The Clerk shall accept for filing only nomination petitions on official blanks which were initialed by him at the time of issuance from his office and which contain the required number of signatures for candidates having those qualifications required for elective City officers by this Charter. When petitions are filed by persons other than the person whose name appears thereon as a candidate, they may be accepted for filing only when accompanied by the written consent of the person in whose behalf the petition or petitions were circulated. The Clerk shall, within five days after receipt thereof, determine the sufficiency of the signatures on each petition filed, and if he finds that any petition does not contain the required number of legal signatures of registered electors, he shall immediately notify the candidate in writing of the insufficiency of his petition.