THE CARRIERS OF THE HERITAGE 225 



stant in each germ-cell respectively until the division 

 of second spermatocytes into spermatids which are 

 subsequently transformed into spermatozoa, and of the 

 second oocytes into mature eggs and second polar cells, 

 when it is reduced to one half the normal number. As 

 spermatozoan and mature egg unite in fertilization, 

 the original number of chromosomes is restored in the 

 fertilized egg (zygote). 



7. FERTILIZATION 



The stages concerned in a typical case of fertiliza- 

 tion, according to Boveri, are illustrated in Figures 

 59 to 67. 



In Figure 59 the "head" and the "middle piece" 

 of the sperm-cell have penetrated into the egg cyto- 

 plasm, while in Figure 60 the tail of the sperm-cell 

 has become lost and the middle piece, which furnished 

 the centrosome, has rotated 180 so that it lies between 

 the nucleus, or head, of the sperm-cell and that of the 

 egg-cell. Figure 61 shows an increase in the size of 

 the sperm nucleus and a division of the centrosome into 

 two parts which begin to migrate towards the poles. 

 This process of polar migration of the centrosomes 

 is carried further in Figure 62 as well as the increase in 

 the size of the sperm nucleus, until in Figure 63 the 

 process is complete so that the centrosomes have as- 

 sumed a polar position and the sperm nucleus is equal 

 in size to the egg nucleus and lies in contact with it. 

 In Figure 64 the chromatin network of the two nuclei 

 has formed into an equal number of chromosomes which 



