164 THE CHROMOSOMES 
sends out half of its chromatin into the first polar 
body (Fig. 46, c). Without a resting stage a new 
spindle is formed around the chromosomes in the egg 
and a second polar body is thrown off, asin e. The 
first polar body may also divide. The three polar 
bodies and the egg, f, are comparable to the four 
spermatozoa. All four spermatozoa are functional, 
but only one product of the two divisions of the egg 
is functional. Unless the tetrad is specifically 
oriented upon the polar spindle of the egg the chance 
is equally good that any one of the four threads that 
make up the tetrad will be the one that remains in 
the egg. 
CROSSING OVER 
If the traditional view of the maturation of the 
egg and of the sperm were accepted as covering the 
entire behavior of .the chromosomes during this 
period, there would be no possibility for an interchange 
between the members of a pair. But there are 
several stages in the ripening of the germ cells when 
an interchange between homologous chromosomes 
might possibly take place. For instance, when the 
thin threads are coming together (Fig. 43, e, f, g, h) 
several observers have described them as twisting 
around each other (synaptic twisting) as represented 
in these figures. If where the threads cross a part of 
one thread becomes continuous with the remainder of 
the other thread (Fig. 24) an interchange of pieces 
will have been accomplished. If, as shown in Fig. 
24, B, the chromosomes are represented as a linear 
