Cytological aspects of fertilization in the lamellibranch, Mytilus edulis. I. Polar body formation and development of the female pronucleus.

Abstract

AbstractAspects of fertilization encompassing the meiotic events of the maternal chromatin, the formation of the polar bodies, and the development of the female pronucleus have been investigated in the lamellibranch, Mytilus edulis.The cromosomes of the mature egg of Mytilus edulis at the time of fertilization are organized on the first metaphase plate of meiosis. The first meiotic metaphase figure, oriented normal to the egg's cortex, consists of two asters, each of which contains paired centrioles, microtubules, and some smooth endoplasmic reticulum. Subsequent to sperm incorporation, the first polar body is produced by a process akin to cytokinesis following the movement of the chromosomes at anaphase. Later, the metaphase plate of the second meiotic apparatus is formed. Morphologically, the second metaphase figure is similar to the first; however, each aster appears to contain only one centriole. The second polar body, produced in the same manner as the first, remains associated with the zygote via a cytoplasmic bridge which persists up to the time of the first cleavage. Both polar bodies contain the same cellular inclusions as the zygote; however, only the second polar body has its chromatin delimited by a perforated nuclear envelope. After the formation of the second polar body, the maternal chromatin is organized into chromosomal vesicles. The chromosomal vesicles later fuse to form the female pronucleus.