=Fertilization.--=In fertilization (Fig. 12, p. 48) the spermatozoon penetrates the wall of the ovum and after undergoing considerable alteration its nucleus fuses with the nucleus of the egg. In some forms only the head (nucleus) and middle-piece enter, the tail being cut off by a so-called fertilization membrane which forms at the surface of the egg and effectually blocks the entrance of other spermatozoa. Thus normally only one spermatozoon unites with an egg. In some forms while several may enter the egg only one becomes functional. As soon as the nucleus of the spermatozoon, now known as the male _pronucleus_, reaches the interior of the egg, it enlarges and becomes similar in appearance to the female _pronucleus_. It swings around in such a way (Fig. 12_b_, p. 48) that the middle piece, now transformed into a centrosome, lies between it and the female pronucleus. The two pronuclei (_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 individual chromosomes do not intermingle their substance at this time, but that each apparently retains its own individuality. There is considerable evidence which indicates that throughout life the chromosomes 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. [Illustration: FIG. 12 Diagram to illustrate fertilization; [male], male pronucleus; [female], female pronucleus; observe that the chromosomes of maternal and paternal origin respectively do not fuse.] =Significance of the Behavior of the Chromosomes.--=The question confronts us as to what is the significance of this elaborate system which keeps the chromosomes of constant size, shape and number; which partitions them so accurately in ordinary cell-divisions; and which provides for a reduction of their numbers by half in the germ-cell while yet securing that each mature gamete gets one of each kind of chromosome. Most biologists look on these facts as indicating that the chromosomes are specifically concerned in inheritance.