{HEAD ~ Male Hartley guinea pigs} from Charles River, weighing 250 275 grams on arrival and 300 600 grams at testing, serve as {TAIL ~ subjects} in the experiment 
serve as
{ HEAD ~ Male Hartley guinea pigs } from Charles River , weighing 250 275 grams on arrival and 300 600 grams at testing , serve as { TAIL ~ subjects } in the experiment
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH