CYTOLOGICAL ASPECTS OF FERTILIZATION IN LAMELLIBRANCH, MYTILUS-EDULIS .2. DEVELOPMENT OF MALE PRONUCLEUS AND ASSOCIATION OF MATERNALLY AND PATERNALLY DERIVED CHROMOSOMES

The events of fertilization involving the modifications of the incorporated spermatozoon and the association of the paternally and maternally derived pronuclei have been studied by techniques of light and electron microscopy in the lamellibranch, Mytilus edulis. At the site of gamete plasma membrane fusion a small fertilization cone is produced. As the spermatozoon moves through the fertilization cone, its nuclear envelope degenerates. The degeneration of the nuclear envelope is followed by the dispersion of the sperm chromatin and the formation of the pronuclear envelope. The sperm aster, produced during the formation of the male pronucleus, consists of fasicles of microtubules and endoplasmic reticulum which radiate from a centrosphere region containing two centrioles. When the pronuclei complete their migration toward each other they are structurally similar and contain a fibro‐granular nucleoplasm. Eventually, the pronuclei become closely apposed, their proximal surfaces flatten, and produce an array of interdigitating nucleoplasmic projections. Concomitantly, the chromatin condenses and forms large reticular aggregations. Association of the parentally derived chromosomes is effected when the two pronuclear envelopes are dismantled. Microtubules that comprise a portion of the mitotic spindle become associated with the chromosomes as they move together in preparation for the first cleavage division. Hence, like many vertebrates, but unlike sea urchins, no zygotic nucleus is produced. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company