i8o SOME RECENT DEVELOPMENTS 



fuses with part of the spermatozoon, this fusion 

 constituting the act of fertilisation. Auerbach was 

 the first to show, in 1874, that in the egg of 

 Ascaris two nuclei may be seen shortly after the 

 spermatozoa reach the egg, and that these two 

 nuclei fuse together. In 1875 Oscar Hertwig 

 stated, as the result of a careful series of observa- 

 tions on the eggs of Echini, that one of the two 

 nuclei seen by Auerbach is the head of the sper- 

 matozoon, and that the other is part of the nucleus 

 or germinal vesicle of the egg, probably the nucleolus 

 or germinal spot. In 1875 and 1876 van Beneden 

 saw the fusion of two nuclei, or pronuclei as he 

 named them, in the ovum of the rabbit, as a result 

 of fertilisation ; one of these pronuclei he found to 

 arise from the egg nucleus, though not as Hertwig 

 had supposed from the nucleolus: the other, or 

 male pronucleus, van Beneden recognised as in 

 some way connected with the spermatozoon, though 

 he failed to trace it directly to the head of the 

 spermatozoon as Hertwig had done in the sea 

 urchins. These discoveries naturally attracted 

 great attention, as they for the first time rendered 

 possible a theory of fertilisation based on the 

 changes actually known to occur during the pro- 

 cess. A number of other investigators, prominent 

 among them being Fol, Greeff, Selenka, Flemming, 

 Hensen, and Boveri, studied the phenomena in 

 various groups of animals and definitely established 

 the fact that the essence of the act of fertilisation 

 consists in fusion of part of the egg nucleus with 

 the head or nucleus of the spermatozoon i.e. t 



