The Collaborative International Dictionary of English v.0.48:
Pronucleus \Pro*nu"cle*us\, n.; pl. Pronuclei (-[imac]). [NL.
See Pro-, and Nucleus.] (Biol.)
One of the two bodies or nuclei (called male and female
pronuclei) which unite to form the first segmentation nucleus
of an impregnated ovum.
Note: In the maturing of the ovum preparatory to
impregnation, a part of the germinal vesicle (see
Polar body, under Polar) becomes converted into a
number of small vesicles, which aggregate themselves
into a single clear nucleus. which travels towards the
center of the egg and is called the female pronucleus.
In impregnation, the spermatozoon which enters the egg
soon loses its tail, while the head forms a nucleus,
called the male pronucleus, which gradually travels
towards the female pronucleus and eventually fuses with
it, forming the first segmentation nucleus.