o 3 . 2 ▁to ▁facil itate ▁the ▁rep uls ive ▁Sl it / Rob o ▁signal ing ▁through ▁c is ▁interactions ▁with ▁Rob o 1 / 2 . ▁ ▁Sl it / Rob o ▁signal ing ▁is ▁seen ▁throughout ▁the ▁developing ▁nerv ous ▁system ▁and ▁is ▁demonstr ative ▁of ▁the ▁importance ▁of ▁rep uls ive ▁c ues ▁in ▁growth ▁cone ▁migration ; ▁the ▁a for ement ioned ▁reg ulation ▁of ▁these ▁rep uls ive ▁bar riers ▁determ ines ▁the ▁path ▁of ▁the ▁comm iss ural ▁ax on . ▁ ▁E phr ins ▁▁ ▁In ▁the ▁ 1 9 4 0 s ▁Roger ▁S per ry ▁was ▁conduct ing ▁experiments ▁on ▁new ts ▁and ▁fro gs ▁to ▁understand ▁how ▁ax ons ▁are ▁gu ided ▁to ▁their ▁top ographic ▁locations ; ▁he ▁did ▁this ▁by ▁cutting ▁the ▁opt ic ▁n erve ▁and ▁rot ating ▁the ▁det ached ▁eye ▁ 1 8 0 ° . ▁What ▁he ▁observed ▁was ▁that ▁the ▁animals ▁behav ed ▁as ▁if ▁their ▁visual ▁world ▁was ▁back - to - front ▁and ▁ups ide - down ▁when ▁presented ▁with ▁a ▁l ure ▁in ▁front ▁of ▁them . ▁He ▁explained ▁this ▁behavior ▁by ▁the ▁existence ▁of ▁two ▁or ▁more ▁grad ients ▁" that ▁spread ▁across ▁and ▁through ▁each ▁other ▁with ▁their ▁axes ▁roughly ▁per pend icular "; ▁this ▁became ▁known ▁as ▁the ▁chem o aff inity ▁hypothesis . ▁This ▁subsequently ▁lead ▁to ▁extensive ▁research ▁and ▁the ▁discovery ▁of ▁two ▁rep uls ive ▁factors , ▁E ph rin - A 5 ▁and ▁E