FERTILISATION. 63 



In the course of spermatogenesis as well as of oogenesis, it has been 

 shown by van Beneden, O. Hertwig, and others that the number of 

 chromosomes is reduced to one half the number characteristic of the body 

 cells of the animal. Suppose that number be 4, the mature ovum has 

 only 2 and the mature sperm has only 2. Thus the fertilisation 

 process does not double the number of chromosomes, as it would do 

 if there were no reducing divisions. It seems likely that the reduction 

 process is of advantage in affording opportunity for new permutations 

 and combinations in the hereditary qualities, of which the chromosomes 

 are probably the vehicles. It is one of the vital complications which 

 make for variation. 



Fertilisation. In the seventeenth and eighteenth cen- 

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

 the following are the most important : 



(1) 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. 



(2) The union of spermatozoon and ovum is very intimate ; 

 the nucleus of the spermatozoon and the reduced nucleus of 

 the ovum approach one another, combining to form a unified 

 nucleus. 



(3) When this combined or segmentation nucleus begins 



