TEM of longitudinal section (left image) of the junction of midpiece and principal piece in the spermatozoon of a chinchilla (Chinchilla laniger). On the right is a comparable region of the spermatozoon of a suni (Nesotragus moschatus). At the junction of the midpiece and principal piece of mammalian sperm is the annulus (Jensen's ring), a dense structure firmly attached to the overlying flagellar membrane. The configuration of this region shows considerable species variation. In some there is a groove or recess between the annulus and the motor apparatus. In others, the surface contour is flat and the annulus is triangular in section, with the base at the plasma membrane and the apex projecting beneath the last gyre of the mitochondrial sheath. It is speculated that the annulus is a stabilizing structure preventing caudal displacement of mitochondria in actively motile sperm tails.