The vessel is then positioned in {HEAD ~ a holder} on {TAIL ~ the perimeter} of a centrifuge rotor 
The vessel is then positioned in { HEAD ~ a holder } on { TAIL ~ the perimeter } of a centrifuge rotor
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH