302 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
(c,d,e), each containing the reduced number of chromosomes, approach, 
the centrosome divides, the nuclear walls disappear, the typical 
division spindle forms, and the chromosomes of paternal and maternal 
origin respectively come to lie side by side at the equator of the spindle 
ready for the first division or cleavage (f,g). It will be noted that the 
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Fic. 50.—Diagram to illustrate fertilization; ¢, male pronucleus; ¢, female 
pronucleus; observe that the chromosomes of maternal and paternal origin 
respectively do not fuse. (From Guyer.) 
individual chromosomes do not intermingle their substance at this 
time, but each apparently retains its own individuality. There is 
considerable evidence which indicates that throughout life the chro- 
mosomes contributed by the male parent remain distinct from those 
of the female parent. Inasmuch as each germ-cell, after maturation, 
contains only half the characteristic number of chromosomes, the 
original number is restored in fertilization. 
Significance of the behavior of chromosomes.—The question 
confronts us as to what is the significance of this elaborate system 
