ?” she muttered. The words were all but inaudible, and she didn’t know how to finish the plea. Did she want to be separated from Zaleh, or would it be better just to ask for the juice straight away? She’d always told Sarah not to try to intervene; her job was to support her once the exchange was complete.

Ruth willed herself to speak again, but this time her lips didn’t respond at all. She had always imagined that if an exchange went badly she would see her flesh melting away, but still have the energy to bellow for help. She tried to raise her hand and make a gesture, but nothing happened.

Was this fatigue, or paralysis? Were her cytes starving as the new ones fed too greedily, or had some small but crucial population defected and was yet to be replaced?

Darkness slipped across her vision, but she couldn’t tell if she’d let her eyes close or if she was staring blindly. The notion that she might be losing her sight made her want to shout at the top of her lungs – but not only did she fail to make a sound, the impulse itself suddenly lost momentum, as if some load-bearing part of her emotional machinery had crumbled. She could no more enact the choreography of fear than she could have danced across the room if every bone in her legs had shattered at the first footfall.

She tried to take stock of what remained, but all her faculties had grown equally brittle. Each line of thought she set out to pursue splintered under the weight of her attention. She was a truck that broke every road it traversed, a piano that severed every struck string.

She thought: if she stopped thinking, at least she might do no more damage. The notion hung suspended in her mind for a moment, intact long enough for her to contemplate it clearly and endorse the goal. And then – whether she had willed a self-protective slumber into being, or simply snapped the last struts propping up her awareness – she reached surcease.