Spermatogenesis in testes of the free-living marine nematode Pontonema vulgare was studied with electron microscopy. The nucleus of spermatocytes has a large nucleolus; the cytoplasm is filled with numerous ribosomes, mitochondria, cisternae of the rough endoplasmic reticulum (RER), Golgi bodies, and flattened osmiophilic cisternae, which are interpreted as the modified membranous organelles (MO) of the spermatozoa of other nematodes studied. After completion of the second meiotic telophase, the nucleus is surrounded by a newly formed nuclear envelope. Further nucleus transformation includes condensation of the chromatin and shrinkage of the nuclear envelope. The deep infoldings of the nuclear envelope give a starlike shape to the nuclei. The cytoplasm of the early spermatids contains the same organelles as in the late spermatocytes, including MO. Many of the latter assume a cuplike or pocketlike shape. During spermatogenesis the peripheral cytoplasm containing the ribosomes, RER, Golgi bodies, and transparent vesicles moves to one pole of the cell forming the residual body. The main cell body of the late spermatid includes the nucleus, mitochondria, and MO embedded in a dense filamentous matrix. The fibrous bodies (FB) of a paracrystalline structure occur in spermatids throughout their developmental transformations. The central part of the spermatozoa contains a starlike nucleus with a nuclear envelope. The filamentous cytoplasm of the spermatozoa includes mitochondria and MO. The spermatozoa extracted from the testes form numerous long filopodia. The dense filamentous cytoplasm of the spermatozoa is continuous with the content of the filipodia. The reconstitution of the nuclear envelope and separate development of MO and FB described in P. vulgare spermatogenesis are the special characters of enoplid nematodes. The reduced character of FB development and simplified structure of MO differentiate P. vulgare from other nematodes studied.
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