It shall not be necessary to summon all the jurors contained in any list of jurors under this Part of this Act, but it shall be competent to summon such jurors only, commencing from the top of the list as may be necessary to ensure a sufficient number for the trial of the cases which shall remain for trial at the date of the citation of the jurors, and such number shall be fixed by the clerk of the court in which the second diet is to be called, or in any case in the High Court by the Clerk of Justiciary, and where jurors are not summoned, from the whole jurors in any list not being required, such jurors shall be placed upon the next list issued, until they have attended to serve.