FUSION OF PRONUCLEI 259 



angles to the surface of the egg and gradually works its way 

 through the vitelline membrane until its head lies within the 

 egg protoplasm (D, sp). The tail is then cast off, and the 

 head, penetrating deeper into the protoplasm, takes on the 

 form of a rounded nucleus-like body, the male pronudeus 

 (E 1 , pr. nu. ) 



The two pronuclei, each surrounded, in some cases at any 

 rate, with an investment of protoplasm, approach one 

 another (E 1 , E 2 ) and finally unite to form the single nucleus 

 (p 1 F 3 ) of what is now not the ovum but the oosperm the 

 impregnated egg or unicellular embryo. The fertilizing 

 process is thus seen to consist of the union of two nuclear 

 bodies, one contributed by the male gamete or sperm, the 

 other by the female gamete or ovum. It follows from this 

 that the essential nuclear matter or chromatin of the oosperm 

 is derived in equal proportions from the two parents. 



In connection with each pronucleus is a sun-like figure 

 (E 1 , E 2 ) produced by part of the cell-protoplasm becoming 

 arranged in the form of threads radiating from a common 

 centre as in the ordinary dividing cell (p. 64) : after the 

 fusion of the pronuclei one of these " suns " is found at each 

 end of the resultant oosperm-nucleus (r 1 ). The radiating 

 threads appear to have an important action in connection 

 with the characteristic movements of the chromatin during 

 karyokinesis (Fig. 10, p. 64). 



The precise mode of union of the two pronuclei is even 

 now hardly certain : some observations seem to show that it 

 consists in an interweaving of the two chromatin filaments 

 (p 1 p 3 ) : others appear to indicate that an actual fusion of 

 male and female chromatin takes place (G). 



Fertilization being thus effected the process of segmentation 

 or division of the oosperm takes place as described in the 

 preceding lesson (p. 245). 



s 2 



