B 



D 



G 



I 



fl 



IF 



fl 



H 



L 



I u 



J 



Figure i8. Diagram to illustrate the nature of the changes in a 

 pair of chromosomes as germ cells are formed. A, The two chromosomes 

 (one of paternal origin, the other of maternal origin) are paired. B, 

 They have shortened and thickened. C, Each has split lengthwise. D, 

 After further shortening and thickening, the four lobes are pressed 

 together to form a tetrad. E, F, Cell division has occurred ; two of the 

 lobes have passed into the cell E, two others into the cell F. G, H, I, J, 

 A second division has occurred : one lobe has passed into each of the 

 four germ cells G, H, I, J, where they transform into chromosomes of 

 the usual type. Thus each germ cell contains either the maternal or the 

 paternal chromosome of the pair. 



49 



