72 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



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 meiotir division is that it affords oppor- 

 tunity 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 each daughter-cell gets 

 half of the total number. 



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 naturaUsts, nicknamed " ovists," beHeved that 

 the ovum was all-important, only needing the sperm's 

 awakening touch to begin unfolding the miniature model 

 which it contained. Others, nicknamed " animalculists," 

 were equally confident that the sperm was essential, though 

 it required to be fed by the ovum. Even after it was 

 recognised that both kinds of reproductive elements were 

 essential, many thought that their actual contact was un- 

 necessary, that fertilisation might be effected by an aura 

 seminalis. Though spermatozoa were distinctly seen by 

 Hamm and Leeuwenhoek in 1679, their actual union with 

 ova was not observed till 1843, when Martin Barry detected 

 it in the rabbit. 



Of the many facts which we now know about fertilisa- 

 tion, the following are the most important : — 



(i) Apart from the occurrence of parthenogenesis in a 

 few of the lower animals, an ovum begins to divide only 

 after a spermatozoon has united with it. After one sper- 

 matozoon has entered the ovum, the latter ceases to be 

 receptive, and other spermatozoa are excluded. If, as 

 rarely happens, several spermatozoa effect an entrance into 

 the ovum, the result is usually some abnormality. It is 

 said, however, that the entrance of numerous spermatozoa 

 (polyspermy) is frequent in insects and Elasmobranch fishes. 



