THE CHROMOSOMES 131 



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, as in e. The 

 first polar body may also divide. The three polar 

 bodies and the egg, /, are comparable to the four 

 spermatozoa. All four spermatozoa are functional, 

 but onl}" 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 preceding account of the maturation of the 

 egg and of the sperm w^ere accepted as covering the 

 entire behavior of the chromosomes during this 

 period, there w^ould 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 

 thiii threads are coming together (Fig. 43, 6, /, 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, J5, the chromosomes are represented as a linear 



