FER TILISA TION. 



61 



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. 



cs 



Fig. 28. — Fertilisation in Ascaris megalocephala, 

 — After Boveri. 



Spermatozoon (sp.) entering ovum, which contains reduced nucleus 

 (JV), having given off two polar bodies (p.b. i and 2). 



Sperm nucleus (the upper), and ovum nucleus (N), each with two 

 chromatin elements or idants, and with centrosom^s (c.j.). 



Centrosomes (c. s.) with " archoplasmic " threads radiating outwards 

 in part to the chromosomes of the two approximated nuclei. 



Segmentation spindle before first cleavage. 



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 



