Ultrastructural observations of the in utero sperm of Ascaris lumbricoides reveal that it consists of a relatively clear, ameboid anterior region and a conical posterior region containing numerous surface membrane specializations, dense mitochondria, a lipid-like refringent body of variable size, and a dense nucleus which lacks an apparent nuclear envelope. No acrosomal complex was observed. Pseudopods emanating from the anterior cytoplasm make first contact with the primary oocytes and appear to be responsible for the localized removal of the extraneous coat covering the oolemma. Subsequently the gamete membranes interdigitate and finally fuse. Because this pseudopodial action appears similar to that reported for the acrosomal filaments in flagellated sperm, the anterior region of the Ascaris sperm is thought to serve an acrosomal function. Following gamete-membrane fusion, the sperm nucleus acquires a particulate appearance and becomes disorganized. Once inside the oocyte, the sperm cytoplasm consists of dense mitochondria, ribosomes, and vesicles derived from the surface membrane specializations. The refringent body, whose contents possibly contribute to the synthesis of ribosomes, is usually absent by the time the sperm cytoplasm attains a central position in the egg.

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