The course of spermiogenesis in Arenicola brasiliensis was observed with the electron microscope. The spermatogonia floating in the body cavity seem to proliferate and differentiate to mature spermatozoa in the coelomic fluid. More than a hundred spermatids are connected t o one large central mass of cytoplasm and spermiogenesis proceeds synchronously in one cluster, which changes into a sperm-disc during maturation. The pre-acrosomal vesicle originates from the Golgi-body and gradually changes into the acrosomal vesicle of peculiar structure like a cup upside down. In the process of differentiation of the acrosome, a part of the material in the acrosomal vesicle is transferred into the space between the vesicle and the nucleus. The posterior one-third of the cylindrical nucleus is surrounded by four middle-piece mitochondria. The flagellar axoneme originates from one of the centrioles, which is located near a posterior pit in the nucleus.Arenicola brasiliensis (formerly A . cristata) is a very common species distributed widely in the world. The processes of gametogenesis and early development of this annelid have been described in detail by OKADA (1 941 ), using the light microscope. The early spermatogonia are liberated from gonads into the coelomic fluids as clusters, which he termed "spermatophores", and they develop in the form of sperm-discs until these are disintegrated to free the spermatozoa in the process of discharge from the nephridia.In the present paper, I will report observations on the ultrastructure of the spermatozoon and the process of its formation, obtained with the electron. microscope. Since the structural changes take place during spermiogenesis, my interest is mainly directed to the stages following sperm-disc formation. MATERIALS AND METHODSArenicola brasiliensis were collected from the seashore near Matsuyama City in June when the worms shed egg clusters very frequently. Some worms had many fully formed sperm-discs and others had many immature spermatids in the coelomic fluids. The sperm-discs and spermatids 89
Sound recordings of courtship in Drosophila mercatorum were analyzed with an oscilloscope. Sounds in this species consist of two kinds of pulses, referred to as the A and B sounds, respectively. These differ from each other in their oscilloscope pattern and in the behavior accompanying them. A comparative study of three strains from widely separated geographical regions (New York, El Salvador, and Hawaii) revealed significant differences among strains in the interpulse interval (ipi) of the A sound. The ipi of the B sound increased as it proceeded for New York and Hawaii males but not for males from El Salvador. These characteristics may influence mating success in interstrain crosses. The present results suggest that the first steps toward divergence in the nature of the sound are quantitative; this may affect the response threshold of the female. Genetic systems responsible for female receptivity may have been independently developed in the two sexes.
The ultrastructure 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 platelike 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.In the phylum Annelida, the ultrastructural changes in spermiogenesis have been reported in some species of Oligochaeta by GATENBY and DALTON (1959), FERRAGUTI and LANZAVECCHIA (1971), ANDERSON et al. (1967ANDERSON et al. ( , 1969, and SHAY (1 972). In the class Echiuroidea, TYLER (1 965) reported briefly the ultrastructure of the spermatozoa of Urechis caupo and its changes in fertilization, but no electron microscope study has yet been performed on sperm formation in echiuroid worms.We intend to investigate the ultrastructural changes in gametogenesis of various marine Annelida, and in the present paper, we will report the process of the sperm formation as revealed by the electron microscope in an echiuroid species, Ikedosoma gogoshimense.As we reported previously (1962), the germ cells develop floating in the coelomic fluid in this worm. Male sexual cells make a cluster from the early stage 77
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