Morphology of the "spindle-shaped body" in the developing tail of human spermatids.

At the beginning of the late cap phase of the normal differentiation of human spermatids, a unique structure, the "spindle shaped body", occupies part of the future middle-piece. This spindle-shaped body was followed throughout its development, and its functional relationahip to the formation of the fibrous sheath is discussed. The spindle-shaped body disappears when the fibrous sheath is completed and the annulus moves down, giving rise to the middle-piece.