ity ▁gen es ▁help ▁to ▁define ▁the ▁anterior ▁and ▁posterior ▁polar ities ▁within ▁each ▁emb ry onic ▁par as eg ment ▁by ▁reg ulating ▁the ▁transmission ▁of ▁signals ▁via ▁the ▁W nt ▁signal ing ▁path way ▁and ▁H edge h og ▁signal ing ▁path way . ▁ ▁Seg ment ▁polar ity ▁gen es ▁are ▁expressed ▁in ▁the ▁emb ry o ▁following ▁expression ▁of ▁the ▁gap ▁gen es ▁and ▁pair - rule ▁gen es . ▁The ▁most ▁commonly ▁c ited ▁examples ▁of ▁these ▁gen es ▁are ▁eng ra iled ▁and ▁go ose berry ▁in ▁D ros oph ila ▁mel an og aster . ▁The ▁segment ▁polar ity ▁is ▁the ▁last ▁step ▁in ▁emb ry onic ▁development ▁and ▁a ▁repeated ▁pattern ▁where ▁each ▁half ▁of ▁each ▁segment ▁is ▁deleted ▁and ▁a ▁mirror - image ▁is ▁dup licated ▁and ▁revers ed ▁to ▁replace ▁that ▁half ▁segment ; ▁thus , ▁forming ▁a ▁pattern ▁element . ▁ ▁Seg ment ▁polar ity ▁in ▁D ros oph ila ▁Seg ment ation ▁polar ity ▁occurs ▁during ▁the ▁release ▁of ▁morph og ens , ▁which ▁functions ▁to ▁differenti ate ▁patterns ▁within ▁sections . ▁The ▁development ▁of ▁a ▁pattern ▁depends ▁on ▁the ▁grad ients ▁of ▁these ▁morph og ens . ▁ ▁Eng ra iled ▁In ▁D ros oph ila , ▁the ▁eng ra iled ▁gene ▁is ▁expressed ▁only ▁in ▁cells ▁within ▁the ▁posterior ▁section ▁of ▁every ▁segment . ▁Its ▁role ▁is ▁to ▁distinguish ▁posterior ▁from ▁anterior ▁sections ▁of ▁each ▁segment . ▁Eng ra iled ▁expression ▁is ▁generally ▁restricted ▁to ▁cells ▁in ▁the ▁posterior