pronucleus (pro-nu'kle-us)

A primordial or germinal nucleus. female p. A small body, lying at first near the periphery, just beneath the extrusion globules, and representing the portion of the nucleus of the ovum (see germinal vesicle, under vesicle) which remains after the extrusion of the polar globules. It sinks to or toward the center of the vitellus, where it usually remains, and must fuse with the male p. before segmentation can take place. A system of radiating lines runs out from it into the yolk, forming a starlike figure, the so-called female aster, male p. The p. into which the head or some other portion of a spermatozoid is transformed after it has imbedded itself in the vitellus and become enlarged, clear, and spheroidal. It finally unites with the female p., and, like the latter, is surrounded by radiating lines forming the so-called male aster. [Lat., pro, before, + nucleus, from nux, nut.]