In our judgment, the District Court had jurisdiction of the case against petitioner. It may not be prevented from exercising that jurisdiction by mandamus or prohibition.

Whether the District Court erred in permitting the complaint to be amended cannot be determined in this mandamus action. Petitioner has an adequate remedy by way of appeal to review the action of the District Court when a final appealable order has been entered in the case.

Mandamus is an extraordinary remedy reserved for really extraordinary causes. It may not be used as a. substitute for appeal. Beneke v. Weick, 6 Cir., 275 F.2d 38.

Leave to file the mandamus petition is, therefore, denied.