PER TILISA TION. 63 



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 : 



^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. 



(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) The ovum centrosome disappears before fertilisa- 

 tion, and it is a centrosome introduced by the spermato- 

 zoon that divides into the two which play an important 

 role in the cleavage or segmentation of the fertilised 

 ovum. 



(4) When the combined or segmentation nucleus begins 

 the process of development by dividing, each of the two 

 daughter nuclei which result consists partly of material 

 derived from the sperm nucleus, partly of material derived 

 from the ovum nucleus. In other words, the union is 

 orderly as well as intimate, and the subsequent division is 

 so exact*, that the qualities marvellously inherent in the 

 sperm nucleus (those of the male parent), and in the ovum 



