FERTILISATION. 57 



vesicle is somehow a necessary preparation for its union with the fertilis- 

 ing nucleus of the sperm. 



In the differentiation of some male-cells, processes somewhat analogous 

 to the formation of polar bodies have been observed. 



Fertilisation. — Inthe seventeenth and eighteenth centuries, 

 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 con- 

 tained. 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 unnecessary, that fertilisation 

 might be effected by an aura seminalis. Though sperm- 

 atozoa were distinctly seen by Hamm and Leeuwenhoek in 

 1677, their actual union with ova was not observed till 

 1843, when Dr. 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 sperm- 

 atozoon 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 pathological. 



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

 the nucleus of the spermatozoon and the modified nucleus 

 of the ovum move or are drawn to one another,- combining 

 in a very orderly way to form a single nucleus. 



(3.) When this combined or segmentation nucleus starts 

 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 so marvellously inherent in the 

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

 nucleus (those of the mother animal), are diffused through- 

 out the body of the offspring, and persist in its reproductive 

 cells. 



