(3) This provision shall also apply in cases where an infant is served with a petition or notice of motion, or a summons, in any matter. 13. Before the name of any person is used in any action as next friend of any infant or other party, or as relator, that person shall sign a written authority for that purpose, and the authority shall be filed in the Registry. 14. (1) Trustees, executors, and administrators may sue and be sued on behalf of or as representing the property or estate of which they are trustees or representatives, without joining any of the persons beneficially interested in the trust or estate, and shall be considered as representing such persons; but the Court or a Judge in chambers may at any stage of the proceedings, order any such persons to be made parties either in addition to or in lieu of the previously existing parties. (2) The provisions of paragraph (11) of this rule shall also apply to trustees, executors and administrators sued in proceedings to enforce a security by foreclosure or otherwise. 15. Where a defendant is added or substituted, the writ of summons shall be amended accordingly and the plaintiff shall, unless otherwise ordered by the Court or a Judge in chambers, file an amended writ and cause the new defendant to be served in the same manner as original defendants are served and the proceedings shall be continued as if the new defendant had originally been made a defendant. 16. Any application to add, strike out or substitute a plaintiff or defendant may be made to the Court or Judge in chambers at any time before trial by motion or summons, or in a summary manner at the trial of the action. 17. (1) Where in any action, a defendant claims as against any person not already a party to the action (in this Order called "the third party") that(a) he is entitled to contribution or indemnity; or (b) he is entitled to any relief or remedy relating to, or connected with the original subject-matter of the action and substantially the same as some relief or remedy claimed by the plaintiff; or (c) any question or issue relating to or connected with the said subject-matter is substantially, the same as some question or issue arising between the plaintiff and the defendant and should properly be determined not only as between the plaintiff and the defendant and the third party or between any or either of them, the Court or a Judge in chamber may give leave to the defendant to issue and serve a third-party notice. (2) The Court or a Judge in chambers may give leave to issue and serve a third party notice on an ex parte application supported by affidavit or where the Court or Jud e in chambers directs a summons to the plaintiff to be issued, upon the hearing of the summons: Provided that leave shall not be granted in cases where the action was begun and an order for pleadings made before the date of the commencement of this rule. 18. (1) The notice shall(a) state the nature and grounds of the claims or the nature of the question or issue sought to be determined and the nature and extent of any relief or remedy claimed; (b) be in accordance with Form 23 or Form 24 in the Appendix to these Rules with such variations as circumstances may require; and (c) be sealed and served on the third party in the same manner as writ of summons is sealed and served. (2) The notice shall, unless otherwise ordered by the Court or by a Judge in chambers, be served within the time limited for delivering the defence, or, where the notice is served by a defendant to a counter-claim, the reply and with it also shall be served a copy of the writ of summons or originating summons and of any pleadings filed in the action.