Viruses produced by this method are advantageously free of {HEAD ~ any infectious agents} comprised in {TAIL ~ animal sera} 
comprised in
Viruses produced by this method are advantageously free of { HEAD ~ any infectious agents } comprised in { TAIL ~ animal sera }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL