The City of the Sun Read Online Free PDF

stimuli provided by other tissues developing in the immediate environment.
    “These parasites, being communal pseudo-organisms, retain essential versatility in each and every cell. Most of the species don’t make a lot of use of that versatility—parasitism is a relatively simple way of life, which doesn’t demand a great deal of differentiation of functions. But this one is a very highly developed parasite...a super-parasite. You can think of the others as plant-like things, sending tap roots down into the flesh of their hosts to soak up moisture and nourishment. It’s essentially a crude business, like drilling oil wells. The parasites are fairly discreet—they use thin drills, strands of cells only two or three thick—but what they do is nevertheless a fairly straightforward job of boring and mining.
    “The odd man out is cleverer than that. His cells make use of their versatility by mimicking the cells of the host. Thus, when he sends a tap root down through skin tissue to the wall of a blood vessel, the strand cells take on many of the characteristics of dermal cells, and the cells which actually do the thieving take on many of the characteristics of blood-vessel-wall cells. This parasite then has a much higher degree of integration with his host. The host no longer recognizes him as an invader, and thus he becomes immune to the body’s natural tendency to reject foreign matter. The extra functions fulfilled by these cells—the parasite functions—are masked by the apparent conformity of the cells to their immediate tissue environment.
    “I can’t tell from this report how far the mimicry goes. But if this parasite is really clever—and we have grounds to suspect that it is—then the mimic cells might actually carry out the functions of the tissues they mimic, so that as well as the tap root cells being indistinguishable from host-tissue cells by the host’s bodily defenses, they actually do the job they ought to be doing if they were host-tissue cells. That way, this particular parasite could maintain a much more extensive internal network than its relatives. It wouldn’t have to limit itself to a few discreet strands of cells—it could ramify much more extensively inside its host. And that would mean that it could support a much greater biomass all told—something like the formations we could see on these people, instead of just a little thing like a spider web on the back of a rabbit.
    “Also, of course, this could explain why the colonists might have been unable to muster any kind of medical defense against this parasite. If its internal ramifications can mimic host cells well enough to fool the host body itself, no external antibiotic would get close to it...not without attacking the host tissues too. The external dendrites—the black cells—are probably fairly easy to dispose of...but if the colonists dispose of them they simply grow back from inside. The roots can’t be touched by any normal methods.
    “In brief, I suspect that this is the most efficient parasite I’ve ever come across. Maybe it’s so efficient that it doesn’t deserve to be called a parasite—maybe just a commensal. It really cooperates with the host body, taking the nourishment it needs with the absolute minimum of biotic vandalism. Maybe the only thing we can say against it is that it isn’t very pretty. Maybe...I think I’ll reserve judgment on that until I get a much closer, much longer look.”
    There was a respectful pause.
    “You may applaud,” I told them.
    They didn’t. Not that it mattered. I hadn’t planned an encore.
    “If you’re right,” said Nathan, “then the obvious question is...can we find any way to attack such a parasite?”
    “Oh yes,” I assured him. “Genetic engineering gives us much more subtle routes of attack than any antibiotic drug. We can actually attack the thing in its genes—the very genes which give it its versatility and its ability to mimic specialized cells. The