sentence: The nonpermissiveness of older bowel is relative , rather than absolute , as illustrated by Tcof1 ( Treacher Collins-Franceschetti syndrome 1 ) mutant mice , where heterozygous mice do not fully colonize the bowel at embryonic day 14.5 because of depletion of early NC precursors but continue to migrate , fully colonizing the colon by embryonic day 18.5 ( 9 ) .