726 LIFE : OUTLINES OF GENERAL BIOLOGY 



torially into two. Meanwhile the reduced nucleus of the egg-cell, 

 without the resting phase which usually follows a division, gives 

 off equatorially another polar body, usually called the second polar 

 body. Thus the oocyte with n chromosomes has given rise to four 



cells with - chromosomes: (1-2) the first polar body and its neigh- 

 bour if it divides, (3) the second polar body, and (4) the ripe ovum 

 whose reduced nucleus retires from the periphery to the centre, 

 and is known as the female pronucleus. The ripe ovum is the only 

 viable cell of the four — a contrast to what occurs in spermatogenesis. 

 The typical history of an ovum includes the extrusion of polar 

 bodies and the associated reduction of the chromosomes to half the 

 normal number, but there is exceptional behaviour in partheno- 

 genesis. In some parthenogenetic ova, only one polar body is formed, 



Fig. 114. 



Reducing or Meiotic Division. The diploid number of eight chromosomes in 

 cell I, is reduced to the haploid number of four chromosomes in each 

 of the cells ia and ib. 



and there is no reduction in the number of chromosomes. In some 



other cases the parthenogenetic ovum exhibits the usual reduction 



division and forms two polar bodies. But the second of these is not 



set adrift ; it remains within the ovum and reunites with the reduced 



nucleus, thus restoring the normal number of chromosomes. In 



ordinary cases the large fact is that the ripe ovum and the ripe 



n 

 spermatozoon contain, as the result of meiosis, the haploid or - 



number of chromosomes. 



REDUCING OR MEIOTIC DIVISION.— If it were not for the re- 

 ducing division during maturation, the number of chromosomes would 

 be doubled at each fertiHsation, which is absurd. But the reducing 

 division has a deep biological significance of its own, and this 

 makes it desirable to go into some detail as to what occurs. It may 

 be enough, however, to keep to what happens in a large number of 

 cases. 



