We describe the initial stages of the sperm--egg interaction in the human--hamster heterospecific system. The ultrastructural study reveals that fusion is initiated on both sides of the spermatozoon in the region extending from the anterior part of the post-acrosomal sheath (PAS) to the acrosome equatorial segment (AES). The splitting of the PAS and the appearance of a particular material within the oocyte subsisting until the pronuclear formation are described. The observation of the available electron micrographs suggests that this material may originate from the PAS. The role, the origin and the nature of this material are discussed.
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Courtot et al. (1988) studied this question.