Lee nodded, so Letzmiller continued. "Replacive surgery is actually quite old. Old as medicine itself, I suppose. Very early attempts at dentures were tried, though with little success. And, of course, peg legs and hooks for persons who had lost their hands might be called replacive surgery, though they were very crude. Later on came more refined dentures, artificial limbs, corrective lenses, skull plates, hearing aids, plastic or cosmetic surgery, blood transfusions, all types of skin grafts, et cetera. "The 1950s saw the beginning of bone and corneal transplants, use of plastics in arteries, those huge heart-lung and kidney machines, implantation of electrodes in the heart to steady its beat--many things which were mostly emergency or stop-gap measures. All through the late 1900s refinements continued to be made, but it wasn't until 1988 that the fathers of replacive surgery, Doctors Mills, Levinson and McCarty made the breakthrough that revolutionized the whole concept. In very simplified language they unlocked the key to producing specialized living tissue through a bombardment of an extremely complex carbon compound with amino acids and electricity, then making it selective in function by a fantastically intricate application of radiation. "That pulmonary replacement you received in 1991 was undoubtedly one of the first successes. You were quite lucky, you know. Up until 2017, only about five per cent of their synthesized hearts lasted more than thirty days. At any rate, the principle was established, and it was proven that it could work. Most of our work from then till a few years ago has been in improving and refining the work those three good doctors did over three hundred years ago." Letzmiller's cigar had gone out, and he discarded it in favor of a cigarette. "That would be the end of my history lecture, if it were not for the nature of your trouble." Lee looked at him closely. "Why's that?" "Well, Mr. Lee, the big thing missing in that summation is the seemingly impossible task of synthesizing nerve tissue, especially that of the cerebral cortex. It's been approximated, at any rate closely enough to give us good enough results to allow an artificial tissue to respond to brain signals about ninety-eight per cent as well as the original would. But actual duplication? No. At least not until about three years ago. To tell you the truth, it is barely out of the experimental stage." "Experimental!" * * * * * "Yes, this will be the first complete replacement of a human brain. Oh, of course it has been done with animals, and it has been successful with partial replacements on humans. But you will have the honor of being the first human with a complete substitution." Lee could not contain himself. "Doc, that's just it! There won't be a single atom of me except what you fellows have conjured up--" Letzmiller broke in mildly. "I think 'conjured' is hardly the proper word, Mr. Lee."