24 GENETICS 



ing cell), while the fertilized egg which is formed by 

 the union of two gametes (mature egg- and sperm- 

 cell), and which consequently has the characteristic 

 number of chromosomes, is called a zygote (yoked 

 cell). 



A diagrammatic representation of the process of 

 maturation is shown in Figure 14. 



The number of chromosomes (not shown in the 

 diagram) remains constant in each germ-cell respec- 

 tively until the division of spermatids into sperma- 

 tozoa, 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 chromo- 

 somes is restored in the fertilized egg (zygote) . 



8. Fertilization 



The stages concerned in a typical case of fertiliza- 

 tion, according to Boveri, are illustrated in Figures 

 15 to 23. 



In Figure 15 the "head" and the "middle piece" 

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

 plasm, while in Figure 16 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 17 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 18 as 



f JWPEMT UBRARt 



