user: Question: Who directed the episode that was written by Jackson Gillis and Originally aired on May 21, 1955?
Columns: ['Series #', 'Season #', 'Title', 'Directed by:', 'Written by:', 'Original air date']
Types: ['real', 'real', 'text', 'text', 'text', 'text']
Sample row: [53.0, 1.0, '"Through the Time Barrier"', 'Harry Gerstad', 'David Chantler', 'April 23, 1955']
SQL:

assistant: SELECT "Directed by:" FROM "Table" WHERE "Written by:" = 'Jackson Gillis' AND "Original air date" = 'May 21, 1955';