GNU Collaborative International Dictionary of English

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1.       
   Pronucleus , n.; pl. Pronuclei . [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.
   1913 Webster
   ☞ 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.
   1913 Webster