302 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



(c,d,e),ea,ch 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 (/, g). It will be noted that the 



FIG. 50. Diagram to illustrate fertilization; 3, male pronucleus; ?, female 

 pronucleus; observe that the chromosomes of maternal and paternal origin 

 respectively do not fuse. (From Gityer.) 



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 



