THE CELLULAR BASIS 143 



In this way the number of chromosomes in 

 the mature egg or sperm comes to be one-half 

 the number present in other kinds of cells, 

 and when the egg and sperm unite in fertili- 

 zation the whole number is again restored. 

 The double set of chromosomes is known as 

 the diploid number, the single set as the hap- 

 loid number, and the maturation division in 

 which this reduction from the double to the 

 single set takes place is the reduction division. 

 It is generally held that this reduction takes 

 place in the first of the two maturation di- 

 visions (Fig. 32, C, D), and that the second 

 of these divisions is like an ordinary mitosis 

 in that each chromosome splits into two and 

 the halves move apart, such a division being 

 known as an equation division (Fig. 32 E), 

 but it is possible that some chromosome pairs 

 undergo an equation division in the first ma- 

 turation mitosis and a reduction division in 

 the second, while other chromosome pairs may 

 reverse this order. 



It is an interesting fact that long before 

 the reduction of chromosomes had been actu- 

 allv seen Weismann maintained on theoret- 



