bustrophedon ← {(((1↑⍴⍵)⍴0 1)/⍳1↑⍴⍵){,⍵[⍺;]←⌽⍵[⍺;]}⍵}

M ← 4 5 ⍴ 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

,((⍴M)⍴ bustrophedon M),' '

#apl #bustrophedon