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 respectively until the divi- 

 sion 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). 



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 



