FERTILIZATION OF THE OVUM. 39 



the male element it undergoes certain changes, called matura- 

 tion. The nucleus of the ovule divides and gives off a portion 

 of its substance, which forms a small body, the polar body, 

 which proceeds to the surface of the cell and is there ex- 

 truded. This process is repeated, a second polar body being 

 given off from the nucleus and expelled from the cell in the 



FIG. 10. 



Maturing ovum of physa (fresh-water snail) (Kostanecki and Wierzejski). Above 

 is the first polar body extruded from the ovule and dividing into two bodies. 

 Just beneath it is the nucleus of the ovule, dividing to form the second polar 

 body. At the centre of the ovule is the male pronucleus with its centrosome 

 and attraction-sphere, derived from the spermatozoon. 



same way. By this process the nucleus is " reduced/' or 

 divested of a portion of its chromatin, so that when the chro- 

 matin of the spermatozoon, also reduced, is united with it the 

 number of chromosomes normal to the cells of the species is 

 present. The portion of the original nucleus now remaining 

 in the ovule is called the female pronucleus. 



Fertilization of the ovum: The ovule is not capable of under- 



