Section 2. The clerk of the superior court for criminal business in Suffolk county shall, not less than twenty-eight days before the first Mondays of January and July, respectively, issue writs of venire facias for forty-five veniremen of whom thirty-nine shall be from Boston and two each from Chelsea, Revere and Winthrop. From these forty-five veniremen the court shall then select twenty-three grand jurors to serve in said court, who shall serve for each sitting thereof for six months and until another grand jury has been impanelled in their stead.