The tube probably contains the metol. Many, many years ago, developers were packaged like this. Some will remember the two separate foil envelopes (or it you are really old, glass tubes) that made up Kodak M-Q developer packages. Unmodified metol reacts with sulfite to form an insoluble compound. Modern formulations use a phthalate coating that gets around the problem, but that came after your developer was made. You dissolve the metol first, then add the other stuff.