The differentiation process of the
Perinereis brevicirris spermatozoa was observed by the electron microscope to examine the mode of intercellular communication at different stages of synchronized germ cell differentiation. Spermatogonia in large clusters were framed by a thin cytoplasmic envelope and appeared to be divided into several blocks by cytoplasmic processes. Spermatogonia in one block were connected to each other by desmosomes and intercellular bridges. At the spermatocyte stage, the blocks in large clusters became independent small clusters from which cells detached by meiotic divisions. These cells showed neither connection with the cytoplasmic central mass nor cytoplasmic communications, except for desmosomes, and accordingly no synchrony was observed in cell differentiation. In spermatids, the smallest group was composed of four cells which were connected by a cytoplasmic bridge. During spermiogenesis, a filamentous acrosomal rod extended through the nucleus, and nuclear condensation followed. In spermatozoa, a dense material was observed in the intranuclear space near the central axis on the condensed chromatin. Mg
2+-ATPase activity was found on the dense material. The functional significance of ATPase is discussed.
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