62 REPRODUCTION AND LIFE HISTORY. 



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 maturation there is a conjugation 

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

 maternal and paternal origin. 



In the reducing, meiotic, or maturation division each daughter-cell 

 gets one or the other member of each pair of homologous chromosomes. 



In the case of the ovum the meiotic division usually occurs in the 

 formation of the first polar body, so that it and the reduced nucleus of 



the ovum have each chromosomes. There is no further reduction in 



2 . 



the formation of the second polar body, which involves an ordinary 

 equation-division. The first polar body often divides into two. Thus 

 the result is one viable cell (the mature ovum) and three non-viable 



cells (the polar bodies), each with ^ chromosomes. 



In the spermatogenesis or production of spermatozoa the meiotic 

 division is usually the second-last. A " mother-sperm cell " or 

 spermatogonium divides into spermatocytes with n chromosomes, each 



of these divides into 2 spermatocytes with ~ chromosomes, and these 



again divide into spermatocytes which differentiate into spermatozoa. 

 The result is that from each of the penultimate generation of spermato- 

 cytes there arise four spermatozoa, each with * chromosomes. Thus 



there is a close parallelism in the maturation process in the two sexes. 

 That the fertilisation of the ovum restores the number to the normal n 

 is obvious. 



Part of the significance of meiotic division is that it affords opportunity 

 for fresh permutations and combinations of hereditary qualities, for 

 chromosomes are the bearers of at least some of these. 



It is important to understand that in ordinary mitosis or cell-division, 

 each daughter-cell gets an absolutely similar half of each chromosome 

 of the mother-cell, whereas in meiotic division the daughter-cells get 

 dissimilar halves. 



If we compare the nucleus and its chromosomes to such a common- 

 place thing as a box of matches we may make the difference between 

 the two kinds of division obvious. We might halve the matches by 

 putting half of them into another box (meiotic division) ; or we might 

 take a knife and split each match longitudinally and put one of the sets 

 of halves into another box (ordinary equation division). 



Fertilisation. In the seventeenth and eighteenth cen- 

 turies, some naturalists, nicknamed "ovists," believed that 



