62 REPRODUCTION AND LIFE HISTORY OF ANIMALS. 



recognised that both kinds of reproductive elements were 

 essential, many thought that their actual contact was un- 

 necessary, that fertilisation might be affected by an aura 

 seminalis. Though spermatozoa were distinctly seen by 

 Hamm and Leeuwenhoek in 1677, 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 



FIG. n. Fertilisation in Ascaris megalocephala. 

 (After BOVERI.) 



1. Spermatozoon (s.) entering ovum, which contains reduced 

 nucleus (A^), having given off two polar bodies (/.. i and 2). 



2. Sperm nucleus (n), and ovum nucleus (A r ), each with two 

 chromatin elements or idants, with centrosomes (c.s.). 



3. Centrosomes (c.s.) with " archoplasmic " threads radiating out- 

 wards, in part to the chromosomes of the two approximated nuclei. 



4. Segmentation spindle before first cleavage. 



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 pathological. It is said, however, 



