258 THE EVOLUTION OF CONTINUITY 



the second polar body. From the half of the spindle which 

 remains within the yelk after the extrusion of the second 

 polar body, a small nucleus is formed which retires towards 

 the centre of the yelk and remains as the nucleus of the 

 unimpregnated ovum, the female pronucleus of V. Beneden, 

 a structure comparable in size rather to the germinal spot 

 than to the germinal vesicle." (Cleland and Mackay.) 



After polar extrusion the final nucleus of the ovum is 

 ready to unite with the male pronucleus ; this latter being 

 what remains (it is reasonable to suppose) after the sperma- 

 tozoal head has itself performed what seems to be a form 

 of polar extrusion. For on entering the ovum the sperma- 

 tozoon first of all loses its tail, and then a small globule, 

 " the seminal granule," is discarded by what remains. The 

 result is the male pronucleus. The protoplasm surrounding 

 this then assumes a markedly radiating appearance, and 

 the male and female pronuclei approach each other, fusion 

 takes place, and fertilisation is complete. 



Polar extrusion apparently represents the second modi- 

 fication of karyokinesis occurring in the growth-cycle, the 

 first being reduction of chromosomes. For the phenomena 

 seem clearly to be karyokinetic. Achromatic poles oppose 

 each other and draw to themselves their subject chromo- 

 somes, though this is done in a simpler manner than in 

 ordinary karyokinesis. There is no monaster formation, 

 and apparently no loops are formed, but straight rods which 

 lie between the poles in the lines of attraction revealed by 

 the spindle, and which divide transversely at the equatorial 

 plane. 



When the two pronuclei approach each other a new kind 

 of spindle formation occurs along the lines of mutual attrac- 

 tion. It is exactly the opposite of that exhibited in karyo- 

 kinesis, for the pronuclei represent, so to speak, dissimilar 

 poles, and the lines of force pass directly without break 

 from the male pronucleus to the female, exactly as repre- 

 sented in Fig. 87 of two dissimilar magnetic poles facing 

 each other. 



The true significance of polar extrusion is not known, 

 but it is reasonable to regard the phenomena as those of 

 two final acts of cell-division. It is not equal division in 



