or later； it would stop； and its acid…base balance would return to normal。 Then it would be vulnerable to Andromeda。〃
〃True。〃
〃But it didn't die。〃
〃Perhaps some rapid form of immunity。〃
〃No。 Impossible。 There are only two explanations。 When the child stopped crying； either the organism was no longer there…had been blown away； cleared from the air…or else the organism…〃
〃Changed；〃 Stone said。 〃Mutated。〃
〃Yes。 Mutated to a noninfectious form。 And perhaps it is still mutating。 Now it is no longer directly harmful to man； but it eats rubber gaskets。〃
〃The airplane。〃
Hall nodded。 〃National guardsmen could be on the ground； and not be harmed。 But the pilot had his aircraft destroyed because the plastic was dissolved before his eyes。〃
〃So Burton is now exposed to a harmless organism。 That's why the rat is alive。〃
〃That's why Burton is alive；〃 Hall said。 〃The rapid breathing isn't necessary。 He's only alive because Andromeda changed。〃
〃It may change again；〃 Stone said。 〃And if most mutations occur at times of multiplication； when the organism is growing most rapidly。。。〃
The sirens went off； and the puter flashed a message in red。
GASKET INTEGRITY ZERO。 LEVEL V CONTAMINATED AND SEALED。
Stone turned to Hall。 〃Quick；〃 he said； 〃get out of here。 There's no substation in this lab。 You have to go to the next sector。〃
For a moment； Hall did not understand。 He continued to sit in his seat； and then； when the realization hit him； he scrambled for the door and hurried outside to the corridor。 As he did so he heard a hissing sound； and a thump as a massive steel plate slid out from a wall and closed off the corridor。
Stone saw it and swore。 〃That does it；〃 he said。 〃We're trapped here。 And if that bomb goes off； it'll spread the organism all over the surface。 There will be a thousand mutations； each killing in a different way。 We'll never be rid of it。〃
Over the loudspeaker； a flat mechanical voice was saying； 〃The level is closed。 The level is closed。 This is an emergency。 The level is closed。〃
There was a moment of silence； and then a scratching sound as a new recording came on； and Miss Gladys Stevens of Omaha； Nebraska； said quietly； 〃There are now three minutes to atomic self…destruct。〃
29。 Three Minutes
A NEW RISING AND FALLING SIREN CAME ON； AND all the clocks snapped their hands back to 1200 hours； and the second hands began to sweep out the time。 The stop…clocks all glowed red； with a green line on the dial to indicate when detonation would occur。
And the mechanical voice repeated calmly； 〃There are now three minutes to self…destruct。〃
〃Automatic；〃 Stone said quietly。 〃The system cuts in when the level is contaminated。 We can't let it happen。〃
Hall was holding the key in his hand。 〃There's no way to get to a substation？〃
〃Not on this level。 Each sector is sealed from every other。
〃But there are substations； on the other levels？〃
〃Yes。。。〃
〃How do I get up？〃
〃You can't。 All the conventional routes are sealed。
〃What about the central core？〃 The central core municated with all levels。
Stone shrugged。 〃The safeguards 。
Hall remembered talking to Burton earlier about the central…core safeguards。 In theory； once inside the central core you could go straight to the top。 But in practice； them were ligamine sensors located around the core to prevent this。 Originally intended to prevent escape of lab animals that might break free into the core； the sensors released ligamine； a curare derivative that was water…soluble； in the form of a gas。 There were also automatic guns that fired ligamine darts。
The mechanical voice said； 〃There are now two minutes forty…five seconds to self…destruct。〃
Hall was already moving back into the lab and staring through the glass into the inner work area； beyond that was the central core。
Hall said； 〃What are my chances？〃
〃They don't exist；〃 Stone explained。
Hall bent over and crawled through a tunnel into a plastic suit。 He waited until it had sealed behind him； and then he picked up a knife and cut away the tunnel； like a tail。 He breathed in the air of the lab； which was cool and fresh； and laced with Andromeda organisms。
Nothing happened。
Back in the lab； Stone watched him through the glass。 Hall saw his lips move； but heard nothing； then a moment later the speakers cut in and he heard Stone say； 〃 best that we could devise。〃
〃What was？〃
〃The defense system。〃
〃Thanks very much；〃 Hall said； moving toward the rubber gasket。 It was circular and rather small； leading into the central core。
〃There's only one chance；〃 Stone。 said。 〃The doses are low。 They're calculated for a ten…kilogram animal； like a large monkey； and you weigh seventy kilograms or so。 You can stand a fairly heavy dose before〃
〃Before I stop breathing；〃 Hall said。 The victims of curare suffocate to death； their chest muscles and diaphragms paralyzed。 Hall was certain it was an unpleasant way to die。
〃Wish me luck；〃 he said。
〃There are now two minutes thirty seconds to self…destruct；〃 Gladys Stevens said。
Hall slammed the gasket with his fist； and it crumbled in a dusty cloud。 He moved out into the central core。
***
It was silent。 He was away from the sirens and flashing lights of the level； and into a cold； metallic； echoing space。 The central core was perhaps thirty feet wide； painted a utilitarian gray； the core itself； a cylindrical shaft of cables and machinery； lay before him。 On the walls he could see the rungs of a ladder leading upward to Level IV。
〃I have you on the TV monitor； 〃 Stone's voice said。 〃Start up the ladder。 The gas will begin any moment。〃
A new recorded voice broke in。 〃The central core has been contaminated；〃 it said。 〃Authorized maintenance personnel are advised to clear the area immediately。〃
〃Go！〃 Stone said。
Hall climbed。 As he went up the circular wall； he looked back and saw pale clouds of white smoke blanketing the floor。
〃That's the gas；〃 Stone said。 〃Keep going。〃
Hall climbed quickly； hand over hand； moving up the rungs。 He was breathing hard； partly from the exertion； partly from emotion。
〃The sensors have you；〃 Stone said。 His voice was dull。
Stone was sitting in the Level V laboratory； watching on the consoles as the puter electric eyes picked up Hall and outlined his body moving up the wall。 To Stone he seemed painfully vulnerable。 Stone glanced over at a third screen； which showed the ligamine ejectors pivoting on their wall brackets； the slim barrels ing around to take aim。
〃Go！〃
On the screen； Hall's body was outlined in red on a vivid green background。 As Stone watched； a crosshair was superimposed over the body； centering on the neck。 The puter was programmed to choose a region of high blood flow； for most animals； the neck was better than the back。
Hall； climbing up the core wall； was aware only of the distance and his fatigue。 He felt strangely and totally exhausted； as if he had been climbing for hours。 Then he realized that the gas was beginning to affect him。
〃The sensors have picked you up；〃 Stone said。 〃But you have only ten more yards。〃
Hall glanced back and saw one of the sensor units。 It was aimed directly at him。 As he watched； it fired； a small puff of bluish smoke spurting from the barrel。 There was a whistling sound； and then something struck the wall next to him； and fell to the ground。
〃Missed that time。 Keep going。〃
Another dart slammed into the wall near his neck。 He tried to hurry； tried to move faster。 Above； he could see the door with the plain white markings LEVEL IV。 Stone was right； less than ten yards to go。
A third dart； and then a fourth。 He still was untouched。 For an ironic moment he felt irritation： the damned puters weren't worth anything； they couldn't even hit a simple target。。。
The next dart caught him in the shoulder； stinging as it entered his flesh； and then there was a second wave of burning pain as th