Genetics 



171 



X-chromosome. Below this, the diagrams of the first (unequal) and the 

 second (equal) spermatocyte divisions, leading to the ultimate sperm- 

 cell with three chromosomes and a small degenerate cell with two. 

 When the egg and sperm again unite in fertilization, the original six 

 chromosomes are restored, and the egg is again as we see it at the top. 

 From what has been said, it can be plainly seen that in all organ- 



^> 

 ft 



io«tv @@ 



9 



o 



% 



£)•••• 



(7 



Fig. 83. 



Diagram of chromosome-cycle of Phyllaphis coweni. 

 (After Doncaster.) 



See text for explanation. 



isms where there is an X-chromosome, this extra particle (as it does not 

 divide as do the other chromosomes) must result in some sperm-cells 

 having an even number of chromosomes and others an odd number. 



For example, let us say there are twenty-one chromosomes in the 

 original germ-cell from which the sperm is to develop. One of the newly 



