The minutes passed, and Dixon felt his strength rapidly ebbing, both from his herculean exertions and from the paralyzing rays of the green moon beating down upon his unprotected head. As his speed of foot lessened the Centaurians began inexorably pressing their advantage. Dixon was no longer escaping unscathed. In spite of his frantic efforts to dodge, twice the violet bolts grazed his body in searing flashes of exquisite agony. His muscles stiffened still more in the attack of the Green Sickness. Desperately dodging a Centaurian bolt, he stumbled and nearly fell. As he staggered to regain his balance, one of his antagonists scrambled to the coveted position behind him. It was only Ruth's scream of warning that galvanized Dixon's numbed brain into action in time to meet the imminent peril. In one mighty effort he flung his ax at the Centaurian in front of him. The heavy blade cut deep into the thinly armored body. Mortally wounded, the creature collapsed. Dixon whirled and flung up his shielded left arm just in time to intercept the violet bolt of the other Centaurian. Warily backing away, Dixon succeeded in retrieving his ax from beside the twitching body of the fallen invader. Then, with the heavy weapon again in his hand, he remorselessly charged his remaining foe. The Centaurian's tube flashed in a veritable hail of hurtling violet bolts, but Dixon caught the flashes upon his shield and closed grimly in. One final leap brought him to close quarters. The heavy ax whistled through the air in a single mighty stroke that cleft the Centaurian's frail body nearly in two. Then Ruth's excited scream came again. "Bruce--the other one! Get it quick!" * * * * * Dixon turned. The wounded invader, taking advantage of their preoccupation in the final struggle with its mate, had dragged its crippled body over to the instrument enclosure. Dixon staggered toward it as fast as his half-paralyzed muscles would permit. He was just too late. The Centaurian jerked a lever home a fraction of a second before Dixon's smashing ax forever ended his activities. The lever's action upon the pen of inert hybrids was immediate. The sweeping lances of light vanished in a brief sheet of vivid flame which kindled the dark globes on the hybrids' gruesome heads to steady opalescence--and the dread horde came to life! Sprawling from the pen, they came scuttling toward Dixon in a surging flood--a scene out of a nightmare. Dixon faced the oncoming horde in numb despair, knowing that his nearly-paralyzed body had no chance in flight. Then, just as the hybrids were nearly upon him, he heard Ruth's encouraging voice again. "There's still one chance left, Bruce," she cried, "and I'll take it!" Dixon turned. Ruth had in her hand the tiny contact grenade he had given her for a last emergency. She snapped the safety catch on the little bomb, then hurled it squarely at the giant opalescent globe looming close beside her.