ELECTRON MICROSCOPE STUDIES ON SPERM DIFFERENTIATION IN MARINE ANNELID WORMS. I. SPERM FORMATION IN IKEDOSOMA GOGOSHIMENSE

The ultrastructur of spermatozoa and the changes through which they are differentiated during sperm formation in an echiuroid were observed under the electron microscope. Many spermatids are connected to one central cytoplasmic mass and the sperm differentiation proceeds synchronously in one sperm-ball. Dense plate-like structures appear in the cytoplasm of early spermatids and disappear soon. In the process of nuclear condensation, many electron-dense aggregates appear in homogeneously textured chromonema and the aggregates are packed together to form a uniformly dense nucleus. Near the centriole at the opposite side from the central mass, the mitochondria fuse together to form one large middle-piece mitochondrion and the acrosomal vesicle is formed from the Golgi-complex. The differentiating acrosome in the late spermatid moves to the anterior tip of the head. In the completed acrosome, a flocculent substance accumulates in the conspicuously expanded invaginated pocket of the acrosomal vesicle and two kinds of material of different electron density fill the inside of the acrosomal vesicle. The spermatozoa remain connected to the central mass at the lateral side of the head until they become fully mature and are packed into the nephridia before spawning.