A party may amend his pleading as a matter of course at any time before a responsive pleading is filed and served, or, if the pleading is one to which no responsive pleading is required and the action has not been placed upon the trial calendar, he may so amend it at any time within thirty days after it is filed. Otherwise a party may amend his pleading only by leave of court or by written consent of the adverse party; and leave shall be freely given when justice so requires. A party shall plead in response to an amended pleading within the time remaining for response to the original pleading or within ten days after service of the amended pleading, whichever period may be the longer, unless the court otherwise orders.