The events of fertilization involving the incorporation of the spermatozoa, the development of the male pronucleus, and the association of the maternally and paternally derived pronuclei have been investigated in the egg of Spisula solidissima. At the site of gamete plasma membrane fusion, a small fertilization cone is produced through which the contents of the spermatozoon passes. Entrance of the spermatozoon is accompanied by the vesiculation of sperm nuclear envelope. Subsequently the sperm chromatin is dispersed. A male pronuclear envelope and sperm aster are not formed until the maternal chromatin completes meiosis. Following these events, migration occurs and the male and female pronuclei come into proximity with one another. Later the pronuclear envelopes vesiculate and, concomitantly, the chromosomes condense into large reticular aggregates. The two groups of chromosomes become associated on what will become the metaphase plate of the first cleavage spindle. Thus, as in the rabbit and the mussel, no zygote nucleus is produced.