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 spermatozo["o]n 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.