He could not actually get a clear look at the tiling, either, for the red corpuscles were in the way. They were now soft bags nearly the size of the ship. Occasionally, one ballooned past the ship at close quarters and was pushed elastically inward at the point of contact, without undergoing any visible harm.

One time a small smear was left behind. Perhaps the contact had been just a little too forceful and a line of miniaturized molecules had been formed against the hull, Morrison thought. The smear lifted off quickly, however, and dissolved in the surrounding fluid.

The platelets were another story, since by their very nature they were much more fragile than the red corpuscles.

One made a head-on collision with the ship. Or perhaps it had been slowed by a collision with a red corpuscle so that the ship had overtaken it. The prow of the ship penetrated deeply and the skin of the platelet punctured. Its contents oozed out slowly, mixing with the plasma and then forming into two or three long strings that tangled with each other. They clung to a portion of the ship's hull for quite a time, trailing behind.

Morrison waited to see any evidence of a clot forming. None did.

Minutes later Morrison saw, up ahead, a milky fog that seemed to fill the blood vessel from wall to wall, pulsating and undulating. Inside it were dark granules that moved steadily from one side to the other. To Morrison, it looked like a malignant monster and he couldn't help but cry aloud in a moment of terror.

Dezhnev turned his head, startled, and said, "It's a white cell, Albert, a leucocyte. It is nothing to be bothered about."

Morrison swallowed and felt distinctly annoyed. "I know it's a white cell. It just caught me by surprise. It's bigger than I thought it would be."

"It's nothing," said Dezhnev. "A piece of pumpernickel, really, and no bigger than it should be. We're just smaller. And even if it were as big as Moscow, so what? It's just floating along in the bloodstream as we are."

"As a matter of fact," said Kaliinin gently, "it doesn't even know we're here - I mean, that we're anything special. It thinks we're a red corpuscle."

Konev seemed to be addressing the air in front of him in an abstracted sort of way, saying, "White cells do not think."

A flash of resentment crossed Kaliinin's face, flushing it slightly, but her voice remained even. "By saying 'think,' Albert, I am merely using a figure of speech. What I mean is that the white cell's behavior toward us is that which it would display toward a red corpuscle."

Morrison cast another look toward the large billowing cell up ahead and decided that, harmless or not, he found its appearance distasteful. He looked with much appreciation at the contrast made by Kaliinin's pretty high-cheekboned face, and wondered why she had never had that little mole under the left corner of her lip removed. Then he wondered if it didn't add just the right trifle of piquancy to a face that might otherwise be considered too pretty to possess character.

That moment of beside-the-point speculation effectively removed the uneasiness that the white cell's appearance had introduced and Morrison returned, in his mind, to Kaliinin's statement.

"Does it act as though we're a red corpuscle because we're the right size for it?"

"That may help," said Kaliinin, "but it's not the real reason. You judge a red corpuscle to be a red corpuscle because you see it. The white cell judges a red corpuscle to be one because it senses the characteristic pattern of the electromagnetic pattern on its surface. White cells are trained - that is just another figure of speech - let us say, adapted - to ignore that."

"But this ship doesn't have the electromagnetic pattern of a red corpuscle… Ah, but I guess you've taken care of that."

Kaliinin smiled in gentle self-satisfaction. "Yes, I have. It is my speciality."

Dezhnev said, "That is it, Albert. Our little Sophia knows, completely in her head" - he tapped his right temple - "the exact electromagnetic pattern of every cell, every bacterium, every virus, every protein molecule, every -"

"Not quite," said Kaliinin, "but those I forget, my computer can supply. And I have a device here that can use the energy of the microfusion motors to place positive and negative electric charges on the ship in whatever pattern I choose. The ship has the charge pattern of a red corpuscle on itself as best as I can duplicate it, and that is close enough to cause the white cell to react - or, rather, not to react - accordingly."

"When did you do that, Sophia?" asked Morrison with interest.

"When we were reduced to the size that would make us a potential object of interest for a white cell or for the immune apparatus generally. We don't want antibodies swarming over us, either."

A thought occurred to Morrison. "Since we're talking about being reduced in size, why hasn't the Brownian motion gotten worse? I should think it would batter us more as we got smaller."

Boranova put in from behind, "So it would if we were unminiaturized objects of this size. Since we are miniaturized, there are theoretical reasons that prevent Brownian motion from getting very bad. It's nothing to worry about."

Morrison thought about it, then shrugged. They weren't going to tell him anything they thought might make him too knowledgeable in the matter of miniaturization and what did that matter? The Brownian motion had not grown worse. In fact, it had grown less troublesome (or was he just getting used to it?) and he had no objection to that. That made it, as Boranova said, nothing to worry about.

His attention shifted back toward Kabinin. "How long have you been training in this field, Sophia?"

"Since my graduate days. Even without Shapirov's coma, we all knew the time would come when a trip through a bloodstream would become necessary. We've been planning something like this for a long time and we knew that this skill of mine would be needed."

"You might have planned an automated crewless ship."

"Someday, perhaps," said Boranova, "we will, but not yet. We cannot, even now, make the automation equivalent to the versatility and ingenuity of a human brain."

"That's true," said Kaliinin. "An automated pattern maker would place us in the red corpuscle pattern as a way of following the path of least resistance, and it would do little more than that. After all, it would be a useless expense and perhaps an impractical exercise altogether to try to instil in an automated pattern maker the ability to change appropriately in response to all sorts of improbable conditions. When I am present, however, I have the capacity to do almost anything. I can change the pattern to meet an unlikely emergency, to test the value of something earlier unthought of, or simply to suit a whim. - For instance, I could change the ship's pattern to that of an E coli bacterium and the white cell would attack at once."

"I'm sure of that," said Morrison, "but don't do it, please."

"No fear," said Kaliinin. "I won't."

But Boranova's voice sounded in sudden - and uncharacteristic - excitement. "On the contrary, Sophia, do that!"

"But, Natalya -"

"I mean it, Sophia. Do it. We haven't tested your instrument under field conditions, you know. Let's try it."

Konev muttered, "That's a waste of time, Natalya. Let us first get to where we're going."

Boranova said, "It won't do us any good to get there - if we can't enter a cell. Here is an immediate opportunity at hand to see if Sophia can control the behavior of a cell."