19. Intervention not otherwise allowed

19. No person shall be allowed to intervene in a pending action otherwise than in pursuance of, and in conformity with, the provisions of the last preceding section. And no person shall be added as plaintiff, or as the next friend of a plaintiff, `without his own consent thereto: 

Provided however that any person on whose behalf an action is instituted or defended under section 16 may apply to the court to be made a party, and all parties whose names are so added as defendants shall be served with a summons in the manner hereinafter mentioned, and the proceedings as against them shall be deemed to have begun only on the service of such summons.