☞ In the maturing of the ovum preparatory to impregnation, a part of the germinal vesicle (see , 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.