Cells and Their Activities 277 



At the time that the sperm enters the egg, it has, of course, gone 

 through the whole process of spermatogenesis and has but a haploid 

 number of chromosomes. Such is not necessarily the case with the 

 egg. In some cases, maturation divisions are not initiated until after the 

 sperm penetrates; in others, they are but partially completed. In 

 mammals, it appears that the second polar body is forming when the 

 sperm enters. 



The events following sperm penetration have been intensively 

 studied in the roundworm, Ascaris. In the eggs of this animal, the 

 maturation divisions do not occur until after the sperm enters. The 

 sperm upon entering moves toward the center of the cell and rounds 

 up to form the mule pronucleus. The egg nucleus moves toward the 

 periphery of the cell, and, for the first meiotic division, the spindle is 

 so arranged that half of the homologous chromosomes are discarded in 

 the first polar body. Following this reductional division, the remain- 

 ing split chromosomes rotate 90 degrees and the second spindle is formed, 

 followed by the second maturation division. The egg nucleus with its 

 haploid number of chromosomes is now known as the female pronucleus. 

 It moves toward the male pronucleus, the homologous chromosomes 

 pair, and the spindle of the first embryological division is formed. Here- 

 after cell divisions are by mitosis with the resulting cells always having 

 the diploid or 2N number of chromosomes. Meiosis will not appear again 

 in this individual until it reaches sexual maturity. 



