Parboil, blanch, and mince sweetbreads. Put over the fire with just stock enough to cover them, season to taste, and bring to a boil. Thicken well with a white roux, heat again, stir in a beaten egg for each cupful of sweetbread dice, and pour out upon a dish to cool. When stiff, make, with floured hands, into croquettes, roll in egg and fine cracker-crumbs and set in a cold place for at least two hours before frying them in deep fat.