The formation of the narrow stripe of cells that lies between the prospective dorsal and ventral thalami. This boundary contains signals that pattern the prethalamic and thalamic territories of the future mid-diencephalon. [ GOC:dgh GOC:jid http://www.ncbi.nlm.nih.gov/pubmed/11425897 GOC:dph GOC:cls http://www.ncbi.nlm.nih.gov/pubmed/16452095 http://purl.obolibrary.org/obo/go/references/0000021 ]