Every collection made under the authority of the act and of this part shall be preserved in the public museum designated in the permit and shall be accessible to the public. No such collection shall be removed from such public museum without the written authority of the Secretary of the Smithsonian Institution, and then only to another public museum, where it shall be accessible to the public; and when any public museum, which is a depository of any collection made under the provisions of the act and this part, shall cease to exist, every such collection in such public museum shall thereupon revert to the national collections and be placed in the proper national depository.