THE CELL. 33 



changes which are spoken of collectively as egg ripening or the 

 maturation of the egg. These changes consist of the so-called 

 reduction of chromosomes. The process of ripening may go on 

 and be completed before the spermatozoon enters the egg or 

 indeed approaches it. This is not the same in different animals. 

 A similar reduction of chromosomes takes place in the sperma- 

 tozoa, during their formation from the so-called spermatogonia, 

 which will be discussed in its proper place. Here it is only 

 necessary to say that the spermatozoon is a flagellated cell which 

 possesses all the essential constituents of other cells. The 

 anterior large part of the spermatozoon, the so-called head, 

 represents the nucleus ; the so-called intermediate or middle 

 piece is the centrosome. The flagellum or tail represents the 

 protoplasmic part of the cell. 



The process of maturation and fertilization has been worked 

 out in detail in a considerable number of animals. We shall 

 describe this process as it occurs in Physa fontinalis, a mollusk, 

 in which the clearness of the microscopical picture allows of 

 exact observation of both processes in all their minuteness 

 (Kostanecki and Wierzejski). Here the process of maturation 

 does not take place until after the entrance of the spermatozoon 

 into the egg ; so that the so-called extrusion of both polar bodies 

 occurs simultaneously with the first stages of the true process of 

 -fertilization. 



The process of maturation consists of two unequal karyo- 

 kinetic divisions of the egg cell, which have all the character- 

 istics of the cell division described above (Fig. 12). The kary- 

 okinetic figure now moves toward the surface of the egg, and in 

 consequence of the splitting of the chromosomes (metakinesis) 

 the mother star is transformed into two daughter stars. A 

 round elevation is formed on the surface of the egg, and this 

 becomes occupied by one-half of the chromosomes and a centro- 

 some with one-half of the central spindle (polar spindle). By 

 the formation of an intermediate body the separation of the first 

 polar body is completed (Fig. 13). 



This same process is gone through again in the following 

 way. When the first polar body is not entirely separated off 



