Thereafter, a pouring aperture is cut in {HEAD ~ the barrier layers} which span {TAIL ~ the opening} 
span
Thereafter , a pouring aperture is cut in { HEAD ~ the barrier layers } which span { TAIL ~ the opening }
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL