Although it is now almost universally accepted that Calabar swellings are in some way connected with infections with the filarial worm, Loa loa, the method by which the swellings are produced is still a matter of speculation. The idea has been advanced that the swellings may be caused by direct irritation of the subcutaneous tissues by the migrating worms; the rare occurrence of moving swellings gives some support to this view, but as a rule no swellings occur in places where the movements of the worm can be plainly seen. Another idea, suggested by Manson, is that the swellings result from the periodic deposition of embryos in the subcutaneous tissue by the adult females. In one instance he found large numbers of microfilariae in a Calabar swelling, but in another case he failed to find any. Furthermore, the swellings may occur within a few months after arrival in an endemic area before any microfilariae can be found and before it is likely that the worms have been able to become mature.