An ultrastructural analysis of polyspermy in the surf clam, Spisula solidissima

Spisula eggs made polyspermic by insemination with high concentrations of sperm were prepared for light and electron microscopy. In eggs that had been inseminated by at least eight spermatozoa, incorporation and male pronuclear development were morphologically similar to events observed in monospermic zygotes. Subsequent to their migration to the central region of the zygote the female pronucleus and the male pronuclei become closely associated. This is followed by chromosome condensation and vesiculation of the pronuclear envelopes. The chromosomes become arranged on a multipolar spindle in preparation for the first cleavage division. In approximately 50%zygotes one of the pronuclei appears to be asynchronous in its morphogenesis; the vesiculation of its envelope and the condensation of its chromatin taking place at a later stage than in adjacent pronuclei. This pronucleus fails to form discrete chromosomes and is recognized as a large spheroidal body which becomes aligned on the multipolar spindle. The fate of this mass of condensed chromatin is unknown.