Rule 5.01

5.00—Putting the Ball in Play. Live Ball. 

Rule 5.01 
At the time set for beginning the game the umpire-in-chief shall call “Play.” 

DISCUSSION:
The umpire--in-chief stands behind the catcher. He is the "plate umpire." The base umpire does not put the ball into play.

CROSS-REFERENCE:
9.04 - umpire-in-chief