70 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



from its place of formation, and in favourable conditions 

 meets either within or outside the body with a spermatozoon 

 from another animal. Before the union between ovum and 

 spermatozoon is effected, generally indeed before it has 

 begun, the nucleus or germinal vesicle of the ovum moves 

 to the periphery and divides twice. This division results in 

 the formation and extrusion of two minute cells or polar 

 bodies, which come to nothing, though they may linger for 

 a time in the precincts of the ovum, and may even divide. 

 The second division follows the first without the inter- 

 vention of the " resting stage " which usually succeeds a 

 nuclear division. In most cases the division which forms 

 the first polar body is a reducing or meiotic division, the 

 number of chromosomes being reduced to half the number 

 characteristic of the cells of the body. The extrusion of 

 polar globules and the associated reduction is almost 

 universal in the history of ova, but in some parthenogenetic 

 ova only one polar body is formed, and there is no reduc- 

 tion in the number of chromosomes. In some other cases 

 the parthenogenetic ovum passes through the meiotic 

 phase and forms two polar bodies. The second of these, 

 however, is not liberated, but remains within the ovum and 

 re-uniting with the reduced nucleus restores the normal 

 number of chromosomes. 



Reducing or Meiotic Division. — In each kind of 

 animal there is a definite number of chromosomes, say n, 

 in each of the body-cells. In the ripe germ-cells, however, 



there is half the normal number, ", so that when spermato- 

 zoon and ovum unite in fertilisation the normal number is 

 restored. 



In the history of the germ-cells, therefore, in one way or 

 another, at one stage or another, the number of chromo- 

 somes undergoes reduction to half the normal. In many 

 cases this reduction comes about through a " heterotypic " 

 or meiotic division. We give a condensed account of what 

 happens in a large number of cases. 



The immature germ-cells, whether oocytes or spermatocytes, show n 

 chromosomes, half of which are of paternal, and half of maternal, origin. 

 At a certain stage in the ripening or inatnration there is a conjugation 

 of the chromosomes in pairs, and the two forming a pair seem to be of 

 maternal and paternal origin. 



