The mucosa of the urinary bladder is protected from herniation through the bladder wall when the intracystic pressure is suddenly raised by the arrangement of the three interlacing muscle layers. The outer longitudinal layer has large loosely woven bundles, as has the inner longitudinal layer; the middle circular layer has small bundles and is much more closely woven. In sectioning bladder walls, it was found that frequently there are left loose fibrous tissue pathways extending entirely through the bladder wall when the intracystic pressure is suddenly raised, bundle interruption to cause them to be oblique (fig. 1). Such pathways, I consider to be the necessary congenital etiologic factor in the formation of diverticula. The acquired factor is the intracystic pressure which varies according to the strength and irritability of the bladder wall, the strength being influenced chiefly by any degree of muscular hypertrophy secondary to an obstruction. Opinion in late