Half-dragged and half-floating they were led toward an opening in the roof from which a stair led downward. They passed down thus into the building's interior, lit by many windows. Norman glimpsed long halls ending in barred doors, guards here and there. Tube-lines ran along the walls and somewhere machines were throbbing dully. They came at last to a barred door whose guard opened it at the croaking order of the frog-men who held the two, and they were thrust inside, as the door clanged. They turned, and exclaimed in amazement. The room held fully a half-hundred men! They were men such as the two fliers had never seen before, like humans except that their skins were a light green instead of the normal white and pink. They were dressed in dark short tunics, and kept talking to each other in a tongue quite unintelligible to Norman and Hackett. They came closer, flocking curiously around the two men, with a babel of voices quite meaningless to the two. Then one of the men uttered an exclamation, and all turned. * * * * * The barred door had swung open and a half-dozen frog-guards entered, followed by two frog-men carrying a square little mechanism from which tubing led back out through the door. "Norman--these men--" Hackett was whispering rapidly. "If there are men in this world too, it may be that--" "Quiet, Hackett--look at what they're doing." The two frog-men had set their mechanism in place and then croaked out a brief word or order. Slowly, reluctantly, one of the green men moved toward them. Quickly they removed a metal disk fastened to his arm, exposing a small orifice like an unhealed wound. Onto this they fastened a suckerlike object from which a transparent tube led back through the mechanism. The machine hummed and at once a red stream pulsed through the tube and back through the mechanism. The man to whom it was attached was growing rapidly pale! Norman, sick with horror, clutched his companion. "Hackett--these frog-men are sucking his blood from him!" "Good God! And look--they're doing it with another!" "All of these men--kept prisoners to furnish them with blood. It must be the damned creatures' food! And we here with the others--" A common horror shook the two. It did not seem to affect the green men in the room, though, who advanced to the mechanism one by one with a reluctant air as of cows unwilling to be milked. Each was attached to the mechanism by the sucking disk on his arm, and out of each the blood poured through the tube. The metal disk was replaced on his arm then and he went back to the others. Norman saw that the frog-men took only from each an amount of blood that they could lose and yet live, since, though each came back pale and weak from the mechanism, they were able to walk.